Basic knowledge of wastewater treatment--Several indicators for evaluating activated sludge

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Several indicators for evaluating activated sludge: (1) MLSS (Mixed Liquid Suspended Solid) refers to the dry weight of suspended solids in 1 liter of the aeration tank mixture; it is an indicator used to measure the amount of activated sludge present in the reactor. It includes microbial biomass (Ma), naturally produced oxidation products of microorganisms (Me), organic matter (Mi) and inorganic substances (Mii) adsorbed on sludge flocs that cannot be degraded by microorganisms. Since MLSS is convenient to measure, it is often used in engineering as an indicator to estimate the number of microorganisms in activated sludge. During engineering design, it is desirable to maintain a high MLSS level in order to reduce the volume of the aeration tank and save space and investment. However, the MLSS concentration cannot be too high, as this may lead to insufficient oxygen supply. In typical reactors, the sludge concentration is controlled at 2000–6000 mg/L. (2) MLVSS (Mixed Liquid Volatile Suspended Solid) refers to the amount of volatile suspended solids contained in 1 L of the aeration tank mixture. It includes only microbial cells (Ma), products of microbial autolysis (Me), and organic substances adsorbed on sludge flocs that cannot be degraded by microorganisms (Mi); it does not include inorganic substances (Mii). Therefore, MLVSS can reflect the number of microorganisms in the reactor with relatively high accuracy. Under normal circumstances, the MLVSS/MLSS ratio of activated sludge used for treating domestic wastewater is around 0.75; for industrial wastewater, this ratio varies depending on the quality of the water, with significant differences observed. (3) SV%: Sludge settling ratio, which is the ratio of the volume occupied by the sludge to the original volume of the mixed liquid after the mixture has been left to stand in a measuring cylinder for 30 minutes. After 30 minutes of normal activated sludge settling, it can approach its maximum density; therefore, under normal operation, SV% roughly reflects the amount of sludge in the reactor and can be used to control sludge discharge. The normal value of SV% in a typical aeration tank is 20% to 30%. Changes in SV% can also promptly reflect abnormalities such as sludge bulking. Therefore, SV% is an important indicator for controlling the operation of activated sludge processes. (4) SVI, the sludge volume index, refers to the volume occupied by 1 g of dry sludge after the mixed liquid in the aeration tank has settled statically for 30 minutes, with the unit being ml/g. SVI = volume of sludge after 30 minutes of settling of the mixture / dry weight of sludge = (SV% × 100) / MLSS. SVI reflects the degree of looseness and coagulation properties of the sludge. A low SVI indicates that the sludge particles are small and compact, contain a high amount of inorganic substances, and have few microorganisms; in such cases, the sludge lacks activity and adsorption capacity. An excessively high SVI indicates that the sludge structure is loose, making it difficult to settle and separate; it is either about to expand or has already expanded. (5) SDI stands for Sludge Density Index, which refers to the number of grams of active sludge contained in 100 ml of the mixture after it has been left to stand for 30 minutes. The unit is g/ml. Generally, SVI