Aerobic Sludge Digestion

Sludge Digestion Calculations:

Oxygen requirement calculation for aerobic sludge digesters can be estimated based on information in the Metcalf & Eddy handbook. First, the mass of oxygen required per mass of sludge digested is based on the following stoichiometry:

The following overall reaction applies

C₅H₇NO₂ + 7O₂ 5CO₂ + NO₃ + 3H₂ O + H

The weight ratio (O₂ Ratio) for this overall reaction is 2:1  O₂ / Biomass digested. This ratio applies for waste activated sludge from the activated sludge process.

The typical solids loading to an aerobic digester is 1.5% to 3%, depending on the degree of thickening.

The handbook states that the reduction in Volatile Suspended Solids (VSS) by aerobic digestion is 40% to 50%. An experienced wastewater engineer and plant superintendent states that it is typically closer to 30%. Typical hydraulic retention times employed are as follows:

Waste Activated Sludge Only: 10-15 Days
Waste Activated Sludge from Plant without Primary Clarification: 12-18 Days
Primary + Waste Activated Sludge or Trickling Filter Sludge: 15-20 Days

The oxygen requirement for an aerobic digester can be estimated as follows:

If the digester is to be filled and aerated over several days, the hydraulic retention time (HRT) during the fill period can be counted as about half the effective time if the basin was full. For example, if the digester is to be filled/aerated over seven days, and then aerated for seven more days the effective total HRT will be about 10 days.

Effective HRT = (Fill Day)/2 + Full Days

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Click to see more information about each application below:

Sewer Main Aeration	Activated Sludge Aeration	Post Aeration
Odor Control	Pure Oxygen Injection	Sludge Digestion
		Ozone Disinfection

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