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.
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), SCH
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.
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:
Reference Monod growth rate expression for fermenting bacteria:
μF = μ̂F SP / (KP + SP)
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:
Reference Monod growth rate expression for acetogenic bacteria:
μSAB = μ̂SAB SVA / (KVA + SVA) KA / (KA + SA)
Product (acetate) inhibition switching function (when SA >> KA, µSAB << μ̂SAB)
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:
μM = μ̂M SA / (KA + SA) 1 / (log([H+] / 10-7) + 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.
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) Methane, SCH (mg/L COD) ρj
Hydrolysis -1 1 qH*XF
Fermentation 1 YVA/YF -1/YF µF*XF
Acetogenesis YA/YSAB -1/YSAB 1 µSAB*XSAB
Methanogenesis -1/YM 1 YCH/YA µM*XM
YVA = g-volatile acids (COD) produced/g-particulate COD consumed
YF = g-fermenting biomass (COD) grown/g-hydrolysis products
YA = g-acetate produced/g-volatile acid COD consumed
YSAB = g-SAB cell growth (COD)/g-volatile acids (COD) consumed
YM = g-archaea cell growth (COD)/g-acetate (COD) consumed
YCH = g-methane produced (COD)/g-acetate (COD) consumed
qH = kH (XS/XF) (XS / (KXS + XS/XF)) (mg-COD-SP/mg-COD-XF/d) Hydrolysis product formation rate
µF = μ̂F (SP / (KP + SP)) (mg-COD-XF/mg-COD-SP/d) Fermenting bacteria growth rate
µSAB = μ̂SAB (SVA / (KVA + SVA)) (KA / (KA + SA)) (mg-COD-XSAB/mg-COD-SVA/d) Acetogenic bacteria growth rate
µM = μ̂M (SA / (KA + SA)) (1 / (log([H+] / 10-7) + 1)) (mg-COD-XM/mg-COD-SA/d) Methanogenic archaea growth rate
Summary of microbial process issues:
1. Anaerobic digestion depends on coordination of three trophic groups of microorganisms.
2. The rate determining step depends on digester conditions: carbon substrates, temperature, pH, etc. Often, it is hydrolysis.
3. Metabolite inhibition (especially pH, acetate) can determine process performance. Methanogens need neutral pH.
4. Spatial organization of populations is important, especially SAB and archaea for interspecies metabolite transfer. Flocculant and mixed suspensions favor optimal spatial organization.
5. Speculation that significant hydrogen resides in micro-environment rather than bulk liquid or gas phases and may not be measurable even though it is a substrate.
Example. Anaerobic digester is CSTR, T = 35 C, Q = 3,000 m3/d, influent is CODXS = 10,000 g/m3. Overall yield for mixed population, Y = 0.04 g cells produced/g-COD destroyed, 50% of influent COD is destroyed, 70% of the digester gas produced is methane (CH4) and 30% is CO2.
Find the rate of methane production in m3/day under steady-state condition.
Steady-state mass balance for COD on digester CSTR:
0 = CODXS,IN – COD XS,OUT – COD – CODCH4,OUT
0 = Q(CODXS,IN) – Q(0.5 CODXS,IN) – QY(1-0.5)CODXS,IN – RCOD-CH4
RCOD-CH4 = Q CODXS,IN (1 – 0.5 -0.02) = 3,000(10,000)(0.48)
= 1.44 x 107 g-COD-CH4/day
Assume CH4 is ideal gas p = 1 atm, T = 35 C.
RV, COD-CH4 = 1.44 x 107 g CODCH4/day (0.4 LCH4/g CODCH4)10-3 m3/L
= 5,800 m3/day CH4
Total volume of gas produced/d = 5,800/0.7 = 8,200 m3/day digester gas.
GENERAL ANAEROBIC DIGESTER PROCESS CHARACTERISTICS
1. Suspended growth, mixed system (CSTR no recycle. Feed rate is too low compared with process volume to bother with batch or semi-batch simulation)
2. Low growth rate and low cell yields for bacteria and archaea, compared with aerobic heterotrophs.
3. Heated: 35 C for mesophilic, 55 C for thermophilic
4. Reducing environment: ORP = -200 to -400 mV
5. Floating cover for gas separation and storage
6. Mixing and heating operation often use same equipment
Typical design parameters for typical mesophilic high rate digester:
15 d < Θ = τ < 20 d
Process Control:
1. Consistency in sludge feed (primary and secondary fractions) and feed rate. Spikes in COD loading can produce excess acid due to rapid growth of fermenting bacteria.
2. Alkalinity, typically 2 to 5 g-CaCO3/L. Source is biological reactions, but can be added if pH drops too low.
Typical high rate mesophilic digester performance
1. Typical TS reduction range = 45 – 50%
2. Typical VS reduction range = 55 – 65%
3. Biogas production ≅ 0.5 m3/kg-COD consumed
4. Methane production ≅ 0.35 m3/kg-COD consumed
BioWin Anaerobic Digester Example
Steady state solution
SRT: 20 days
Temperature: 35.0
Flowsheet
Configuration information for all Anaerobic Digester units
Physical data
Element name Volume [m3] Area [m2] Depth [m] Head space volume
Anaerobic Digester0 60000 10000. 6.0 20000.
Operating data Average (flow/time weighted as required)
Element name Pressure pH
Anaerobic Digester0 103.0 7.0
Element name Average Temperature
Anaerobic Digester0 35.0
Configuration information for all COD Influent units
Element name Flow (m3/d) COD mg/L TKN mg N/L Total P mgP/L pH Alk mM Inorg S.S. mgTS S/L Ca mg/L Mg mg/L DO mg/L
COD in 3000 10000 600. 50. 7.30 10 2000. 160 25. 0.
Anaerobic Digester Effluent, BioWin Simulation
Water Quality Components Conc. (mg/L) Mass rate (kg/d) Notes
Volatile suspended solids 2237.18 6711.53
Total suspended solids 4253.85 12761.54
Particulate COD 3544.99 10634.98
Filtered COD 1172.99 3518.96
Total COD 4717.98 14153.94
Soluble PO4-P 17.56 52.68
Total P 50.00 150.00
Filtered TKN 478.94 1436.82 All NH4-N
Particulate TKN 113.88 341.65
Total Kjeldahl Nitrogen 592.82 1778.47
Filtered Carbonaceous BOD 122.13 366.39
Total Carbonaceous BOD 899.51 2698.52
Nitrite + Nitrate 0.00 0.00
Total N 592.82 1778.47
Total inorganic N 476.72 1430.17
Alkalinity 16.28 48.83 mmol/L, kmol/d
pH 6.49 A bit low
Volatile fatty acids 134.95 404.85
Total precipitated solids 0 0.00
Total inorganic suspended solids 2016.67 6050.01
Ammonia N 476.72 1430.17
Nitrate N 0.00 0.00
Operation and Performance Value Units
Hydraulic residence time 480.00 hours Compare with
Flow 3000.00 m3/d Pg 9 example
Gas flow rate (dry) 8368.77 m3/d 8,200 m3/d
Methane content 72.63 % 70%
Carbon dioxide content 26.31 % 30%
Hydrogen content 0.09 %
Ammonia content 0.46 %
VSS destruction 44.96 % 50%