Urban sewage treatment process?

With the acceleration of urbanization and industrialization, domestic sewage pollution is becoming more and more serious, and the amount of urban sewage treatment is increasing. How to effectively treat domestic sewage has become an important factor in urban development and social and economic sustainable development. This paper mainly expounds the technology and method of sewage treatment.

I. Background

Urban domestic sewage is the product of urban development. With the acceleration of urbanization and industrialization, its output is increasing and pollution is becoming more and more serious, which seriously restricts the sustainable development of urban society and economy. With the rapid development of global economy, environmental protection, especially urban sewage treatment, has become a hot topic in various countries. Urban sewage treatment plays a key role in improving urban water environment and ensuring urban economic development. The economic development of western developed countries in 1950s led to serious environmental pollution in 1960s. By the end of 1970s, more than18,000 urban sewage treatment plants had been built in the United States, with an investment of trillions of dollars. Britain, France and Germany each invested huge sums of money to build 7000~8000 urban sewage treatment plants. Sewage treatment in China began in 1970s. According to statistics, by the end of 2000, 427 sewage treatment plants had been built in China, and the total investment in urban sewage treatment projects was about 654.38+0.5 billion yuan.

Second, the urban sewage treatment process overview

Typical urban sewage treatment processes mainly include mechanical treatment, biochemical treatment and sludge treatment. The system consisting of mechanical treatment and biochemical treatment belongs to the secondary treatment system, in which the removal rate of BOD5 and SS can reach 90%-98%. The treatment effect between primary treatment and secondary treatment is generally called enhanced treatment, primary and semi-treatment or incomplete secondary treatment, and there are two main types: high-load biological treatment and chemical treatment. The removal rate of BOD5 is 45%-75%. The secondary treatment system with biological phosphorus and nitrogen removal function is usually called advanced secondary treatment. In order to remove specific substances, the treatment system set after the secondary treatment belongs to the tertiary treatment, such as chemical phosphorus removal and activated carbon adsorption.

2. 1 pollutant classification

From the perspective of sewage treatment, pollutants can be divided into suspended solid pollutants, organic pollutants, toxic substances, polluted organic substances and polluted nutrients. A large amount of organic matter contained in urban sewage is discharged into water body, which will reduce the content of dissolved oxygen in water body, even reach anoxic state, seriously pollute water body, and make fish in water unable to 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 multiply and form "water bloom" and "red tide"

2.2 sewage treatment methods

Sewage treatment methods can be divided into physical treatment, biological treatment, sludge disposal and chemical treatment according to water quality types, and can also be divided into primary treatment, secondary treatment and tertiary treatment according to treatment degree. Physical treatment of urban sewage is a method to separate and remove pollutants from sewage through physical action. Commonly used methods include screen filtration interception, gravity separation and centrifugal separation. The corresponding treatment equipment mainly includes grid, grit chamber, sedimentation tank and centrifugal oxygen, among which the sedimentation tank is the same as that in urban water supply treatment. Biological treatment is a method to remove organic matter from sewage by using the metabolism of microorganisms. Commonly used are activated sludge method, biofilm method, oxidation pond and sewage land treatment method.

Chemical treatment is rarely used in urban sewage treatment, and generally involves other chemical methods in urban water supply treatment, such as neutralization redox, ion exchange, electrolysis, etc., which are mainly used in industrial wastewater treatment and rarely used in urban sewage treatment. Sludge needs to be treated to prevent secondary pollution, and its disposal methods are often concentration, anaerobic digestion, dehydration and heat treatment. Primary treatment is mainly aimed at suspended solids in water, and physical methods are often used. After primary treatment, the suspended solids in sewage can be removed by about 40%, and the organic matter attached to the suspended solids can also be removed by about 30%. The secondary treatment mainly removes colloid and dissolved organic pollutants from sewage.

Third, sewage treatment technology.

At present, the more popular sewage treatment processes are AB method, SBR method, oxidation ditch method, ordinary aeration method, membrane separator and so on, each with its own characteristics.

3. 1 AB method

In this process, the aeration tank is divided into two levels of oxygen supply according to the load. Class A load is high, aeration time is short, and the amount of sludge produced is large. The sludge load is above 2.5 kgbod/(kgmlss d), and the tank volume load is above 6 kgbod/(m3 d). Low grade B load and long sludge age. Grade A and Grade B can also be built by stages, and there is an intermediate sedimentation tank between Grade A and Grade B. The F/M (ratio of pollutant amount to microbial amount) of the two ponds is different, forming different microbial groups. Although AB method has the advantage of energy saving, it is not suitable for low concentration water quality.

3.2 SBR method

In this method, water inlet, aeration, sedimentation and effluent are completed in the same tank. Usually, 3-4 tanks are a group and run in turn, and they run intermittently from one tank to another, so it is called sequencing batch activated sludge process. This integrated process is characterized by simple process. Because there is only one reaction tank, there is no need for secondary sedimentation tank, return sludge and related equipment, and there is generally no regulating tank. In most cases, the primary sedimentation tank can be omitted. Therefore, it can save land occupation and investment, resist impact load and operate flexibly, and can arrange different states of aeration, anoxia and anaerobic in time to achieve the purpose of phosphorus and nitrogen removal.

3.3 Ordinary aeration method

Its variant process, ordinary aeration method, appeared the earliest, and the actual treatment effect is good, which can treat a large amount of sewage. For Jc-r plant, sludge digesters can be built centrally, and the generated biogas can be used as energy. The disadvantage of traditional general aeration method is that it can only be used as a conventional secondary treatment, and it has no function of nitrogen and phosphorus removal. In recent years, in engineering practice, nitrogen removal can be realized by reducing the volume load of ordinary aeration tanks; Phosphorus can be removed by setting an anaerobic zone in front of an ordinary aeration tank or by chemical methods. Commonly used aeration method to remove BOD, there are many forms in the pool type, such as oxidation ditch, which is called the variant process of commonly used aeration method in engineering, and can also be collectively called commonly used aeration method.

3.4 oxidation ditch method

It was developed in the early 1950s. Because of its simple structure and easy management, it was quickly popularized and innovated. At present, various forms of oxidation ditch have been developed in its application. The representative ones are: ① Pa type, referred to as single ditch type, and rotating brush aeration is used for surface aeration. The water depth is generally 2.5-3.5m② ② Auschwitz formula, referred to as concentric circle type, is mostly composed of elliptical third ring road, and the third ring road adopts different ones. Do, such as outer ring 0, middle ring 1 and inner ring 2, is beneficial to nitrogen and phosphorus removal. Rotating disc aeration is adopted, and the water depth is generally 4.0-4.5m(3), which is referred to as circulating baffled flow for short. Inverted umbrella impeller is adopted for aeration, and the water depth is generally about 3.0 m, but sludge is easy to precipitate. (4) Three-ditch oxidation ditch (T-shaped oxidation ditch) is composed of three tanks, with an aeration tank in the middle and two tanks on the left and right serving as sedimentation tanks. Aeration tank. It is characterized by rotating brush aeration, shallow water, large floor area, no anaerobic tank and no phosphorus removal function port J.

3.5 Membrane separation technology

Using membrane separation instead of precipitation to separate mud from water can bring the following changes to the activated sludge process: ① There is no longer the problem of sludge bulking. When adjusting the activated sludge system, it is not necessary to consider the settling performance of sludge, which greatly simplifies the process control; (2) The sludge concentration in the aeration tank will be greatly increased, and the MLSS can reach more than 20 g/L, so that the system can run under the conditions of over-sludge age and ultra-low load, which fully meets the needs of removing various pollutants; (3) Under the same treatment requirements, the volume of the aeration tank can be greatly reduced, saving the occupied area of the treatment plant; ④ The increase of sludge concentration requires a higher aeration rate, so pure oxygen aeration will be widely used in membrane separation.

3.6 Process optimization

Comparison of traditional activated sludge process with oxidation ditch and SBR process. ① Conventional activated sludge process is suitable for large sewage treatment plants with medium load. ② Oxidation ditch method and SBR method have low capital cost and high operation cost. If the treatment scale is 654.38+100000 t/d, the total treatment cost of oxidation ditch and SBR is almost the same as that of conventional activated sludge process (treatment cost = operating cost+depreciation+fixed assets investment loan interest). The smaller the scale, the lower the total treatment cost of oxidation ditch and SBR. Therefore, for small and medium-sized sewage treatment plants, oxidation ditch and SBR are economically beneficial. (3) oxidation ditch and SBR process generally do not have primary sedimentation tank and sludge digestion tank, and the treatment unit is reduced by more than 50% compared with the conventional activated sludge process, which simplifies the operation and management; The equipment has a high degree of localization and low price.

Four. Suggestions and countermeasures

(1) Omission of some processes and equipment. For small and medium-sized sewage plants, the sludge reflux equipment and sludge digestion process should be omitted when selecting the process, and the sludge should be directly dehydrated after treatment and concentration, and then sent to the garbage plant or used as agricultural fertilizer. Chlorine disinfection process can be omitted, but this process is set for accidental discharge. If an accident occurs, temporary measures can be taken to save investment.

(2) The determination of wastewater inflow indexes BOD5 and CODcr should not be too large, otherwise the wastewater retention time will be too long and the investment will increase.

Establish a sewage charge system and appropriately raise the price of tap water to compensate the operating cost of sewage treatment plants. The effluent index of sewage treatment plant should meet the relevant local standards so as to be reused as industrial cooling water, urban clean water and farmland irrigation water.

③ Raise construction funds through multiple channels, including bank loans, national debt and public burden, and establish a stable source of debt repayment funds. It can be invested by the government first, and then operated by the sewage treatment company as an enterprise. With the support of laws and policies, it can operate with high efficiency and low cost and collect sewage charges.

(4) Build-operate-hand-over with bot mode. Through the concession agreement, the government will award the project to the project company established for the concession project within a certain period of time, and the project company will be responsible for the financing, construction, operation and maintenance of the project; After the franchise expires, the company will return the project to the government department free of charge.

⑤ Strengthen exchanges and cooperation, introduce foreign advanced technology, and establish and develop sewage treatment technology and environmental protection equipment suitable for the national conditions. Sewage treatment is a municipal infrastructure, although the environmental and social benefits are remarkable, it is difficult to have considerable economic benefits at present. Therefore, it needs financial and policy support from the government.

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