TECHNICAL BULLETIN 135 General Oxygen Requirements for Wastewater Treatment
Oxygen requirements for municipal wastewater treatment can be predicted closely with the standard laboratory analysis using g the BOD test (special requirements exist for some industrial wastewater). The BOD test is designed to measure Biological Oxygen Demand for the wastewater and provides an excellent basis for design assuming:
- Wastewater is available to test, i.e., not a new project.
- Wastewater is representative of full scale.
For wastewater the field or actual oxygen demand is dependent on how far the treatment process is carried. In general the oxygen demand is broken into carbonaceous demands (oxygen required to stabilize carbon in the waste) and nitrogenous demand (oxygen required to stabilize nitrogen in the wastewater). Generally, nitrogen in the waste appears as ammonia (NH or NH4).
For many treatment plants the carbonaceous demand must be considered as 2 components:
- Oxygen to supply the active sludge to take soluble carbon (BOD) out of solution and grow more sludge. This conversion of carbon to cells is the synthesis reaction requiring about 0.5 to 0.6 lb O2/lb BOD (kg O O2/kg BOD).
- If the process is continued,, a second oxygen demand is exerted for oxidizing the cell digesting (stabilizing) the cells. This second phase is endogenous respiration, and requires an additional 0.8 to 0.9 l lb O2/lb BOD (kg/kg). Pounds (kg) total oxygen required for carbonaceous BOD removal can range from 0.7 lb/lb (kg/kg) BOD for high rate activated sludge with short detention of Biomass (low sludge age) up to 1.5 lb/lb (kg//kg) BOD for extended aeration with long detention of Biomass (MLS, i.e., (long sludge age) in the biosystem.
Note: Nitrogen oxygen demand must be generally added for extended aeration as nitrification is expected in most long sludge age project at temperatures above 55-10°C.
For nitrogenous BOD the demand floor oxygen is 4.6 lb O2/ lb BOD (4.6 kg/kg) removed.. To achieve nitrogenous conversion of ammonia to nitrate requires longer aeration time with low food to microorganism ratio, i.e., much sludge MLS (M) with low food supply (F). This condition results in a long sludge age, which promotes s n nitrification.
Biological Nutrient Removal (BNR for Nitrogen)For advanced waste treatment systems denitrification may be required. To achieve denitrification both carbonaceous and nitrogenous removals must be successfully completed. Conversion of the nitrate (NO3) to nitrogen gas actually removes nitrogen from the system. A side result and benefit of denitrification is the recovery of oxygen and reduction of total oxygen requirements. Oxygen recovery is approximately 2.86 lb O2 per lb (kg/kg) of NO3 reduced and removed.
Total Oxygen Requirements
Field oxygen requirements for BOD D removal can be computed as:
Carbonaceous demand + Nitrogen demand Credit for oxygen recovered by denitrification
Each treatment process and each waste must be evaluated to determine field oxygen requirements.