Anaerobic Digestion

Benefits

1. Pollution abatement
2. Energy production
3. Fertilizer production

 Anaerobic Digestion

Drawbacks

• High capital cost
• sensitive process
• requires daily attendance and close monitoring

 Anaerobic Digestion

Microbiology

• Complex Organic Wastes (enymatic hydrolysis)
• Soluble cmpds (acid bacteria)
• Organic acids ( methane bacteria)
• CH₄ , CO₂

• Methanogens depend on acid bacteria (facultative heterotrophs) for their substrates (acids)
• Acidogens depend on methanogens to use acids up. Excess acid concentration causes severe inhibition

1. Acetogenic
2. Hydrogenogenic
3. Homoacetogenic
4. Methanogen Archaea

• Mathanogens produce methand by 2 pathways 

1. Acetic Acid Oxidation (decarboxylation - demethylization)
2. Reduction of CO₂ with H₂ (alpha carbon saturation. approx 25-30 strains identified. all strains capable of both rxns)

• Facultative, heterotrophic, common
• converts insoluble CHONPS to soluble monosaccharides, amino acids, and fatty acids (extracellular)
• Converts solule components to volatile organic acids, alcohols, NH₃, H₂S (intracellular)

• Obligate anaerobes, highly specialized
• Only known substrate and metabolic function is conversion of longer chain VOA's to acetate, formate, H₂, CO₂ (intracellular)

• Obligate anaerobes, highly specialized
• only function is conversion of H₂ and CO₂ to acetate (intracellular)

• Highly obligate anaerobes
• O₂ is toxic and oxidation reduction potential must be below -400mv (intracellular)

3 temp zones

(temp must not cary more than +/- 2 C for methanogens) 

• Psychrophilic <25C - inhibit methanogenesis, not acetogenesis
• Mesophilic 30-40C - anaerobic digesters must operate in mesophilic or thermophilic ranges
• Thermophilic 50-65C

VOAs act as an inhibitor if concentration is high and pH is low enough

• only un-ionized form inhibits
• determined by pH

Ammonia affected by pH like VOA

• in aqueous soln, ionizes to form equilibrium
• only un-ionized form causes toxicity
• toxicity begins as pH approaches 9.24

Alkalinity

• pH of 6.8-7.5, carbonate system is primary buffer
• all buffering depends on bi-carbonate alkalinity in this range

• pH should drop
• VOAs should increase
• Small chain acids allow for methanogens to do work, organic acids finish it

• Concentration of VS
• Detention Time (dayS)
• Organic Loading Rates