Amount of oxygen consumed by oxidation of organic substrate (BOD5) per day = minimal necessary
amount of oxygen put into activated sludge tank:
rox [kg O2/d ] = SOU [kg O2/kg BOD5]. BOD5 [kg/d]
Values of specific oxygen uptake SOU depending on sludge age and temperature (in [kg O2/kg BOD5]):
sludge
age [days]
sludge
age [days]
Optimal operation of activated sludge tank and life conditions of bacteria require to put more oxygen than
will be consumed, so the concentration of oxygen in the activated sludge tank would never reach zero
value. Optimal equilibrium concentration of oxygen c+ is approximately 2 mg L–1. In some cases this value
can be lower but it should never get lower than 1 mg L–1).
Input of oxygen to the activated sludge tank per day = optimal amount of oxygen put into activated sludge
tank
OC´ [kgO2/d] = rox [kg O2/d ]. cS' / (cS' – c+)
c+ = 2 mg L–1
cs‘ = saturation concentration of oxygen in activated sludge (depending on temperature) [mg L–1]
cs‘ = 0,95 · cs
cs = saturation concentration of oxygen in pure water (tabulated values, depending on
temperature) [mg L–1]
Intensity of aeration – volume of air that is necessary to put into activated sludge tank
Iaer [m3·h–1] = 13.9 · OC‘ / E
efficiency of oxygen absorption at standard conditions, in pure water – 5.5 % for each meter of
depth (h) of activated sludge tank Ew [%] = 5.5 · h [m]
efficiency of oxygen absorption in activated sludge tank: E [%]= α · Ew = α · 5.5 · h
value of coefficient α is 0.6 – 0.7
Values of specific production of surplus sludge SPS at 10 °C [kg sludge/kg BOD5]
sludge age [days]
SS / BOD5
sludge age [days]
SS / BOD5
SS / BOD5 = ratio of concentration of suspended solids and BOD5 at the inflow to the activated sludge tank
Production of surplus sludge [kg/d] = SPS [kg sludge/kg BOD5] · BOD [kg/d]