What is the 'three stage' treatment of the wastewater treatment process?
Sewage treatment is a systematic project. The removal of pollutants in water body is usually the result of multiple mechanisms.
Sewage treatment technology is divided into three stages
One stage treatment: by mechanical treatment, such as grille, precipitation or air flotation, to remove the stone, sand, fat and grease contained in sewage.
Two stage treatment: Biological treatment, in which pollutants in sewage are degraded and converted into sludge under the action of microorganisms.
Three stage treatment: Advanced treatment of sewage, which includes the removal of nutrients and disinfection of sewage by chlorination, ultraviolet radiation or ozone technologies.
Some wastewater treatment processes may not include all of these processes, depending on the target of treatment and the quality of the water.
One stage treatment (mechanical treatment)
The mechanical (level 1) treatment section includes structures such as grills, sand settling ponds, and initial settling ponds to remove coarse particles and suspended matter for the purpose of treatment. The principle of treatment lies in the separation of solid and liquid through physical method to separate pollutants from sewage, which is a commonly used sewage treatment method.
Mechanical (One stage) treatment is required for all wastewater treatment processes (although some processes sometimes omit the One stage sedimentation tank). The typical BOD5 and SS removal rates for One stage treatment of municipal wastewater are 25% and 50%, respectively. In biological phosphorus and nitrogen removal sewage treatment plants, aerated sand sedimentation tank is generally not recommended, in order 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, whether or not to set the initial sedimentation and how to set it should be carefully analyzed and considered according to the subsequent processes of water quality characteristics, so as to ensure and improve the water quality of the subsequent processes such as phosphorus and nitrogen removal.
Two stage treatment (biochemical treatment)
Sewage biochemical treatment belongs to Two stage treatment, to remove unsinkable suspended matter and soluble biodegradable organic matter as the main purpose, its process composition is diverse, can be divided into biofilm method and activated sludge method (AB method, A/O method, A2/O method, SBR method, oxidation ditch method) stability pond method, land treatment method and other treatment methods.
The influence of temperature on microorganisms is very extensive. Although some types of bacteria are also active in high temperature environment (50℃ ~ 70℃) and low temperature environment (-5 ~ 0℃), the most suitable temperature range for the growth of most microorganisms in sewage treatment is 20-30℃. In the appropriate temperature range, the physiological activity of microorganisms is vigorous, and its activity is enhanced with the increase of temperature, and the treatment effect is better. Beyond this range, the activity of microorganisms becomes worse, and the biological reaction process will be affected.
Generally, the maximum and mini limits for controlling the reaction process are 35℃ and 10℃ respectively.
PH value
The optimal PH value range for microorganisms in activated sludge system is 6.5-8.5, and the acidic or alkaline environment is not conducive to the survival and growth of microorganisms. In severe cases, the sludge floc will be destroyed, the bacteria micelle will be disintegrated, and the treatment effect will deteriorate sharply.
Dissolved oxygen
It is important to maintain a certain concentration of dissolved oxygen in the mixture for aerobic biological reactions. When the dissolved oxygen in the environment was higher than 0.3mg/l, both facultative bacteria and aerobic bacteria performed aerobic respiration. When the dissolved oxygen is lower than 0.2-0.3mg/l and close to zero, facultative bacteria will turn into anaerobic respiration, and most aerobic bacteria basically stop breathing, while some aerobic bacteria (mostly filamentous bacteria) may grow well, which often leads to sludge swelling after dominating the system. Generally, it is appropriate to maintain about 2mg/l of dissolved oxygen at the outlet of aeration tank. If it is too high, energy consumption will be increased, which is economically uneconomical.
Among all the influencing factors, matrix factors and pH value are determined by influent quality. The control of these factors mainly depends on daily monitoring and strict implementation of relevant regulations and regulations. For the general municipal sewage, most of these factors will not constitute a great impact, the parameters can be basically maintained within the appropriate range. The change of temperature is related to the climate. For the 10,000 ton municipal sewage treatment plant, especially the activated sludge process, the control of temperature is difficult to implement, which is not very feasible in economics and engineering. Therefore, it is generally through the appropriate selection of design parameters to meet the treatment requirements of different temperature changes, in order to achieve the treatment objectives.
Therefore, the main objective of process control falls on the activated sludge itself and the 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 play its role continuously and stably.
The key to process control of biological reaction system lies in the selection of control object or control parameter, which is closely related to the processing technology or processing target.
Previously mentioned dissolved oxygen is a very important indicator parameter in the type and process of biological reaction, it can directly and relatively quickly reflect the operating status of the whole system, convenient operation and management, the installation and maintenance of instruments and meters are also relatively simple, which is also the reason why in recent ten years our new sewage treatment plants have basically realized dissolved oxygen on-site and online monitoring.
Three stage processing (advanced processing)
Three stage treatment is the advanced treatment of water, which is the highest treatment measure of sewage after Two stage treatment. Current Chinese sewage treatment plant into practical application is not much. It will be after the Two stage treatment of water for denitrification, dephosphorization treatment, with activated carbon adsorption or reverse osmosis method to remove the remaining pollutants in the water, and ozone or chlorine disinfection kill bacteria and viruses, and then the treatment of water into the waterway, as flushing toilets, spraying streets, watering green belt, industrial water, fire prevention and other water sources.
Thus, it can be seen that the function of sewage treatment process is only to purify sewage while enriching pollutants into sludge through biodegradation and transformation and solid-liquid separation, including the One stage sludge produced in the One stage treatment stage, the residual activated sludge produced in the Two stage treatment stage and the chemical sludge produced in the Three stage treatment stage. Because these sludge contains a lot of organic matter and pathogens, and is very easy to rot and stink, it is easy to cause Two stage pollution, the task of eliminating pollution has not been completed. Sludge must be disposed of properly by a certain volume reduction, reduction and stabilization of harmless disposal Wells. The success of sludge treatment and disposal has an important impact on sewage plants, which 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 cancelled out. So in the actual application process, sludge treatment in the sewage treatment process is also quite critical.
