Anaerobic Treatment Shihwu Sung, Ph.D., PE Department of Civil, Construction & Environmental Engineering Iowa State University Anaerobic Treatment Short Course Part 1
Fundamentals • * Wastewater Characteristics Analysis • * Anaerobic Fundamentals • Anaerobic Treatment Processes • (Traditional vs. High-Rate) • AD Applications to Sewage Sludge • AD Applications to Animal Wastes • AD Applications to Industrial Wastewaters • Beneficial Use of Biosolids & Regulations* • AD Bio-refinery Concept
Wastewater is water (H2O) plus “something” • “something” or “pollutants” in most of the cases • only count for 1% or less by weight in the wastewater • 1% = 10 g pollutants / Kg of water = 10,000 mg/L • All pollutants can be measured in Solid Matrix Tests
particulate soluble organic 550oC in Temp. inorganic 1μm in size Solid Matrix TS = TSS + TDS ║ ║║ VS = VSS + VDS +++ TFS = FSS + FDS particulate organic soluble organic particulate inorganic soluble inorganic sand, silt salts T : Total S : Solids or Suspended D : Dissolved V : Volatile F : Fixed
Example:A water sample contains: Sugar: 100 mg/L Fine sand: 60 mg/L Bacteria: 25 mg/L Salts e.g., NaCl, KHCO3: 125 mg/L What is the TDS and VS/TS of this sample? T : Total S : Solids or Suspended D : Dissolved V : Volatile F : Fixed TS = TSS + TDS ║ ║ ║ VS = VSS + VDS + + + TFS = FSS + FDS 310 85 225 125 25 100 185 60 125
Organic content in wastewater • Organics can be subdivided into: • Carbohydrate, Protein, Lipid (Oil & Fat) • Biodegradability: • Carbohydrate>Protein>Lipid • As a “rule of thumb” in Anaerobic Digestion • Higher protein generates more alkalinity • Maximum 30% of COD from Lipid • Thermophilic digestion preferred in lipid digestion • due to higher lipid solubility in higher temperature
(BODult / BOD5) = 1.5 – 2.0 Biodegradable COD (BCOD) Estimation: 1. BCOD = BOD5 * (BODult / BOD5) 2. BCOD (Oil & Grease) = FOG * 2.88 3. BCOD (Protein) = ( TKN – NH3-N) * 6.29 * 1.50 4. BCOD (Carbohydrate) = (1) – (2) – (3) Grease: C8H16O C8H16O + 23/2 O2→ 8 CO2 + 8 H2O 2.88 g COD / g grease Protein: C16H24O5N4 (12x16 + 24 + 16x5 + 14x4) / (14x4) = 6.29 1.50 g COD / g protein BODult = Ultimate BOD ~ BCOD FOG = Fat, Oil & Grease TKN = Total Kjeldahl Nitrogen = organic-N + ammonia-N See Handout: Table 3.1 COD Mass Equivalents of Some Common Constituents
COD / BOD5 ratio => Biodegradability Lower ratio => Higher biodegradability Typical value of raw wastewater = 1.5 – 2.0 Typical value of biologically treated effluent = 4 - 8 NH4-N / TKN ratio => Freshness Lower ratio => Fresh sample pH issues: chemical usage, VFA production, etc.
Wastewater Treatment Options • aerobic treatment for soluble chemical oxygen demand (COD) in 50 - 40,000 mg/L range • anaerobic treatment for high CODs (4000 - 50,000 traditionally) • alternative processes for CODs < 50 mg/L (e.g., carbon adsorption, ion exchange) and > 50,000 mg/L (e.g., evaporation and incineration)
Historical development: Mainly used for reducing mass of high solids wastes, e.g. human waste (nightsoil), animal manure, agricultural waste and sludge. • Early applications of anaerobic waste treatment include: • Mouras automatic scavenger - cited in French journal cosmos in 1881 • Septic tank- developed by Donald Cameron in 1895 (England) Anaerobic Waste Treatment : An Overview
Imhoff tank: developed by Karl Imhoff in 1905 (Germany) Imhoff tank is a modified version of septic tank consisted of two - story in which sedimentation was allowed in upper tank and digestion of settled solids in the lower compartment. Upper tank Lower tank Imhoff tank Cont..
No. of plants (Source: Frankin, 2001) Cumulative Number of AD Plants for Industrial Applications
Renewable Energy Food CH4, H2 Liquid Wastes (Industrial, Domestic etc.) Fish ponds Scrubbing Slurries (Sewage sludge, Liquid manure) CH4, H2S, H2, CO2 Anaerobic bioconversion NH4+, PO43-, S2- Post- treatment Treated effluent Solid Wastes (Manure, Organic Refuse) Irrigation Agri- residues (Crops residues etc.) Biosolids Food Organic Fertilizer Food AD – Waste Treatment and Resources Recovery Micro-aerobic So recovery
Anaerobic Digestion (1) Complex Organics Carbohydrates Proteins Lipids 1. Hydrolysis Simple Organics (2) Volatile Organic Acids Propionate, Butyrate, etc. 2. Acidogenesis Acetate H2 + CO2 28% 72% (3) 3. Methanogenesis CH4 + CO2
Definition: Anaerobic treatment is a biological process carried out in the absence of O2 for the stabilization of organic materials by conversion to CH4 and inorganic end-products such as CO2 and NH3. Anaerobic microorganisms Organic materials + Nutrients CH4 + CO2 +NH3 + Biomass Anaerobic processes Anaerobic fermentation Anaerobic respiration Anaerobic Waste Treatment
Anaerobic fermentation In anaerobic fermentation, there is no external electron acceptor. The product generated during the process accepts the electrons released during the breakdown of organic matter. Thus, organic matter acts as both electron donor and acceptor. The process releases less energy and the major portion of the energy is still contained in the fermentative product such as ethanol. Anaerobic fermentation of glucose to ethanol Energy Ethanol Pyruvate Glucose Electron
Anaerobic respiration Anaerobic respiration on the other hand requires external electron acceptor. The electron acceptors in this case could be SO42-, NO3- or CO2. The energy released under such a condition is higher than anaerobic fermentation. Energy CO2 + H2O Pyruvate Glucose Electron H2S CH4 N2 SO42- CO2 NO3- Anaerobic respiration of glucose Electtron acceptors: O2 > NO3- > SO42- > CO2
Less energy requirement as no aeration is needed 0.5-0.75 kWh energy is needed for every 1 kg of COD removal by aerobic process 2. Energy generation in the form of biogas 1.16 kWh electricity is produced for every 1 kg of COD removal by anaerobic process 3. Less biomass (sludge) generation Anaerobic process produces only 20% of sludge that of aerobic process Advantages of AD
CO2 + H2O 0.5 kg Aerobic process Biodegradable organic 1 kg New biomass 0.5 kg Biogas > 0.9 kg Anaerobic process Biodegradable organic 1 kg New biomass < 0.1 kg Waste + O2 CO2+H2O + new cells Waste CH4 + new cells
4. Less nutrients (N & P) requirement Lower biomass synthesis rate also implies less nutrients requirement: 20% of aerobic 5. Application of higher organic loading rate 5-10 times higher organic loading rates than aerobic processes 6. Space saving Higher loading rate requires smaller reactor volume Advantages of AD - continued • Ability to transform several hazardous solvents including • chloroform, trichloroethylene and trichloroethane
1. Long start-up time Lower biomass synthesis rate -> longer start-up time to attain a biomass concentration 2. Long recovery time Subjected to disturbances and take longer time to return to normal operating condition 3. Specific nutrients/trace metal requirements Methanogens have specific nutrients e.g. Fe, Ni, and Co requirements 4. More susceptible to changes in environmental conditions Methanogens are prone to changes in conditions suchas temperature, pH, redox potential, etc. Limitations of AD
5. Treatment of sulfate rich wastewater The presence of sulfate reduces the methane yield could also inhibit the methanogens due to sulfide toxicity 6. Effluent quality of treated wastewater May not able to degrade the organic matter to the level meeting the discharge limits Limitations of AD – continued 7. Treatment of high protein & nitrogen containing wastewater High ammonia may cause inhibition
Organic loading rate: Low loading rates:0.5-1.5 kg COD/m3-day High loading rates:10-40 kg COD/m3-day (for activated sludge process) (for high rate reactors, e.g. AF, & UASB) Biomass yield: High biomass yield:0.35-0.45 kg VSS/kg COD Low biomass yield:0.05-0.15 kg VSS/kg COD (biomass yield is not constant but depends on types of substrates metabolized) (biomass yield is fairly constant irrespective of types of substrates metabolized) Specific substrate utilization rate: High rate: 0.75-1.5 kg COD/kg VSS-day Low rate: 0.15-0.75 kg COD/kg VSS-day Start-up time: Long start-up: 1-2 months for mesophilic Short start-up: 1-2 weeks : 2-3 months for thermophilic Comparison between Anaerobic & Aerobic processes Anaerobic Aerobic
SRT: Longer SRT is essential to retain the slow growing methanogens SRT of 4-10 days is enough Microbiology: Multi-step process and diverse group of microorganisms degrade organic matters in a sequential order Mainly a one-species phenomenon Environmental factors: Less susceptible to changes in environmental conditions. Highly susceptible to changes in environmental conditions Anaerobic Aerobic
Biogas Content • Methane, CH4 : 50 – 75% • Carbon Dioxide, CO2: 25 – 50% • Nitrogen, N2 : 1 – 5% • Hydrogen Sulfide, H2S < 1% • Hydrogen, H2 - trace
KH CO2 (g) CO2(l) + H2O H2CO3 Ka1 Ka2 H2CO3H+ + HCO3- H+ + CO3-2 Methane Content • Carbon Dioxide content can be estimated by reactor pH • and bicarbonate concentrations KH = Henry’s Law constant pKa1 = 6.33 pKa2 = 10.33 at 20oC • Methane content is the balance of carbon dioxide
Relationship between pH, bicarbonate and carbon dioxide at 35oC and 1 atm pressure
Step 1: Calculation of COD equivalent of CH4 • CH4 + 2O2 --------------> CO2 + 2H2O • 16 g 64g • 16 g CH4 ~ 64 g O2 (COD) • => 1 g CH4 ~ 64/16 = 4 g COD ------------ (1) Step 2: Conversion of CH4 mass to equivalent volume => 1 Mole CH4 ~ 22.4 L CH4 => 16 g CH4 ~ 22.4 L CH4 => 1 g CH4 ~ 22.4/16 = 1.4 L CH4------------ (2) Theoretical methane yield per kg COD at STP Assumption: No oxygen demand could be satisfied in an anaerobic reactor but production of methane
Step 3: CH4 generation rate per unit of COD removed From eqs. (1) and (2), we have => 1 g CH4 ~ 4 g COD ~ 1.4 L CH4 => 4 g COD ~ 1.4 L CH4 => 1 g COD ~ 1.4/4 = 0.35 LCH4 or 1 Kg COD ~ 0.35 m3 CH4 ----------- (3) Complete anaerobic degradation of 1 Kg COD produces 0.35 m3 CH4 at STP
Kinetics • Rate of reaction(s) will determine • Process efficiency • Effluent quality • Reactor volume requirements (SRT & HRT) • Ultimately one or several of the rate limiting reactions will control the overall rate of conversion of organics to methane and CO2
0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Monod (non-inhibitory) Kinetics ^ µ Maximum specific growth rate (h-1) Ks Half saturation constant, mg/L S (g/L as COD)
0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Andrews (inhibitory) Kinetics (h-1) S (g/L as COD)
Kinetics relate to SRT • Specific growth rate is a function of SRT • Food to microorganism (F/M) ratio is a function of 1/SRT • SRT will control process performance note: solids retention time (SRT) and hydraulic retention time (HRT) are the same for a conventional digester without solids recycle
Particulate Hydrolysis steps Complex Biodegradable Particulates 1 1 2 3 Lipids Proteins and Carbohydrates Long Chain Fatty Acids Amino Acids and Simple Sugars 1 1 Acidogenesis anaerobic oxidation Volatile Acids (propionic, butyric, etc.) fermentation Methane Forming Acid Forming Hydrolysis 2 2 Homoacetogenesis Hydrogen Acetic Acid 3 Methanogenesis 5 4 aceticlastic methanogens hydrogen oxidizing methanogens Methane
Fermentative bacteria (1) This group of bacteria is responsible for the first stage of anaerobic digestion - hydrolysis and acidogenesis. These bacteria are either facultative or strict anaerobes. The anaerobic species belonging to the family of Streptococcaceae and Enterobacteriaceae and to the genera of Bacteroides, Clostridium, Butyrivibrio, Eubacterium, Bifidobacterium and Lactobacillus are most common. Process Microbiology The anaerobic degradation of complex organic matters is carried out by a series of bacteria as indicated in the figure (with numbers). There exists a coordinated interaction among these bacteria. The process may fail if a certain group of these bacteria is inhibited.
This group of bacteria metabolizes propionate and other organic acids (> 2-C), alcohols and certain aromatic compounds (i.e. benzoate) into acetate and CO2. CH3CH2COO - CH3COO - + CO2 + H2 Hydrogen producing acetogenic bacteria (2) Syntrophic association of acetogenic organisms with methanogenic H2- consuming bacteria helps to lower the concentration of H2 below inhibitory level so that propionate degrading bacteria are not suppressed by excessive H2 level. H2 partial pressure < 10-2 (100 ppm)
Homoacetogenes (3) Homoacetogenesis has gained much attention in recent years in anaerobic processes due to its final product: acetate, which is the important precursor to methane generation. The bacteria are, H2 and CO2 users. Clostridium aceticum and Acetobacterium woodii are the two homoacetogenic bacteria isolated from the sewage sludge sample. Homoacetogenic bacteria has a high thermodynamic efficiency as a result there is no accumulation H2 and CO2 during growth on multi-carbon compounds. CO2 + H2 CH3COOH + 2H2O
Methanogens (4 and 5) Methanogens are unique group of microbes classified as Archaebacteria, that are distinguished from the true bacteria by a number of characteristics, including the possession of membrane lipids, absence of the basic cellular characteristics (e.g. peptidoglycan) and distinctive ribosomal RNA. Methanogens are obligate anaerobes and considered as a rate limiting species in anaerobic treatment of wastewater. Moreover, methanogens co-exist or compete with sulfate reducing bacteria for the substrates in anaerobic treatment of sulfate-laden wastewater. Two classes of methanogens that metabolize acetate to methane are: • Methanosaeta (old name Methanothrix): Rod shape, low Ks, high affinity • Methanosarcina (also known as M. mazei ): Spherical shape, high Ks, • low affinity
5 Methanosaeta 4 Methanosarcina kmax = 10 d-2 Ks = 400 mg/L 3 Specific Utilization rate, k,d-1 2 kmax = 2 d-2 Ks = 20 mg/L Methanosaeta 1 Growth kinetics of Methanosarcina and Methanosaeta
Avoid excessive air/O2 exposure • No toxic/inhibitory compounds present in the influent • Maintain pH between 6.8 –7.2 • Sufficient alkalinity present • Low volatile fatty acids (VFAs) • Temperature around mesophilic range (30-38 oC) or • thermophilic range (50-60 oC) • Enough nutrients (N & P) and trace metals especially, Fe, Co, Ni, etc. • COD:N:P = 350:7:1 (for highly loaded system) 1000:7:1 (lightly • loaded system) • SRT/HRT >> 1 (high rate anaerobic reactors) Essential Conditions for Anaerobic Treatment
The successful operation of anaerobic reactor depends on maintaining the environmental factors close to the comfort of the microorganisms involved in the process. Environmental factors Temperature • Anaerobic processes like other biological processes strongly depend on temperature. • In anaerobic system: three optimal temperature ranges; • Psychrophilic (5 - 15oC) • Mesophilic (30 – 38 C) • Thermophilic (50 - 60 oC)
Effect of Temperature on Anaerobic Activity Rule of thumb: Rate of a reaction doubles for every 10 degree rise in temperature up to optimal temp.
pH There exist two groups of bacteria in terms of pH optima namely acidogens and methanogens. The best pH range for acidogens is 5.5 – 6.5 and for methanogens is 7.6 – 8.0. The optimal pH for combined cultures is 6.8 - 7.2. Low pH reduces the activity of methanogens causing accumulation of VFA and H2. At higher partial pressure of H2, propionic acid degrading bacteria will be severely inhibited thereby causing excessive accumulation of higher molecular weight VFAs such as propionic and butyric acids and the pH drops further. If the situation is left uncorrected, the process may eventually fail. This condition is known as a “SOUR” or STUCK” The remedial measures: Reduce the loading rates and supplement chemicals to adjust the pH. Chemicals such as NaHCO3, NaOH, Na2CO3, Quick lime (CaO), Slaked lime Ca(OH)2, NH3 etc.
An anaerobic treatment system has its own buffering capacity against pH drop because of alkalinity produced during waste treatment: e.g. the degradation of protein present in the waste releases NH3 which reacts with CO2 forming ammonium bicarbonate as alkalinity. NH3 + H2O +CO2 NH4HCO3 The degradation of salt of fatty acids may produce some alkalinity. CH3COONa + H2O CH4 + NaHCO3 Sulfate and sulfite reduction also generate alkalinity. CH3COO - + SO42- HS- + HCO3- + 3H2O Cont..
Nutrients and trace metals Anaerobic process requires macro (N, P and S) and micro (trace metals) nutrients in sufficient concentration to support biomass synthesis. In addition to N and P, anaerobic microorganisms especially methanogens have specific requirements of trace metals such as Ni, Co, Fe, Mo, Se, etc. The nutrients and trace metals requirements for anaerobic process are much lower as only 4 - 10% of the COD removed is converted biomass. Inhibition/Toxicity The toxicity is caused by the substance present in the influent waste or byproducts of the metabolic activities. Ammonia, heavy metals, halogenated compounds, cations, etc. are the examples of the former type whereas ammonia, sulfide, VFAs belong to latter group. Cont.. COD:N:P = 350:7:1 (for highly loaded system) 1000:7:1 (lightly loaded system)
Effect of Sulfate on Methane Production When the waste contains sulfate, part of COD is diverted to sulfate reduction and thus total COD available for methane production would be reduced greatly. Sulfide will also impose toxicity to methanogens at Concentration of 50 to 250 mg/L as free sulfide.