WASTEWATER TREATMENT-IV

SOLUTION

1. Consider a wastewater flow of 10 MLD with a BOD 5 value of 600 mg/L. It is proposed to treat this waste using an activated sludge process (ASP). The size of the proposed aeration tank will such that the water is detained for 4 hours. Complete mixing is assumed in the aeration tank.

Design the complete system for an effluent BOD (S e ) concentration of 100 mg/L. Complete design means the values of V, X, R, Q_w and X_r . Assume X₀ and X_e to be negligible. Assume X_r to be 10000 mg/L. Y_T = 0.5; K_d = 0.05 /d; K =0.1 L/mg/d.

Express, for the whole system, the substrate and biomass input/output and formation/destruction rates in Kg/d. Show these values on the process schematic and demonstrate that the mass balance is maintained for both substrate and biomass.
Calculate the oxygen requirement for this system

In addition to the BOD₅ concentration of 600 mg/L, this water also contains nitrogen and phosphorus. The nitrogen concentration expressed as Total Kjeldahl Nitrogen (TKN) is 20 mg/L as N, the phosphorus concentration is 5 mg/L as P. Calculate the nitrogen and phosphorus requirement for this system. Hence find out the effluent nitrogen and phosphorus concentration in the effluent from the system.

Express the nitrogen and phosphorus input/output and formation/destruction rates in Kg/d. Show these values on the process schematic and demonstrate that the mass balance is maintained for both nitrogen and phosphorus.

NOTE: Biomass may be represented as either: C₅H₇O₂N or C₆₀H₈₇O₂₃ N₁₂ P.

Solution:

S_e = 10 mg/L; q = K.S_e = 0.1.(10) = 1/d; Therefore μ = Y_T .q - K_d = (0.5)(1)-0.05= 0.45/d

Also,  

Hence,

Also

Since X_r = 10000 mg/L

Also (1 + R - R. ) =, ;     or,

Therefore,   

Substrate Balance:

Biomass Balance: