Sludge Age vs. Solids Retention and Mean Cell Residence Time

An age old debate of sludge wasting can become a long one. As I was doing some research on this topic, I found some interesting facts, circumstances and scenarios surrounding the issue. The very first obstacle that I encountered was getting a consistent definition, equation or explanation of sludge age. Usually this would be a frustrating situation, but it actually fueled me to dig deeper, and here are the results of my search.

One publication stated:

“Sludge age is also commonly termed as the solids retention time (SRT) or mean cell residence time (MCRT). Historically, sludge age has been calculated as a ratio of the total solids in aeration to the weight of total solids in the aeration tank influent. Average sludge age will be simply the total amount of solids in the system divided by the amount leaving the system each day.”

This one statement shows why there are problems when it comes to defining and determining these terms. Although all three of these methods can be used as process control techniques pertaining to wasting sludge, they are distinctly different.

Sludge age is actually the average time a particle of suspended solids remains in the activated sludge system. So in essence, the sludge age strictly refers to the amount of time the particle spends inside the activated sludge system. Thus, the mathematical equation is based on the daily amount of suspended solids, which is calculated from the daily influent flow and concentration to the primary clarifier or the daily influent flow and concentration to the plant if there is no primary clarifier and the total amount of solids in the aerator or the aeration basin. This is determined by using the Mixed Liquor Suspended Solids (MLSS).

Sludge age      =    Total lbs of MLSS in aeration basin
(days)                   Daily lbs of TSS in the influent

Solids Retention Time (SRT) on the other hand is the average time a unit of cell mass stays in the activated sludge system. It is based on the suspended solids. The difference in the SRT equation and the sludge age equation is that while sludge age is based on what is in the aerator, solids retention is based on what is leaving the activated sludge process, including the solids in the clarifiers.

Mean Cell Residence Time uses both the MLSS and the suspended solids (SS) to determine the time in days. It also uses a combination of what is in the system and what exits the system as effluent and as wasted suspended solids.

One of these three solids balances involved in the wastewater treatment process can give you a representation of either how old the sludge is or how long it has been in the system. No matter which equation your particular facility chooses to use, remember that you are simply finding the mass of solids in the plant divided by the mass of new solids made each day.

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