Sewage treatment process

Sewage treatment generally includes the following three treatment stages: The first stage is the removal of stones, gravel, fat, grease, etc. It is contained in the sewage by mechanical treatment (such as grid, sedimentation or air flotation). The secondary treatment is a biological treatment. Pollutants in sewage are degraded by microorganisms and converted into sludge. Tertiary treatment is the advanced treatment of sewage, including the removal of nutrients and disinfection of sewage by 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.

Mechanical processing

The mechanical (primary) processing part includes structures such as grids, grit chambers and primary sedimentation tanks. The purpose of the treatment is to remove coarse particles and suspended solids. The treatment principle is to achieve solid-liquid separation through physical methods to separate pollutants from sewage. This is a commonly used sewage treatment method. Mechanical (primary) treatment is a necessary item for all wastewater treatment processes (although some processes sometimes omit the primary sedimentation tank). The typical removal rates of BOD5 and SS in primary sewage treatment are 25% and 50%, respectively. Aerated sedimentation tanks are generally not recommended for biological phosphorus and nitrogen removal wastewater treatment plants to avoid rapid degradation of organic matter removal. In the case that the quality characteristics of the raw sewage are not conducive to the removal of phosphorus and nitrogen, according to the subsequent process of the special injection of water quality, it is necessary to carefully analyze and consider the settings and methods of the initial sedimentation to ensure and improve the inflow of the subsequent processes such as phosphorus and nitrogen quality.

Biochemical wastewater treatment

Sewage biochemical treatment is a secondary treatment. Its main purpose is to remove insoluble suspended matter and soluble biodegradable organic matter. Its process composition is various and can be divided into activated sludge method, activated sludge method, activated sludge method, activated sludge (A2 / O) method, activated sludge method, oxidation ditch method, stable pond method, soil and Other processing methods. Recently, the activated sludge process has been used in most municipal sewage treatment plants. 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, and convert organic pollutants into harmless gaseous products (CO2), liquid products (water) and organic-rich substances Solid product (microbial population or biological sludge); the remaining biological sludge is separated from solids and liquids in sedimentation tanks and removed from the purified sewage.
In sewage biochemical treatment, the factors that affect the activity of microorganisms can be divided into two categories: substrate and environment.
The matrix includes nutrients, such as organic compounds mainly containing carbon elements, that is, nutrients such as carbon source materials, nitrogen sources, and phosphorus sources, and trace elements such as iron, zinc, and manganese. In addition, it includes some toxic and harmful chemicals such as phenol, benzene and other compounds, as well as some heavy metal ions such as copper, cadmium and lead ions.

Environmental impact factors include:

(1) Temperature The effect of temperature on microorganisms is very extensive. Although some bacteria are active in high temperature environments (50 ° C to 70 ° C) and low temperature environments (-5 ° C to 0 ° C), the most suitable temperature range for most microorganisms in wastewater treatment is 20 to 30 ° C. In a suitable temperature range, the physiological activity of the microorganism is strong, and its activity increases with increasing temperature, the better the treatment effect. Beyond this range, the activity of microorganisms will deteriorate and the biological reaction process will be affected. Generally, the maximum and minimum limits for controlling the reaction process are 35 ° C and 10 ° C, respectively.

(2) pH. The most suitable pH range of microorganisms in activated sludge system is 6.5-8.5. An acidic or alkaline environment is not conducive to the survival and growth of microorganisms. In severe cases, sludge flocs will be destroyed, bacterial micelles will be decomposed, and the treatment effect will be dramatically deteriorated.

(3) Dissolved oxygen. For aerobic biological reactions, it is important to maintain a certain concentration of dissolved oxygen in the mixed solution. When the dissolved oxygen in the environment is higher than 0.3 mg / L, both facultative and aerobic bacteria breathe aerobic; when the dissolved oxygen is lower than 0.2-0.3 mg / L and close to zero, the facultative bacteria turn to anaerobic breathing . Most aerobic bacteria basically stop breathing, and some aerobic bacteria (mainly filamentous bacteria) may also grow well and often cause sludge swelling after dominating the system. Generally speaking, the dissolved oxygen at the outlet of the aeration tank should be maintained at about 2 mg / liter. If the dissolved oxygen is too high, energy consumption will increase, which is uneconomical.

Among all influencing factors, matrix factors and pH depend on the quality of the incoming water. The control of these factors mainly depends on daily monitoring and strict implementation of relevant regulations. For general urban sewage, these factors will not have much impact, and the parameters can basically be kept in an appropriate range. Changes in temperature are related to climate. For a 10,000-ton municipal wastewater treatment plant, especially when activated sludge is used, temperature control is difficult to implement and economically and engineeringly not feasible. Therefore, the processing requirements of different temperature changes generally achieve the processing goals through proper selection of design parameters. Therefore, the main goals of process control fall on the activated sludge itself and environmental factors that can be changed by means of regulation. The main task of control is to take appropriate measures to overcome the influence of external factors on the activated sludge system so that it can continue to operate stably.
The key to the process control of the biological response system lies in the choice of control objects or control parameters, which is closely related to the processing process or target.
As mentioned earlier, dissolved oxygen is a very important indicator parameter in the type and process of biological reactions. It can intuitively and relatively quickly reflect the operating status of the entire system. Convenient operation and management, relatively simple installation and maintenance of instruments. This is why China’s new sewage treatment plant has basically realized on-line monitoring of dissolved oxygen on-site in the past ten years.

Level 3 processing:

Tertiary treatment is the advanced treatment of water, and there are not many sewage treatment plants put into practical application in China. It removes nitrogen and phosphorus from the secondary treated water, removes pollutants remaining in the water through activated carbon adsorption or reverse osmosis, disinfects and sterilizes the virus with ozone or chlorine gas, and then sends the treated water to the middle channel for flushing , Spraying streets, watering green belts, industrial water, fire protection and other water sources.

It can be seen that the role of the sewage treatment process is to enrich pollutants into the sludge, and at the same time purify the sewage through biodegradation and solid-liquid separation, including the first-stage sludge produced in the first-stage treatment section and the second-stage treatment section Residual activated sludge and chemical sludge from tertiary treatment. Because these sludges contain a large amount of organic matter and pathogens, they are easy to rot and odor, and easily cause secondary pollution, and the task of eliminating pollution has not been completed. Sludge must be properly disposed of through certain reduction, reduction and stabilization harmless treatment wells. The success of sludge treatment and disposal has an important impact on sewage treatment plants, and must be paid attention to. If the sludge is not treated, the sludge will have to be discharged together with the treated sewage, and the purification effect of the sewage treatment plant will be offset. Therefore, in actual application process, the sludge treatment in the sewage treatment process is also quite critical.

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