I. Background
Urban sewage is a product of urban development, with the acceleration of urbanization and industrialization, its generation is increasing, pollution is becoming more and more serious, has seriously constrained the sustainable development of urban socio-economic. In the rapid development of the global economy today, environmental issues, especially urban wastewater treatment has become a hot spot of research in various countries. The treatment of urban wastewater plays a key role in improving urban water environment and safeguarding urban economic development. Western developed countries in the 1950s economic development, had led to the 1960s serious environmental pollution. To the end of the 1970s, the United States built more than 18,000 urban sewage treatment plants, investing trillions of dollars in capital; Britain, France, Germany, each cost a huge amount of money to build 7,000 to 8,000 urban sewage treatment plants. China's sewage treatment began in the 1970s. According to statistics, as of the end of 2000, the country has built 427 sewage treatment plants, for the construction of urban sewage treatment works for a total investment of about 15 billion yuan.
Two, urban sewage treatment process overview
The typical urban sewage treatment process mainly includes mechanical treatment, biochemical treatment, sludge treatment and other sections. Mechanical treatment and biochemical treatment system is a secondary treatment system, which BOD5 and SS removal rate of up to 90% -98%. Treatment effect between the primary and secondary treatment in the middle is generally known as strengthened and treatment, a half-treatment or incomplete secondary treatment, mainly high-load biological treatment and chemical treatment method of two categories, BOD5 removal rate of 45% -75%. The secondary treatment system with biological phosphorus removal and nitrogen removal 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 and so on.
2.1 Classification of pollutants
From the point of view of wastewater treatment, pollutants can be divided into suspended solid pollutants, organic pollutants, toxic substances, polluting organisms and polluting nutrients. Urban sewage contains a large number of organic matter discharged into the water body, will make the water body to reduce the content of dissolved oxygen, and even reach a state of hypoxia, serious pollution of water bodies, so that the fish in the water can not survive. The concentration of organic matter in sewage is generally expressed by biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total oxygen demand (TOD) and total organic carbon (TOC). Nutrients mainly refer to nitrogen and phosphorus, which can make algae and plankton bloom, forming "water bloom" and "red tide".
2.2 Wastewater Treatment Methods
Wastewater treatment methods can be divided into physical treatment, biological treatment, sewage sludge disposal and chemical treatment methods according to the type of water quality, but also according to the degree of treatment can be divided into the primary treatment, secondary treatment, and tertiary treatment processes. Physical treatment of municipal wastewater is the use of physical separation and removal of pollutants in the sewage method. Commonly used methods are sieve filter retention, gravity separation, centrifugal separation, etc., the corresponding treatment equipment are mainly grids, sand sinks, sedimentation tanks and centrifuges oxygen which sedimentation tank with the urban water treatment in the sedimentation tank. Biological treatment method is the use of microbial metabolism, the removal of organic matter in the sewage method. Commonly used activated sludge, biofilm method, there are oxidation ponds and sewage land treatment method.
Chemical treatment method is used less in urban sewage treatment, generally involves urban water treatment in other chemical methods such as neutralization and oxidation reduction, ion exchange, electrolysis is mainly used for industrial wastewater treatment, rarely used in urban sewage treatment. Sludge needs to be treated in order to prevent secondary pollution, and its disposal methods are often thickening, anaerobic digestion, dehydration and heat treatment. Primary treatment of suspended solids in water, often using physical methods, after primary treatment, the removal of suspended solids in sewage up to about 40%, attached to the suspended organic matter can also be removed about 30%; secondary treatment of sewage to remove colloidal and dissolved organic pollutants.
Three, sewage treatment process technology
The current popular sewage treatment process: AB method, SBR method, oxidation ditch method, ordinary aeration, membrane separator, etc., each with its own characteristics.
3.1 AB method
The process of aeration tank according to the high and low load is divided into two levels of oxygen supply. A high load, short aeration time, the amount of sludge produced, sludge load 2.5 kg BOD / (kg MLSS-d) or more, the pool volume load of 6 kg BOD / (m3-d) or more; B level load is low, the age of the sludge is longer. a level and B level can also be built in stages The intermediate sedimentation tank is set up between level A and level B. The F／M ratio of the two levels of tanks is 1:1. Two levels of pools F / M (pollutant volume and microbial volume ratio) is different, the formation of different microbial groups. AB method, despite the advantages of energy saving, but not suitable for low concentration of water quality.
3.2 SBR method
This method of water intake, aeration, precipitation, water in the same pool to complete, often by 3-4 pools constitute a group, rotating operation, a pool of pools to run intermittently, so it is known as sequential batch activated sludge method. This process is characterized by a simple process, because there is only - a reaction tank, do not need a secondary sedimentation tank, reflux sludge and related equipment, generally do not set up a regulating tank, in most cases can be omitted in the primary sedimentation tank, so it saves the area and investment, shock load and flexible operation, can be arranged from time to time, aeration, anoxia and anaerobic Different states, to achieve the purpose of phosphorus denitrogenation.
3.3 General aeration method
The variant process of general aeration method appeared the earliest, and its actual treatment effect is good, can deal with large amounts of sewage, for Jc-r plant can be centralized construction of sludge digestion tanks, biogas can be produced for energy use. The shortcomings of the traditional Pu aeration method is only as a conventional secondary treatment, does not have the function of nitrogen and phosphorus removal. In recent years, in engineering practice, by reducing the volume load of the general aeration tank, can achieve the purpose of nitrogen removal; set anaerobic zone in front of the general aeration tank, can remove phosphorus, can also be used to remove phosphorus by chemical method. Ordinary aeration method to remove BOD, there are a variety of forms in the pool type, such as oxidation ditch, engineering known as ordinary aeration method of the variant process, can also be collectively referred to as ordinary aeration method.
3.4 Oxidation ditch method
It was developed in the early 1950s and formed, because of its simple structure, easy to manage, and soon be popularized and applied, and continuous innovation. At present, the oxidation ditch in the application of the development of a variety of forms, more representative of: ① Pa type, referred to as a single ditch type, surface aeration using rotary brush aeration, the depth of water is generally in the 2.5-3.5 m. ② Ao type, referred to as concentric circles, the actual application of more than oval three-ring road composition, three ring road using different DO, such as the outer ring of 0, the ring for the 1, the inner ring for the 2, which is conducive to the removal of nitrogen and phosphorus. 2, which is conducive to the removal of nitrogen and phosphorus. The use of rotating disk aeration, water depth is generally 4.0-4.5 m. ③ card type, referred to as circular folding type, the use of inverted umbrella impeller aeration, water depth is generally about 3.0 m, but the sludge is easy to deposit. ④ three ditch oxidation ditch (T-type oxidation ditch), the process consists of three pools, the middle for the aeration pool, the left and right two pools as a sedimentation pool and. Aeration pool. It is characterized by the use of rotary brush aeration, shallow water, large footprint, no anaerobic tanks, does not have the function of phosphorus removal mouth J.
3.5 Membrane Separation Technology
Membrane separation instead of sedimentation for mud-water separation can bring about the following changes in the activated sludge process: ① No longer exists the problem of sludge expansion. In regulating the activated sludge system, no longer have to consider the settlement performance of sludge, thus greatly simplifying the process control; ② aeration tank sludge concentration will be greatly increased, MLSS can be greater than 20 g / L, so that the system can be in the super-large sludge age, ultra-low load state operation, to fully meet the need to remove a variety of pollutants; ③ in the same treatment requirements, the aeration tank can be greatly reduced in size, saving the Treatment plant footprint; ④ the increase in sludge concentration, requiring a higher aeration rate, and thus pure oxygen aeration will be used in large quantities with the separation of the membrane.
3.6 Process Preference
Comparison of conventional activated sludge method with oxidation ditch and SBR process. ① Conventional activated sludge method is suitable for large-scale sewage treatment plants with medium load. ② Oxidation ditch method, SBR method of low capital costs, high operating costs. If the scale of treatment for 100,000t / d, depreciation to 20 years, oxidation ditch, SBR and conventional activated sludge method of total treatment costs are roughly equivalent (treatment costs = operating costs + depreciation + fixed asset investment loan interest). The smaller the scale, the lower the total treatment cost of oxidation ditch, SBR. Therefore, for small and medium-sized sewage treatment plants, oxidation ditch, SBR in the economic benefits. ③ oxidation ditch, SBR process is generally not set up primary sedimentation tank and sludge digestion tank, treatment unit than conventional activated sludge method to reduce more than 50%, simplified operation and management; and high degree of equipment localization, low price.
Four, suggestions and countermeasures
①Omission of certain processes and equipment. For small and medium-sized sewage plant, the selection process should be considered to omit the sludge reflux equipment and sludge digestion process, sludge treatment is used by the concentration of direct dewatering, and then sent to the garbage plant or used for agricultural fertilizer. Chlorine disinfection process can be omitted, but the process is set up for accidental discharge, if there is an accident, temporary measures can be taken, thus saving investment.
②The sewage intake indicators BOD5, CODcr should not be too large, otherwise the sewage retention time is too long, the investment will increase.
Establishment of sewage charging system, appropriately raise the price of tap water to compensate for the operating costs of sewage treatment plants. Sewage treatment plant effluent indicators should meet the relevant local standards for reuse in industrial cooling water, urban clean water and agricultural irrigation water.
3 adopt multi-channel to raise funds for construction, including bank loans, national debt and public burden, and establish a stable source of debt service funds. Can be used first by the government to invest in construction, after completion of the sewage treatment company entrepreneurial operation, in regulations, policy support, so that it is efficient, low-cost operation, and sewage charges.
④The use of construction - operation - transfer of the "bot" model. The government, through a concession agreement, awards the project to a project company set up for the concession project for a certain period of time, which is responsible for the financing, construction, operation and maintenance of the project; after the expiration of the concession period, the company returns the project to the government department without any compensation.
⑤ Strengthen exchanges and cooperation, the introduction of foreign advanced technology, the establishment and development of sewage treatment process technology and environmental protection equipment suitable for national conditions. Sewage treatment is a municipal infrastructure, although the environmental and social benefits are significant, but at present it is difficult to have a sizable economic benefits. Therefore, the government needs to give financial and policy support.
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