A brief introduction to wastewater treatment processes-Hezhong Environment (Beijing) Co., Ltd. was established in 2003

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A brief introduction to wastewater treatment processes

Published Time:

2022-07-21

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Wastewater treatment processes are divided into three levels:

Primary treatment: Physical treatment, Through mechanical treatment, such as screens, sedimentation or flotation, to remove stones, sand, grease, and oil from the wastewater.

Secondary treatment: Biochemical treatment, Pollutants in wastewater are degraded and converted into sludge under the action of microorganisms.

Tertiary treatment: Advanced treatment of wastewater, It includes the removal of nutrients and disinfection of wastewater through chlorination, ultraviolet radiation, or ozone technology. Depending on the treatment objectives and water quality, some wastewater treatment processes may not include all of the above processes.

01 Primary treatment

Mechanical (primary) treatment sections include structures such as screens, grit chambers, and primary sedimentation tanks, aiming to remove coarse particles and suspended solids. The principle of treatment is to achieve solid-liquid separation through physical methods, separating pollutants from wastewater. This is a commonly used wastewater treatment method.

Mechanical (primary) treatment is a necessary process in all wastewater treatment processes (although sometimes some processes omit the primary sedimentation tank). The typical removal rates of BOD5 and SS in primary treatment of urban wastewater are 25% and 50%, respectively.

In biological phosphorus and nitrogen removal wastewater treatment plants, aerated grit chambers are generally not recommended to avoid rapid degradation of organic matter removal; when the characteristics of raw wastewater are not conducive to phosphorus and nitrogen removal, the setting and method of primary sedimentation need to be carefully analyzed and considered according to the characteristics of the subsequent process to ensure and improve the influent water quality of subsequent processes such as phosphorus and nitrogen removal.

02 Secondary treatment

Wastewater bio-treatment belongs to secondary treatment, mainly aiming to remove non-settleable suspended solids and biodegradable dissolved organic matter. Its process composition is diverse and can be divided into activated sludge process, AB process, A/O process, A2/O process, SBR process, oxidation ditch process, stabilization pond process, CASS process, land treatment process, etc. Currently, most urban wastewater treatment plants use the activated sludge process.

The principle of biological treatment is to complete the decomposition of organic matter and the synthesis of organisms through biological action, especially the action of microorganisms, converting organic pollutants into harmless gaseous products (CO2), liquid products (water), and solid products rich in organic matter (microbial populations or biological sludge); excess biological sludge is separated from the purified wastewater in sedimentation tanks through solid-liquid separation.

03 Tertiary treatment

Tertiary treatment is an advanced treatment of water, a wastewater treatment process following secondary treatment, and the highest level of wastewater treatment. It is not widely used in China's wastewater treatment plants.

It removes nitrogen and phosphorus from the water that has undergone secondary treatment, removes remaining pollutants from the water using activated carbon adsorption or reverse osmosis, and disinfects bacteria and viruses using ozone or chlorine, then sends the treated water to the secondary water system for use in flushing toilets, spraying streets, irrigating green belts, industrial water, fire protection, etc.

Therefore, the role of wastewater treatment processes is only to enrich pollutants into sludge through biodegradation and solid-liquid separation, purifying wastewater while enriching pollutants into sludge, including primary sedimentation sludge produced in the primary treatment section, excess activated sludge produced in the secondary treatment section, and chemical sludge produced in the tertiary treatment.

Since these sludges contain a large amount of organic matter and pathogens, and are easily putrefied and odorous, they easily cause secondary pollution, and the task of eliminating pollution has not yet been completed. Sludge must undergo certain volume reduction, weight reduction, and stabilization and harmless treatment and proper disposal. The success or failure of sludge treatment and disposal has an important impact on wastewater treatment plants and must be taken seriously.

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

04 Odor control technology

Physical methods mainly include dilution and adsorption; chemical methods include absorption and combustion; biological methods include bio-agent method, biofiltration, packed tower bio-deodorization, and bioscrubbing, plant extract liquid atomization spraying, etc.