Operational Conditions for Anaerobic Digesters Quanbao Zhao Washington State University 8/1/2013

Operational Conditions Temperature pH Hydraulic retention time (HRT) Solid retention time (SRT) Toxicants Flow and Mix

Temperature

Mesophilic condition Medium temperature loving bacteria/archae Preferred temperature range: 20 and 45 °C (68 and 113, °F) Best performance: 38°C (100 °F) Moderate degradation rate Moderate biogas production rate Moderate heat demand

Thermophilic condition Heat-loving bacteria/archea Preferred temperature: 45 - 122 °C (113 - 252 °F) Typical temperature: 55°C (131°F) High degradation rate High biogas production rate High heat demand, ammonia inhibition

Phychrophilic condition Cold-loving bacteria/archae Preferred temperature: 0°C to +25°C (32-68 °F) Low degradation rate Low biogas production rate No heat demand

pH

pH Optimum pH for the methanogens: 6.5-8.0 Optimum pH for the acid-forming bacteria: 5.8-6.2 Buffering system in the digester: bicarbonate/carbonate, ammonium/ammonia, Phosphate Why pH drops in the digester? Volatile fatty acid accumulation What happens if pH goes higher than 8? Ammonia inhibition

Hydraulic retention time

Hydraulic retention time (HRT) HRT means how long the liquid will stay in the reactor before it comes out. HRT=Volume of the reactor/Flow rate of the influent; Unit: day. Proper HRT is important to fully digest the organic materials in the waste. Dairy manure: above 20 days Longer HRT -> Larger reactor volume -> Higher construction cost

Solid Retention Time

Solid retention time (SRT) Solid retention time is the average time microbes remain in the digester How to keep a high SRT: Sludge recirculation Reactor design: UASB, ASBR For CSTR, SRT= HRT if no recycle the sludge For CSTR, SRT> HRT if recycle the sludge

Toxicants 1 Volatile fatty acids (VFAs) Metals Hydrogen Sulfide (H2S) Acetic acid, Propionic acid, Butyric acid etc. Inhibit methanogens, could cause the fail Methods for detection: titration, gas chromatography Metals Heavy metals: Copper, Chromium, Cadmium, Nickel et al. Light metals: Aluminum, Potassium, Sodium, Calcium and Magnesium. Hydrogen Sulfide (H2S)

Toxicants 2 Ammonia Organic chemicals Free ammonia is the major problem High pH release free ammonia from ammonium Organic chemicals Long chain fatty acids phenols

Flow and Mix Problems: Sand/heavy solids will build up at the bottom Grease/fiber will build up at the top Mixing is a key factor to enhance the kinetics and prevent settling/crusting Remove sand before feeding into the digester