What are the stages of sewage treatment? (Sewage treatment)

Sewage treatment process is divided into three stages:

Primary treatment: physical treatment, through mechanical treatment, such as grille, precipitation or air flotation, remove the stone, sand and fat, grease contained in the sewage.

Secondary treatment: Biochemical treatment in which contaminants in sewage are degraded and converted into sludge under the action of microorganisms.

Tertiary treatment: Advanced treatment of sewage, which includes the removal of nutrients and the disinfection of sewage by chlorination, ultraviolet radiation or ozone technology. Depending on the target of the treatment and the quality of the water, some sewage treatment processes do not include all of the above processes.

1. First-level treatment

The mechanical (first-level) treatment section includes structures such as grilles, sand settling ponds, and initial settling ponds to remove coarse particles and suspended matter for the purpose of treatment. The principle of treatment is to achieve solid-liquid separation by physical method and separate pollutants from sewage, which is a commonly used sewage treatment method.

Mechanical (primary) treatment is essential for all wastewater treatment processes (although some processes sometimes omit the primary sedimentation tank), and the typical removal rates of BOD5 and SS for municipal wastewater primary treatment are 25% and 50%, respectively.

In biological phosphorus and nitrogen removal wastewater treatment plants, aerated sand sedimentation tanks are generally not recommended to avoid the removal of rapidly degrading organic matter. In the case that the water quality characteristics of the original sewage are not conducive to phosphorus and nitrogen removal, the setting of the initial sedimentation and the setting mode need to be carefully analyzed and considered according to the subsequent process of water quality characteristics, so as to ensure and improve the influent water quality of the subsequent process such as phosphorus and nitrogen removal.

2. Secondary treatment

The biochemical treatment of sewage belongs to the secondary treatment, with the main purpose of removing unsinkable suspended matter and soluble biodegradable organic matter. Its process is varied, which can be divided into activated sludge method, AB method, A/O method, A2/O method, SBR method, oxidation ditch method, stabilization pond method, CASS method, land treatment method and other treatment methods. At present, most municipal sewage treatment plants use activated sludge process.

The principle of biological treatment is to complete the decomposition of organic matter and the synthesis of biological organisms through biological action, especially the role of microorganisms, and transform organic pollutants into harmless gas products (CO2), liquid products (water) and solid products rich in organic matter (microbial communities or biological sludge); The excess biological sludge is separated by solid and liquid in the sedimentation tank and removed from the purified sewage.

3, three level treatment

Tertiary treatment is the deep treatment of water, which is the wastewater treatment process after secondary treatment, and is the highest treatment measure of sewage. At present, there are not many sewage treatment plants put into practical application in China.

It will be the secondary treatment of water denitrification, dephosphorization treatment, with activated carbon adsorption or reverse osmosis method to remove the remaining pollutants in the water, and ozone or chlorine disinfection to kill bacteria and viruses, and then the treated water into the waterway, as flushing toilets, spraying streets, watering green belt, industrial water, fire prevention and other water sources.

It can be seen that the role of the sewage treatment process is only through biodegradation and conversion and solid-liquid separation, while purifying the sewage, the pollutants are enriched into the sludge, including the primary sludge generated by the primary treatment section, the residual activated sludge generated by the secondary treatment section and the chemical sludge generated by the tertiary treatment.

Because these sludge contains a lot of organic matter and pathogens, and it is easy to F·B stink, it is easy to cause secondary pollution, and the task of eliminating pollution has not been completed. Sludge must be properly disposed of through certain capacity reduction, reduction and stabilization harmless treatment Wells. The success of sludge treatment and disposal has an important impact on sewage plants and must be paid attention to.

If the sludge is not treated, the sludge will have to be discharged with the treated effluent, and the purification effect of the sewage plant will be offset. Therefore, in the actual application process, the sludge treatment in the sewage treatment process is also quite critical.

4, deodorization process

The physical method mainly includes dilution method, adsorption method, etc. Chemical method includes absorption method, combustion method, etc.; Biological methods include biological agent method, biological filtration method, filled tower biological deodorization method and biological washing method, plant extract atomization spray method.