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Tweet MyronLMeters.com attempts to provide its customers with the latest in water quality research and industry updates. Find more at https://www.myronlmeters.com/. Abstract This research was conducted essentially to treat fresh peat water using a series of adsorbents. Initially, the characterization of peat water was determined and five parameters, including pH, colour, COD, turbidity, and iron ion [...]
MyronLMeters.com attempts to provide its customers with the latest in water quality research and industry updates. Find more at https://www.myronlmeters.com/.
This research was conducted essentially to treat fresh peat water using a series of adsorbents. Initially, the characterization of peat water was determined and five parameters, including pH, colour, COD, turbidity, and iron ion exhibited values that exceeded the water standard limit. There were two factors influencing the adsorption capacity such as pH, and adsorbent dosages that were observed in the batch study. The results obtained indicated that the majority of the adsorbents were very efficient in removing colour, COD, turbidity at pH range 2-4 and Fe at pH range 6-8. The optimum dosage of cationic surfactant modified zeolite (CSMZ) was found around 2 g while granular activated carbon (GAC) was exhibited at 2.5 g. In column study, serial sequence of CSMZ, GAC, and limestone showed that the optimal reduction on the 48 hours treatment were found pH = 7.78, colour = 12 TCU, turbidity = 0.23 NTU, COD = 0 mg/L, and Fe= 0.11 mg/L. Freundlich isotherm model was obtained for the best description on the adsorption mechanisms of all adsorbents.
Keywords: cationic surfactant modified zeolite, granular activated carbon, limestone, peat water
Water is essential and fundamental to all living forms and is spread over 70.9% of the earth’s surface. However, only 3% of the earth’s water is found as freshwater, of which 97% is in ice caps, glaciers and ground water (Bhatmagar & Minocha, 2006). In Malaysia, more than 90% of fresh water supply comes from rivers and streams. The demand for residential and industrial water supply has grown rapidly coupled with an increase in population and urban growth (WWF Malaysia, 2004). Water demand in affected populations such as rural areas also demands that attention is paid to providing more sustainable solutions rather than transporting bottled water (Loo et al., 2012). For this reason, it is essential to ensure availability of local sources of water supply and even develop new potential sources of water such as from peat swamp forest to overcome future water shortages.
River water surrounded by peat swamp forest is defined as peat water and is commonly available as freshwater since it has a low concentration of salinity. The previous study shows that peat swamp forest has high levels of acidity and organic material depending on its region and vegetation types (Huling et al., 2001). Under natural conditions, tropical peat lands serve as reservoirs of fresh water, moderate water levels, reduce storm-flow and maintain river flows, even in the dry season, and they buffer against saltwater intrusion (Wosten et al., 2008).
Due to the acidity and high concentration of organic material, selective treatment of peat water must be conducted prior to its use as water supply. Recently, many methods have been designed and have proven their effectiveness in treating raw water such as coagulation and flocculation (Franceschi et al., 2002; Liu et al., 2011; Syafalni et al., 2012a), absorption (Ćurković et al., 1997), filtration (Paune et al., 1998) and combining (Hidaka et al., 2003). Careful consideration of the most suitable method is important to ensure that the adsorption process is the most beneficial, economically feasible method as well as easy to operate for producing high quality of water in a particular location.
Many researchers have shown that activated carbon is an effective adsorbent for treating water with high concentrations of organic compounds (Eltekova et al., 2000; Syafalni et al., 2012b). Its usefulness derives mainly from its large micropore and mesopore volumes and the resulting high surface area (Fu & Wang, 2011). However, its high initial cost makes it less economically viable as an adsorbent. Low cost adsorbent such as zeolite nowadays has been explored for its ability in many fields especially in water treatment. Natural zeolite has negative surface charge which gives advantages in absorbing unwanted positive ions in water such heavy metal. These ions and water molecules can move within the large cavities allowing ionic exchange and reversible rehydration (Jamil et al., 2010). The effectiveness of zeolite has been improvised by modified zeolite with surfactant in order to achieve higher performance in removing organic matter (Li & Bowman, 2001). Among tested cationic surfactants, hexa-decyl-tri-methyl ammonium (HDTMA) ions adsorbed onto adsorbent surfaces are particularly useful for altering the surface charge from negative to positive (Chao & Chen, 2012). Surfactant modified zeolite has been shown to be an effective adsorbent for multiple types of contaminants (Zhaohu et al., 1999).
Zeolite is modified to improve its capability of exchanging the anion by cationic surfactants, called CSMZ. CSMZ adsorbs all major classes of water contaminants (anions, cations, organics and pathogens), thus making it reliable for a variety of water treatment applications (Bowman, 2003). Nowadays, interest in the adsorption of anions and neutral molecules by surfactant modified zeolite has increased (Zhang et al., 2002). Modification of zeolite by surfactant is commonly done by cationic or amphoteric surfactants. By introducing surfactant to the zeolite, an organic layer is developed on the external surfaces and the charge is reversed to positive (Li et al., 1998). However, the present study used zeolite that had been modified using Uniquat (QAC-50) as cationic surfactant (CSMZ) and their performance towards the removal of color, COD, turbidity and iron ion from peat water were investigated.
Four adsorbents were used in these experiments which are natural zeolite, zeolite modified by cationic surfactant, activated carbon and limestone. All adsorbents were prepared with equivalent sizes of 1.18 mm – 2.00 mm. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used for polishing zeolite during the preparation phase and for pH adjustment of the sample. Furthermore, potassium dichromate (K2CrO7), silver sulphate (Ag2SO4), sulphuric acid (H2SO4) and mercury (II) sulphate (HgSO4) were used as digestion solution reagents and acid reagents for COD analysis. Lastly, Uniquat (QAC-50) was used as cationic surfactant to modify the zeolite.
2.1 Preparation of Surfactant Modified Zeolite
In these studies, 100 g of prewashed natural zeolite was contacted with 5.6 ml/l Uniquat (QAC-50) as cationic surfactant (CSMZ). The mixture was then stirred at room temperature for 4 hours at 300 rpm (Karadag et al., 2007). The zeolite then was filtered and washed with distilled water several times. After that, the absorbent was dried in an oven at a temperature of 105 °C for 15 hours.
2.2 est Procedures
2.2.1 Batch Studies
Serial batch studies were conducted at room temperature (28 ± 1 °C) to investigate the influence of pH and dosage for removing colour, COD, turbidity and iron ion from peat water. Shaking speed of 200 rpm for 20 minutes were fixed and operated respectively. A working volume of 150ml peat water sample was set up in 250 ml conical flasks. Preceding the batch studies, initial concentration for those parameters was determined. The optimum pH and dosage of absorbent were determined. Subsequently, the percentage of removal was finally determined, plotted, and compared.
2.2.2 Batch Column Studies
Column studies were carried out using a plastic column with dimensions: 5.4 cm diameter and 48 cm length. Three adsorbents were filled inside the column at a specific depth with the supporting layers of marbles, cotton wool, and perforated net. Total volume of 2000 ml peat water was pumped in the up flow mode from the vessel into the column by using a Masterflex peristaltic pump at a minimum flow rate of (30, 60, 90) ml/min. In this study, however, column studies were performed un-continuously (batch) due to limitations of time. All parameters related to the column design are summarized in the following Table 1.
Table 1. Column studies parameters
|Horizontal Surface Area, A
|Column volume, V
||30, 60, 90
|Surface Loading Rate, SLR= Q/A
||1.31, 2.62, 3.93
The serial sequence arrangements of adsorbents were conducted as shown in Figure 1 below. Effluent samples were collected at various time intervals, whilst maintaining room temperature, and analysed.
Figure 1. Schematic diagrams of lab-scale column studies
3. Results and Discussion
3.1 eat Water Characterization
Surface water originating from the peat swamp forest was taken from Beriah peat swamp river along the Kerian River on several occasions as the main sample. The characterization of peat water was carried out at the sampling point (in-situ measurement) using a multi-parameter probe as well as in the environmental laboratory of civil engineering, USM. Fundamentally, the characterization procedures were based on the Standard Methods for the Examination of Water and Wastewater (APHA, 1992). Table 2 represents the peat water characteristics in average value and the comparison to the standard drinking water quality in Malaysia.
Table 2. The characteristics of peat water sample from Beriah Peat Swamp Forest
|4.67 – 4.98
Thirteen parameters were successfully determined where the first six parameters, including pH, temperature, TDS, DO, conductivity, and salinity were measured at the sampling point, whilst the rest of the parameters, including colour, turbidity, COD, iron ion, Ammoniacal Nitrogen, NH3-N, Ammonia (NH3), and Ammonium (NH4+) were examined from the sample brought to the environmental laboratory on the same day.
Acidic pH of the peat water was predicted due to the composition of the surrounding peat soil itself which had been formed by decaying material possessing humic substances (Rieley, 1992). Besides that, humic substances also lead to the high organic content as humic substances are comprised of numerous oxygen containing functional group and fractions (humic acid, fulvic acids and humin) with different molecular weights which mean yielding high concentration of turbidity and COD as well as coloured water (Torresday et al., 1996). Moreover, composition of peat soil may also have an impact on the iron ion concentration of peat water (Botero et al., 2010).
From the thirteen parameters, five parameters were indicated exceeding the standard limit. These parameters were pH, colour, turbidity, COD, and iron ion that showed values of 4.67 – 4.98, 224.7 TCU, 20.8 NTU, 33.3 mg/l, and
1.24 mg/l respectively while the standard limit of these parameters are 6.5 – 9.0, 15 TCU, 5 NTU, 10 mg/l, and 0.3 mg/l accordingly.
3.2 Effect of Initial pH on the Efficiency of Colour, COD, Turbidity, and Iron Ion (Fe) Removal
Influence of initial pH on the adsorption capacity for removing colour, COD, turbidity, and iron ion were investigated.
Figure 2(a) to Figure 2(d) below, displayed the percentage removal of colour, COD, turbidity, and iron ion against pH of adsorbents respectively.
Figure 2(a) shows the maximum removal percentage of colour that was removed by natural zeolite, CSMZ, and granular activated carbon (GAC) which were 79%, 90%, 82% respectively. This adsorption is depended on the characteristic of adsorbents itself. For zeolite and CSMZ were related to the amount of cationic ions (Al3+) increased, resulting in high reaction activity and GAC was related to the adsorption capacity. It was observed that the adsorption capacity was highly dependent on the pH of the solution, and indicated that the colour removal efficiencies decreased with the increase of solution pH.
The pH of the system exerts profound influence on the adsorptive uptake of adsorbate molecules presumably due to its influence on the surface properties of the adsorbent and ionization or dissociation of the adsorbate molecule. Figure 2(b) represents the percentage removal of natural zeolite and CSMZ where they reach optimum efficiency in removing organic compound (COD) at pH 2 with efficiency of 53% and 60% respectively. Meanwhile, the highest percentage removal of COD for GAC was achieved at pH 4 with efficiency obtained about 61%. Identical trends in colour removal were exhibited in percentage removal of COD for natural zeolite, CSMZ and GAC. In fact, this result also reveals that GAC has the highest percentage removal among natural zeolite and CSMZ yet optimum in difference pH solution. Neutralization mechanism occurs in low pH makes color removal, COD removal and Turbidity removals at pH 2 are higher for most of absorbents in this process.
In Figure 2(c), percentage turbidity removal against pH for each adsorbent revealed that optimal reduction of turbidity was obtained in an acidic environment with efficiency removal of 96%, 98%, 95% for natural zeolite, CSMZ, and GAC respectively. When the pH of the solution was adjusted above pH 6 to pH 12, the tendencies of all adsorption performances were gradually decreased. Moreover, it also showed that the lowest efficiency for the three adsorbents were identified at pH 12 with percentage values removal 55%, 61%, and 59% for natural zeolite, CSMZ, and GAC respectively.
Figure 2(d) demonstrates the removal efficiencies of iron ion as a function of the influent pH. The maximum removal of iron ion was observed at pH 8 for both natural zeolite and CSMZ whereas GAC had its optimum removal at pH 6. Natural zeolite and CSMZ only yielded 73% and 62% removal efficiency while GAC had more significant removal with removal efficiency of 80% to the iron ion concentration. Further, it is evident from the graph that gradual increment of removal efficiency for natural zeolite, CSMZ, and GAC occurred when the initial pH of the solution was increased to higher values. Somehow, at pH values greater than 6 the removal efficiency of GAC reduced slightly while for natural zeolite and CSMZ the reduction occurred from pH values above 8.
3.3 Effect of Adsorbent Dosage on the Efficiency of Colour, COD, Turbidity, and Iron Ion (Fe) Removal
The effect of adsorbent dosage was studied for all adsorbents employed on colour, COD, turbidity, and iron ion removal by varying the dosage of adsorbent and keeping all other experimental conditions constant. The pH was set to acidic conditions which were most favourable in obtaining the highest removal efficiency. In this study, to find optimal adsorbent dosage of natural zeolite and CSMZ, the appropriate experiments were carried out at adsorbent dosages in the range of 0.5 g to 5.0 g while for GAC, the adsorbent dosage was varied from 0.01 g to 4.0
- The experimental results for all the adsorbents are represented by Figure 3(a) to Figure 4(d).
Figure 3. Percentage of color (a), COD (b), turbidity (c), and Fe (d) removal against pH for NZ, and CSMZ
Figure 3(a) displays the relationship between the amount of adsorbent mass (dosage) and adsorption efficiency for natural zeolite and CSMZ in terms of removing colour. The colour removal of peat water increased from about 25% to 52% with increasing adsorbent dosage of natural zeolite from 0.5 g to 3.5 g whereas for CSMZ, removal percentage increased from 41% to 53% with increasing adsorbent dosage from 0.5 g to 2.0 g. However, further increase in adsorbent dosage to 5.0 g only led to slight degradation of removal efficiency to 50% and 41% for natural zeolite and CSMZ respectively. This degradation with further increases in adsorbent dosage was due to the unsaturated adsorption active sites during the adsorption process since the adsorbates in the vessel were only shaken for 20 minutes (insufficient time). Besides, modification of zeolite by cationic surfactant had proven to have better colour removal as presented in the graph.
Percentage removal of COD against the adsorbent dosage is shown in Figure 3(b). It was observed that the highest percentage removal for both natural zeolite and CSMZ to remove COD were 51% and 59%, achieved at adsorbent dosage 3.5 g and 2.0 g respectively.
The variations in removal of turbidity of peat water at various system pH are shown in Figure 3(c). The removal rate of turbidity was highest at the adsorbent dosage of 0.5 g with 70% and 93% removal efficiency for respective natural zeolite and CSMZ. The removal rate showed a smooth downward trend with the increase in adsorbent dosage. Concurrently, the adsorption capacity gradually decreased with the increasing adsorbent dosage. The least efficient removal of turbidity was noted at dosage 5.0 g with percentage removal recorded for natural zeolite and CSMZ only 57% and 70% respectively.
Figure 3(d) demonstrates the percentage iron ion removal of natural zeolite and CSMZ with respect to their dosage. The result shows that there was a significant difference trend in iron ion adsorption efficiencies between natural zeolite and CSMZ. For natural zeolite, it was shown that the removal percentage of iron ion had increased until it reached 1.0g of dosage with 72% of removal efficiency. On the other hands, CSMZ was only able to remove about 63% of iron ion when its dosage was increased to 2.5 g. The lowest percentage removals were 47% and 57% recognized at the adsorbent dosage 5.0 g for respective natural zeolite and CSMZ.
Figure 4. Percentage of color (a), COD (b), turbidity (c), and Fe (d) removal against dosage for GAC
The result illustrated in Figure 4(a) shows the maximum removal percentage of colour for GAC at 2.5 g dosage was 62%. Moderate increment in colour removal was identified along with the addition dosage of 2.5 g whilst abatement of removal efficiency began subsequently at adsorbent dosage of 3.0 g to 4.0 g.
The results from Figure 4(b) indicated that increasing the GAC dosage would increase the efficiency in removing COD respectively. The optimum dosage was recorded at 3.0 g with 72% of removal efficiency. Meanwhile, increasing the dosage above 3.0 g exhibited a slight decrease in removal efficiency with 67% to 61% for COD removal. A better result in removing COD was also shown by GAC compared to the natural zeolite and CSMZ.
The percentage of turbidity removed by GAC in different dosages is described in Figure 4(c). The highest removal was indicated at adsorbent dosage 2.5 g with removal efficiency of 70% while the minimum removal was 52% recorded at the adsorbent dosage 0.01 g. However, starting from adsorbent dosage of 3.0 to 4.0 g, removal efficiency began to decrease to 68%, 67%, and 69% respectively.
The result of percentage removal of iron ion by GAC in peat water is presented in Figure 4(d). It was found that the rate of removal was rapid in the initial dosage between 0.01 g to 3.0 g at which the removal efficiency increased from 28% to 71% accordingly. Subsequently, a few significant changes in the rate of removal were observed. Possibly, at the beginning, the solute molecules were absorbed by the exterior surface of adsorbent particles, so the adsorption rate was rapid. However, after the optimum dose was reached, the adsorption of the exterior surface becomes saturated and thereby the molecules will need to diffuse through the pores of the adsorbent into the interior surface of the particle (Ahmad & Hameed, 2009).
3.4 Batch Column Experiment
On the first running, the column was packed with natural zeolite (1st layer), limestone (2nd layer), and GAC (3rd layer) as shown in Figure 5(a). Removal efficiency for colour, COD, turbidity, and iron ion was recognized to be increased when the contact time was increased. At the time interval 1 hour to 6 hours, however, the increment was not so significant. The removal efficiency at 1 hour treatment was 39%, 21%, 54%, 36% while at 6 hours treatment was 77%, 65%, 73%, 60% recorded for respective colour, COD, turbidity, and iron ion. Poor removal efficiency at 1 hour treatment indicated that the required time to remove all parameters were insufficient. It is evident that if the adsorption process is allowed to run for 24 hours on the column, the removal efficiency shows notable removal. Percentage removals of colour, COD, turbidity, and iron ion at 24 hours were 83%, 72%, 76%, 65% respectively. Furthermore, the highest removal for respective colour, COD, turbidity, and iron ion were obtained at 48 hours treatment with 87%, 81%, 86%, and 79% of removal efficiency.
Figure 5. Percentage removal of color, COD, turbidity, and Fe for 1st run(a), 2nd run(b), and 3rd run (c) at flowrate 30 ml/min
On the second running, the column was packed with CSMZ (1st layer), limestone (2nd layer), and GAC (3rd layer) as presented in Figure 5(b). The removal percentages of colour, COD, turbidity, and iron ion were noticed after 1 hour to be 52%, 49%, 71%, and 30% respectively. The time of contact between adsorbate and adsorbent is proven to play an important role during the uptake of pollutants from peat water samples by adsorption process. In addition, the development of charge on the adsorbent surface was governed by contact time and hence the efficiency and feasibility of an adsorbent for its use in water pollution control can also be predicted by the time taken to attain its equilibrium (Sharma, 2003). Removal efficiency of 90% for colour, 81% for COD, 91% for turbidity, and 57% for iron ion were obtained at 24 hours of contact time.
On the third running, the column was packed with a difference sequence of CSMZ (1st layer), GAC (2nd layer), and limestone (3rd layer) demonstrated in Figure 5(c). It can be seen that the adsorption of these four parameters were slightly rapid at time interval 1 hour to 6 hours treatment. Further gradual increment with the prolongation of contact time form 24 hours to 48 hours has also occurred. Observation at 1 hour treatment recorded the removal efficiency of 62%, 58%, 87%, and 48% for respective colour, COD, turbidity, and iron ion. Whereby, 6 hours treatment had yielded higher removal percentage removal of 75%, 77%, 93%, and 58% respectively for colour, COD, turbidity, and iron ion. Further removal of colour, COD, turbidity, and iron ion was recorded when the treatment was run for 24 hours which exhibited 92%, 91%, 98%, 77% of removal efficiency respectively. Prolonged time to 48 hours indeed showed better removal of colour, COD, turbidity, iron ion with percentage removal of 95%, 100%, 99%, and 89% respectively. It can be seen that the arrangement of CSMZ, GAC, and limestone has the highest removal efficiency for all parameters at the flow rate influent of 30 ml/min.
Figure 6. Percentage removal of color, COD, turbidity, and Fe against contact time for 2nd run(a) at flow rate 60 mL/min and at flowrate 90 mL/min (b)
The experimental adsorption behaviour was further seen for its adsorption capacity during 60 ml/min and 90 ml/min flow rate. In addition, the flow rate adjustment had also resulted in differences in surface loading rate in which the sample going through the surface area of adsorbent bed (horizontal surface area, A= 22.9 cm2) for 30 ml/min equals to 1.31 cm/min while the flow rate of 60ml/min equals to 2.62 cm/min, and the flow rate of 90 ml/min equals to 3.93 cm/min. The percentage removal for both flow rate adjustments of CSMZ, GAC, and limestone arrangement were exhibited in Figure 6 (a) and Figure 6 (b). Based on these Figures, lower removal efficiencies were indicated at 1 hour time interval of 6 hours of contact time. The percentage removals for both 60 ml/min and 90 ml/min flow rate at 1 hour were 57%, 56%, 80%, 38% and 49%, 58%, 61%, 35% for colour, COD, turbidity, and iron ion respectively. Subsequently, when the contact time was at 6 hours, the removal percentage were 70%, 79%, 88%, 56%, and 60%, 77%, 70%, 47%. However, the maximum removal efficiency at 48 hours for both flow rates was not much different from the 30ml/min flow rate.
3.5 Adsorption Isotherm
In the present investigation, the experimental data were tested with respect to both Freundlich and Langmuir isotherms. Based on the linearized Freundlich isotherm models for natural zeolite, CSMZ, GAC in terms of adsorptive capacity to remove colour, COD, turbidity, and iron ion, the majority of them exhibited fits for all adsorbate with regression value (R2) above 0.6, except for iron ion and turbidity for respective CSMZ, and GAC. On the other hand, the linearized Langmuir isotherm models for natural zeolite, CSMZ, GAC in terms of adsorptive capacity to remove colour, COD, turbidity, and iron ion, had exhibited fits for all adsorbate with regression value (R2) was at range of 0.242 to 0.912. The Langmuir isotherm model for all adsorption mechanisms were identified to have smaller R2 values compared to the Freundlich isotherm model. Thereby, it can be concluded that the Freundlich isotherm model was more applicable in determining the adsorption mechanisms for this study.
3.6 Peat Water Quality Post Column Treatment
Peat water treatment in column with serial sequence of natural zeolite, CSMZ, and limestone had exhibited the highest removal with percentage removal at 48 hours at 95%, 100%, 99%, and 89% for colour, COD, turbidity, and iron ion respectively. Final readings at 48 hours treatment on pH, TDS, DO, conductivity, salinity, colour, turbidity, COD, and iron ion were 7.78, 74 mg/l, 4.03 mg/l, 137 uS/cm, 0.05 ppt, 12 TCU, 0.23 NTU, 0 mg/l, and 0.11 mg/l respectively (see Table 3). These findings, on the other hand, have indicated that peat water treatment had successfully produced water which satisfied the standard drinking water quality.
Table 3. The characteristics of results of peat water treatment from Beriah Peat Swamp Forest
Note: 1. *)Malaysian standard for drinking water quality;2. NA = Not analyzed.
From the results presented in this paper, the following conclusions can be drawn:
1) The optimum removal of colour, COD, and turbidity for all adsorbents were observed to occur during acidic conditions at pH range 2 – 4 whereas for iron ion, the maximum removal was noted at pH range 6 – 8.
2) At pH 2, CSMZ yielded the highest removal for colour and turbidity with removal efficiency of 90% and 98% respectively. Meanwhile, GAC has the highest percentage removal of COD at pH 4 with removal efficiency obtained about 61% while at pH 6, GAC exhibited the best removal of iron ion with percentage removal around 80%.
3) CSMZ revealed stronger adsorptive capacity for colour, COD, and turbidity compared to natural zeolite.
4) The optimal removal was achieved for the serial sequence of CSMZ (1st layer), GAC (2nd layer), and Limestone (3rd layer) with the adsorbent media at 30 ml/min of flow rate.
5) Freundlich isotherm was more reliable to describe the adsorption mechanisms of colour, COD, turbidity, and iron ion for natural zeolite, CSMZ, and GAC.
The authors wish to acknowledge the financial support from the School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia and Universiti Sains Malaysia (Short Term Grant No. 304/PAWAM/60312015).
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Wosten, J. H. M., Clymans, E., Page, S. E., Rieley, J. O., & Limin, S. H. (2008). Peat- Water interrelationships in a Tropical Peatland Ecosystem in Southeast Asia. Catena, 73, 212-224. http://dx.doi.org/10.1016/j.catena.2007.07.010
Zhang, P., Tao, X., Li, Z., & Bowman, R. S. (2002). Enhanced Perchloroethylene Reduction in Column Systems Using Surfactant Modified Zeolite/zero-valent Iron Pellets. Environmental Science and Technology, 36, 3597-3603. http://dx.doi.org/10.1021/es015816u
Modern Applied Science; Vol. 7, No. 2; 2013
ISSN 1913-1844 E-ISSN 1913-1852
Published by Canadian Center of Science and Education
S. Syafalni1, Ismail Abustan1, Aderiza Brahmana1, Siti Nor Farhana Zakaria1 & Rohana Abdullah1
1 School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia. Correspondence: S. Syafalni, School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia,
Nibong Tebal 14300, Penang, Malaysia. E-mail: firstname.lastname@example.org
Received: December 3, 2012 Accepted: January 14, 2013 Online Published: January 22, 2013 doi:10.5539/mas.v7n2p39 URL: http://dx.doi.org/10.5539/mas.v7n2p39
Shared via Creative Commons Attribution 3.0 Unported license
admin 21 Mar, 2013
TweetAbstract Access to safe drinking water is important as a health and development issue at national, regional, and local levels. About one billion people do not have healthy drinking water. More than six million people (about two million children) die because of diarrhea which is caused by polluted water. Developing countries pay a high cost [...]
Access to safe drinking water is important as a health and development issue at national, regional, and local levels. About one billion people do not have healthy drinking water. More than six million people (about two million children) die because of diarrhea which is caused by polluted water. Developing countries pay a high cost to import chemicals including polyaluminium chloride and alum. This is the reason why these countries need low-cost methods requiring low maintenance and skill. The use of synthetic coagulants is not regarded as suitable due to health and economic considerations. The present study was aimed to investigate the effects of alum as coagulant in conjunction with bean, sago, and chitin as coagulants on the removal of color, turbidity, hardness, and Escherichia coli from water. A conventional jar test apparatus was employed for the tests. The study was taken up in three stages, initially with synthetic waters, followed by testing of the efficiency of coagulants individually on surface waters and, lastly, testing of blended coagulants. The experiment was conducted at three different pH conditions of 6, 7, and 8. The dosages chosen were 0.5, 1, 1.5, and 2 mg/l. The results showed that turbidity decrease provided also a primary E. coli reduction. Hardness removal efficiency was observed to be 93% at pH 7 with 1-mg/l concentration by alum, whereas chitin was stable at all the pH ranges showing the highest removal at 1 and 1.5mg/l with pH 7. In conclusion, using natural coagulants results in considerable savings in chemicals and sludge handling cost may be achieved.
Alum; Chitin; Sago; Bean; Coagulation; Turbidity
The explosive growth of the world’s human population and subsequent water and energy demands have led to an expansion of standing surface water . Nowadays, the concern about contamination of aquatic environments has increased, especially when water is used for human consumption. About one billion people do not have healthy drinking water. More than six million people (about two million children) die because of diarrhea which is caused by polluted water[2,3].
In most of the cases, surface water turbidity is caused by the clay particles, and the color is due to the decayed natural organic matter. Generally, the particles that determine the turbidity are not separated by settling or through traditional filtration. Colloidal suspension stability in surface water is also due to the electric charge of particle surface. Thus, there is great importance in either the development of more sophisticated treatments or the improvement of the current ones .
The production of potable water from most raw water sources usually entails the use of a coagulation flocculation stage to remove turbidity in the form of suspended and colloidal material. This process plays a major role in surface water treatment by reducing turbidity, bacteria, algae, color, organic compounds, and clay particles. The presence of suspended particles would clog filters or impair disinfection process, thereby dramatically minimizing the risk of waterborne diseases [5,6].
Many coagulants are widely used in conventional water treatment processes, based on their chemical characteristics. These coagulants are classified into inorganic, synthetic organic polymers, and natural coagulants . Alum has been the most widely used coagulant because of its proven performance, cost effectiveness, relatively easy handling, and availability. Recently, much attention has been drawn on the extensive use of alum. Aluminum is regarded as an important poisoning factor in dialysis encephalopathy. Aluminum is one of the factors which might contribute to Alzheimer’s disease [7-9]. Alum reaction with water alkalinity reduces water pH and its efficiency in cold water [10,11]. However, some synthetic organic polymers such as acrylamide have neurotoxicity and strong carcinogenic effect [8,12].
In addition, the use of alum salts is inappropriate in some developing countries because of the high costs of imported chemicals and low availability of chemical coagulants . This is the reason why these countries need low-cost methods requiring low maintenance and skill.
For these reasons, and also due to other advantages of natural coagulants/flocculants over chemicals, some countries such as Japan, China, India, and the United States have adopted the use of natural polymers in the treatment of surface water for the production of drinking water . A number of studies have pointed out that the introduction of natural coagulants as a substitute for metal salts may ease the problems associated with chemical coagulants.
Natural macromolecular coagulants are promising and have attracted the attention of many researchers because of their abundant source, low price, multi-purposeness, and biodegradation[11,14,15]. Okra, rice, and chitosan are natural compounds which have been used in turbidity removal [16-18]. The extract of the seeds has been mentioned for drastically reducing the amount of sludge and bacteria in sewage .
In view of the above discussion, the present work has been taken up to evaluate the efficiency of various natural coagulants on the physico-chemical contaminant removal of water. To date, most of the research has been concentrated on the coagulant efficiencies in synthetic water, but in this study, we move ahead making an attempt to test the efficiency of the natural coagulants on surface water. The efficiencies of the coagulants as stated by  might alter depending on many factors: nature of organic matter, structure, dimension, functional groups, chemical species, and others.
Natural coagulants and their preparation
Sago is a product prepared from the milk of tapioca root. Its botanical name is ‘Manihot esculentaCrantz syn. M. utilissima’. Hyacinth bean with botanical name Dolichos lablab is chosen as another coagulant. Both the coagulants were used in the form of powders (starches). Starch consists mainly of a homopolymer of α-D-glucopyranosyl units that comes in two molecular forms, linear and branched. The former is referred to as amylose and the latter as amylopectin . These have the general structure as per  (Figure 1) .
Figure 1. General structure of amylose and amylopectin.
The third coagulant was chitin ([C8H13O5N]n), which is a non-toxic, biodegradable polymer of high molecular weight. Like cellulose, chitin is a fiber, and in addition, it presents exceptional chemical and biological qualities that can be used in many industrial and medical applications. The two plant originated coagulants were taken in the form of powder or starch. Chitin was commercially procured.
The first stage included testing the efficiency of the four coagulants on the synthetic waters. Synthetic waters with turbidity of 70 and 100 nephelometric turbidity units (NTU) were prepared with fuller’s earth in the laboratory and were used in this part of the study. The experiment was carried out using a jar test apparatus. The experiments were conducted in duplicates to eliminate any kind of error. Efficiency was evaluated by determination of reduction in turbidity of both the synthetic samples.
In the second stage of the experiment, the individual coagulants were evaluated for their efficiency on the surface waters. The water samples for this stage and the preceding stage were collected from the surface reservoir, Mudasarlova, located at a distance of 5 km from the Environmental Monitoring Laboratory, GITAM University, where the experiments were carried out. This is the reservoir which serves as a source of domestic water for the nearby residents.
Care was taken while collecting the samples so that a representative sample is obtained. All samples were collected in sterile plastic containers. The samples were transported to the laboratory, and all the experiments were conducted within a duration of 24 h. The physical parameters like temperature and color were noted at the point of sample collection. The water samples were analyzed for the following parameters pre- and post-treatment with the coagulants (Table 1).
Table 1. Physico-chemical parameters tested (stage II)
The coagulants were tested at various concentrations like 0.5, 1, 1.5, and 2 mg/l at three pH ranges of 6, 7, and 8.
The results obtained from the second stage of the study have encouraged us to further extend the study in terms of blended coagulants. The blending of coagulants was taken up from the fact that alum was the most widely used coagulant, and hence, it was taken as one part. The remaining combinations were 2, 3, 4, and 5 parts of the natural coagulants, i.e., 1:2, 1:3, 1:4, and 1:5.
Testing of the following parameters was adopted for evaluating the efficiency of the blended coagulants (pre- and post-coagulation) (Table 2). All the analysis has been performed as per the standard methods given by APHA, 2005 .
Table 2. Physico-chemical parameters tested (stage III)
E. coli presence
The E. coli bacterial presence and absence were determined in the pre- and post-coagulated water using H2S strip bottle. The water sample was filled into the bottle and allowed to stand for 24 h at room temperature. After 24 h, the water sample was observed for color change; black color change indicates the presence of E. coli.
Coagulant actions onto colloidal particles take place through charge neutralization of negatively charged particles. If charge neutralization is the predominant mechanism, a stochiometric relation can be established between the particles’ concentration and coagulant optimal dose.
In the initial stage of the experiment, the coagulants were tested against synthetic turbid samples with 70 and 100 NTU. According to Figure 2a,b, the optimum dosage of alum was observed to be 1mg/l for both the turbid samples, and the optimum pH is observed to be 7.
Figure 2. Turbidity removal efficiency of alum with initial turbidities of (a) 100 and (b) 70 NTU.
It is understood from Figure 3a,b that the optimum dosage for chitin as coagulant is 1.5 mg/l (turbidity to 40 NTU) for 100 NTU, whereas not much difference was observed between pH 7 and 8 for both the turbid samples. The optimum pH is observed to be 7 for both 70 and 100 NTU samples.
Figure 3. Turbidity removal efficiency of chitin with initial turbidities of (a) 100 and (b) 70 NTU.
Figure 4a,b exemplifies the trends of sago on the turbidity removal of the synthetic solutions. It is observed that sago was effective at both 1 and 1.5 mg/l (turbidity reduced to 50 and 45 NTU, respectively) for 100 NTU solution, and the efficiency was stable at pH 7 and 8.
Figure 4. Turbidity removal efficiency of sago with initial turbidities of (a) 100 and (b) 70 NTU.
Figure 5a,b illustrates the effect of bean on the synthetic turbid samples and turbidity removal. It is observed that bean was effective at 1mg/l (turbidity reduced to 55 NTU) for 100 NTU solution, and the efficiency was stable at pH 7 and 8.
Figure 5. Turbidity removal efficiency of bean with initial turbidities of (a) 100 and (b) 70 NTU.
Implications from the stage 1 experiment articulate that the coagulants are quite stable at the pH ranges tested; hence, in the proceeding experiments, all the three pH ranges were considered. In the second stage of experiment, the environmental samples from the surface water source were collected and tested for the removal of turbidity and other chemical parameters. The dosages were the same as the previous stage. The results are graphically represented as shown in Figures 6, 7,8, 9.
Figure 6. Turbidity removal efficiency of individual coagulants.
Figure 7. Total hardness removal efficiency of individual coagulants.
Figure 8. Calcium hardness removal efficiency of individual coagulants.
Figure 9. Chloride removal efficiency of coagulants.
The turbidity removal efficiencies of the individual coagulants are depicted in Figure 6 wherein there was a broad variation among the pH ranges. The maximum reduction was observed with 1 mg/l (87%) of bean at pH 6 followed by 1 mg/l (82%) sago at the same pH. At pH 7, the maximum efficiency was shown by bean with 1.5 mg/l dosage (85.37%) followed by bean and sago with 1 (82.49%) and 1.5 mg/l (82.49%), respectively. Removal efficiencies of 41.46% and 36.59% were reported by 1 mg/l of bean and sago, respectively, at pH 8. The minimum reductions are not reported as there was a negative competence of the coagulants at different doses and pH variations. It can be observed from the graph that there was an increase in the turbidity of the water at these dosages like with 2 g of chitin the turbidity removal was −19.51. In the entire study, the best results were obtained with total hardness removal wherein no negative competence was reported as shown in Figure 7. The utmost removal was observed with 0.5-mg/l (97.67%) sago at pH 7. At pH 6, it was (90.70%) with 1.5 mg/l of bean. At pH 8, the reduction was (93.02%) with 0.5 mg/l of alum. Apart from these, the general observation was that all the coagulants were effective in an average removal of 65% total hardness at all pH variations and doses. The tracking for the least efficiency has showed chitin at pH 6 with 2-mg/l dose (34.88%).
The calcium hardness removal efficiencies are directly proportional with the total hardness removal; the highest removal was recorded by chitin (93.33%) at pH 7 with 1.5-mg/l dose as shown in Figure 8. Removal of 90% is at pH 8 and 7 with 0.5-mg/l alum and 1-mg/l chitin, respectively. Minimum effectiveness was observed by chitin (6.67%) at pH 6 with 2-mg/l dose. On an average, the removal competence was more than 60% with all coagulants at doses at all the pH conditions.
Figure 8 illustrates the chloride removal efficiency of the coagulants tested. The average competence was observed to be 40%. The maximum competence was noted at pH 7 by chitin (83.64%) at 1.5 mg/l followed by sago (81.82%) at 1 mg/l. Indeed at pH 7, the removal was observed to be superior as a whole. Similarly, pH has shown inferior effectiveness in the amputation of chloride. The remarkable point that was noted is that at pH 8, where the removal was superior, the increase in doses of sago and bean (1.5 and 2 mg/l) has shown a depressing outcome.
With the results obtained from the second stage experimentation, the study was carried forward for the evaluation of blended coagulants. From the literature, it was understood that blended coagulants show improved competence than that of the individual ones.
The regular test of turbidity was substituted with conductivity to establish a relation and test the difference with these parameters. The conductivity diminution was observed to be preeminent at the ratio of 1:2 of all the blended coagulants 26.12%, 26.00%, and 21.35% with alum/bean, alum/chitin, and alum/sago, respectively. The highest reduction was observed with alum/sago at pH 8 with 1:2 ratio (32.28%) (Figure 10).
Figure 10. Conductivity removal efficiency of blended coagulants.
The total hardness reduction trend of the blended coagulants was recorded as follows: at pH 7, all combinations of alum/bean have resulted in negative competence. Amputation of 100% was observed with alum/chitin and alum/sago at 1:2 and 1:4 and 1:5 doses, respectively (Figure 11). The overall competence of the alum/chitin and alum/sago were registered to be more than 80%. The calcium hardness efficiencies of the blended coagulants were similar to that of the total hardness. The highest removal efficiency was shown by alum/chitin with 1:5 ratio at pH 7 (Figure 12).
Figure 11. Total hardness removal efficiency of blended coagulants.
Figure 12. Calcium hardness removal efficiency of blended coagulants.
As said earlier, the turbidity was replaced by color determination taking into account the fact that turbidity is directly related to the color. pH 7 has been remarkably effective in the highest removal of color from the water. The blended coagulant alum/sago was found to be very effective with 98% to 100% reduction in color at all the ratios of dosage (Figure 13). The blended coagulants alum/chitin and alum/sago were relatively successful at an average rate of 80% decline in the color at almost all ratios of dosage at pH 7 and 8.
Figure 13. Color removal efficiency of blended coagulants.
Alum/sago blend has a noteworthy effect on the removal of chloride from the water samples in which no negative result was noted. The highest reduction was observed with alum/chitin with dose of 1:5 (85.71%) at pH 7. Indeed, pH 7 can be optimized as perfect pH for this blend as all the ratios of dosages were quite efficient in the removal of chloride (Figure 14).
Figure 14. Chloride removal efficiency of blended coagulants.
Although many studies have used synthetic water in the experiments, this work chose to use raw water collected directly from the surface source. Therefore, it is important to consider that the natural compounds may cause variations in their composition, which interfere in the treatment process. All those factors are taken into account when evaluating the obtained results.
The characteristics of the superficial water used in this study are observed as that the water used has apparent color, turbidity, solids, and amount of compounds with a relatively high absorption in UV (254 nm). It is noticeable that the water has high turbidity and color.
The effectiveness of alum, commonly used as a coagulant, is severely affected by low or high pH. In optimum conditions, the white flocs were large and rigid and settled well in less than 10 min. This finding is in agreement with other studies at optimum pH [24,25]. The optimum pH was 7 and was similar to the obtained results by Divakaran . At high turbidity, a significant improvement in residual water turbidity was observed. The supernatant was clear after about 20-min settling. Flocs were larger and settling time was lower. The results showed that above optimum dosage, the suspensions showed a tendency to restabilize.
The effectiveness of the chitin in the present study in the removal of various contaminants with varied pH individually and also in blended form can be traced to the explanation from the literature that chitin has been studied as biosorbent to a lesser extent than chitosan; however, the natural greater resistance of the former compared to the last, due to its greater crystallinity, could mean a great advantage. Besides, the possibility to control the degree of acetylation of chitin permits to enhance its adsorption potential by increasing its primary amine group density. Recent studies regarding the production of chitin-based biocomposites and its application as fluoride biosorbents have demonstrated the potential of these materials to be used in continuous adsorption processes. Moreover, these biocomposites could remove many different contaminants, including cations, organic compounds, and anions .
Chitosan has high affinity with the residual oil and excellent properties such as biodegradability, hydrophilicity, biocompability, adsorption property, flocculating ability, polyelectrolisity, antibacterial property, and its capacity of regeneration in many applications . It has been used as non-toxic floccules in the treatment of organically polluted wastewater .
The effects of coagulation process on hardness are observed for varying levels of hardness, which resulted in significant decrease of hardness removal. The study correlates with the results obtained by , wherein they had a maximum hardness removal of 84.3% by chitosan in low turbid water with initial hardness of about 204 mg/l as CaCO3.
Several experiments were carried out to determine the comparative performance of chitosan on E. coli in different turbidities. E. coli negative is present in the chitin-treated waters in all of the turbidities. The conclusive evidence was found for the negative influence of chitosan on E. coli. The regrowth of E. coli was not observed in the experiments after 24 h, which was similar to the observations by .
As far as sago is considered, the starch was effective both individually and as blended coagulant. Unlike polyaluminium chloride, the efficiency of the natural coagulants is not affected by pH. The pH increased their efficiency, which is one of the advantages of natural coagulants. The principle behind the efficiency of the sago from the literature can be stated as follows: Sago starch is a natural polymer that is categorized as polyelectrolyte and can act as coagulant aid. Coagulant aid can be classified according to the ionization traits, which are the anions, cations, and amphoteric (with dual charges). Bratskaya et al.  mentioned that among the three groups, cation polymer is normally used to remove adsorbed negatively charged particles by attracting the adsorbed particles through electrostatic force. They discovered that anion polymer and those non-ionized cannot be used to coagulate negatively charged particles.
The chemical oxygen demand (COD) reduction is influenced by the concentration of sago used; the lower the concentration the better the removal of the COD. Using less than 1.50 g L-1, better COD reduction is observed. At this low concentration, settling time did not influence the COD reduction. Similarly, concentration of sago used at lower than 1.50 g L-1 reduced the turbidity in less than 15 min of settling time. Sago concentration higher than 1.50 g L-1 increased the turbidity; however, settling time has an influence on the turbidity reduction at higher sago concentrations. This pattern is congruent with the COD removal .
The sago starch-graft-polyacrylamide (SS-g-PAm) coagulants were found to achieve water turbidity removal up to 96.6%. The results of this study suggest that SS-g-PAm copolymer is a potential coagulant for reducing turbidity during water treatment .
At its optimum concentration, D. lablab seed powder does not affect the pH of the water. Total and calcium hardness remained almost constant and were within acceptable levels according to World Health Organization standards for drinking water. Moreover, coagulation of medium to high turbidity water with D. lablab seed powder with the finest grain size reduced turbidity further. The best performance of the finest seed powder could be due to its large total surface area, whereby most of the water-soluble proteins are at the solid–liquid interface during the extraction process as stated by Gassenschmidtet al. . This might have increased the concentration of active coagulation polymer in the extract, which improved the coagulation process. The coagulant extract from seeds has shown antimicrobial activity in the comparative culture test, which was also observed in the study of Tandonet al. .
D. lablab demonstrated the best performance with turbid water, in which a turbidity removal efficiency of 87% was observed. The restabilization of destabilized colloidal particles, which was associated with higher residual turbidities, occurred at dosages above the optimum. It is commonly observed that particles are destabilized by small amounts of hydrolyzing metal salts and that optimum destabilization corresponds with neutralization of the particles’ charge. Larger amounts of coagulants cause charge reversal so that the particles become positively charged and, thus, restabilization occurs, which results in elevated turbidity levels . It has also been observed that the reduction in turbidity is associated with significant improvements in bacteriological quality. The effect of natural coagulants on turbidity removal and the antimicrobial properties against microorganisms may render them applicable for simultaneous coagulation and disinfection of water for rural and peri-urban people in developing countries .
It is observed that blended coagulants gave utmost efficiency as compared to the traditional alum coagulants. Here in this blending process, we reduce the alum dose up to 80%; thus, we reduce the drawbacks of the alum. Also, we can reduce the cost of the treatment using the natural coagulants instead of the traditional coagulant.
E. coli is the best coliform indicator of fecal contamination from human and animal wastes. E. colipresence is more representative of fecal pollution because it is present in higher numbers in fecal material and generally not elsewhere in the environment . Results showed the absence of E. coli increases with increasing time. A greater percentage of E. coli was eliminated in higher turbidities. The aggregation and, thus, removal of E. coli was directly proportional to the concentration of particles in the suspension. Chitosan and other natural coagulants showed antibacterial effects of 2 to 4 log reductions.
Antimicrobial effects of water-insoluble chitin and coagulants were attributed to both its flocculation and bactericidal activities. A bridging mechanism has been reported for bacterial coagulation by chitosan . Especially with reference to chitosan, molecules can stack on the microbial cell surface, thereby forming an impervious layer around the cell that blocks the channels, which are crucial for living cells . On the other hand, cell reduction in microorganisms, such as E. coli, occurred without noticeable cell aggregation by chitosan.
This indicates that flocculation was not the only mechanism by which microbial reduction occurred. It was found that when samples were stored during 24 h, regrowth of E. coli was not observed for all turbidities. It should be noted that the test water contained no nutrient to support regrowth of E. coli, and chitosan is not a nutrient source for it. Another experiment was designed to check the effect of alum alone. Regrowth of E. coli was not observed for unaided alum after 24 h. The number of E. coli after resuspension of sediment reached to the initial numbers after 24 h and showed that it cannot be inactivated by alum. Such findings have been previously reported by Bina.
Access to clean and safe drinking water is difficult in rural areas of India. Water is generally available during the rainy season, but it is muddy and full of sediments. Because of a lack of purifying agents, communities drink water that is no doubt contaminated by sediment and human feces. Thus, the use of natural coagulants that are locally available in combination with solar radiation, which is abundant and inexhaustible, provides a solution to the need for clean and safe drinking water in the rural communities of India. Use of this technology can reduce poverty, decrease excess morbidity and mortality from waterborne diseases, and improve overall quality of life in rural areas.
The application of coagulation treatment using natural coagulants on surface water was examined in this study. The surface water was characterized by a high concentration of suspended particles with a high turbidity. At a varied range of pH, the suspended particles easily dissolved and settled along with the coagulants added. Research has been undertaken to evaluate the performance of natural starches of sago flour, bean powder, and chitin to act as coagulants individually and in blended form. In all three cases, the main variable was the dosage of the coagulant. The study shows that natural characteristics of starch and other coagulants can be an efficient coagulant for surface water but would need further study in modifying it to be efficient to the maximum. Thus, it can be concluded that the blended coagulants are the best which give maximum removal efficiency in minimum time.
It is chitin and chitosan which can readily be derivatized by utilizing the reactivity of the primary amino group and the primary and secondary hydroxyl groups to find applications in diversified areas. In this work, an attempt has been made to increase the understanding of the importance and effects of chitin at various doses and pH conditions, upon the chemical and biological properties of water. In view of this, this study will attract the attention of academicians and environmentalists.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
International Journal of Energy and Environmental Engineering 2012, 3:29 doi:10.1186/2251-6832-3-29
Department of Environmental Studies, GITAM Institute of Science, GITAM University, Visakhapatnam, Andhra Pradesh 530045, India
The electronic version of this article is the complete one and can be found online at:http://www.journal-ijeee.com/content/3/1/29
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© 2012 Vara; licensee BioMed Central Ltd.
admin 9 Jan, 2013
TweetIntroduction Water desalination via reverse osmosis (RO) technology provides a solution to the world’s water shortage problem. Until now, the production of fresh water from seawater has reached 21-million cubic meter per day all around the world (Wangnick, 2005). However, the success of RO technology is subject to improvement as the technology is challenged by [...]
Water desalination via reverse osmosis (RO) technology provides a solution to the world’s water shortage problem. Until now, the production of fresh water from seawater has reached 21-million cubic meter per day all around the world (Wangnick, 2005). However, the success of RO technology is subject to improvement as the technology is challenged by a biofouling problem –a problem related to biological material development which forms a sticky layer on the membrane surface (Flemming, 1997; Baker and Dudley, 1998).
Continuous biofouling problems in RO lead to higher energy input requirement as an effect of increased biofilm resistance (Rf) and biofilm enhanced osmotic pressure (BEOP), lower quality of product water due to concentration polarization (CP) – increased concentration due to solutes accumulation on the membrane surface, (Herzberg and Elimelech, 2007), and thus significant increase in both operating and maintenance costs.
Recent studies and objectives
Recent studies show the importance of the operating conditions (e.g. flux and cross flow velocities) in RO biofouling. The presence of feed channel spacers has also been getting more attention as it may have adverse effects. A previous study (Chong et al., 2008) without feed channel spacers showed that RO biofouling was a flux driven process where higher flux increased fouling rate. It was also shown that biofouling caused a BEOP effect due to elevated CP of solutes at the membrane surface, thus resulted in loss of driving force. The BEOP effect was more severe at high flux and low crossflow operation.
In another recent study (Vrouwenvelder et al., 2009a) involving feed channel spacers suggested that flux did not affect fouling and biofouling was more severe when the crossflow velocity was higher. However, these studies were conducted on river water at low level of salinity and under no/very low flux conditions, which may suggested that BEOP effect was not observed in the above studies. These contradictory observations relating to the biofouling process in RO need to be systematically addressed as it is critical to understand the mechanism for sustainable operation of RO technology.
The objective of this study was to observe the impact of spacer towards RO biofouling as well as to investigate the development of biofilm in a spacer filled channel. The experiments were conducted at constant flux and biofouling was observed by the increase of transmembrane pressure (TMP). Observation with confocal light scanning microscope (CLSM) method was conducted to the fouled membrane and spacers to provide information of biofilm development inside the membrane module.
Materials and methods
A lab-scale set-up was arranged to resemble the real RO operation where experiments were performed with elevated salinity, high pressure, imposed flux, and permeation. The schematic diagram of the set-up is depicted in Figure 1. It is a fully-recycled system with two identical RO modules running in series. Feed solution contained constant amounts NaCl and nutrient broth (NB) to provide sufficient TOC level.
The study was conducted in the constant flux mode and biofouling was measured via the rise in TMP. A mass-flow controller was installed at the permeate side to maintain the amount of permeate withdrawn. A bacteria solution was injected into the system before the feed solution entered the RO modules and a set of microfilters (5 μm and 0.2 μm) were installed at downstream to prevent excess bacteria from entering the feed tank and turning the feed tank into an “active bioreactor”.
Model bacteria Pseudomonas aeruginosa (PAO1) was used in the experiment. Bacteria stock solution used in the biofouling tests was prepared in batch and the stock solution was replenished every 24 hours. Bacteria were grown in mixture of NB and NaCl solution where they were harvested after 24 hours and diluted into autoclaved salt solution. The concentration of bacteria was controlled and measured by optical density (OD) using UV spectrophotometer at 600 nm. Batch prepared bacteria stock solution has some advantages over using continuous feed from a chemostat (Chong et al., 2008). A more consistent and fresh bacteria load and without excess nutrient was introduced into the system as nutrient content was completely removed in the harvesting step.
Prior to every experiment, cut RO membranes (DOW Filmtec, BW-30) were soaked in Milli-Q water and sterilized in 70% ethanol solution. Similar pretreatment procedures were applied to membrane support layers and feed channel spacers prior every experiment. The spacers used in the experiments are obtained from unused Hydranautics LFC-1 spiral wound module (Figure 2).
The membranes were compacted at a maximum flux (~65 L/m2.h) overnight with Milli-Q water until a stable flux was achieved. Following compaction, the flux was set to the desired values and NaCl solution was added into the feed tank until the desired concentration was achieved. The system was let to mix for 1.5 hours. NB solution was then added into the feed tank to provide an average background nutrient concentration of 6.5 mg/L TOC. The system was allowed to well-mix for 1.5 hours.
The biofouling test was initiated by continuous injection of bacteria stock solution into the flow line at a dilution rate of 1:500 based on RO cross-flow rate. Biofilm was allowed to grow on the RO membranes. TMP rise due to biofouling was measured over time. The solution in the feed tank was removed and replaced with a fresh solution at the same NaCl and NB concentration twice per day in order to maintain the freshness level of the feed solution.
Upon completion of the fouling test, the RO system was cleaned with:
Tap water adjusted to pH 2 with HNO3 for 1.5 hours
Tap water adjusted to pH 11 with NaOH for 1.5 hours
Flowing tap water for rinsing for 1.5 hours
Final rinsing with Milli-Q water at unadjusted pH
The fouled membranes were removed from the RO cells for membrane autopsy. In this analysis, fluorescence staining methods and confocal laser scanning microscope (CLSM) were used to detect the biofilm.
Biofilms were prepared for CLSM by staining with the LIVE/DEAD BacLight Bacterial Viability Kits (Molecular Probes, L7012). It consists of SYTO 9 green-fluorescent nucleic acid stain and the red-fluorescent nucleic acid stain, propidium iodide (PI). These stains possess different spectral characteristics and different ability to penetrate healthy bacterial cells. When used alone, the SYTO 9 stain generally labels all bacteria in a population — those with intact and damaged membranes. In contrast, propidium iodide penetrates only bacteria with damaged membranes, causing a reduction in the SYTO 9 stain fluorescence when both dyes are present. Thus, with an appropriate mixture of the SYTO 9 and propidium iodide stains, bacteria with intact cell membranes stain fluorescent green, whereas bacteria with damaged membranes stain fluorescent red.
Microscopic observation and image acquisition of biofilms were performed using a confocal laser scanning microscope (ZEISS, model LSM710), equipped with Argon laser at 488 nm and DPSS561-10 laser at 561 nm. Images were captured using confocal microscope bundled program ZEN 2009.
Results and discussions
The cross-flow velocity (CFV) in RO membrane operations is known to affect fouling rate. At higher CFV, the flow causes scouring effects which results in slower fouling (Koltuniewicz et al., 1995). On the other hand, experiments of RO modules without the presence of flux shows that a higher cross-flow velocity may increase biofouling due to more nutrients supply (Vrouwenvelder et al., 2009b).
In our study, the investigation was carried out by varying the cross-flow velocity (CFV) from
0.1, 0.17, to 0.34 m/s. The NaCl concentration used was constant at 2000 mg/L and the applied flux was constant at 35 LMH. TMP values were measured overtime and normalized to the initial TMP.
Figure 3 shows the normalized TMP profiles. Faster TMP rise was observed at lower CFV and both operation with and without spacer show similar profiles. The delay of TMP rise caused by spacer was quantified by measuring the time needed for the TMP to increase by 10 % (Table 1). The effect of spacer was higher at higher CFV where the percentage of the delay was 21.21 % and 42.87 % at 0.10 m/s and 0.17 m/s respectively. An interesting phenomenon was observed during the earlier TMP rise (0-3 days) where change in CFV gives little effect on TMP profiles. Similar phenomenon was observed for operation with and without spacer. A possible explanation for this phenomenon is that during this period bacterial attachment was dominant and therefore operation at constant flux gives similar initial TMP rise. Previous studies (Chong et al., 2008) have shown previously that membrane biofouling is a flux driven process where higher flux increases the TMP rise. However, their study did not include spacers and did not focus on initial TMP rise.
Table 1. The delay of biofouling rate caused by spacer at different CFV
The effect of different salt concentrations was also investigated. In this experiment the flux and CFV were fixed at 35 LMH and 0.17 m/s respectively. Figure 4 shows the normalized TMP profile of three different NaCl concentrations in the feed solution. When the feed channel spacer was absent it was very obvious that faster TMP rise was observed at higher salt concentration. This suggests that the effect of concentration polarization (CP) increases with the salt concentration and confirms the presence of the biofilm enhanced osmotic pressure (BEOP) effect (Herzberg and Elimelech, 2007; Chong et al., 2008). This phenomenon however, was less obvious when the spacer was present on the membrane. The spacer appears to provide flow eddies thus reducing the effect of CP and to be useful to prevent biofouling on the membrane which was indicated with slower TMP rise. The spacer gives bigger effect at higher salt concentration where the time to reach 10 % TMP rise was delayed by 30 % at 100 mg/L and 2000 mg/L NaCl, and 95.7 % at 4000 mg/L (Table 2).
4.2 Biofilm development in spacer-filled RO membrane channel The development of biofilm in spacer-filled channel was observed via microscopic and microscopic method. Macroscopic images are to show overall uniformity of biofilm distribution, while the microscopic images are able to show a more detailed biofilm patterns. All of the images in this study were taken from separate experiments as the samples were unable to be reused after analysis, however all the conditions for the experiments were maintained the same.
Figure 5 shows the macroscopic images of biofilm development. The biofilm sample on the membranes and spacers were stained with 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) dye. CTC stains bacteria with respiration activity and stained cells appear in red colour. Analysis was done after 0, 3, 6, and 10 days, the condition was 35 LMH flux, 0.17 m/s CFV, and 4000 mg/L NaCl concentration. Longer experiment duration gives thicker and denser biofilm, which can be seen from higher red colour intensity. The biofilms have also shown overall uniformity across the membrane area where similar patterns were observed among each spacer squares.
Figure 5. Macroscopic images of biofilm development on membranes and spacers. (A) 0-day, (B) 3-day, (C) 6-day, (D) 10-day. Biofilms stained with CTC dye and images taken with SONY NEX-5 digital camera.
Confocal laser scanning microscope (CLSM) provides a more detailed analysis of biofilm development (Figure 6). Based on the images, it appears that biofilm was initiated on the membrane; it later covered more areas and started to appear on the spacer. Areas behind the attached filaments of the spacer fiber seem to be suitable for the initial bacterial attachments rather than the centre of the spacer. Biofilm build-up observed on areas under the detached filaments was caused by higher shear due to accelerated CFV. Our experiments confirmed that biofouling in RO is a flux driven process. A lower TMP rise was observed at lower flux, which means slower biofouling rate. This is also supported with the biofilm coverage data where less coverage was observed at lower flux.
From the findings above, several conclusions can be drawn. The hydrodynamic condition of the flow is affecting the biofouling process. Cross flow velocity (CFV) is an important parameter and lower fouling can be achieved at higher CFV. Having feed channel spacers on the membrane is advantageous as it provides a more well-mixed flow, reduces concentration polarization and reduces TMP increase. Biofilm enhanced osmotic pressure (BEOP) was another phenomenon observed in this study. Due to the BEOP effect, a faster TMP rise was achieved at higher salinity. However, with the presence of the spacer the BEOP effect was reduced significantly.
From our microscopic analysis of biofilm shows that initial bacterial deposition and biofilm development was started on the membrane especially on areas behind the attached spacer filaments. Biofilm develops over time to cover more areas and starts to grow on the spacer at the later stages. Imposed flux also influences the biofilm development where lower biofouling is achieved at lower flux.
Baker, J. S. and Dudley, L. Y. (1998), “Biofouling in membrane systems – a review”, Desalination, Vol. 118, No. 1-3, pp. 81-90.
Chong, T. H., Wong, F. S. and Fane, A. G. (2008), “The effect of imposed flux on biofouling in reverse osmosis: Role of concentration polarisation and biofilm enhanced osmotic pressure phenomena”, Journal of Membrane Science, Vol. 325, No. 2, pp. 840-850.
Flemming, H. C. (1997), “Reverse osmosis membrane biofouling”, Experimental Thermal and Fluid Science, Vol. 14, No. 4, pp. 382-391.
Herzberg, M. and Elimelech, M. (2007), “Biofouling of reverse osmosis membranes: Role of biofilm-enhanced osmotic pressure”, Journal of Membrane Science, Vol. 295, No. 1-2, pp.
Koltuniewicz, A. B., Field, R. W. and Arnot, T. C. (1995), “Cross-flow and dead-end microfiltration of oily-water emulsion. Part I: Experimental study and analysis of flux decline”, Journal of Membrane Science, Vol. 102, No. 1-3, pp. 193-207.
Suwarno, S. R., Puspitasari, V. L., Chong, T. H., Fane, A. G., Chen, X., Rice, S. A., Mcdougald, D. and Cohen, Y. (2010) “The hydrodynamic effect on biofouling in reverse osmosis membrane processes”, IWA International Young Water Professionals Conference, Sydney,
Vrouwenvelder, J. S., Hinrichs, C., Van Der Meer, W. G., Van Loosdrecht, M. C. and Kruithof, J. C. (2009b), “Pressure drop increase by biofilm accumulation in spiral wound RO and NF membrane systems: role of substrate concentration, flow velocity, substrate load and flow direction”, Biofouling, Vol. 25, No. 6, pp. 543-555.
Wangnick (2005), 2004 Worldwide Desalting Plants Directory, Global Water Intelligence, Oxford, England.
Publication Date: Jan 2014 – ISBN – 9781780404769
admin 9 Dec, 2012
TweetDesalination refers to processes that remove some amount of salt and other minerals from saline water. Salt water is desalinated to produce fresh water suitable for human consumption or irrigation. One potential byproduct of desalination is salt. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused [...]
Desalination refers to processes that remove some amount of salt and other minerals from saline water.
Salt water is desalinated to produce fresh water suitable for human consumption or irrigation. One potential byproduct of desalination is salt. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on developing cost-effective ways of providing fresh water for human use. Along with recycled wastewater, this is one of the few rainfall-independent water sources.
Large-scale desalination typically uses large amounts of energy and specialized, expensive infrastructure, making it more expensive than fresh water from conventional sources, such as rivers or groundwater.
Desalination is particularly relevant to countries such as Australia, which traditionally have relied on collecting rainfall behind dams to provide their drinking water supplies.
According to the International Desalination Association, in 2009, 14,451 desalination plants operated worldwide, producing 59.9e6 cubic meters (2.12×109 cu ft) per day, a year-on-year increase of 12.3%. It was 68 million m3 in 2010, and expected to hit 120 million m3 by 2020; some 40 million m3 is planned for the Middle East. The world’s largest desalination plant is the Jebel Ali Desalination Plant (Phase 2) in the United Arab Emirates.
Schematic of a multistage flash desalinator
A – steam in
B – seawater in
C – potable water out
D – waste out
E – steam out
F – heat exchange
G – condensation collection
H – brine heater
Plan of a typical reverse osmosis desalination plant
The traditional process used in these operations is vacuum distillation—essentially the boiling of water at less than atmospheric pressure and thus a much lower temperature than normal. This is because the boiling of a liquid occurs when the vapor pressure equals the ambient pressure and vapor pressure increases with temperature. Thus, because of the reduced temperature, energy is saved. Multistage flash distillation, a leading method, accounted for 85% of production worldwide in 2004.
The principal competing processes use membranes to desalinate, principally applying reverse osmosis technology. Membrane processes use semipermeable membranes and pressure to separate salts from water. Reverse osmosis plant membrane systems typically use less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade. Desalination remains energy intensive, however, and future costs will continue to depend on the price of both energy and desalination technology.
Cogeneration is the process of using excess heat from power production to accomplish another task. For desalination, cogeneration is the production of potable water from seawater or brackish groundwater in an integrated, or “dual-purpose”, facility in which a power plant becomes the source of energy for desalination. Alternatively, the facility’s energy production may be dedicated to the production of potable water (a stand-alone facility), or excess energy may be produced and incorporated into the energy grid (a true cogeneration facility). Cogeneration takes various forms, and theoretically any form of energy production could be used. However, the majority of current and planned cogeneration desalination plants use either fossil fuels or nuclear power as their source of energy. Most plants are located in the Middle East or North Africa, which use their petroleum resources to offset limited water resources. The advantage of dual-purpose facilities is they can be more efficient in energy consumption, thus making desalination a more viable option for drinking water.
The Shevchenko BN350, a nuclear-heated desalination unit
In a December 26, 2007, opinion column in The Atlanta Journal-Constitution, Nolan Hertel, a professor of nuclear and radiological engineering at Georgia Tech, wrote, “… nuclear reactors can be used … to produce large amounts of potable water. The process is already in use in a number of places around the world, from India to Japan and Russia. Eight nuclear reactors coupled to desalination plants are operating in Japan alone … nuclear desalination plants could be a source of large amounts of potable water transported by pipelines hundreds of miles inland…”
Additionally, the current trend in dual-purpose facilities is hybrid configurations, in which the permeate from a reverse osmosis desalination component is mixed with distillate from thermal desalination. Basically, two or more desalination processes are combined along with power production. Such facilities have already been implemented in Saudi Arabia at Jeddah and Yanbu.
A typical aircraft carrier in the US military uses nuclear power to desalinate 400,000 US gallons (1,500,000 l; 330,000 imp gal) of water per day.
Factors that determine the costs for desalination include capacity and type of facility, location, feed water, labor, energy, financing, and concentrate disposal. Desalination stills now control pressure, temperature and brine concentrations to optimize efficiency. Nuclear-powered desalination might be economical on a large scale.
While noting costs are falling, and generally positive about the technology for affluent areas in proximity to oceans, a 2004 study argued, “Desalinated water may be a solution for some water-stress regions, but not for places that are poor, deep in the interior of a continent, or at high elevation. Unfortunately, that includes some of the places with biggest water problems.”, and, “Indeed, one needs to lift the water by 2,000 meters (6,600 ft), or transport it over more than 1,600 kilometers (990 mi) to get transport costs equal to the desalination costs. Thus, it may be more economical to transport fresh water from somewhere else than to desalinate it. In places far from the sea, like New Delhi, or in high places, like Mexico City, high transport costs would add to the high desalination costs. Desalinated water is also expensive in places that are both somewhat far from the sea and somewhat high, such as Riyadh and Harare. In many places, the dominant cost is desalination, not transport; the process would therefore be relatively less expensive in places like Beijing, Bangkok, Zaragoza, Phoenix, and, of course, coastal cities like Tripoli.” After being desalinated at Jubail, Saudi Arabia, water is pumped 200 miles (320 km) inland through a pipeline to the capital city of Riyadh. For coastal cities, desalination is increasingly viewed as an untapped and unlimited water source.
In Israel as of 2005, desalinating water costs US$ 0.53 per cubic meter. As of 2006, Singapore was desalinating water for US$ 0.49 per cubic meter. The city of Perth began operating a reverse osmosis seawater desalination plant in 2006, and the Western Australian government announced a second plant will be built to serve the city’s needs. A desalination plant is now operating in Australia’s largest city, Sydney, and the Wonthaggi desalination plant was under construction in Wonthaggi, Victoria.
The Perth desalination plant is powered partially by renewable energy from the Emu Downs Wind Farm. A wind farm at Bungendore in New South Wales was purpose-built to generate enough renewable energy to offset the Sydney plant’s energy use, mitigating concerns about harmful greenhouse gas emissions, a common argument used against seawater desalination.
In December 2007, the South Australian government announced it would build a seawater desalination plant for the city of Adelaide, Australia, located at Port Stanvac. The desalination plant was to be funded by raising water rates to achieve full cost recovery. An online, unscientific poll showed nearly 60% of votes cast were in favor of raising water rates to pay for desalination.
A January 17, 2008, article in the Wall Street Journal stated, “In November, Connecticut-based Poseidon Resources Corp. won a key regulatory approval to build the $300 million water-desalination plant in Carlsbad, north of San Diego. The facility would produce 50,000,000 US gallons (190,000,000 l; 42,000,000 imp gal) of drinking water per day, enough to supply about 100,000 homes … Improved technology has cut the cost of desalination in half in the past decade, making it more competitive … Poseidon plans to sell the water for about $950 per acre-foot [1,200 cubic metres (42,000 cu ft)]. That compares with an average [of] $700 an acre-foot [1200 m³] that local agencies now pay for water.” Each $1,000 per acre-foot works out to $3.06 for 1,000 gallons, or $.81 per cubic meter.
While this regulatory hurdle was met, Poseidon Resources is not able to break ground until the final approval of a mitigation project for the damage done to marine life through the intake pipe is received, as required by California law. Poseidon Resources has made progress in Carlsbad, despite an unsuccessful attempt to complete construction of Tampa Bay Desal, a desalination plant in Tampa Bay, FL, in 2001. The Board of Directors of Tampa Bay Water was forced to buy Tampa Bay Desal from Poseidon Resources in 2001 to prevent a third failure of the project. Tampa Bay Water faced five years of engineering problems and operation at 20% capacity to protect marine life, so stuck to reverse osmosis filters prior to fully using this facility in 2007.
In 2008, a San Leandro, California company (Energy Recovery Inc.) was desalinating water for $0.46 per cubic meter.
A Jordanian-born chemical engineering doctoral student at University of Ottawa, Mohammed Rasool Qtaisha, invented a new desalination technology that is alleged to produce between 600% and 700% more water output per square meter of membrane than current technology. General Electric is looking into similar technology, and the U.S. National Science Foundation funded the University of Michigan to study it, as well. Patent issues and details of the technology were unresolved as of 2008.
While desalinating 1,000 US gallons (3,800 l; 830 imp gal) of water can cost as much as $3, the same amount of bottled water costs $7,945.
In the United States, due to a recent court ruling under the Clean Water Act, ocean water intakes are no longer viable without reducing mortality of the life in the ocean, the plankton, fish eggs and fish larvae, by 90%. The alternatives include beach wells to eliminate this concern, but require more energy and higher costs, while limiting output.
All desalination processes produce large quantities of a concentrate, which may be increased in temperature, and contain residues of pretreatment and cleaning chemicals, their reaction byproducts, and heavy metals due to corrosion. Chemical pretreatment and cleaning are a necessity in most desalination plants, which typically includes the treatment against biofouling, scaling, foaming and corrosion in thermal plants, and against biofouling, suspended solids and scale deposits in membrane plants.
To limit the environmental impact of returning the brine to the ocean, it can be diluted with another stream of water entering the ocean, such as the outfall of a wastewater treatment or power plant. While seawater power plant cooling water outfalls are not as fresh as wastewater treatment plant outfalls, salinity is reduced. If the power plant is medium-to-large sized and the desalination plant is not enormous, the power plant’s cooling water flow is likely to be at least several times larger than that of the desalination plant. Another method to reduce the increase in salinity is to mix the brine via a diffuser in a mixing zone. For example, once the pipeline containing the brine reaches the sea floor, it can split into many branches, each releasing brine gradually through small holes along its length. Mixing can be combined with power plant or wastewater plant dilution.
Brine is denser than seawater due to higher solute concentration. The ocean bottom is most at risk because the brine sinks and remains there long enough to damage the ecosystem. Careful reintroduction can minimize this problem. For example, for the desalination plant and ocean outlet structures to be built in Sydney from late 2007, the water authority stated the ocean outlets would be placed in locations at the seabed that will maximize the dispersal of the concentrated seawater, such that it will be indistinguishable beyond between 50 and 75 meters (160 and 246 ft) from the outlets. Typical oceanographic conditions off the coast allow for rapid dilution of the concentrated byproduct, thereby minimizing harm to the environment.
The Kwinana Desalination Plant opened in Perth in 2007. Water there and at Queensland’s Gold Coast Desalination Plant and Sydney’s Kurnell Desalination Plant is withdrawn at only 0.1 meters per second (0.33 ft/s), which is slow enough to let fish escape. The plant provides nearly 140,000 cubic meters (4,900,000 cu ft) of clean water per day.
Alternatives without pollution
Some methods of desalination, particularly in combination with evaporation ponds and solar stills (solar desalination), do not discharge brine. They do not use chemicals in their processes nor the burning of fossil fuels. They do not work with membranes or other critical parts, such as components that include heavy metals, thus do not cause toxic waste (and high maintenance). A new approach that works like a solar still, but on the scale of industrial evaporation ponds is the Integrated Biotectural System. It can be considered “full desalination” because it converts the entire amount of saltwater intake into distilled water. One of the unique advantages of this type of solar-powered desalination is the feasibility for inland operation. Standard advantages also include no air pollution from desalination power plants and no temperature increase of endangered natural water bodies from power plant cooling-water discharge. Another important advantage is the production of sea salt for industrial and other uses. Currently, 50% of the world’s sea salt production still relies on fossil energy sources.
Alternatives to desalination
Increased water conservation and efficiency remain the most cost-effective priorities in areas of the world where there is a large potential to improve the efficiency of water use practices. Wastewater reclamation for irrigation and industrial use provides multiple benefits over desalination. Urban runoff and storm water capture also provide benefits in treating, restoring and recharging groundwater.
A proposed alternative to desalination in the American Southwest is the commercial importation of bulk water from water-rich areas either by very large crude carriers converted to water carriers, or via pipelines. The idea is politically unpopular in Canada, where governments imposed trade barriers to bulk water exports as a result of a claim filed in 1999 under Chapter 11 of the North American Free Trade Agreement (NAFTA) by Sun Belt Water Inc., a company established in 1990 in Santa Barbara, California, to address pressing local needs due to a severe drought in that area.
Experimental techniques and other developments
Many desalination techniques have been researched, with varying degrees of success.
One such process was commercialized by Modern Water PLC using forward osmosis, with a number of plants reported to be in operation.
The US government is working to develop practical solar desalination.
The Passarell process uses reduced atmospheric pressure rather than heat to drive evaporative desalination. The pure water vapor generated by distillation is then compressed and condensed using an advanced compressor. The compression process improves distillation efficiency by creating the reduced pressure in the evaporation chamber. The compressor centrifuges the pure water vapor after it is drawn through a demister (removing residual impurities) causing it to compress against tubes in the collection chamber. The compression of the vapor causes its temperature to increase. The heat generated is transferred to the input water falling in the tubes, causing the water in the tubes to vaporize. Water vapor condenses on the outside of the tubes as product water. By combining several physical processes, Passarell enables most of the system’s energy to be recycled through its subprocesses, namely evaporation, demisting, vapor compression, condensation, and water movement within the system.
Geothermal energy can drive desalination. In most locations, geothermal desalination beats using scarce groundwater or surface water, environmentally and economically.
Nanotube membranes may prove to be effective for water filtration and desalination processes that would require substantially less energy than reverse osmosis.
Biomimetic membranes are another approach.
On June 23, 2008, Siemens Water Technologies announced technology based on applying electric fields that purports to desalinate one cubic meter of water while using only 1.5 kWh of energy. If accurate, this process would consume only one-half the energy of other processes. Currently, Oasis Water, which developed the technology, still uses three times that much energy.
Freeze-thaw desalination uses freezing to remove fresh water from frozen seawater.
In 2009, Lux Research estimated the worldwide desalinated water supply will triple between 2008 and 2020.
Desalination through evaporation and condensation for crops
The Seawater Greenhouse uses natural evaporation and condensation processes inside a greenhouse powered by solar energy to grow crops in arid coastal land.
Low-temperature thermal desalination
Originally stemming from ocean thermal energy conversion research, low-temperature thermal desalination (LTTD) takes advantage of water boiling at low pressures, potentially even at ambient temperature. The system uses vacuum pumps to create a low-pressure, low-temperature environment in which water boils at a temperature gradient of 8–10 °C (46–50 °F) between two volumes of water. Cooling ocean water is supplied from depths of up to 600 meters (2,000 ft). This cold water is pumped through coils to condense the water vapor. The resulting condensate is purified water. LTTD may also take advantage of the temperature gradient available at power plants, where large quantities of warm wastewater are discharged from the plant, reducing the energy input needed to create a temperature gradient.
Experiments were conducted in the US and Japan to test the approach. In Japan, a spray-ﬂash evaporation system was tested by Saga University. In Hawaii, the National Energy Laboratory tested an open-cycle OTEC plant with fresh water and power production using a temperature difference of 20 C° between surface water and water at a depth of around 500 meters (1,600 ft). LTTD was studied by India’s National Institute of Ocean Technology (NIOT) from 2004. Their first LTTD plant opened in 2005 at Kavaratti in the Lakshadweep islands. The plant’s capacity is 100,000 liters (22,000 imp gal; 26,000 US gal)/day, at a capital cost of INR 50 million (€922,000). The plant uses deep water at a temperature of 7 to 15 °C (45 to 59 °F). In 2007, NIOT opened an experimental, floating LTTD plant off the coast of Chennai, with a capacity of 1,000,000 litres (220,000 imp gal; 260,000 US gal)/day. A smaller plant was established in 2009 at the North Chennai Thermal Power Station to prove the LTTD application where power plant cooling water is available.
In October 2009, Saltworks Technologies, a Canadian firm, announced a process that uses solar or other thermal heat to drive an ionic current that removes all sodium and chlorine ions from the water using ion-exchange membranes.
Shared under the Creative Commons Attribution-ShareAlike License, original text and illustrations found here: http://en.wikipedia.org/wiki/Desalination
admin 28 Oct, 2012
TweetSince the 1960s, Myron L products have led the industry in high quality, simple to operate conductivity and pH instrumentation for municipal, commercial and industrial water quality control, chemical concentration testing and process control. Today, Myron L meters are more convenient than ever to research and buy right here at MyronLMeters.com. Industrial Wastewater Treatment Industrial [...]
Since the 1960s, Myron L products have led the industry in high quality, simple to operate conductivity and pH instrumentation for municipal, commercial and industrial water quality control, chemical concentration testing and process control. Today, Myron L meters are more convenient than ever to research and buy right here at MyronLMeters.com.
Industrial Wastewater Treatment
Industrial wastewater treatment covers the mechanisms and processes used to treat waters that have been contaminated in some way by human industrial or commercial activities prior to its release into the environment or its re-use.
Most industries produce some wet waste although recent trends in the developed world have been to minimize such production or recycle waste within the production process. However, many industries remain dependent on processes that produce wastewaters.
Sources of industrial wastewater
Beer is a fermented beverage with low alcohol content made from various types of grain. Barley predominates, but wheat, maize, and other grains can be used. The production steps include:
• Malt production and handling: grain delivery and cleaning; steeping of the grain in water to start germination; growth of rootlets and development of enzymes (which convert starch into maltose); kilning and polishing of the malt to remove rootlets; storage of the cleaned malt
• Wort production: grinding the malt to grist; mixing grist with water to produce a mash in the mash tun; heating of the mash to activate enzymes; separation of grist residues in the lauter tun to leave a liquid wort; boiling of the wort with hops; separation of the wort from
the trub/hot break (precipitated residues), with the liquid part of the trub being returned
to the lauter tub and the spent hops going to a collection vessel; and cooling of the wort
• Beer production: addition of yeast to cooled wort; fermentation; separation of spent yeast
by filtration, centrifugation or settling; bottling or kegging.
Water consumption for breweries generally ranges 4–8 cubic meter per cubic meter (m3/m3) of beer produced.
Breweries can achieve an effluent discharge of 3–5 m3/m3 of sold beer (exclusive of cooling waters). Untreated effluents typically contain sus-pended solids in the range 10–60 milligrams per liter (mg/l), biochemical oxygen demand (BOD) in the range 1,000–1,500 mg/l, chemical oxygen demand (COD) in the range 1,800–3,000 mg/l,
and nitrogen in the range 30–100 mg/l. Phosphorus can also be present at concentrations of the order of 10–30 mg/l. Effluents from individual process steps are variable. For example, bottle washing produces a large volume of effluent that, however, contains only a minor part of the total organics discharged from the brewery. Effluents from fermentation
and filtering are high in organics and BOD but low in volume, accounting for about 3% of total wastewater volume but 97% of BOD. Effluent pH averages about 7 for the combined effluent but can fluctuate from 3 to 12 depending on the use of acid and alkaline cleaning agents. Effluent temperatures average about 30°C.
The dairy industry involves processing raw milk into products such as consumer milk, butter, cheese, yogurt, condensed milk, dried milk (milk powder), and ice cream, using processes such as chilling, pasteurization, and homogenization. Typical by-products include buttermilk, whey, and their derivatives.
Dairy effluents contain dissolved sugars and proteins, fats, and possibly residues of additives. The key parameters are biochemical oxygen demand (BOD), with an average ranging from 0.8 to 2.5 kilograms per metric ton (kg/t) of milk in the untreated effluent; chemical oxygen demand (COD), which is normally about 1.5 times the BOD level; total suspended solids, at 100–1,000 milligrams per liter (mg/l); total dissolved solids: phosphorus (10–100 mg/l), and nitrogen (about 6% of the BOD level). Cream, butter, cheese, and whey production are major sources of BOD in wastewater. The waste load equivalents of specific milk constituents are: 1 kg of milk fat = 3 kg COD; 1 kg of lactose = 1.13 kg COD; and 1 kg protein = 1.36 kg COD. The wastewater may contain pathogens from contaminated materials or production processes. A dairy often generates odors and, in some cases, dust, which need to be controlled. Most of the solid wastes can be processed into other products and byproducts.
Pulp and Paper industry
The pulp and paper industry is one of worlds oldest and core industrial sector. The socio-economic importance of paper has its own value to the country’s development as it is directly related to the industrial and economic growth of the country. Paper manufacturing is a highly capital, energy and water intensive industry. It is also a highly polluting process and requires substantial investments in pollution control equipment.
The pulp and paper mill is a major industrial sector utilizing a huge amount of lignocellulosic materials and water during the manufacturing process, and releases chlorinated lignosulphonic acids, chlorinated resin acids, chlorinated phenols and chlorinated hydrocarbons in the effluent. About 500 different chlorinated organic compounds have been identified including chloroform, chlorate, resin acids, chlorinated hydrocarbons, phenols, catechols, guaiacols, furans, dioxins, syringols, vanillins, etc. These compounds are formed as a result of reaction between residual lignin from wood fibres and chlorine/chlorine compounds used for bleaching. Colored compounds and Adsorbable Organic Halogens (AOX) released from pulp and paper mills into the environment poses numerous problems. The wood pulping and production of the paper products generate a considerable amount of pollutants characterized by Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Suspended Solids (SS), toxicity, and colour when untreated or poorly treated effluents are discharged to receiving waters. The effluent is toxic to aquatic organisms and exhibits strong mutagenic effects and physiological impairment.
Iron and steel industry
The production of iron from its ores involves powerful reduction reactions in blast furnaces. Cooling waters are inevitably contaminated with products especially ammonia andcyanide. Production of coke from coal in coking plants also requires water cooling and the use of water in by-products separation. Contamination of waste streams includes gasification products such as benzene, naphthalene, anthracene, cyanide, ammonia, phenols, cresols together with a range of more complex organic compounds known collectively as polycyclic aromatic hydrocarbons (PAH).
The conversion of iron or steel into sheet, wire or rods requires hot and cold mechanical transformation stages frequently employing water as a lubricant and coolant. Contaminants include hydraulic oils, tallow and particulate solids. Final treatment of iron and steel products before onward sale into manufacturing includes pickling in strong mineral acid to remove rust and prepare the surface for tin or chromium plating or for other surface treatments such as galvanisation or painting. The two acids commonly used arehydrochloric acid and sulfuric acid. Wastewaters include acidic rinse waters together with waste acid. Although many plants operate acid recovery plants, (particularly those using Hydrochloric acid), where the mineral acid is boiled away from the iron salts, there remains a large volume of highly acid ferrous sulfate or ferrous chloride to be disposed of. Many steel industry wastewaters are contaminated by hydraulic oil also known as soluble oil.
The principal waste-waters associated with mines and quarries are slurries of rock particles in water. These arise from rainfall washing exposed surfaces and haul roads and also from rock washing and grading processes. Volumes of water can be very high, especially rainfall related arisings on large sites. Some specialized separation operations, such as coalwashing to separate coal from native rock using density gradients, can produce wastewater contaminated by fine particulate haematite and surfactants. Oils and hydraulic oils are also common contaminants. Wastewater from metal mines and ore recovery plants are inevitably contaminated by the minerals present in the native rock formations. Following crushing and extraction of the desirable materials, undesirable materials may become contaminated in the wastewater. For metal mines, this can include unwanted metals such aszinc and other materials such as arsenic. Extraction of high value metals such as gold and silver may generate slimes containing very fine particles in where physical removal of contaminants becomes particularly difficult.
Mines and quarries
The principal wastewater associated with mines and quarries are slurries of rock particles in water. These arise from rainfall washing exposed surfaces and haul roads and also from rock washing and grading processes. Volumes of water can be very high, especially rainfall related arisings on large sites. Some specialized separation operations, such as coal washing to separate coal from native rock using density gradients, can produce wastewater contaminated by fine particulate hematite and surfactants. Oils and hydraulic oils are also common contaminants. Wastewater from metal mines and ore recovery plants are inevitably contaminated by the minerals present in the native rock formations. Following crushing and extraction of the desirable materials, undesirable materials may become contaminated in the wastewater. For metal mines, this can include unwanted metals such as zinc and other materials such as arsenic. Extraction of high value metals such as gold and silver may generate slimes containing very fine particles in where physical removal of contaminants becomes particularly difficult.
Wastewater generated from agricultural and food operations has distinctive characteristics that set it apart from common municipal wastewater managed by public or private wastewater treatment plants throughout the world: it is biodegradable and nontoxic, but that has high concentrations of biochemical oxygen demand (BOD) and suspended solids(SS). The constituents of food and agriculture wastewater are often complex to predict due to the differences in BOD and pH in effluents from vegetable, fruit, and meat products and due to the seasonal nature of food processing and postharvesting.
Processing of food from raw materials requires large volumes of high grade water. Vegetable washing generates waters with high loads of particulate matter and some dissolved organics. It may also contain surfactants.
Animal slaughter and processing produces very strong organic waste from body fluids, such as blood, and gut contents. This wastewater is frequently contaminated by significant levels of antibiotics and growth hormones from the animals and by a variety of pesticides used to control external parasites. Insecticide residues in fleeces is a particular problem in treating waters generated in wool processing.
Processing food for sale produces wastes generated from cooking which are often rich in plant organic material and may also contain salt, flavourings, colouring material and acidsor alkali. Very significant quantities of oil or fats may also be present.
Complex organic chemicals industry
A range of industries manufacture or use complex organic chemicals. These include pesticides, pharmaceuticals, paints and dyes, petro-chemicals, detergents, plastics, paper pollution, etc. Waste waters can be contaminated by feed-stock materials, by-products, product material in soluble or particulate form, washing and cleaning agents, solvents and added value products such as plasticisers.
The waste production from the nuclear and radio-chemicals industry is dealt with as Radioactive waste.
Many industries have a need to treat water to obtain very high quality water for demanding purposes. Water treatment produces organic and mineral sludges from filtration and sedimentation. Ion exchange using natural or synthetic resins removes calcium, magnesium and carbonate ions from water, replacing them with hydrogen and hydroxyl ions. Regeneration of ion exchange columns with strong acids and alkalis produces a wastewater rich in hardness ions which are readily precipitated out, especially when in admixture with other wastewaters.
Treatment of industrial wastewater
The different types of contamination of wastewater require a variety of strategies to remove the contamination.
Most solids can be removed using simple sedimentation techniques with the solids recovered as slurry or sludge. Very fine solids and solids with densities close to the density of water pose special problems. In such case filtration or ultrafiltration may be required. Although, flocculation may be used, using alum salts or the addition of polyelectrolytes.
Oils and grease removal
Many oils can be recovered from open water surfaces by skimming devices. Considered a dependable and cheap way to remove oil, grease and other hydrocarbons from water, oil skimmers can sometimes achieve the desired level of water purity. At other times, skimming is also a cost-efficient method to remove most of the oil before using membrane filters and chemical processes. Skimmers will prevent filters from blinding prematurely and keep chemical costs down because there is less oil to process.
Because grease skimming involves higher viscosity hydrocarbons, skimmers must be equipped with heaters powerful enough to keep grease fluid for discharge. If floating grease forms into solid clumps or mats, a spray bar, aerator or mechanical apparatus can be used to facilitate removal.
However, hydraulic oils and the majority of oils that have degraded to any extent will also have a soluble or emulsified component that will require further treatment to eliminate. Dissolving or emulsifying oil using surfactants or solvents usually exacerbates the problem rather than solving it, producing wastewater that is more difficult to treat.
The wastewaters from large-scale industries such as oil refineries, petrochemical plants, chemical plants, and natural gas processing plants commonly contain gross amounts of oil and suspended solids. Those industries use a device known as an API oil-water separator which is designed to separate the oil and suspended solids from their wastewater effluents. The name is derived from the fact that such separators are designed according to standards published by the American Petroleum Institute (API).
The API separator is a gravity separation device designed by using Stokes Law to define the rise velocity of oil droplets based on their density and size. The design is based on thespecific gravity difference between the oil and the wastewater because that difference is much smaller than the specific gravity difference between the suspended solids and water. The suspended solids settles to the bottom of the separator as a sediment layer, the oil rises to top of the separator and the cleansed wastewater is the middle layer between the oil layer and the solids.
Typically, the oil layer is skimmed off and subsequently re-processed or disposed of, and the bottom sediment layer is removed by a chain and flight scraper (or similar device) and a sludge pump. The water layer is sent to further treatment consisting usually of an Electroflotation module for additional removal of any residual oil and then to some type of biological treatment unit for removal of undesirable dissolved chemical compounds.
A typical API oil-water separator used in many industries
Parallel plate separators are similar to API separators but they include tilted parallel plate assemblies (also known as parallel packs). The parallel plates provide more surface for suspended oil droplets to coalesce into larger globules. Such separators still depend upon the specific gravity between the suspended oil and the water. However, the parallel plates enhance the degree of oil-water separation. The result is that a parallel plate separator requires significantly less space than a conventional API separator to achieve the same degree of separation.
A typical parallel plate separator
Removal of biodegradable organics
Biodegradable organic material of plant or animal origin is usually possible to treat using extended conventional wastewater treatment processes such as activated sludge ortrickling filter. Problems can arise if the wastewater is excessively diluted with washing water or is highly concentrated such as neat blood or milk. The presence of cleaning agents, disinfectants, pesticides, or antibiotics can have detrimental impacts on treatment processes.
Activated sludge process
A generalized, schematic diagram of an activated sludge process.
Activated sludge is a biochemical process for treating sewage and industrial wastewater that uses air (or oxygen) and microorganisms to biologically oxidize organic pollutants, producing a waste sludge (or floc) containing the oxidized material. In general, an activated sludge process includes:
An aeration tank where air (or oxygen) is injected and thoroughly mixed into the wastewater.
A settling tank (usually referred to as a “clarifier” or “settler”) to allow the waste sludge to settle. Part of the waste sludge is recycled to the aeration tank and the remaining waste sludge is removed for further treatment and ultimate disposal.
Trickling filter process
A cross-section of the contact face of the bed media in a trickling filter
A trickling filter consists of a bed of rocks, gravel, slag, peat moss, or plastic media over which wastewater flows downward and contacts a layer (or film) of microbial slime covering the bed media. Aerobic conditions are maintained by forced air flowing through the bed or by natural convection of air. The process involves adsorption of organic compounds in the wastewater by the microbial slime layer, diffusion of air into the slime layer to provide the oxygen required for the biochemical oxidation of the organic compounds. The end products include carbon dioxide gas, water and other products of the oxidation. As the slime layer thickens, it becomes difficult for the air to penetrate the layer and an inner anaerobic layer is formed.
The components of a complete trickling filter system are: fundamental components:
- A bed of filter medium upon which a layer of microbial slime is promoted and developed.
- An enclosure or a container which houses the bed of filter medium.
- A system for distributing the flow of wastewater over the filter medium.
- A system for removing and disposing of any sludge from the treated effluent.
The treatment of sewage or other wastewater with trickling filters is among the oldest and most well characterized treatment technologies.
A trickling filter is also often called a trickle filter, trickling biofilter, biofilter, biological filter or biological trickling filter.
Treatment of other organics
Synthetic organic materials including solvents, paints, pharmaceuticals, pesticides, coking products and so forth can be very difficult to treat. Treatment methods are often specific to the material being treated. Methods include Advanced Oxidation Processing, distillation, adsorption, vitrification, incineration, chemical immobilization or landfill disposal. Some materials such as detergents may be capable of biological degradation and in such cases, a modified form of wastewater treatment can be used.
Treatment of acids and alkalis
Acids and alkalis can usually be neutralized under controlled conditions. Neutralisation frequently produces a precipitate that will require treatment as a solid residue that may also be toxic. In some cases, gasses may be evolved requiring treatment for the gas stream. Some other forms of treatment are usually required following neutralisation.
Waste streams rich in hardness ions as from de-ionisation processes can readily lose the hardness ions in a buildup of precipitated calcium and magnesium salts. This precipitation process can cause severe furring of pipes and can, in extreme cases, cause the blockage of disposal pipes. A 1 metre diameter industrial marine discharge pipe serving a major chemicals complex was blocked by such salts in the 1970s. Treatment is by concentration of de-ionisation waste waters and disposal to landfill or by careful pH management of the released wastewater.
Treatment of toxic materials
Toxic materials including many organic materials, metals (such as zinc, silver, cadmium, thallium, etc.) acids, alkalis, non-metallic elements (such as arsenic or selenium) are generally resistant to biological processes unless very dilute. Metals can often be precipitated out by changing the pH or by treatment with other chemicals. Many, however, are resistant to treatment or mitigation and may require concentration followed by landfilling or recycling. Dissolved organics can be incinerated within the wastewater by Advanced Oxidation Processes.
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