CHEE 370 Waste Treatment Processes Lecture #22 Anaerobic Digestion

Sludge Handling “50% of the cost, 90% of the headache” Process Objectives: Reduce the volume (Remove water) Reduce the organic content Reduce # of micro-organisms and pathogens “Stabilize the Sludge”

Sludge Thickening Wasted sludge (both primary and waste-activated) is “thickened” - the solids content is increased by removing some of the liquid Makes handling the sludge more manageable Reduces the required size for the anaerobic digesters and storage tanks Minimizes the energy requirements for subsequent processes such as heat drying Typically accomplished by physical means

Read textbook pages 1488 - 1489. Metcalf and Eddy

Gravity Thickening Metcalf and Eddy

Centrifugal Thickening Metcalf and Eddy

Centifugal Thickening

Sludge Primary sludge ~ 45,000 mg/L solids Contains untreated biodegradable material Thickened wasted activated sludge ~ 40,000 mg/L solids Contains active biomass

Anaerobic Digestion Involves methanogenic bacteria which grow on very simple carbon sources 2 General Types of Methanogens: H2-utilizing methanogens 4 H2 + CO2 CH4 + 2H20 Acetoclastic methanogens CH3COOH  CH4 + CO2 Strict anaerobes (O2 is lethal)

Anaerobic Digestion Anaerobic growth is a very slow process Typically operate at higher temperatures (35 °C is optimal for methanogenic bacteria) to get high levels of conversion and minimize the reactor volume Methanogens grow best at pH 7.0 - control the pH closely By thickening the solids before digestion, the amount of water that needs to be heated is minimized Reduces energy requirements and cost

Anaerobic Digestion Objectives Destruction of organic material Reduce the oxygen demand of the sludge - thereby making it more “stable” and suitable for release to the environment Accomplished through oxidation Methane is formed in the process Methane can be used to run the heating system for the anaerobic digester Pathogen Destruction Anaerobic digestion at 35 °C and at a solids retention time of 15 days will lead to a high level of pathogen destruction

Anaerobic Digestion Objectives Increase “dewaterability” of the final sludge Wasted activated sludge cannot be thickened to more than 5% (50,000 mg/L) Primary sludge can be thickened to as much as 6% Sludge after anaerobic digestion can be thickened to as much as 130,000 mg/L If dewatering is applied after digestion, the sludge can be thickened to as much as 25%

Anaerobic Digestion Methanogenic degradation of complex substrates requires a concerted effort of many bacterial species Three Stage Process: Hydrolysis and Fermentation Acetogenesis and dehydrogenation Methanogenesis

Step 1 Hydrolysis and Fermentation Performed by various facultative bacteria Does not reduce the COD of the sludge Carbohydrates  simple sugars Proteins  amino acids Sugars and amino acids  fatty acids and alcohols (fermentation) Lipids  long chain fatty acids Hydrolysis of lipids is the rate-limiting step Important for the design of the digesters

Step 2 Acetogenesis and Dehydrogenation Organic acids and alcohols are further degraded by bacteria to produce acetic acid and H2 4 H2 + CO2 CH4 + 2H20 CH3COOH  CH4 + CO2 Virtually all COD into the digester ends up as methane Methanogenic bacteria have a very low yield - only a small amount of biomass is produced Step 3 Methanogenesis

Anaerobic Digestion Decomposition of organic and inorganic matter in the absence of oxygen Main process used for the stabilization of sludge from municipal WW treatment Advances in the design of digesters have made the process relatively economical There are beneficial uses for the digested sludge Methane produced in the digestion can be used as fuel

Heterotrophs vs Methanogens Activated Sludge (Heterotrophs) Anaerobic Digestion (Methanogens) Bacteria Archaea Assume they require aerobic growth conditions Strict anaerobic growth conditions Typical yield range: 0.3 - 0.5 g VSS/g bCOD 0.05 - 0.10 g VSS/g COD max at 20 C: 3 - 13.2 d-1 max at 35 C: 0.3 - 0.38 d-1 Tables 8-10 and 10-10 in the text provide ranges for other parameters.

Effect of H2 Effective anaerobic digestion requires a diverse microbial community Hydrogen gas partial pressure is the key to balancing the reactions and populations present in the reactor Formation of acetic acid and H2 by anaerobic oxidation is inhibited by high ppH2 Methanogens cannot use organic acids other than acetic acid H2-utilizing methanogens must remove H2 as fast as it is produced to allow anaerobic oxidation to proceed

Effect of pH Acid forming bacteria (pH 4.5 - 5) are much more tolerant of low pH than methanogens (pH 7.0) Production of volatile fatty acids (VFAs) decreases the sludge pH Normally counterbalanced by buffering associated with cellular CO2 production Imbalances reduce the pH of the system, impairing methanogenesis “Stuck” or “Sour” digester Compounding problem - requires immediate attention

Effect of Temperature Acid forming bacteria have a much higher maximum specific growth rate (max) that changes more dramatically with temperature, as compared to methanogens Where: kT = reaction-rate coefficient at temp. T (C) k20 = reaction-rate coefficient at 20 C  = temperature-activity coefficient T = temperature (C)

Digester Design Factors ******Solids retention time****** Hydraulic retention time Temperature Alkalinity pH Presence of inhibitory substances Nutrient availability Methane production

Single-Stage Digestion Uniform feeding is important Total solids are reduced by ~ 50 % May have fixed or floating covers (Methane + Oxygen = Trouble) Metcalf and Eddy Figure 14-20

Two-Stage Digestion Not common in current practice First tank is for digestion Second tank is primarily for storage Metcalf and Eddy Figure 14-20

Basic Model for Anaerobic Digestion (AD) Want to know: Quantity of solids leaving the digester VSS and TSS destruction in the digester as a function of the SRT How much methane is produced Simple model: CSTR with no recycle HRT = SRT ( = c = V/Q) No additional equations will be provided on the equation sheet - develop the relationships from basic mass balances and understanding of the process

AD Model Assumptions Design based on the rate-limiting step - breakdown of volatile fatty acids (VFAs) Non-biodegradable fractions of COD remain unchanged by the digestion process Heterotrophic bacteria only decays and the COD associated with decay will be accumulated as VFAs available to the methanogens Complete hydrolysis and fermentation of biodegradable organic matter -> fully available to methanogens Use the kinetics for the growth of the methanogens to determine the minimum SRT, then use this value with a safety factor to determine the operating conditions

Minimum SRT Calculation Where: umax,m = maximum specific growth rate for the methanogens Kd,m = decay rate for the methanogens

Factor of Safety for Growth It is necessary to provide a factor of safety for methanogen growth (prevent “stuck” digester) and headspace Use a factor of safety of at least 2.5 The ministry of the environment requires at least 15 days SRT at 35 C Compare with your calculation and select the larger value

Heterotroph Mass Balance Assume there is no growth - only decay Perform a mass balance on the digester for the heterotrophic bacteria: As the SRT increases, the amount of active heterotrophic biomass in the effluent decreases

Debris Mass Balance Debris (XD) can enter the digester in the influent (XDo) stream and is also generated during biomass decay Perform a debris mass balance on the digester : Where fd = debris fraction of the degraded biomass (fd ranges from ~ 0.08 - 0.20)

VFAs for Methanogens Multiple Sources: Soluble biodegradable COD (Ss) Biodegradable particulate COD (Xs) Decay of heterotrophic biomass

Effluent VFA and Formation of Methanogenic Bacteria CSTR without recycle

CODin = CODout + CODmethane produced Methane Production COD balance can be performed in order to determine the amount of methane produced CODin = Q(SSo + XSo + XHo + XDo) CODout = Q(Svfa + XH + Xm + XD) CODin = CODout + CODmethane produced CH4 + 2O2  CO2 + 2H2O 64 g COD/mol CH4

Methane Production Use the ideal gas law to calculate the volume produced per day (V=nRT/P) Textbook example 7-9, p. 633 - Effect Of Temp! Volume of methane produced per day: Where mCH4 is mass-COD of CH4 produced/time

Methane Gas Production Conversion of COD to methane gas Consider glucose (C6H12O6) Show: 0.35 L CH4/g COD consumed at STP

Anaerobic Treatment of a Single Compound Assume a single substrate is fed to an anaerobic digester at a flowrate of 1 L/day and at a concentration of 10,000 mg COD/L. The effluent COD concentration is to be less than 500 mg-COD/L and the transformation will be carried out by methanogens. REMEMBER: ALWAYS DRAW A DIAGRAM!

Determine: Given: The SRT [50 days] The reactor volume [50 L] The recommended design volume [125 L] The volume of methane produced per day under standard conditions in the designed reactor [3.3 L/d] Volume of methane produced at 35 ºC [3.72 L/d] Given: μmax= 0.15 d-1 kd=0.03 d-1 Kvfa=1000 mg COD/L Ym=0.03 mg VSS/mg COD

Previously: If Svfa,available >> Kvfa: And:

Digester Volume When designing the digester, it is important to include additional “head space” for the methane gas that is produced during the anaerobic digestion Typically, an additional 25% volume is included in the design This increase in volume does not influence the SRT

Anaerobic Treatment of Mixed Composition Sludge Consider a waste treatment plant where the primary sludge and wasted activated sludge (WAS) are blended and sent to an anaerobic digester. The combined sludge is determined to have the following characteristics: Combined Sludge Flow m3/day 210 TSS mg-TSS/L 21619 VSS mg-VSS/L 17073 Soluble biodegradable COD mg-COD/L 87 Soluble non biodegradable COD 29 Particulate biodegradable COD 18733 Active biomass concentration 1256 Biomass debris 989 Particulate non-biodegradable COD 9413

Design of the Anaerobic Digester The digester is to be operated at an SRT of 15 days, according to the Ministry of the Environment’s minimum regulations Assume the following parameters apply: max, m= 0.27 d-1, kd,m= 0.03 d-1 Kvfa = 2000 mg COD/L Ym= 0.03 mg COD/mg COD kd,h = 0.22 d-1 fd = 0.2 VSS/TSS for particulate organic fraction = 0.90 mg VSS/mg TSS

Calculate: Svfa,available [19590 mg-COD/L] Confirm that the ministry guideline for the SRT is the appropriate choice for operating the system The recommended design volume for the digester [3940 m3] Volume of methane produced per day in the system at 35 C and 1 atm (Careful - biomass is already in COD units!) [1500 m3/d] Svfa [1115 mg-COD/L] Xm [382 mg-COD/L] XD [1182 mg-COD/L]

% VSS Destruction VSSin = 17073 mg/L (In this example) VSSout = XH + XD + Xm + Xnon-biodeg particultes Remember to convert from COD to VSS units % VSS destruction ~ 54 %

% TSS Destruction TSSin = 21619 mg/L (In this example) TSSout = FSS + XH + XD + Xm + Xnon-biodeg particultes Remember to convert appropriately to TSS units (given 0.9 mg VSS/mg TSS for the particulate organic fraction) % TSS destruction ~ 47 %

Methane is a usable product, and the amount of biomass produced through AD is low, so why don’t we use it for WW treatment in general? Slow growth kinetics requires long SRT (large reactor volumes) High temperatures required - COD present in WW will not generate sufficient methane to heat water to 35 °C Effluent is not of sufficient quality Nitrifiers do not grow under anaerobic conditions Effluent from the digester usually contains high ammonia concentrations Liquid stream from sludge processing is usually fed back into the AS system

Sludge Handling and Disposal Heat Drying Used to prepare the sludge for incineration or for sale as fertilizer Sludge moisture content after drying is ~ 10% Dried sludge is termed “biosolids” The high cost of drying and relatively low levels of nutrients in the biosolids have limited its use as a fertilizer

Sludge Handling and Disposal Incineration Complete oxidation of the biosolids to produce CO2, H20, and ash Advantages: Maximum volume reduction Destruction of persistent pathogens and toxins Potential to obtain energy Limitations: Expensive Requires trained operators and constant monitoring Environmental impact Concerns with the disposal of the ashes

Ultimate Disposal Ocean dumping of sludge is discouraged or prohibited Lagoons can be used for sludge disposal in remote locations Excess liquid from the lagoon is returned back to the WW treatment plant Landfills or land application are the most commonly used methods of disposal Landfills are used for disposing sludge, grease, grit, and other solids Wastes are deposited in a designated area, compacted with a tractor or roller, and covered with a 30 cm layer of clean solid or composted sludge to minimize odours and prevent attracting flies, rodents etc.

Land Application Agricultural lands, forests, golf courses, parks … Concerns with public health risk through direct exposure or consumption of contaminated crops and groundwater Controlling factors: Utilization rate of nutrients by crops and vegetation Potential of plants to uptake toxic components (mainly metals) from the sludge Accumulation of metal and salts in the soil Aesthetic Standards and guidelines are developed based on toxicity studies and bioaccumulation within individual species and through food chains

Review Anaerobic Digestion Decomposition of primary and wasted activated sludge (WAS) in the absence of oxygen Uses methanogenic bacteria Low biomass yields and methane gas production Digester modelled as a CSTR without recycle Design based on volatile fatty acids (from a variety of sources) as the limiting substrate

Review Anaerobic Digester Design Relevant Design Questions: How much methane is produced? Solve using COD balance (64 g COD/mol CH4) Quantity of solids leaving the digester? What is the % VSS and % TSS destruction across the digester? Important to always keep track of your units!