Introduction of the method and process flow of sewage treatment

According to the treatment degree, sewage treatment can be divided into primary, secondary and tertiary treatment.



The first-class treatment, which belongs to physical treatment, mainly removes the suspended solid pollutants in the sewage. Most of the physical treatment methods can only meet the requirements of the first-class treatment. After the first-class treatment of sewage, BOD can generally remove about 30%, which can not meet the discharge standard. The first stage treatment belongs to the pretreatment of the second stage treatment.



The second stage treatment mainly removes the organic pollutants (BOD and COD) in colloid and dissolution state in the sewage, and the removal rate can reach more than 90%, which makes the organic pollutants meet the discharge standard.



Third, further treatment of the refractory organic matter, nitrogen and phosphorus can lead to eutrophication of the water soluble inorganic substances. The main methods are biological denitrification and phosphorus removal, coagulation precipitation, sand filtration, activated carbon adsorption, ion exchange and electroosmotic analysis.



The whole process is that the raw sewage through the coarse grid is lifted by the sewage lifting pump, and then flows through the grid or sand filter, and then into the sand settling tank. The sewage separated by sand and water enters the primary sedimentation tank. The above is the first-class treatment. The effluent from the primary sedimentation tank enters the biological treatment equipment, including the activated sludge method and the biofilm method (the reactor of the activated sludge method includes aeration tank, oxidation ditch, etc, Biofilm method includes biofilter, biological rotary table, biological contact oxidation method and biological fluidized bed). The effluent of biological treatment equipment enters secondary sedimentation tank. The effluent from secondary sedimentation tank is discharged through disinfection or into three-stage treatment. The first stage treatment is finished to be secondary treatment. The third stage treatment includes biological denitrification and phosphorus removal, coagulation sedimentation, sand filtration, activated carbon adsorption, ion exchange The method of exchange and electrodialysis. Part of sludge in secondary sedimentation tank is returned to primary sedimentation tank or biological treatment equipment, and part of sludge enters sludge concentration tank, then enters sludge digestion tank. After dehydration and drying equipment, sludge is finally used.

Five typical processes



(1) Batch activated sludge process (SBR)



Batch activated sludge process, also known as sequencing batch reactor (SBR), is composed of several or more SBR tanks. During operation, wastewater enters the tank in batches and goes through five independent stages successively, namely, influent, reaction, sedimentation, drainage and idle. Water level control for water inlet and drainage, time control for reaction and sedimentation. The time of an operation cycle varies according to the load and effluent requirements, which is generally 4-12h, in which the reaction accounts for 40%. The effective tank volume is the sum of the water inflow and the required sludge volume in the cycle. Compared with the continuous flow process, the reaction speed is faster, the treatment efficiency is high, and the ability of load impact resistance is strong; due to the high substrate concentration and concentration gradient, the alternate anoxic and aerobic state can inhibit the excessive reproduction of specific aerobic bacteria, which is conducive to biological nitrogen and phosphorus removal. Moreover, due to the short sludge age, filamentous bacteria can not be the advantage, so the sludge is not easy to bulking; Compared with the continuous flow method, SBR process has the advantages of short process, simple structure and low operation cost. When the water volume is small, only one batch reactor is needed, no special sedimentation tank and regulating tank are needed, no sludge reflux is required, and the operation cost is low.



(2) Adsorption regeneration (contact stabilization) method



This method makes full use of the initial removal capacity of activated sludge. In a short time (10-40min), suspended and colloidal organic matter in wastewater can be removed by adsorption, and then through liquid-solid separation, the wastewater can be purified, and about 85% - 90% of BOD5 can be removed. Among the activated sludge saturated with adsorption, some of which need to be recycled are introduced into the regeneration tank for further oxidation and decomposition to restore its activity; the other part of the excess sludge is discharged into the sludge treatment system without oxidative decomposition. It is carried out in two pools (adsorption tank and regeneration tank) or in two sections of the same tank. It has strong ability to adapt to the impact of load, and the primary sedimentation tank can be omitted. The main advantage is that it can greatly save the capital construction investment, and is most suitable for the treatment of wastewater containing more suspended and colloidal substances, such as tannery wastewater and coking wastewater. However, due to the short adsorption time, the treatment efficiency is not as high as the traditional method.



(3) Oxidation ditch oxidation ditch is a special type of delayed aeration method.



Oxidation ditch oxidation ditch is a special type of delayed aeration method. Its plane is like a runway. Two aeration rotary brushes (discs) are arranged in the groove. There are also surface aerators, jet machines or riser type aeration devices. When the aeration equipment works, it can promote the rapid flow of the ditch liquid to realize oxygen supply and mixing. Compared with the conventional aeration process, the oxidation ditch has the advantages of less capital investment, easy maintenance and management, stable treatment effect, good effluent quality, less sludge output, better removal of N and P, and strong adaptability to load impact.



(4) Continuous influent cycle extended aeration activated sludge process (ICEAS)



There is a pre reaction zone (10% of the tank volume) in the front of ICEAS reactor. The reaction tank is composed of pre reaction zone and main reaction zone, and continuous water inflow and intermittent drainage are realized. The pre reaction zone is generally in anaerobic and anoxic state, where organic matter is adsorbed by activated sludge. This zone also has the function of biological selection to inhibit the growth of filamentous bacteria and prevent sludge bulking. The adsorbed organic matter is oxidized and decomposed by activated sludge in the main reaction zone. Continuous influent of water into the reaction solves the contradiction between the incoming water and the intermittent influent. However, the sedimentation effect of the process is poor, the purification effect is poor, sludge bulking is easy to occur, the sludge load is low, the reaction time is long, the equipment volume is increased and the investment is large.



(5) Biological nitrogen and phosphorus removal process (A / A / O)



The wastewater first enters the anaerobic tank and mixes with the return sludge. Under the action of facultative anaerobic fermentation bacteria, the biodegradable macromolecular organic matter in the wastewater is transformed into polyphosphate accumulating bacteria, which can absorb small molecular organic matters (such as VFA) and store them in the body in the form of PHB, and the energy needed is from the decomposition of the polyphosphate chain. Then, the wastewater enters the anoxic zone, and the denitrifying bacteria use the organic matrix in the wastewater to denitrify the NO3 - brought with the reflux mixture. When the wastewater enters the aerobic tank, the concentration of organic matter in the wastewater is low. The phosphorus accumulating bacteria mainly obtain energy by decomposing the PHB in the body for bacterial proliferation. At the same time, the dissolved phosphorus in the surrounding environment is absorbed into the body and stored in the form of polyphosphate chain, and then discharged into the system in the form of residual sludge. The lower concentration of organic matter in the aerobic zone of the system is conducive to the growth of autotrophic nitrifying bacteria. Anaerobic, anoxic, aerobic three different environmental conditions and the organic combination of different types of microbial flora can simultaneously remove organic matter, nitrogen and phosphorus; the process is simple, hydraulic retention time is short; SVI is generally less than 100, no sludge bulking; high phosphorus content in sludge, generally more than 2.5%; The anaerobic anoxic tank only needs to be stirred gently to make it mixed without increasing dissolved oxygen; the anaerobic anoxic state should be avoided in the sedimentation tank to avoid phosphorus release by phosphorus accumulating bacteria, which will reduce the effluent quality and the denitrification can produce N2 to interfere with the sedimentation; The effect of nitrogen and phosphorus removal is affected by the reflux ratio of mixed liquor, while the phosphorus removal effect is affected by do and nitrate oxygen carried in the sludge, so it is impossible to improve the effect of nitrogen and phosphorus removal. 

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