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 kg_VS/m³d
Oxygen Demand	Endogenous respiration	∼2.3 kg_O2 for 1 kg_CELL removed
Oxygen Demand	BOD in primary sludge	1.6–1.9 kg_O2 for 1 kg_VS 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

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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