BACKGROUND

Technical Field

The present invention is related to a method for treatment of landfill leachate and for obtaining useful materials therefrom. The present invention relates to a method of treating a landfill leachate containing ammoniacal nitrogen and phosphate by mixing with seawater to precipitate magnesium ammonium phosphate (MAP), also known as struvite.

Description of Related Art

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.

Landfilling is the most commonly used waste disposal system in many countries because of its simplicity, minimal required ongoing supervision, technical feasibility, and low cost [Kamaruddin, M. A., et. al., Applied Water Sciences, 2015, 5, 113-126 and Aziz, S. Q., et. al., J Environment Management, 2010, 91, 2608-2614]. However, landfills have a huge drawback in that each requires a very large site. Further, ideal sites must have specific geographic, hydrologic, or geologic properties. The availability of such areas is becoming limited due to factors such as increasing population and urban and suburban expansion. Due to the undesirable nature of living near a landfill, the area allocated for landfill is being pushed away from populated areas. In some places landfills are being located nearer to coastal areas. This has the potential to cause undesirable environmental consequences stemming from contamination of the coastal water by leachate or leachate infiltration into land surface water and groundwater. Further, coastal areas are more prone to flooding, decreasing the viability of time-intensive leachate treatment methods or evaporation to solid. To properly manage leachate in such areas, as well as effectively prevent contamination of surrounding water bodies, leachate should be removed from landfills and treated such that the decontaminated water may be returned to the environment.

The chemical constituents present in landfill leachate are generated by the degradation of solid waste in the landfill. The quantity and composition of the landfill leachate are affected by several factors including waste from water percolation, biochemical process in wastes, and the degree of waste compaction [Abbas, A. A., et. al., American Journal of Applied Sciences, 2009, 6, 4, 672-684]. Further, the age of landfill contributes to differences in compositions of landfill leachate. A young leachate (that is, leachate from a landfill 1-2 years old) is characterized by high organic fraction of relatively low molecular weight volatile organic acids, high chemical oxygen demand (COD), total organic carbon (TOC), BOD5 and a BOD5/COD>0.6 [Umar, M., et. al., Waste management, 2010, 30, 11, 2113-2121]. Usually, landfills which are more than 10 years old are characterized by relatively low chemical oxygen demand (COD), e.g., less than 4,000 mg/L, a slightly basic pH, e.g., more than 7.5 and a low biodegradability (BOD5/COD<0.1) [Li, W, et. al., Desalination, 2010, 264, 56-62]. Generally, there is a wide variation of semi-aerobic and anaerobic landfill leachate composition in terms of leachate parameters indicating that aeration process plays a significant role in reducing the concentration of several contaminants.

Within leachate, ammoniacal nitrogen is one of the major pollutants that must be removed prior to release into the environment. Ammoniacal nitrogen concentration in leachates varies from hundreds to thousands of milligrams per liter depending on the factors such as the age of the landfill, solid waste types and loadings, landfill site meteorological, geological, and hydrological conditions, and others. In most situations, the ammoniacal nitrogen concentration tends to increase substantially with the age of the landfill because of the advanced degradation of biological compounds which give rise to ammonia in leachate. Various types of treatments including biological, physical, chemical, and physico-chemical techniques have been explored for the removal of ammoniacal nitrogen.

The most common method for removing ammoniacal nitrogen from landfill leachate is nitrification-denitrification, typically performed as part of activated sludge treatment. Activated sludge plants, which typically treat carbon-based contaminants, often discharge ammonia as an effluent not only as an unresolved constituent of wastewater but also as a byproduct of the treatment process itself. Typically, the concentration of the domestic wastewater ammonia treated in activated sludge process is in the range from 10 to 50 mg N/L. Biological nitrification-denitrification has the potential to mitigate this ammonia release by converting the ammonia to nitrate, then removing the nitrate. However, nitrification-denitrification reactor conditions must be conducive to the growth and activity of both heterotrophic and nitrifying bacteria for simultaneous carbon and ammonia oxidation to occur. Factors including sludge age (Mean Cell Residence Time (MCRT)), pH, temperature, dissolved oxygen (DO) concentration, electron donor substrate concentration, substrate composition, loading rate, and the presence of toxic substances are considered relevant to the growth and activity of heterotrophs and to nitrifiers.

Conventional nitrification-denitrification processes are unable to effectively remove the high levels of ammoniacal nitrogen present in landfill leachate, and the decrease of other readily biodegradable compounds in the leachate limits the ability of microbes to mitigate nitrogen-containing compounds in other ways. These methods also require high energy for aeration. Short-cut nitrification/denitrification and anaerobic ammonium oxidization are other biological techniques that have shown promise in solving the above problems. However, the results have shown that these related biological treatments are also negatively affected by high concentration of ammonium-nitrogen in leachate. Also, chemical precipitation of ammoniacal nitrogen as pre-treatment to biological treatment has shown to cause a decrease in efficiency of COD removal and the dehydrogenase activity while the NH4+concentration increased [Li, X. Z., et. al., Chemosphere, 2001, 44, 37-43]. Therefore, it is not possible to constantly control the nitrification-denitrification process to the required degree in high ammonia leachates. Further, these nitrification-denitrification processes decrease the efficiency of other necessary leachate treatment steps, dramatically limiting their utility.

A promising route to the removal of ammoniacal nitrogen is chemical treatment through the formation of Magnesium-Ammonia-Phosphate (MAP). MAP, also known as struvite, precipitation is able remove ammonia by effectively trapping it inside a solid precipitate that can be easily separated from the leachate solution. The struvite is a white insoluble crystalline compound produced from the treatment reaction shown in the equation below:
Mg2+(aq)+NH4+(aq)+HPO43−(aq)+OH−(aq)+5H2O→MgNH4PO4.6H2O↓  (1)

MAP precipitation has proven effective for treating wastewater having high ammonia content [Crutchik, D. & Garrido, J. M., Water Sci Technol., 2011, 64, 12, 2460-7; and Kumashiro K., Ishiwatari H and Nawamura Y., Second International Conference on Recovery of Phosphates from sewage and animal wastes. 12-13 Mar. 2001. Noordwijkerhout, Holland] and also from landfill leachate [Camargo, C., et. al., Water S A, 2014, 40, 3, 491-494; Di Iaconi, et. al., Bioresource technology, 2010, 101,6, 1732-1736; Huang, H., et. al., J Environ Management, 2014, 145, 191-8; and Xiu-Fen, L., et. al., Environmental Chemistry Letters, 2011, 9, 1, 71-75] using various chemical additives as the magnesium and/or phosphate source. Additionally, struvite has potential as a slow release biofertilizer. The main disadvantage of these processes is the requirement of high-grade additive chemicals needed to react with ammoniacal nitrogen. It is estimated that about 75% of the operation cost of MAP precipitation treatment of leachate is the use of high-grade magnesium sources [Kataki, S., et. al., Conservation and Recycling, 2006, 107, 142-156]. Furthermore, the use of pure compounds and transportation of these chemicals for the treatment is costly and dramatically increases the risk of environmental contamination. Additionally, the leachate may contain heavy metals that are toxic to plants and/or animals and prevent the use of MAP precipitate from landfill leachate as a fertilizer. However, the use of surrounding resources such as seawater would be of great advantage for both operational and transportation costs. For example, seawater can serve as a reliable source of magnesium, particularly in landfills near coastal areas. Further, seawater could be used in sufficient amounts so as to sufficiently dilute heavy metals in the leachate without contaminating the MAP.

The use of seawater as a substitute source of magnesium in leachate treatment via Magnesium Ammonium Phosphate (MAP) precipitation process was previously investigated [M. Shahrir, and M. Zahari, S. I., Potential Use of Sea Water In Leachate Physicochemical Treatment: Process Performance And Struvite Formation. International Conference on Sustainable Development and Livelihoods, ICSDL 2016, Malaysia, which is incorporated herein by reference in its entirety]. A series of batch studies were carried out for the MAP precipitation in the laboratory using a mixture of seawater and leachate but did not lead to a practical and effective treatment method. In order to address the shortcomings of prior work the present disclosure describes a process that utilizes seawater effectively in combination with MAP.

SUMMARY

The present invention relates to a method of treating a landfill leachate containing ammoniacal nitrogen and phosphate by mixing with seawater to precipitate magnesium ammonium phosphate (MAP), also known as struvite.

In an exemplary embodiment, a method of removing ammoniacal nitrogen from a landfill leachate is described. The method comprises adding oxalic acid to seawater from a depletion solution having an oxalic acid concentration of 0.01 to 0.25 M, precipitating a first precipitate comprising calcium oxalate to form a depleted seawater, adding a phosphate source to the depleted seawater to form a supplemented seawater, mixing the supplemented seawater with a landfill leachate having an ammoniacal nitrogen content of 100 to 10,000 mg/L to form a reaction mixture having a molar ratio of Mg2+to NH4+of 1:1 to 1:5, adjusting the pH of the reaction mixture to 7.5 to 11.0 to form an adjusted reaction mixture, aging, for 10 to 120 minutes, the adjusted reaction mixture to form a treated leachate and a second precipitate comprising magnesium ammonium phosphate, and separating the second precipitate and the treated leachate, wherein the second precipitate is substantially free of calcium.

In some embodiments, the method further comprises removing the first precipitate from the depleted sweater.

In some embodiments, the method further comprises measuring the ammonia concentration in the landfill leachate and calculating a volumetric mix ratio of landfill leachate to supplemented seawater.

In some embodiments, the method further comprises adding a second precipitate seed solid to the reaction mixture.

In some embodiments, the second precipitate seed solid comprises magnesium ammonium phosphate.

In some embodiments, the method further comprises adding a coagulation aid to the reaction mixture.

In some embodiments, the coagulation aid is an iron halide.

In some further embodiments, the iron halide is ferric chloride.

In some embodiments, the method further comprises adding a precipitation inhibitor to the reaction mixture.

In some embodiments, the precipitation inhibitor is boric acid.

In some embodiments, the method comprises contacting the landfill leachate with a cation exchange resin prior to mixing with supplemented seawater.

In some embodiments, the contact with the cation exchange resin reduces a metal ion concentration of the landfill leachate by at least 50% of an initial metal ion concentration.

In some embodiments, the metal ion is at least one selected from the group consisting of manganese, calcium, cadmium, zinc, and iron.

In some embodiments, the landfill leachate is contacted with the cation exchange resin in an amount of 0.01 to 1 g cation exchange resin per mL of landfill leachate.

In some embodiments, the method removes at least 90% of an initial amount of ammoniacal nitrogen present in the landfill leachate.

In some embodiments, the aging is performed for 20 to 90 minutes.

In some embodiments, the aging is performed with stirring.

In some embodiments, the aging is performed at 20 to 50° C.

In some embodiments, the method further comprises, prior to the mixing, demethanizing the landfill leachate.

The foregoing general description of the illustrative embodiments and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.

