Sludge Digestion

Aerobic Sludge Digestion with Mazzei Technology

As with Activated Sludge Digestion, Aerobic Sludge Digestion with Mazzei’s AirJection^® Aeration Technology can be made more efficient.

Oxygen Requirement Calculation

To assist plant designers and operators, the following oxygen requirement calculation for aerobic sludge digesters is provided using information from 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.
• Primary sludge will have a lower ratio, which can be estimated from the BOD5 loading of the waste sludge from the primary clarifier. If primary sludge is introduced into the digester, the weight ratio can be estimated from its BOD5 loading and its percentage of the flow into the digester.
• 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 approximately half the effective time if the basin were 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