I. Basic Definitions of SS and MLSS
1. What Is SS (Suspended Solids)
SS is the abbreviation of Suspended Solid or Suspended Substance in English, that is, suspended matter in water quality. It refers to solid substances in a water sample that are retained on a filter membrane with a pore size of 0.45μm and dried to constant weight at 103–105℃.
SS is one of the important indicators for measuring the degree of water pollution. The uppercase letter C is commonly used to represent the content of suspended solids in water quality, and the unit of measurement is mg/L.
2. What Is MLSS (Mixed Liquor Suspended Solids)
MLSS is mixed liquor suspended solids concentration, that is, sludge concentration, indicating the concentration of suspended solids contained in a unit volume of mixed liquor in the aeration tank. It indirectly reflects the amount of microorganisms contained in the mixed liquor.
In order to maintain the purification efficiency of the aeration tank, a certain amount of sludge concentration must be maintained in the tank. For the conventional activated sludge process, the MLSS in the aeration tank is often controlled at 2–3g/L.
3. What Is MLVSS (Mixed Liquor Volatile Suspended Solids)
MLVSS is mixed liquor volatile suspended solids concentration, indicating the concentration of organic solid matter in activated sludge, and can better reflect sludge activity.
In a certain wastewater and treatment system, the proportion of microorganisms in the suspended solids of activated sludge is fixed, and the MLVSS/MLSS value is relatively stable, ranging between 0.75–0.85 for activated sludge in municipal wastewater.
II. Core Differences Between MLSS and SS
1. Different Application Scenarios
- MLSS is generally used in biological tanks and characterizes the sludge concentration of the biological system.
- SS refers to the suspended solids contained in influent and effluent water, carrying many pollutants, including COD, P, etc., and is a characterization of influent water quality.
2. Comparison of Measurement Methods and Requirements
The measurement methods for the two are similar, but the weighing difference requirements for constant weight are different. The specific parameters are shown in the table below:
| Item | SS | MLSS |
|---|---|---|
| Weighing difference for empty bottle to constant weight | ≤0.2mg | ≤0.5mg |
| Weighing difference for sample to constant weight | ≤0.4mg | ≤0.5mg |
| Measurement significance | Determination of suspended solids concentration in wastewater (contains very few inorganic solids) | Determination of sludge concentration in aeration tank (sludge contains more inorganic solids) |
3. Differences in Significance and Composition
The measurement methods for the two are similar, but their significance is different:
- SS is the determination of suspended solids concentration in wastewater, which contains very few inorganic solids.
- MLSS is the determination of sludge concentration in the aeration tank, and the sludge also contains more inorganic solids.
III. Key Conclusions and Selection Recommendations
Understanding the relationship among MLSS, SS, and MLVSS is crucial for the operation control of wastewater treatment systems and instrument procurement. MLSS reflects the sludge concentration of the biological system, SS reflects the degree of water pollution in influent and effluent, and MLVSS reflects sludge activity.
In actual engineering, it is recommended to select appropriate online or laboratory analytical instruments according to process requirements, and strictly follow the constant-weight weighing difference requirements to ensure accurate and reliable data.