1 preface
The development of industry has promoted the further improvement of people's living standards, but it has also caused serious environmental pollution, among which water pollution is one of the main factors of environmental pollution, and all countries in the world attach great importance to it, so controlling water pollution has become one of the important topics of environmental protection. In order to better protect the water environment, China has given great support to urban water supply and drainage as a basic construction project, and set the sewage treatment target. From the treatment rate of 25% in 2000 to 40% in 20 10, China's urban sewage treatment rate has achieved initial results in water pollution control. At the same time, all small and medium-sized cities are required to build sewage treatment plants. During the Tenth Five-Year Plan period, the investment in urban sewage treatment in China reached 654.38+0.2 billion yuan, of which small and medium-sized cities accounted for a large proportion. In order to obtain better investment benefit, according to the present situation of sewage discharge in small and medium-sized cities and the characteristics of urban construction, some suggestions on how to reform the design process of sewage treatment are put forward to promote the sustainable development of small and medium-sized cities in China.
2 Analysis of the problems that should be paid attention to in the design of sewage treatment process in small and medium-sized cities
In the process of sewage treatment design in small and medium-sized cities, many factors should be fully considered, such as the number of treatment plants, the designed water quantity and quality, and clear treatment standards.
2. 1
According to the scale of the city, several sewage treatment plants should be arranged. Usually, large-scale centralized sewage treatment plants will be built in large, medium and small cities, which will help reduce the construction and operation costs.
(1) site selection and planning; According to the urban development plan, sewage treatment plants are usually located in the lower reaches of urban rivers, and the location is far from the urban areas, which leads to the long collection pipe network, difficult supporting and unreasonable utilization of treatment systems; (2) Development trend; With the development of the city, the requirements for the environment are gradually increasing, and villas and some ordinary communities will build sewage treatment systems, thus promoting the development of sewage treatment plants to miniaturization and decentralization; (3) Overall planning; Due to the pollution of water resources, the utilization rate of reclaimed water is getting higher and higher, and the centralized water pollution treatment and reclaimed water system has a large investment and is quite difficult to operate. Therefore, in the process of sewage treatment, the construction of sewage treatment plants will be effectively combined with centralization and decentralization, sewage treatment and reuse, and the layout of sewage treatment plants should be fully considered, including the existing pipeline system, reclaimed water reuse system, urban development and planning, and the layout of sewage treatment plants should be arranged reasonably according to the needs of major cities.
2.2 Determine water quantity and water quality parameters and treatment standards.
In the process of sewage treatment design, it is necessary to analyze and demonstrate the quality and quantity of sewage, which is directly related to the investment, construction and operation costs of sewage treatment plants.
(1) water quantity design; The design sewage quantity should be determined according to urban population, urban economic development and level, urban drainage system, industrial wastewater discharge, sewage pipeline system construction degree, sewage treatment planning period and other factors. After comprehensive analysis of these factors, the scale of sewage treatment plant can be determined.
(2) Water quality design; The quality of sewage is related to the nature of the city, the level of economic development, the nature of industrial water use, other pollution sources and drainage systems. Therefore, in the process of sewage treatment design, the influence of sewage concentration change on actual operation should be fully considered, and the reasonable sewage treatment stages should be carried out by referring to the cycle of similar urban sewage treatment changes. In order to prevent unpredictable factors, the construction of sewage treatment plants should also be carried out in stages, leaving some room and fully considering the rationality of investment.
(3) sewage treatment standards; China has clear regulations on sewage treatment. The effluent quality of sewage treatment plant should conform to the water environment, upstream and downstream water use and self-purification capacity, so that the discharged water can meet the relevant national standards. When determining the effluent standard of sewage plant, we should make a concrete analysis according to the actual situation of the city and give full consideration to its construction, environment, economy and development. Generally speaking, for economically developed cities, the speed of urban construction is fast, and the requirements for environmental protection and water bodies are high. Taking the water body standard as the basis of effluent standard after sewage treatment, the effluent standard is formulated.
3 Small and medium-sized city sewage treatment plant design improvement measures
From the above analysis, it can be seen that the sewage volume and treatment standards in small and medium-sized cities are well determined, but there are great differences in practice. The reason is that we only rely on theoretical knowledge and learn from the experience of the same city in the design process, but fail to combine it with practice, and lack understanding of the relationship between urban construction planning and management, and the future development. Building a systematic, scientific and reasonable sewage treatment plant should not only follow the urban development planning and water supply.
3. 1 Investigation before design
Before the construction of sewage treatment plants in small and medium-sized cities, it is necessary to fully investigate and study the cities and collect useful information, especially the contradiction between the future development of cities and urban planning. Only by fully understanding the distribution of supporting drainage systems and pipe networks in the construction of sewage treatment plants, and understanding the urban functions and the pollution status of discharged water bodies, can we ensure that the planning and design of sewage treatment plants are reasonable and applicable and do not deviate from the future development direction of the city.
3.2 The construction and design of sewage treatment plants should be carried out at different levels, leaving room for overall planning. The construction and design of sewage treatment plants in small and medium-sized cities should be scientific, reasonable and developmental. With the acceleration of the process of urban-rural integration, the development and construction of cities in China have advanced by leaps and bounds, but in this process, urban planning and environmental protection can't keep up with the needs of urban development. With the continuous improvement of economic level, people have higher and higher requirements for quality of life, pay more attention to environmental protection and put forward higher requirements for water pollution control. Therefore, in the process of construction and design, sewage treatment plants should be carried out by stages, and the treatment scale and standards should be expanded according to different stages of urban development. In the process of design, we should leave enough room for future expansion. At the same time, the phased construction should not be too simplistic, and we should consider both the construction stage and the treatment stage. Combined with the actual situation of urban development, the water discharge and water quality in different periods are comprehensively analyzed, and the problems existing in the early stage are improved in the process of phased construction, so that the sewage treatment system is more reasonable.
3.3 The layout design and scheme of the sewage treatment plant should be flexible and designed according to the requirements of sewage discharge and treatment. First of all, it can be used to increase overflow pipes, that is, sewage can surpass some treatment structures under certain conditions. That is to say, when the inflow of sewage is small and the pollution degree is low, the sewage can directly flow into the aeration tank, pass through the primary sedimentation tank, or be directly discharged after passing through the primary sedimentation tank, which can not only meet the discharge standard, but also effectively reduce the operating cost. Secondly, the aeration tank is designed flexibly. In the design process of aeration tank, various modes such as push flow, phosphorus removal, multi-point water inflow, nitrogen removal and complete mixing can be adopted to enhance the flexibility and treatment capacity of the treatment system.
3.4 Design of grit chamber and sedimentation tank.
(1) grit chamber; The main function of grit chamber is to separate larger inorganic particles from wastewater. Generally, grit chamber is designed in front of sedimentation tank and pumping station, which can effectively protect pipelines and components, reduce the load of sedimentation tank, and separate organic particles from inorganic particles for separate treatment. The design of grit chamber can be classified into horizontal flow and vertical flow according to the direction of water flow. Among them, the horizontal flow separation effect is better, and this method is usually adopted. But in 2 1 century, grit chamber was gradually replaced by aerated grit chamber. The design of aerated grit chamber is mainly to introduce aeration at one side, so that the water in sewage tank flows forward in a rotating way, and then a transverse constant-speed circulation is generated. Its advantage is that the cyclone speed of sewage can be controlled by controlling the aeration rate, and the sand removal effect is more stable. It is not affected by the change of flow rate, and can also play the role of pre-aeration of sewage. This aeration makes the organic particles in suspension, and the attached organic matter can be removed through the friction between sand particles, which is convenient for extracting pure sand particles, and at the same time, it can change the water quality of wastewater and make the subsequent treatment easier.
(2) sedimentation tank; The main function of sedimentation tank is to remove solid suspended particles from sewage according to the principle of gravity sedimentation. Sedimentation tanks can be divided into three types: horizontal sedimentation tanks, vertical sedimentation tanks and amplitude sedimentation tanks. According to the sequence of treatment, it can be divided into primary sedimentation tank and secondary sedimentation tank. The primary sedimentation tank mainly removes BOD particles in sewage and SS in water, which can effectively reduce the load of biological treatment unit. The secondary sedimentation tank is mainly used to separate water and sludge, so as to ensure that the treated water can meet the discharge standard, and at the same time, it can concentrate sewage to ensure that the aeration tank has additional sludge and other functions.
4 conclusion
In short, the design of sewage treatment plants in small and medium-sized cities should combine theory with practice, and ensure the rationality, scientificity and predictability of sewage treatment plant construction according to the economic conditions and construction conditions of cities, and at the same time ensure the practicability and operability of the project. The sewage treatment process should also be constantly innovated to realize the synchronous development of cities and environmental protection.
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