Collection! Detailed explanation of sludge concentration MLSS in activated sludge process!

The operation of the activated sludge process requires the reasonable adjustment of numerous control parameters, including the control of mixed liquor suspended solids (MLSS), which is one of the commonly used indicators in the daily operation of wastewater systems. Therefore, today we will provide a detailed introduction to the definition of MLSS and its relationship with other indicators.

　　1. Definition of MLSS (Mixed Liquor Suspended Solids)
　　Mixed liquor suspended solids (MLSS) refers to the content of suspended solids in the mixed liquor at the outlet of the aeration tank. MLSS is represented by the symbol MLSS, with units of mg/L. It is used to measure the amount of activated sludge in the aeration tank. The total amount of MLSS includes the following four aspects:
　　Active microorganisms;
　　Organic matter adsorbed onto the activated sludge that cannot be biodegraded;
　　Residuals from microbial self-oxidation;
　　Inorganic matter.
　　During operation, it is particularly important to note that MLSS refers only to the concentration of mixed liquor in the aeration tank and does not consider the concentration of mixed liquor in the secondary sedimentation tank. When monitoring the concentration of mixed liquor in the aeration tank, it is important to note that the concentration of mixed liquor at the outlet of the aeration tank is used as the standard to measure the concentration of activated sludge in the entire aeration tank.

2. Relationship between MLSS and other control indicators

1) Relationship between MLSS and sludge age
　　Sludge age is achieved through the removal of activated sludge. Reasonable control of sludge age and food-to-microorganism ratio can provide a reasonable range for controlling MLSS. In fact, if the MLSS is increased blindly, when the influent organic matter concentration is not high, the sludge age will be particularly long, exceeding the normal control value of sludge age. This clearly indicates that the MLSS control is too high, which is much more accurate than judging whether to control the MLSS based on the MLSS value.

2) Relationship between MLSS and water temperature
　　The growth, reproduction, and metabolism of activated sludge in the bioreactor are closely related to water temperature. For every 10℃ decrease in water temperature, the activity of activated sludge will decrease by half; when the water temperature is below 10℃, the treatment effect is noticeably poor. Therefore, adjustments to MLSS can be made to address changes in water temperature:
　　When the water temperature is low, the MLSS can be increased to offset the negative impact of reduced activated sludge activity, thereby increasing the removal efficiency of activated sludge at low water temperatures;
　　When the water temperature is high, the activated sludge is highly active, and excessively high MLSS is not conducive to the sedimentation of activated sludge. In this case, reducing the MLSS can help avoid the undesirable situation of un-settled flocs and turbid supernatant.

3) Relationship between MLSS and sludge settling ratio
　　MLSS affects the final settling value of the settling ratio. The higher the controlled MLSS, the greater the final settling ratio, and vice versa. This is because when the MLSS is high, there are more organisms, and after compression and sedimentation, a higher settling ratio will naturally occur. The key difference from other factors that can also increase the settling ratio is to observe whether the settled activated sludge is compact and whether the color is dark brown. In cases where the settling ratio is increased due to factors other than increased MLSS, the sludge is often poorly compacted and dull in color.
　　Of course, excessively low MLSS also significantly affects the settling ratio, but this is often not due to the operator intentionally lowering the MLSS, but rather due to low influent organic matter concentration. In such cases, the operator often feels that the MLSS is too low and tries to increase it, resulting in activated sludge aging. Subsequent observations of the settling ratio will reveal phenomena typical of aged activated sludge, such as high sludge compressibility, dark color, and clear supernatant with small flocs.
　　If the low settling ratio is due to abnormal sludge discharge, observation will also reveal that the settled activated sludge is light in color, poorly compacted, and poorly settled.