3.5 How to design the aerobic digester - Humanitarian Sanitation Hub

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:

  1. Determine Solids Retention Time
  2. Calculate the Digester Volume
  3. Verify the Organic Loading Rate
  4. Calculate the Total Solids Concentration in the Digester
  5. Determine the Area Requirements for the Digester
  6. Determine the Air Requirements for the Digester

The following table presents essential parameters that the design of an aerobic digester can be based on.


ParameterSpecificationValue
Hydraulic Retention
Time
40% Volatile solids reduction
4 – 5°C108 days
15 – 16°C31 days
25 – 26°C16 days
55% Volatile solids reduction
4 – 5°C386 days
15 – 16°C109 days
25 – 26°C64 days
Total Solids Load < 3%
Organic Loading Rate 1.6 – 4.8 kgVS/m3d
Oxygen DemandEndogenous respiration∼2.3 kgO2 for 1 kgCELL removed
BOD in primary sludge1.6–1.9 kgO2 for 1 kgVS removed
Energy for Keeping Solids
in Suspension
Mechanical aerators20 – 40 W/m3
Minimum Airflow
Mixing Capacity
Diffused air20 – 40 L/m3/min
Dissolved Oxygen
in Digester
 1 – 2 mg/L
Design parameters for conventional aerobic sludge digesters (C. V. Andreoli, 2007) (US-EPA)

Let’s apply the key steps we’ve learned to an exercise designing an aerobic digester in the next topic.

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