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Monitoring microbial activity in activated sludge processes is primarily conducted through measuring mixed liquor suspended solids (MLSS). MLSS represents the concentration of suspended solids in the mixed liquor, which includes both the microorganisms and inorganic particles present in the aeration tank.

The reason MLSS is so vital is that it directly correlates with the biomass concentration, which is essential for the biological treatment of wastewater. An optimal level of biomass is necessary for efficient nutrient removal and organic matter degradation, as these organisms play a crucial role in breaking down pollutants in the wastewater.

While other parameters like dissolved oxygen (DO) are important for aerobic processes and suspended solids concentration (SSC) can provide information on the particulate matter, MLSS specifically indicates the quantity of microbial mass available for the treatment process. Therefore, it serves as a primary indicator of the overall microbial population and health in the system, facilitating better control and optimization of the activated sludge process for effective wastewater treatment.