Wastewater New York State Practice Exam

Which type of solids best represent the microorganisms in the activated sludge process?

1. Total suspended solids

2. Mixed liquor suspended solids

3. Mixed liquor volatile suspended solids

4. Fixed solids

The correct answer is: Mixed liquor volatile suspended solids

Mixed liquor volatile suspended solids (MLVSS) are the type of solids that best represent the microorganisms in the activated sludge process. This is because MLVSS specifically measures the portion of suspended solids in the mixed liquor that are organic and capable of biological decay, which predominantly includes the microorganisms responsible for the digestion of organic waste in the activated sludge system. MLVSS is important in evaluating the biomass concentration in the aeration tanks where these microorganisms flourish. The volatile solids component indicates the amount of living microbial mass since these are primarily composed of cells that consume organic material, thereby facilitating the treatment process. Understanding the concentration of MLVSS helps operators optimize aeration processes and improve treatment efficiency. In contrast, total suspended solids include not only the microorganisms but also any inorganic material, which does not contribute to the biological treatment of wastewater. Mixed liquor suspended solids captures both the volatile and fixed solids present in the mixture, but it does not focus exclusively on the active biomass. Fixed solids generally refer to the non-biodegradable components in the system, which provide no direct value to the microbiological processes occurring within the activated sludge system.