The "driving force behind" sludge concentration: exploring the influencing factors

Mixed Liquor Suspended Solids (MLSS) refers to the content of suspended solids in mixed liquor, which is mainly used to characterize the amount of activated sludge in mixed liquor, and the common unit is mg/L. In urban sewage treatment plants, the normal range of MLSS is generally around 2000-4000 mg/L, but this range is not fixed and will be affected by many factors.

In terms of sewage properties, if the organic matter content in sewage is high, microorganisms have sufficient "food" sources, which is conducive to the growth and reproduction of microorganisms, and accordingly a higher MLSS can be maintained; on the contrary, if the organic matter content in sewage is low, the number of microorganisms that can be supported is limited, and the MLSS will be low. For example, sewage with a large proportion of industrial wastewater has complex components and contains many difficult-to-degrade substances or substances that are toxic to microorganisms, which will inhibit the growth of microorganisms, making it difficult to increase MLSS, or even possible to reduce it.

The operating conditions of the aeration tank also have a significant impact. If the aeration is excessive and the dissolved oxygen value is controlled too high, the free bacteria produced during the activated sludge lifting process will be easily oxidized by excessive aeration, making it impossible to further increase the activated sludge concentration; and when the aeration is insufficient, the microorganisms cannot get enough oxygen for metabolic activities, and their growth will also be limited, affecting MLSS. In addition, the sludge return ratio is also a key factor. If the return ratio is too high, that is, the amount of sludge returned is too much, it will dilute the sludge concentration in the aerobic tank, making it difficult to increase the concentration; if the return ratio is too low, it will not be able to add enough activated sludge to the aeration tank, so that the MLSS is maintained at a low level.

For example, when the treatment is mainly domestic sewage and the sewage source is stable and the water quality fluctuates little, the aeration tank operating parameters are adjusted reasonably. At this time, MLSS may stabilize near the higher value of the normal range, the sewage treatment effect is good, and the effluent quality can be stably up to standard. However, if there is heavy rain, the water inflow of the sewage treatment plant suddenly increases significantly, the sewage is diluted a lot, and the concentration of organic matter decreases. At this time, MLSS may be too low, resulting in a decrease in the microorganism's ability to treat pollutants and deterioration of the effluent quality. For another example, when treating some industrial wastewater containing high concentrations of heavy metals, if the pretreatment is not in place, these heavy metals will enter the aeration tank and inhibit the activity of microorganisms. Even if other operating conditions are normal, MLSS will become very low, and may even cause a large number of microorganisms to die, affecting the normal operation of the entire sewage treatment system. On the contrary, if in pursuit of better treatment effects, the amount of sludge discharged is blindly increased, reducing the amount of activated sludge in the aeration tank, the MLSS will be too low, which will weaken the reaction tank's ability to resist shock loads; and excessively reducing the amount of sludge discharged will lead to excessively high MLSS, which will cause problems such as sludge aging, which is also not conducive to sewage treatment.

In summary, sludge concentration, as a key factor in sewage treatment, is closely linked to all aspects of nitrogen removal and phosphorus removal, and is affected by many factors such as sewage properties and aeration tank operating conditions. In actual operation, we need to accurately grasp these factors, always pay attention to changes in sludge concentration, flexibly adjust sewage treatment processes according to different situations, and maintain appropriate sludge concentration to ensure stable and efficient operation of nitrogen removal and phosphorus removal processes, ensure that sewage is discharged in compliance with standards, protect our ecological environment, achieve sustainable use of water resources, contribute to environmental protection, and allow every drop of water to return to the embrace of nature in a clear state.