Page 23 - SIMPLIFIED WASTEWATER TREATMENT FUNDAMENTALS\ Advanced Activated Sludge P3

SRT vs MCRT

Sludge Retention Time (SRT) accounts for the estimated sludge age of the biomass, in

this case, within the Aeration Basin biomass inventory.

Mean Cell Residence Time (MCRT) accounts for the total biomass inventory in both the

Aeration Basin and the solids inventory in the Secondary Clarifier.

Both SRT and MCRT are commonly used.

By design and circumstance, if a wastewater facility typically holds 2 or more feet of

settled solids in their Secondary Clarifiers, MCRT may be the best mechanism for

inventory accountability. If most of the biomass inventory resides within the Aeration

Basin, SRT may be the best monitoring tool.

Note - Once MCRT or SRT are selected as the preferred process inventory monitoring tool,

they should not be interchanged (example - using SRT one week and MCRT another

week).