1. Consider the flow diagram of activated sludge process shown below:

Write a mass balance equation for biomass in a). the whole system, and b). in the settling tank. Combining the above two expressions prove that:


Mass balance for biomass in the whole system:

Mass balance for biomass in the settling tank:

Combining equations 1 and 2 and rearranging,

2.  Consider a wastewater flow of Q MLD with a BOD 5 value of So mg/L. We propose to treat this waste using an activated sludge process (ASP) with no recycle. The size of the proposed aeration tank will such that the water is detained for hours. Complete mixing is assumed in the aeration tank. Write expressions for effluent substrate concentration and the biomass concentration in the aeration tank.

Solution: From biomass balance in aeration tank: 


Therefore,       Equation 1:    

Also,               Equation 2:     

Combining Equations 1 and 2, Equation 3:  

Equation 4:        

Rearranging Equation 4:

From Substrate Balance in aeration tank:

Equation 5:     

Rearranging Equation 5: