Wastewater treatment removes biological and chemical contaminants before water is safely returned to the environment. The Activated Sludge Process (ASP) uses cultivated microorganisms to consume pollutants. Within the aeration tanks of this system, Mixed Liquor Suspended Solids (MLSS) acts as a primary operational indicator. Monitoring and controlling MLSS concentration is integral to ensuring the consistent, efficient performance of the treatment facility.
Defining Mixed Liquor Suspended Solids (MLSS)
Mixed Liquor Suspended Solids (MLSS) is defined as the concentration of suspended solids found in the aeration tank of an activated sludge plant. The term “mixed liquor” refers to the combination of incoming wastewater and recycled, biologically active sludge blended within the tank. MLSS is quantified in units such as milligrams per liter (mg/L) or grams per liter (g/L) and represents the total mass of solids present.
The majority of these suspended solids are living microorganisms, known as biomass, which degrade organic matter. This biomass includes bacteria, protozoa, and fungi, along with non-biodegradable inorganic particles and decomposed organic material. MLSS serves as a measure of the microbial population size available to treat the incoming water. Maintaining the proper MLSS concentration achieves optimal treatment conditions.
The Biological Function of MLSS
The primary function of the biomass within the MLSS is the consumption of soluble organic contaminants, often measured as Biochemical Oxygen Demand (BOD). These microorganisms metabolize the organic compounds, converting them into harmless byproducts like carbon dioxide and water, along with new microbial cell mass. This biological oxidation process is the core mechanism by which the activated sludge system purifies the water.
The microbes within the mixed liquor aggregate to form dense clumps known as flocs. This flocculation is beneficial for subsequent treatment steps. These microbial flocs improve the settleability of suspended solids in the clarifier tanks following aeration. Effective floc formation ensures the treated water is separated from the biological solids before discharge. The MLSS population’s health and density correlates with the system’s capacity for pollution removal.
Measuring and Maintaining MLSS Levels
Plant operators rely on accurate measurement to manage MLSS concentration within the aeration basin. The standard laboratory method involves gravimetric analysis, where a known volume of mixed liquor is filtered through a pre-weighed filter. The filter is then dried at a high temperature, typically 105 degrees Celsius, and re-weighed to determine the mass of the suspended solids captured.
Operational control over the MLSS concentration is achieved primarily by regulating the flow of two sludge streams: Return Activated Sludge (RAS) and Waste Activated Sludge (WAS). RAS recirculates settled, concentrated biomass from the secondary clarifier back into the aeration tank to maintain the active microbial population. This action directly increases the MLSS concentration and ensures a continuous supply of organisms for treatment.
Conversely, Waste Activated Sludge (WAS) removes excess biomass from the system. As microorganisms consume organic matter and reproduce, their total mass increases, necessitating removal to prevent the concentration from becoming too high. By adjusting the flow rates of both RAS and WAS, operators control the MLSS concentration, keeping it within a target range, typically 2,000 to 5,000 mg/L for conventional plants.
MLSS and Activated Sludge Performance Metrics
The measured MLSS concentration is used to calculate performance metrics that guide process control. One such metric is the Food to Microorganism Ratio (F/M Ratio), which measures the amount of incoming organic load (food) relative to the mass of microorganisms (M) in the aeration tank. The MLSS is often used as the proxy for the microorganism mass in this calculation.
A balanced F/M ratio, often targeted between 0.25 and 0.50 for conventional systems, ensures that the biomass is adequately “fed” but not overloaded. If the ratio is too high, the bacteria are under-populated relative to the food, which can lead to poor contaminant removal. If the ratio is too low, the microorganisms may enter a starvation state, potentially leading to poor settling characteristics.
Another performance indicator derived from MLSS is the Mean Cell Residence Time (MCRT), also known as Sludge Age or Solids Retention Time (SRT). MCRT represents the average time the biological solids remain active within the activated sludge system. This metric is calculated by dividing the total MLSS mass in the system by the mass of solids leaving the system daily (via wasting or treated effluent). MCRT dictates the overall microbial community structure, favoring the growth of slower-growing organisms necessary for nutrient removal when the MCRT is longer.