In wastewater treatment a great deal of confusion exists when it comes to the following three terms:

- Sludge age,
- Sludge (or solids) retention time (SRT), and
- Mean cell residence time (MCRT).

To many people the three terms mean the same thing and they are, therefore, all calculated in exactly the same way. And you will find many textbooks that agree with this last statement. As an example, here is a quote from “Activated Sludge Process Design and Control: Theory and Practice” written by W. Wesley Eckenfelder and Petr Grau.

“The principal parameter used to size aeration basins is SRT (solids retention time). There is no 100 percent agreement in the U.S. as to how this is calculated, and how it differs from MCRT (mean cell residence time) and other, “sludge age” calculations.”

In order to make a firmer distinction between the three terms listed above, sludge age is sometimes referred to as the “Gould Sludge Age.” In an effort to reduce confusion this document considers sludge age, SRT, and MCRT to be synonymous and the formulas for these three terms are identical. In contrast, Gould Sludge Age is calculated differently as shown below in Equation 3.

**Equation 3: Gould Sludge Age Formula**

Gould sludge age refers to the average number of days a particle of suspended solids remains under aeration. It is a calculation used to maintain the proper amount of activated sludge in the aeration tank. It takes into account the solids “**entering**” the system, in contrast to mean cell residence time which considers the solids “**leaving**” the system, as discussed below. When considering Gould sludge age you are, in effect, determining how many days of suspended solids are in the aeration tank. Where the F:M ratio calculation is concerned with the **organic loading** to the aeration system the Gould sludge age is only concerned with **solids loading** to the aeration system as indicated by Figure 4.

**Figure 4: Gould Sludge Process Flow Schematic**

In Equation 3, in the denominator, the variable “*Inf TSS*” would normally refer to the total suspended solids leaving a primary clarifier and would be labeled as “*P.E. SS, mg/L*,” for example. This wastewater treatment plant does not have primary clarifiers so in this particular case the *Inf TSS* variable is referring to the solids concentration in the influent to the oxidation ditch. The Gould sludge age formula shows a very basic equation requiring two key inputs: 1) influent total suspended solids concentration and 2) the mixed liquor suspended solids concentration. There are no variations to how this equation is calculated from plant to plant or between municipal and industrial wastewater treatment facilities.

The value of the Gould sludge age formula is limited but it is shown here for the purpose of clarifying what is oftentimes confusing because of the use (or misuse) of terms and acronyms. Of far more value to plant operators is **the solids retention time or mean cell residence time (MCRT) equation discussed next.**