Mean Cell Residence Time

Mean Cell Residence Time

Mean Cell Residence Time (MCRT) is an expression of the average time (days) that a microorganism will spend in the activated sludge process.

In MCRT, the M is for “mean,” which is a substitute for “average.” The remaining letters, CRT, represent cell residence time — how long a particular bacterial cell, or a pound of bacteria, remains in the activated sludge secondary treatment train. MCRT is normally expressed in days.

Calculation and example

At an activated sludge wastewater treatment plant receiving 3.25 MGD, the final effluent suspended solids concentration averages 21.2 mg/L. What would the calculated MCRT value be when the aeration basin carries 2050 mg/L MLSS and wastes 0.0550 MGD. The waste activated sludge has a concentration of 7980 mg/L. The aeration basin has a volume of 1.00 MG and the secondary clarifier has an operational volume of 0.250 MG.

Here,

Plant flow=3.25 MGD

FE SS=21.2 mg/L

WAS flow=0.055 MGD

WAS SS=7980 mg/L

AB volume=1.0 MG

Clarifier volume=0.25 MG           

(AB MG + Clarifier MG) × (MLSS) × 8.34

MCRT=

lb of MLSS leave secondary system (WAS + effluent)

                                         

                                          (1.0 MG + 0.25MG) × (2050 mg/L) × (8.34)

MCRT=

                                [(0.055 MGD × 7980 mg/L) (8.34) + (3.25 MGD) (21.2 mg/L)(8.34)]