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

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

3. Temperature

4. 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

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

6. 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

7. pH

8. pH Optimum pH for the methanogens: 6.5-8.0
Optimum pH for the acid-forming bacteria:
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

9. Hydraulic retention time

10. 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

11. Solid Retention Time

12. 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

13. 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)

14. 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

15. 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