Question

Two types of flow regime are normally used in the design of an activated sludge process,
namely completely mixed and plug flow (both are continuous). Using first order kinetic
reaction, demonstrate which flow regime provide a better performance, given the
followings:
Q = 550 m3/d t = 6 hours Co = 225 mg/L k = 0.15 d-1

Answer 1

Solution:- As the flow of wastewater is continuous, the reactants entering the reactor and the products flowing out from the reactor is considered to be continuous. So, the equation of hydraulic reaction time (HRT) can be obtained for each regime using the mass-balance of a reactant at steady state conditions. The equation of HRT for first order reaction kinetics :-

1) For completely mixed continuous flow regime (CMFR),

t of CMFR = (1/K) × [ (Co / Ce) - 1] ...............(A)

Where, K = reaction rate constant = 0.15 day^-1

Co = initial reactant concentration in influent = 225 mg / L

Ce = final reactant concentration in effluent ( to be calculated)

t (CMFR) = 6hrs = (6/24) days

Putting in equation (A), we get, final reactant concentration in effluent (Ce). Obtained value of Ce in completely mixed flow regime = 216.86747 mg/L.

The efficiency of CMFR = [(Co - Ce) / Co ] × 100

Efficiency of CMFR = [ (225 - 216.86747) / 225] × 100

Efficiency of CMFR = 3.614%

2) For plug-flow continuous flow regime (PFFR),

t for PFFR = (1 / K) × [ ln (Co / Ce)] .............(B)

Putting all respective values in equation (B) and calculate the final reactant concentration in effluent (Ce) in PFFR.

The obtained value of Ce in PFFR = 216.71874 mg/L which is slightly less than obtained in CFFR.

Efficiency of PFFR = [ (Co - Ce) / Co ] × 100

Efficiency of PFFR = [ ( 225 - 216.71874) / 225] × 100

Efficiency of PFFR = 3.681% ( which is slightly higher than CFFR).

So, plug flow continuous flow regime provides a slightly higher and better performance than completely mixed continuous flow regime. Plug flow regime is more efficient than completely mixed flow regime.