In calculating MCRT, what does 'S' represent?

In the context of calculating Mean Cell Residence Time (MCRT), 'S' refers to the pounds of Mixed Liquor Suspended Solids (MLSS) present in the wastewater treatment system. MCRT is a critical operation parameter that helps in understanding the amount of time that microorganisms are retained in the treatment process, which is essential for the biological treatment of wastewater.

The concentration of MLSS is crucial because it reflects the extent of biomass available for the treatment process. A higher MLSS concentration can enhance the treatment efficiency as more microorganisms are present to break down organic matter. By knowing the MLSS, operators can manage their biological processes effectively, adjusting parameters as needed to maintain optimal treatment conditions.

In contrast to this, the other options represent different measures that do not directly relate to MCRT. For example, while pounds of total suspended solids or pounds of microorganisms lost in the effluent may be relevant in broader monitoring of wastewater treatment performance, they do not specify the biomass retained within the operational environment, which is the fundamental purpose of assessing MCRT. Similarly, pounds of organic matter removed is focused on the outcome of the treatment process rather than the retention time of microorganisms within the system.