SELECTED EFFECTS OF ANAEROBIC SLUDGE COMPOSITION ON A BIOGAS PRODUCTION
Intensification of biological treatment processes require more accurate both quantitative and qualitative assessment of a biogas production and factors affecting this process. Better energy recovery is of crucial importance in the overall sludge disposal, however it deeply depends on methane rich gas generation. Increased net biogas production and thus better energetic characteristics of the plant are recognized as the most important reasons to apply disintegration to the wasted activated sludge (WAS) Until now an assessment of efficiency of this process was done by steady state measurements of COD solubilisation, change of sludge flocs' size or change in a protein concentration. Results of tests and full scale operation enlightened some specific problems which may occur in wastewater treatment plants where mesophilic fermentation of sludge was used as a stabilization method a significant decrease of a biogas production was proved. Operational procedure basing on a methanogenic activity respirometric tests was proposed and tested in fullscale design procedure. INTRODUCTION At present multi phase biological reactors are most commonly applied facilities in municipal wastewater treatment. These reactors provide proper technical conditions to perform three main groups of processes in one technical facility as follows (Rybicki, 2009) Conversion of an organic matter contained in raw sewage into a biomass also its respiration products such as: carbon dioxide and water; discard of excess biomass is wasted from a system as so called wasted activated sludge (WAS) Incorporation of surplus amount of phosphorus into a biomass during consecutive exposure of a biomass to anaerobic and aerobic conditions; Conversion of an organic nitrogen and ammonia which are main nitrogen compounds in raw wastewater into their oxidized form: nitrites and nitrates (nitrification) and finally release a gaseous nitrogen into an ambient atmosphere (denitrification) Main byproduct of contemporary wastewater appears to be a specific mixture of solid particles suspended in wastewater which contain solid particles which weren't decomposed during primary treatment (primary sludge PS) and excess biomass from biological processes (WAS) This byproduct has significant potential as an energy source. A large amount of energy, in the form of electricity and oil, is used in municipal wastewater treatment plants (WWTPs) Reduction of fossil fuel consumption by municipal WWTPs should be based on production of renewable energy from biosolids, since large quantities of organic matter are included in biosolids. However, most biosolids are simply incinerated or landfilled so an anaerobic digestion (AD) is one of the most useful technologies for renewable energy production from biosolids (Lettinga 1996, Mizuta et al, 2011).The main aims of anaerobic sludge disposal are: biological sludge stabilization and recovery of highly energetic methane gas. Operational disadvantages of raw wastewater sludges are their hydrophilic character, high viscosity, odour emission and risk of biohazard. Anaerobic stabilization processes is converting sludges to biologically stable product for ultimate disposal, safe storage and Rybicki, Cimochowicz Rybicka, Selected effects of anaerobic sludge composition on a biogas production 214 use. Stabilization also reduces the odours, bacteria levels and amount of solids present in the sludge. Besides it is possible to recover an energy thermal and/or electrical in an anaerobic process. During a methane fermentation approximately 50% of total organic matter content in sludge is decomposed producing a biogas (Cimochowicz Rybicka 1999, CimochowiczRybicka&Rybicki, 2011) The biogas is a mixture which content tends to vary depending mostly on sludge content. Gas composition is influenced by degradable matter content while produced gas volume depends mostly on an amount of mineralised organic matter content. Anaerobic degradation The microbial decomposition of complex organic substrates provides a source of energy and building materials to the bacterial cells which convert the organic material to carbon dioxide and methane. The anaerobic degradation of organic material has been described as a multi step conversion of many, parallel, biochemical reactions. Many groups of anaerobic bacteria are involved in this process. Four main stages of this degradation can be distinguished (CimochowiczRybicka, 1999) a. hydrolysis of complex organic materials to soluble products (hydrolytic fermentative bacteria) b. acidogenesis generation of intermediary products such as short chain fatty acids, (hydrogen producing and acetogenic organisms) c. acetogenesis acetate production (hydrogen producing, hydrogen consuming acetogenic organisms) d. methanogenesis methane production (methane forming bacteria) The nature and chemical composition of the substrates used in anaerobic digestion dictate the type and amount of the products. In case the chemical composition of the organic matter is known, the stoichiometric equation according to McCarty (Pavlostatis et al 1999; Cimochowicz Rybicka,2000) can be used for the description of organic matter conversion to methane: CnHaObNc (2n c b 9sd/20 ed/4) H2O d*e/8 CH4+ (n c s*d/5 de/8) CO2+ sd/20 C5H7O2N (c sd/20) NH4 (c sd/20) HCO3 where: d 4n a 2b 3c, s fraction of waste converted to cells, e fraction of waste converted to methane for energy (s e 1) CnHaObN empirical formula of waste being digested, C5H7O2N empirical formula of bacterial dry mass (VSS) Methane production All biochemical processes taking place within a living cell cause chemical structural changes accompanied by an energetic effects. These effects are strictly related to change of chemical potential of reacting substances. So two main types of such reaction can be recognised: exergonic which progress spontaneously causing decrease of total chemical potential of the system endergonic which cause increase of total chemical potential within the system so this type of reactions require to be supplemented by energy from outside of the system. Methane generation is a final stage of organic matter decomposition in an anaerobic conditions resulting from its chemical structure four valent carbon atoms are saturated with four hydrogen atoms so a methane gas is most stable product of this process. Numerous bacteria species are specialised in methane production from different compounds (Ferry,1993; Cimochowicz Rybicka Rybicki, Cimochowicz Rybicka, Selected effects of anaerobic sludge composition on a biogas production 215 2000, Cimochowicz Rybicka&Rybicki, 2009) Some of these bacteria species utilise substrates to cell growth and reproduction both as an energy source and as the sole carbon source. The catabolic pathways of methanogens are very complicated and usually they are presented as one of three groups: CO2 reducing, methylotrophic and aceticlastic pathways as it was detailed in references .Most of the methane produced in nature originates from acetate. However, the some amounts of methane produced from the methyl group of acetate or reduction of CO2 can vary depending on the presence of other metabolic groups of anaerobes and the environment, this acid decomposition can be described by following equation: CH3COOH CO2 CH4 DG' 36 kJ/mol CH4 (Eq 1) An acetate is a major substrate for methane production, only two genera of methanogenic acetotrophs Methanosarcina and Methanothrix and a few species have been described. MAIN FACTORS AFFECTING FERMENTATION GAS CHARACTERISTICS Typical values being used for design purpose are based on specific unit volume of a gas which can be obtained from each of substrates, typical reference values are Table 1. Table 1. The amount and gas composition produced in anaerobic digestion typical values Substrate Gas amount [L/kg VSS] Gas characteristic Carbohydrates 790 50 CH4 50 CO2 Lipids 1250 68 CH4 32 CO2 Protein 700 71 CH4 29 CO2 These values in real term conditions tend to vary as real sludge is a mixture of substrates in varying proportions. Gas production rates range from 750 1000 L of gas produced per kg volatile solids removed (or from 300 to 600 L of gas per kg of VSS delivered to digestion chamber. Biogas production in relation to COD is about 0.5 m/kg COD removed, corresponding to a methane production of 0.35 m CH4 per kg of COD removed (Grady et al. 1999, 2010; CimochowiczRybicka 2000) Gas composition varies with sludge characteristics. Gas components include methane, carbon dioxide, hydrogen sulfide, nitrogen and hydrogen. The characteristic of digestion gas is generalized in Table 2. Rybicki, Cimochowicz Rybicka, Selected effects of anaerobic sludge composition on a biogas production 216 Table 2. Typical characteristic of anaerobic digester gas Constituent Percentage fraction, Methane Carbon dioxide Hydrogen sulfide Nitrogen Hydrogen 55 75 25 45 0.01 1 2 6 0.1 2 Methane determine the heating value of the gas; carbon dioxide represents the stabilized carbon; hydrogen sulfide determines the corrosivity and odour potential of the gas. Characterisation of components of fed and digested sludge Data presented in Table 3 were obtained from lab tests on sludge from Nowy Sacz WWTP, results reflect 8 grab samples from 2010 year. Numeric data are average values from these samples. Table 3. Characterisation of sludges and total solids components grab samples Parameter Unit Primary sludge WAS Digester chamber feed Digested sludge COD mg O2/L 38000 46000 4100