3.5 How to design the aerobic digester
The design of the aerobic digestion process involves a systematic approach with clearly outlined treatment goals, considering factors such as organic matter stabilization, pathogen reduction and desired effluent quality. The definition of treatment goals is followed by the characterization of the sludge or wastewater to be treated and the selection of an aerobic digestion system based on available space and operational preferences.
The design of the aerobic digester can be carried out by following these key steps:
- Determine Solids Retention Time
- Calculate the Digester Volume
- Verify the Organic Loading Rate
- Calculate the Total Solids Concentration in the Digester
- Determine the Area Requirements for the Digester
- Determine the Air Requirements for the Digester
The following table presents essential parameters that the design of an aerobic digester can be based on.
| Parameter | Specification | Value |
|---|---|---|
| Hydraulic Retention Time | 40% Volatile solids reduction | |
| 4 – 5°C | 108 days | |
| 15 – 16°C | 31 days | |
| 25 – 26°C | 16 days | |
| 55% Volatile solids reduction | ||
| 4 – 5°C | 386 days | |
| 15 – 16°C | 109 days | |
| 25 – 26°C | 64 days | |
| Total Solids Load | < 3% | |
| Organic Loading Rate | 1.6 – 4.8 kgVS/m3d | |
| Oxygen Demand | Endogenous respiration | ∼2.3 kgO2 for 1 kgCELL removed |
| BOD in primary sludge | 1.6–1.9 kgO2 for 1 kgVS removed | |
| Energy for Keeping Solids in Suspension | Mechanical aerators | 20 – 40 W/m3 |
| Minimum Airflow Mixing Capacity | Diffused air | 20 – 40 L/m3/min |
| Dissolved Oxygen in Digester | 1 – 2 mg/L | |
Let’s apply the key steps we’ve learned to an exercise designing an aerobic digester in the next topic.
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