ANAEROBIC DIGESTION PROCESSES.pdf1 ANAEROBIC DIGESTION PROCESSES Functional definition of “Anaerobic” = absence of oxygen or nitrate. Role of anaerobic processes in wastewater treatment 1. Enhanced biological phosphate removal (EBPR) 2. Sludge stabilization = reduction in volatile (bioreactive) solids from primary, biofilm, and waste activated sludge 3. Reduction in pathogens in sludge 4. Energy recovery as biofuels production, primarily methane (CH4) For 2-4, the unit process is the anaerobic digester. General characteristics: mixed suspended solids, complex microorganism communities, long hydraulic and solids residence time (30-60 days), mesophilic temperature (~35 C) Dominant Microbial populations: Bacteria and Archaea Rate and extent of stabilization and methane production depend strongly on population interactions. Three groups of anaerobic microorganisms in digester populations: • Group I: hydrolytic fermentative bacteria • Group II: Syntrophic acetogenic bacteria (SAB) • Group III: Archaea (methanogens) Populations, substrates, products and reaction stoichiometries are shown in Figure 1 and Table 1, following.2 Fig 1. PARTICULATE HYDROLYSIS: HYDROLYTIC FERMENTATIVE BACTERIA (Xf) FERMENTATION, Xf AND ACIDOGENESIS: SYNTROPIC ACIDOGENIC BACTERIA, XSAB METHANOGENESIS: ARCHAEA, XM Complex Biodegradable Particulates, XS Proteins and carbohydrates, SP Lipids, SP Amino acids and simple sugars, SP Long Chain Fatty Acids, SP Volatile (Fatty) Acids: propionate, butyrate, etc. (R-COOH), SVA Acetic Acid (CH3COOH) SA Hydrogen (H2) Methane (CH4), SCH3 Table 1. MICROBIAL REACTIONS (MOLAR STOICHIOMETRIES) I. Fermentation reaction examples (glucose substrate, various VA products) Product Reaction ΔG (kJ/mole glucose) Lactate C6H12O6 à 2CH3CH(OH)COO- + 2H+ -198.1 Butyrate C6H12O6 + 2H2O à CH32(CH2)COO- + 2HCO3- +2H2 + 3H+ -254.4 Propionate + acetate 1.5C6H12O6 à 2CH3CH2COO- + CH3COO- + HCO3- + 3H+ -109.9 Genera: Bacteroides, Clostridium, etc. II. Syntropic acetogenic reaction examples (various VA substrates, acetate, H2 products) Substrate Reaction ΔG (kJ/mole substrate) Lactate CH3CH(OH)COO- + 2H2O à CH3COO- + HCO3- + 2H2 + H+ - 3.96 Butyrate CH32(CH2)COO- + 2H2O à 2CH3COO- + 2H2 + H+ + 48.1 Propionate CH3CH2COO- + 3H2O à CH3COO- + HCO3- + 3H2 + H+ + 76.1 Genus: Acetobacter III. Methanogenic reaction examples (various substrates, CH4 product) Substrate Reaction ΔG (kJ/mole substrate) Acetate CH3COO- + 2H2O à CH4 + HCO3- - 31.0 Hydrogen 4H2 + HCO3- + H+ à CH4 + 3H2O - 33.9 Formate 4COO- + H2O + H+ à CH4 + 3HCO3- - 32.6 Genera: Methanococcus, Methanosarcina, Methanospirillum, etc.4 Group I. Hydrolytic and Fermentative bacteria (Xf) Reactions: a. Hydrolysis of particulate COD (XS) primarily by anaerobic bacteria (not facultative, generally): Bacteroides, Clostridium, Bifidobacteria to produce amino acids, simple sugars, lipids and fatty acids. Hydrolysis reactions are not considered to be growth related: -XS + SP = 0 (COD basis) Where Xf = population of hydrolytic/fermenting bacteria and SP = soluble hydrolysis products -rXS = rSP !!"= −!!!!!!!!+!!!!!! b. Fermentation of hydrolysis products by same strains of anaerobic bacteria (reactions are growth linked). Products are volatile fatty acids (lactate, propionate, butyrate, formate), alcohols, in addition to cells. - SP + YfXf + YVASVA = 0 (COD basis) Where Yf = fermenting bacteria cell growth/hydrolyzed products consumed and YVA = volatile acids produced/hydrolysis products consumed, and SVA = soluble volatile acids Rearranging so Xf is reference component: - SP/Yf + Xf + (YVA/Yf)SVA = 0 (COD basis) Relative rates from stoichiometry: !!"#!!"!!= !!"=−!!"1!!5 Reference Monod growth rate expression for fermenting bacteria: !!"= !!!!!!+ !!!! Fermenting bacteria produce protons (acid). Important factor in keeping digester environment balanced and stable is that consumption of VA’s and available alkalinity matches proton production by fermenters. Group II. Syntropic acetogenic bacteria (XSAB). SAB reduce protons to H2 and produce acetate and formate from fermentation products, as well as CO2. Note that some of these reactions are not thermodynamically favored (ΔG > 0). They rely on product consumption (interspecies hydrogen or acetate transfer) by Group III Archaea to drive the reaction. SAB produce alkalinity, which is useful for buffering digester pH. Some SABs are inhibited by product (acetate) accumulation. - SVA + YSABXSAB + YASA = 0 (COD basis) Where YSAB = SAB cell growth/volatile acids consumed and YA = acetate produced/volatile acids consumed, and SA = soluble acetate. Rearranging so reference component is XSAB: - SVA/YSAB + XSAB + (YA/YSAB)SA = 0 (COD basis) Relative rates from stoichiometry: !!!!!!"#= !!"#=−!!"1!!"# Reference Monod growth rate expression for acetogenic bacteria: !!"#= !!"#!!"!!"+ !!"!!!!+ !!!!"# Product (acetate) inhibition switching function (when SA >> KA, µSAB << !SAB)6 Group III. Archaea (XM). Methane producing microorganisms. Approximately 2/3 of methane is produced from acetate and 1/3 from hydrogen. Acetoclastic methane production (using acetate as substrate) is important because acid is removed and alkalinity is formed. Bicarbonate also acts as electron acceptor for both SAB and Archaea. Acetoclastic Archaea strains: Methanosarcina, Methanothrix. - SA + YMXM + YCHSCH = 0 (COD basis) Rearranging so XM is reference component: - SA/YM + XM + (YCH/YM)SCH = 0 (COD basis) Where YM = archaea cell growth/acetate consumed, YCH = methane produced/acetate consumed, and SCH = methane. Reference Monod growth rate expression for Archaea using acetate: !!"!!"!!= !!"=−!!"1!! !!"= !!!!!!+ !!1!"#!!10!!+ 1!! Note pH switching function as H+ gets larger than 10-7 (more acidic), pH switching function gets smaller and growth rate decreases. Another group, not particularly valued, but always active, are sulfate reducing bacteria (SRB). SRB respire sulfate anaerobically to produce H2S species using soluble organic compounds, especially acetate and H2, as electron donors.7 ANAEROBIC DIGESTION STOICHIOMETRIC AND KINETIC MATRIX Components Rates Process Acetate, SA (mg/L COD) Particulate COD, XS (mg/L COD) Fermenting Bacteria, XF (mg/L COD) Volatile Acids, SVA (mg/L COD) Soluble Substrate, SP (mg/L COD) Syntrophic Acetogenic Bacteria, XSAB (mg/L COD) Methanogen Archaea, XM (mg/L COD)