Why should wastewater be pretreated? What are the common pretreatment processes?

the purpose of pretreatment: to remove ss, remove some refractory substances, adjust pH and reduce load are mainly for the follow-up biochemical treatment, which can reduce the burden of biochemical treatment and investment and operation costs, and ensure that the wastewater after biochemical treatment reaches the standard.

pretreatment unit. Pretreatment processes for municipal wastewater can include: coarse screening (grid), medium screening, crushing, flow measurement, pump lifting, slag removal, pre-aeration, flotation, flocculation and chemical treatment.

generally, flotation, flocculation and chemical treatment are not needed for domestic sewage treatment. The adoption of such methods sometimes depends on industrial wastewater in urban wastewater. Flotation is used to remove fine suspended solids, liposuction and fat, and it is carried out in a separate unit or in a pre-aeration tank for degreasing and sometimes slag removal. If the oil industry and meat processing plants have proper pretreatment, the city treatment plant may not need flotation unit. High-strength municipal wastewater can be flocculated with or without chemicals to improve the primary treatment effect and prevent the overload of secondary treatment process. Chlorine is sometimes added to raw wastewater to control odor and improve the sedimentation properties of wastewater. The change of pretreatment unit layout depends on the characteristics of raw wastewater, the next treatment process and the pretreatment unit adopted. There are some general principles that are often applied to unit layout. Grid is used to protect the water pump and prevent solids from scaling in the sediment tank or metering tank. Small treatment plants normally put a Babbitt metering tank in front of the constant-speed lifting pump. In large treatment plants or where variable speed pumps are used, the metering tank can be placed behind the water pump. In most independent domestic sewage plants, the sediment pool is placed behind the lift pump, but when the sludge load is predicted to be large, the sediment pool should be placed in front of the pump.

the primary treatment of the primary treatment unit is precipitation. However, it is common practice that the so-called primary treatment includes pretreatment process. All the treatment plants in big cities adopt the original sewage precipitation method, and must be located in front of the conventional biological filter. The fully mixed activated sludge process can be used to treat raw wastewater without precipitation. However, due to the reasons of sludge disposal and operation cost, this process is only used in small towns, and the pretreatment data of wastewater treatment chemicals addition and dephosphorization are expected to be adopted.