Sludge Anaerobic Digestion and Dewatering: Mesophilic and Thermophilic Parameters, Biogas Yield and Stabilisation Criteria

1. Mesophilic Anaerobic Digestion

Typical values from the trial Technical Guideline for Sludge Treatment and Disposal of Municipal Wastewater Treatment Plants:

• Digestion temperature held at 35 degrees Celsius plus or minus 2 degrees.
• Solids retention time greater than 20 days.
• Organic volumetric loading generally 2.0-4.0 kg per cubic metre per day.
• Volatile solids destruction of 35%-45%.
• Gas production generally 0.75-1.10 normal cubic metres per kg of VSS destroyed.

2. Thermophilic Anaerobic Digestion

• Temperature controlled at 55 degrees Celsius plus or minus 2 degrees, suited to thermophilic methanogens.
• Faster organic destruction and effective kill of pathogens and parasite eggs.
• Volatile solids destruction generally 35%-45%, with retention time shortened to 10-15 days.
• Drawbacks: higher energy consumption, higher operating cost and tighter operational demands.

3. Digester Design and Operational Control Indices

The Tianjin municipal technical specification for sludge treatment and disposal gives the following mesophilic digestion controls, which can be used directly as operating manual values:

• Feed sludge moisture content 90%-96%.
• Digestion temperature held at 33-37 degrees Celsius.
• Hydraulic retention time 20-30 days.
• Organic volumetric loading 0.6-3.0 kg per cubic metre per day.
• Volatile solids destruction at least 40%.
• Total alkalinity in the digester kept above 2000 mg/L.
• Volatile fatty acid concentration kept at 200-500 mg/L.
• Optimum pH range 6.8-7.2.
• Full-tank sludge mixing at least 8 times per day.

4. High-Solids Digestion

Where high-solids digestion is applied, sludge entering the digester reaches 10%-12% solids content, with digester temperature at 34-36 degrees Celsius and pH 6.5-7.5. This route raises biogas yield and cuts operating energy, but demands better pretreatment and mixing systems.

5. Stabilisation Criteria

• Theoretically, mesophilic digestion destroys about 83% of organic matter in 20 days and about 88% in 30 days, but short-circuiting, feeding pattern and toxic substances typically hold actual destruction to around 60% of the theoretical figure.
• In practice, digestion is generally considered complete when organic destruction reaches 40%-50%, or when organic acid content in the digested sludge falls below 300 mg/L.
• Methane typically makes up about 60% of biogas by volume; in theory, degrading 1 kg of COD yields about 0.35 cubic metres of methane at standard conditions.

6. Biogas System Safety

• Digesters and biogas systems must be fitted with over-pressure relief valves and vacuum relief valves; flame arresters are required at every point where the biogas system communicates with the outside.
• Explosion-hazard zones must have automatic methane and carbon dioxide monitoring with alarms, checked periodically for reliability.
• Before a digester is emptied for cleaning, toxic and hazardous gases must be tested on site, operators must wear protective equipment under reliable supervision, and gas concentrations inside the tank must comply with CJJ 6.
• During sludge withdrawal the gas line must remain connected to the gasholder, and mixing must stop when tank pressure exceeds the design value.

7. Interface with Dewatering

• Sludge from anaerobic processes must be treated and disposed of in accordance with GB 50014, and treated sludge must meet GB 18918.
• Anaerobic sludge is normally discharged to a site sludge collection tank, with downstream handling set by sludge character. Granular sludge should be stored and, after settling and decanting, reused as inoculum.
• Dewatering design should reflect the final receiving facility and include standby capacity.
• Digestate from high-solids digestion typically reaches about 70% moisture content and still needs checking against the moisture and calorific requirements of the disposal route, whether incineration, composting or landfill.

References

• Technical Guideline for Sludge Treatment and Disposal of Municipal Wastewater Treatment Plants (trial, in Chinese)
• Tianjin Technical Specification for Municipal Sludge Treatment and Disposal (mesophilic digestion operating values, in Chinese)
• HJ 2013-2012, Technical Specifications of UASB Reactor for Wastewater Treatment (anaerobic sludge disposal interface)
• Amendment to GB 18918-2002 (in Chinese)