Overview
The BOD test, a routine laboratory assay, can be used to forecast oxygen requirements for municipal wastewater treatment (special requirements exist for some industrial wastewater). The BOD test is used to determine the Biological Oxygen Demand of wastewater and is a good starting point for design assuming:
Wastewater is already available for testing, thus this isn't a new endeavour.Wastewater is a full-scale representation.
How can be the oxygen requirement determined?
The field or actual oxygen requirement for wastewater is determined by how far the treatment procedure is carried out. Carbonaceous needs (oxygen required to stabilise carbon in waste) and nitrogenous demands are the two types of oxygen demands (oxygen required to stabilise nitrogen in the wastewater). Ammonia is the most common form of nitrogen in trash (NH or NH4).
1: Carbonaceous/BOD
Carbonaceous demand must be evaluated in two parts for many treatment plants:
To pull soluble carbon (BOD) out of solution and generate new sludge, oxygen must be supplied to the active sludge.
-This carbon to cell conversion is a synthesis procedure that requires 0.5 to 0.6 lb. O2/lb. BOD (kg O2/kg BOD).
If the process continues, a second oxygen requirement is created to oxidise the cells while they digest (stabilise).
-Endogenous respiration is the second phase, which requires an additional 0.8 to 0.9 lb. O2/lb. BOD (kg/kg) of oxygen.
-Total oxygen required for carbonaceous BOD elimination might range from 0.7 lb. /lb. (kg/kg) to 0.7 lb. /lb. (kg/kg).
-BOD up to 1.5 lb. /lb. (kg/kg) is required for high rate activated sludge with short Biomass retention (low sludge age).
2: Nitrogenous BOD
The demand floor oxygen for nitrogenous BOD is 4.6 lb. O2/lb. BOD (4.6 kg/kg) which is eliminated.
Longer aeration time with low food to microbe ratio, i.e., much sludge MLS (M) with low food supply, is required to achieve nitrogenous conversion of ammonia to nitrate (F). This situation causes a protracted sludge age, which encourages nitrification.
Removal of Biological Nutrients (BNR for Nitrogen)
De-nitrification may be necessary for modern waste treatment systems. Both carbonaceous and nitrogenous reductions must be successful in order to achieve de-nitrification. Nitrogen is removed from the system when nitrate (NO3) is converted to nitrogen gas. The recovery of oxygen and lowering of total oxygen demand is a side effect and advantage of de-nitrification. The oxygen recovery rate is about 2.86 lb. O2 per pound (kg/kg) of NO3 decreased and eliminated.
To establish field oxygen requirements, each treatment process and waste must be analysed.
How to calculate the oxygen requirement in WWTP?
A reliable procedure used to calculate the oxygen required is given step wise as:
1. ETP Capacity in m3/day
2. Inlet BOD in mg/l
3. Desired Outlet BOD in mg/l
4. Total BOD to be removed (Influent) = inlet BOD 8* ETP capacity in kg/Day
5. Hence O2 to be transferred in kg/Day
6. Method of Aeration = Fine Bubble Diffuser
{SOTE (Standard O2 Transfer Efficiency for 300 mm disc type diffuser submergence) =20%}
7.Hence O2 required to be transferred Step 5/ 0.20 kg/day.
Assuming alpha Factor (Ratio of SOTE in clean water to effluent water)=0.8,i.e.
O2 required/0.8in Kg/day
8. Another alpha Factor (Factor for fouling of diffusers) =0.8, i.e.(step 8) /0.8 in Kg/day.
9. Oxygen requirement = (step 8) in kg/day
We know Oxygen Concentration in the Air = 21%,
10. Hence, Air Requirement = oxygen requirement/0.21 in Kg/day
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