The main source of sludge in the water treatment process

     Solid substances are by-products produced in the process of sewage biochemical treatment, mainly including inorganic solids and biological solids. Inorganic solids mainly come from the filter residue of the grid and the mud sand in the grit chamber. Biosolids, that is, domestic sludge, are mainly derived from the denser organic particles settled in the original wastewater in the primary sedimentation tank and the excess biomass formed by the conversion of nutrients in the wastewater through the biochemical metabolism process. In the biological treatment of sewage, microorganisms metabolize the organic matter, nitrogen, phosphorus and other nutrients in the sewage to synthesize biological cells to complete their own growth and reproduction, which is manifested as an increase in sludge concentration on a macroscopic scale. In order to maintain a stable sludge concentration (or microbial biomass) in the system and maintain a reasonable growth load, a sludge return process is often set in the activated sludge wastewater treatment process. Except for part of the activated sludge returning to the biochemical reactor, the excess sludge (often referred to as excess sludge) must be discharged from the system regularly.

The remaining sludge produced in the process of sludge sewage treatment has high water content, unstable performance, and contains a large number of toxic and harmful substances in the original sewage. If it is not treated, it will cause secondary environmental pollution. In view of environmental protection and health and safety requirements, sludge must be effectively treated and properly disposed of. The treatment and disposal of the remaining sludge in the rural domestic sewage treatment system must be based on the characteristics of the rural domestic sewage treatment project scale, sludge production, and sludge resource utilization methods, and an economic, reasonable and safe sludge treatment and utilization plan must be formulated. Regarding the treatment and utilization of sludge in the rural domestic sewage treatment system, the following two modes are recommended:
(1) Sludge treatment and utilization mode of decentralized rural domestic sewage treatment system. Distributed rural domestic sewage treatment systems generally treat domestic sewage from one or several households. The scale is generally small, and the amount of sludge produced is correspondingly small. In the spring and summer crops need fertilizer season, the sludge can be used as fertilizer for farmland, flowers, vegetables, etc. after simple pile-up anaerobic fermentation to reduce organic matter and remove pathogenic bacteria. In the off-season when fertilizer is needed in autumn and winter, the sludge can be collected and processed in a special or domestic waste collection and transportation system after simple air-drying and dehydration.
(2) The sludge treatment and utilization mode of the centralized rural domestic sewage treatment system. The centralized rural domestic sewage treatment plant handles a large amount of sewage and produces a large amount of surplus sludge. Therefore, a special sludge treatment unit must be built inside the sewage treatment plant to treat the sludge.

Sludge treatment should follow the following main principles:
(1) Reduction: Generally, the water content of sludge is above 95%, and the volume is large, which is not conducive to storage, transportation and consumption. Therefore, the purpose of reducing the volume of sludge should be achieved by reducing the water content of sludge. This process is called For reduction.
(2) Stabilization: The organic content of the dry matter of the sludge is generally 60%-70%, which will undergo anaerobic degradation, which is extremely perishable and produces foul smell. Therefore, it is necessary to adopt a biological anaerobic digestion process to convert the organic components in the sludge into stable end products. Chemical agents can also be added to stop the activity of microorganisms in the sludge to stabilize the sludge, such as adding lime to increase the alkalinity, while also killing pathogenic microorganisms in the sludge.
(3) Harmlessness: The sludge produced by domestic sewage treatment contains a large number of pathogenic bacteria, parasite eggs and viruses, which can often cause the spread of infectious diseases. Some sludge also contains a variety of heavy metal ions and toxic and harmful organic matter. Therefore, the sludge must be thoroughly harmlessly treated.

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