Water treatment uses physical, chemical, biological and other methods to improve water quality for source water or water that does not meet the water quality requirements.

Water treatment methods can be summarized in three ways: ①The most commonly used method is to obtain the required water quality by removing some or all of the impurities in the raw water; ②To obtain the required water quality by adding new components to the raw water; Processing does not involve removing impurities or adding new ingredients.

Softened water treatment: treated with chemical “resin”, such as hard water softening.

Commonly used sewage treatment technologies include biochemical methods, such as Activated Sludge Process, Fixed Biofilm Processes, Combined Biological Processes, etc.; physical and chemical methods, such as Granular Media Filtration, Activated Carbon Adsorption, Chemical Precipitation, Membrane Processes, etc.; and natural treatment methods, such as Stabilization Ponds, Aerated or Facultative Lagoons, Constructed Wetlands, chemical color Corset resin treatment method.

Water treatment process:

Sewage treatment generally includes the following three-level treatment: the first-level treatment is to remove stones, sand, fat, grease, etc. contained in the sewage through mechanical treatment, such as grilles, sedimentation or air flotation. The secondary treatment is biological treatment, and the pollutants in the sewage are degraded and converted into sludge under the action of microorganisms. The tertiary treatment is the advanced treatment of sewage, which includes the removal of nutrients and the disinfection of the sewage through chlorination, ultraviolet radiation or ozone technology. Depending on the treatment goals and water quality, some sewage treatment processes do not include all the above processes.

Mechanical processing section

The mechanical (primary) treatment section includes structures such as grids, grit tanks, and primary sedimentation tanks to remove coarse particles and suspended solids. The principle of treatment 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 project for all sewage treatment processes (although some processes sometimes omit the primary sedimentation tank), and the typical removal rates of BOD5 and SS in the primary treatment of urban sewage are 25% and 50% respectively. In biological dephosphorization and denitrification sewage treatment plants, aerated grit tanks are generally not recommended to avoid the removal of rapid degrading organic matter; in the case of raw sewage water quality that is not conducive to dephosphorization and denitrification, whether or not the primary sedimentation is set The method needs to be carefully analyzed and considered according to the follow-up process of water quality special injection to ensure and improve the quality of the influent water of the follow-up process such as phosphorus and nitrogen removal.

Wastewater Biochemical Treatment

The sewage biochemical treatment is a secondary treatment, with the main purpose of removing non-sinkable suspended solids and soluble biodegradable organic matter. Its process composition is diverse and can be divided into activated sludge method, AB method, A/O method, A2/O Various treatment methods such as method, SBR method, oxidation ditch method, stable pond method, and land treatment method. Most urban sewage treatment plants have adopted the activated sludge method recently. 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 gas products (CO2), liquid products (water) and organic-rich products. Solid product (microbial colony or biological sludge); excess biological sludge is separated in the sedimentation tank by solid-liquid separation in the sedimentation tank, and removed from the purified sewage.
In the process of wastewater biochemical treatment, the factors affecting microbial activity can be divided into two categories: matrix type and environmental type:

  1. The matrix category includes nutrients, such as carbon-based organic compounds, namely carbon source materials, nitrogen sources, phosphorus sources and other nutrients, as well as trace elements such as iron, zinc, and manganese; in addition, it also includes some toxic and harmful chemicals Compounds such as phenols and benzene also include heavy metal ions such as copper, cadmium, and lead ions.
  2. Environmental factors mainly include:
  • Temperature. Temperature has a wide range of effects on microorganisms. Although certain types of bacteria are active in high temperature environments (50℃~70℃) and low temperature environments (-5~0℃), most microorganisms are the most active in sewage treatment. The suitable temperature range for growth is 20-30°C. In a suitable temperature range, the physiological activity of microorganisms is vigorous, and its activity increases with the increase of temperature, and the treatment effect is better. 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 progress of the reaction are 35°C and 10°C, respectively.
  • PH value. The most suitable PH value range for activated sludge system microorganisms is 6.5-8.5. The acidic or alkaline environment is not conducive to the survival and growth of microorganisms. In severe cases, the sludge flocs will be destroyed and the bacterial micelles will disintegrate. The treatment effect deteriorated sharply.
  • 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 0.3mg/l, both facultative bacteria and aerobic bacteria carry out aerobic respiration; when the dissolved oxygen is lower than 0.2-0.3mg/l close to zero, the facultative bacteria will turn into anaerobic respiration. Oxygen respiration, most aerobic bacteria basically stop breathing, and some aerobic bacteria (mostly filamentous bacteria) may also grow well, and often lead to sludge expansion after occupying an advantage in the system. Generally, the dissolved oxygen at the outlet of the aeration tank should be kept at about 2mg/l. If it is too high, the energy consumption will increase, which is not economical.

Among all influencing factors, matrix factors and PH value are determined by the quality of influent water. The control of these factors mainly depends on daily monitoring and strict implementation of relevant regulations and laws. For general urban sewage, most of these factors will not have much impact, and the parameters can basically be maintained within an appropriate range. Temperature changes are related to climate. For 10,000-ton municipal sewage treatment plants, especially when activated sludge processes are used, temperature control is difficult to implement, and it is not very feasible economically and engineeringly. Therefore, it is generally through the appropriate selection of design parameters to meet the processing requirements of different temperature changes to achieve the processing goals. Therefore, the main goal of process control falls on the activated sludge itself and the environmental factors that can be changed by regulating and controlling means. 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 Play a role continuously and steadily.
The key to achieving the process control of the biological reaction system lies in the selection of control objects or control parameters, which in turn is closely related to the treatment process or treatment goals.
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 conditions of the entire system, convenient operation and management, and simple installation and maintenance of instruments and meters. This is also the reason why my country’s newly built sewage treatment plants have basically realized dissolved oxygen on-site and online monitoring in the past decade.

Three-level processing:

The tertiary treatment is the advanced treatment of water. At present, there are not many sewage treatment plants in my country that have been put into practical use. It will denitrify and dephosphorize the water after the secondary treatment, use activated carbon adsorption or reverse osmosis to remove the remaining pollutants in the water, and use ozone or chlorine to sterilize bacteria and viruses, and then send the treated water into the Waterways are used as water sources for flushing toilets, spraying streets, watering green belts, industrial water, and fire prevention.

It can be seen that the role of the sewage treatment process is only through biodegradation conversion and solid-liquid separation to purify the sewage while enriching the pollutants into the sludge, including the primary sludge generated in the primary treatment section, The remaining activated sludge produced in the secondary treatment section and the chemical sludge produced in the tertiary treatment. Because these sludge contains a large amount of organic matter and pathogens, it is very easy to decay and smell, and it is easy to cause secondary pollution. The task of eliminating pollution has not been completed. The sludge must be properly disposed of through certain volume reduction, volume reduction and stabilization and harmless treatment wells. The success of sludge treatment and disposal has an important impact on the sewage treatment plant 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 also be offset. Therefore, in the actual application process, the sludge treatment in the sewage treatment process is also quite critical.

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