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Municipal
Municipal wastewater
The typical urban sewage treatment process mainly includes mechanical treatment, biochemical treatment, sludge treatment and so on. The system consisting of mechanical and biochemical treatments is a secondary treatment system with BOD5 and SS removal rates of 90% to 98%. Treatment effect between a level and two levels of treatment in the middle is generally called the enhanced level one treatment, a half treatment or incomplete level two treatment, there are mainly high load biological treatment and chemical treatment of two categories, BOD5 removal rate up to 45%-75%. The secondary treatment system with biological phosphorus and nitrogen removal function is usually called deep secondary treatment. In order to remove specific substances, the treatment system set up after the secondary treatment belongs to the tertiary treatment, such as chemical phosphorus removal, activated carbon adsorption, etc.
The typical urban sewage treatment process mainly includes mechanical treatment, biochemical treatment, sludge treatment and so on. The system consisting of mechanical and biochemical treatments is a secondary treatment system with BOD5 and SS removal rates of 90% to 98%. Treatment effect between a level and two levels of treatment in the middle is generally called the enhanced level one treatment, a half treatment or incomplete level two treatment, there are mainly high load biological treatment and chemical treatment of two categories, BOD5 removal rate up to 45%-75%. The secondary treatment system with biological phosphorus and nitrogen removal function is usually called deep secondary treatment. In order to remove specific substances, the treatment system set up after the secondary treatment belongs to the tertiary treatment, such as chemical phosphorus removal, activated carbon adsorption, etc.
Classification of pollutants
From the point of view of sewage treatment, pollutants can be divided into suspended solid pollutants, organic pollutants, toxic substances, polluting organisms and polluting nutrients. A large number of organic matter contained in urban sewage discharged into the water, will reduce the content of dissolved oxygen in the water, or even reach the state of hypoxia, serious pollution of the water, so that the fish in the water can not survive. The concentration of organic matter in wastewater is generally expressed by biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total oxygen demand (TOD) and total organic carbon (TOC). The nutrients, mainly nitrogen and phosphorus, allow algae and plankton to bloom, creating "blooms" and "red tides".
Sewage treatment method
Sewage treatment methods can be divided into physical treatment method, biological treatment method, sewage treatment sludge disposal and chemical treatment method according to the type of water quality, but also according to the degree of treatment can be divided into primary treatment, secondary treatment and tertiary treatment and other processes. The physical treatment method of municipal sewage is to separate and remove pollutants in sewage by physical action. Commonly used methods are screening and intercepting, gravity separation, centrifugal separation, etc., the corresponding processing equipment is mainly grille, sedimentation tank, sedimentation tank and centrifuge oxygen which sedimentation tank with the town water supply treatment in the sedimentation tank. Biological treatment is the use of microbial metabolism, the removal of organic matter in sewage methods. Commonly used are activated sludge method, biofilm method, as well as oxidation pond and sewage land treatment method. Chemical treatment method is less used in urban sewage treatment, generally involving other chemical methods in the treatment of urban water supply, such as neutralization REDOX, ion exchange, electrolysis, mainly used in industrial wastewater treatment, rarely used in urban sewage treatment. Sludge needs to be treated to prevent secondary pollution. The disposal methods are usually concentrated, anaerobic digestion, dehydration and heat treatment. Primary treatment is mainly for suspended substances in water. Physical methods are often adopted. After primary treatment, the removal of suspended substances in sewage can reach about 40%, and the organic matter attached to suspended substances can also be removed about 30%. Secondary treatment mainly removes colloidal and dissolved organic pollutants in sewage. Usually the method is microbial treatment, specific ways have activated sludge method and biological membrane method. Biological treatment is to use the function of microbial decomposition and oxidation of organic matter, and take certain artificial measures to create an environment conducive to the growth and reproduction of microorganisms, so that microorganisms multiply in large numbers, to improve the efficiency of decomposition and oxidation of organic matter. Sewage passes after one class treatment, had gone apart from floating matter and partial suspended matter, the removal rate of BOD5 is about 25% ~ 30%. After the secondary treatment, the BOD5 removal rate can reach more than 90%, and the water discharged from the secondary sedimentation tank can reach the standard discharge. Activated sludge treatment system is one of the most widely used treatment technologies in the field of sewage treatment. Aeration tank is the reactor. Sewage and sludge are mixed in the aeration tank, and the microorganisms in the sludge degrade the complex organic matter in the sewage, and use the energy released to realize the reproduction and movement of microorganisms themselves.
The technology of sewage treatment
The current popular sewage treatment processes include: AB method, SBR method, oxidation ditch method, ordinary aeration method, membrane separator and so on, each of which has its own characteristics.
AB method
The process divides the aeration tank into two levels of oxygen supply according to high and low load. Class A has high load, short aeration time, and large sludge capacity. The sludge load is above 2.5kg BOD/(kg MLSS•d) and the tank volume load is above 6kg BOD/(m3•d). B - class load is low and sludge age is long. Class A and B can also be built by stages, and intermediate sedimentation tank is set between class A and class B. The F/M(the ratio of the amount of pollutants to the amount of microorganisms) of the two-stage pools are different, forming different microbial communities. Although the AB method has the advantage of energy saving, it is not suitable for low concentration water quality.
SBR method
This method of water intake, aeration, precipitation, effluent in the same tank to complete, usually consists of 3-4 pools constitute a group, take turns, a pool of intermittent operation, so it is called sequential batch activated sludge method. This - is characterized by simple technology process for the body, because only - a reaction pool, do not need the second pond, the backflow sludge and related equipment, usually without adjusting pool, the early saves in most cases the pond, so saving and investment, high impact load and flexible operation mode, can on time to arrange different aeration, anoxic and anaerobic state, To achieve the purpose of phosphorus and nitrogen removal.
Ordinary aeration
The variation process of the common aeration method appeared the earliest, its practical treatment effect is good, can deal with a large amount of sewage, for the JC-R plant can be centralized construction of sludge digestion tank, the generated biogas can be used as energy utilization. The disadvantage of the traditional general aeration method is that it can only be used as a conventional secondary treatment without the function of nitrogen and phosphorus removal. In recent years, in engineering practice, the purpose of nitrogen removal can be achieved by reducing the volume load of common aeration tank. In front of the common aeration tank, an anaerobic zone can be used to remove phosphorus or chemical phosphorus. The common aeration method to remove BOD has many forms in the pool type, such as oxidation ditch, which is known as the variation process of common aeration method in engineering, and can also be collectively referred to as common aeration method.
Oxidation ditch process
It was developed in the early 1950s and formed. Because of its simple structure and easy management, it was soon popularized and applied and innovated constantly. At present, the oxidation ditch has developed a variety of forms in the application, the representative ones are: (1) Patch type, referred to as single ditch type, surface aeration using rotary brush aeration, the water depth is generally 2.5-3.5m. (2) Austrian type, referred to as concentric circle type, the actual application of the elliptical three ring road composition, three ring road using different. Do, such as the outer ring is 0, the inner ring is 1, and the inner ring is 2, which is conducive to nitrogen and phosphorus removal. Adopting rotating disc aeration, the water depth is generally 4.0-4.5m. ③ The card type, referred to as cyclic folding, adopts inverted umbrella impeller aeration, the water depth is generally about 3.0m, but the sludge is easy to deposit. (4) Three ditch oxidation ditch (T type oxidation ditch), the process is composed of three pools, the middle as aeration tank, the left and right two pools as sedimentation tank and aeration tank. It is characterized by rotating brush aeration, shallow water, large area, no anaerobic pool, and no phosphorus removal function.
Membrane separation technology
Membrane separation instead of precipitation in sludge and water separation can bring the following changes in the activated sludge process: ① There is no sludge bulking problem. When controlling the activated sludge system, it is unnecessary to consider the settling property of the sludge, so that the process control is greatly simplified. ② The sludge concentration of the aeration tank will be greatly increased, MLSS can be greater than 20 g/L, so that the system can operate under the condition of super-large sludge age and ultra-low load, and fully meet the needs of removing various pollutants; (3) Under the same treatment requirements, the volume of the aeration tank can be greatly reduced and the area of the treatment plant can be saved; (4) With the increase of sludge concentration, higher aeration rate is required, so pure oxygen aeration will be widely used with the separation of membrane.
Process optimization
Comparison of conventional activated sludge process with oxidation ditch and SBR process. ① Conventional activated sludge method is suitable for large sewage treatment plants with medium load. (2) The construction cost of oxidation ditch method and SBR method is low, and the operation cost is high. If the treatment scale is 100,000 t/d and the depreciation is calculated as 20 years, the total treatment cost of oxidation ditch, SBR and conventional activated sludge process is roughly the same (treatment cost = operation cost + depreciation + interest of fixed asset investment loan). The smaller the scale, the lower the total treatment cost of oxidation ditch and SBR. Therefore, oxidation ditches and SBR are economically beneficial for small and medium-sized sewage treatment plants. (3) The oxidation ditch and SBR process generally do not set up primary settling tank and sludge digester, the treatment unit is reduced by more than 50% compared with the conventional activated sludge process, and the operation and management are simplified; And the equipment of high degree of localization, low price. 3.3.2 Comparison of oxidation ditch and SBR process. ① Capital cost: SBR is a joint construction type. The high land price is favorable to SBR, and the cost of civil engineering is lower, but the cost of equipment is higher than that of oxidation ditch. ② in terms of influent water,BOD5 concentration, high, conducive to oxidation ditch; Low, favorable for SBR. Generally, BOD5=150mg/L is taken as the boundary, and the cost of oxidation ditch construction is lower than that of SBR when the value is higher. Less than this value, vice versa. (3) operating costs in terms of aeration, oxidation ditch commonly used mechanical type, SBR is usually used in the blast type, the latter than the former energy saving; The SBR process is operated with variable water level, which increases the head, so the power consumption is smaller than that of the oxidation ditch, and the operation cost is also lower. (4) SBR process has higher requirements for automatic control. In terms of effluent quality, oxidation ditch is dynamic precipitation, while SBR is static precipitation. The latter has higher precipitation efficiency and better effluent quality.
