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Several issues in the design of sewage treatment plants in small and medium-sized cities Author: Summary of Yin Wenxuan and Yin Weihong: In order to achieve good benefits from limited investment, combined with the current situation and construction characteristics of drainage projects in small and medium-sized cities, I would like to share some personal opinions on the design of sewage treatment plants in small and medium-sized cities. keywords: In order to protect the water environment, sewage treatment plants * * Urban water supply and drainage has been listed as a key supported industry in the field of infrastructure construction, and the overall goal of reaching 25% of my country's sewage treatment rate by 2000 and 40% by 2010 is required. Urban sewage treatment plants are required to be built in the "seven major river basins", the "three major lakes", key coastal cities and their offshore cities, and cities with a non-agricultural population of more than 500,000. According to relevant data, my country's urban sewage discharge reached 420×108m3 in 2005. * * During the "Tenth Five-Year Plan" period, investment in urban drainage facilities will reach 120 billion yuan, and sewage treatment projects in small and medium-sized cities will account for a high proportion. In order to achieve good benefits from limited investment, combined with the current situation and construction characteristics of drainage projects in small and medium-sized cities, I would like to share some personal opinions on the design of sewage treatment plants in small and medium-sized cities. 2 Drainage pipe network 2.1 Drainage system The drainage system is the primary issue faced in the design of sewage treatment plants. It not only involves project investment, environmental protection, and the ease of project implementation, but also directly affects the process selection of sewage treatment plants. In the long-term development process of my country's small and medium-sized cities, due to restrictions on investment factors and the influence of development models, most of the current built-up areas have a combined rainwater and sewage system. The combined area accounts for at least 80% of the built-up area. Moreover, the built-up area before the 1980s had dense buildings and crowded underground pipelines. To transform it into a diversion system, it is very difficult to add a sewage pipe network system. Based on the construction situation of domestic urban sewage treatment plants in recent years, I believe that it is appropriate to adopt a mixed drainage system, that is, new urban areas and built-up areas with conditions for reconstruction adopt a diversion system, and most existing built-up areas adopt an interception-type combined system. It is not advisable to blindly pursue a diversion system. 2.2 The overflow problem of combined sewage. Traditional drainage concepts believe that combined pipes and canals are diluted by heavy rains and overflow. When the runoff-to-sewage ratio reaches 5-7 times, it will not cause harm to the water body. This is not the case. The interception-type combined pipe network system should also consider the impact of combined sewage overflow on the water environment during heavy rains. For the design of the combined pipe network system, the following issues should be paid attention to: 2.2.1 The interception multiple should be determined based on the environmental capacity of the receiving water body and the composition of the drainage system to determine the reasonable interception multiple to reduce the overflow frequency and overflow volume of the combined sewage. Generally, n0=1-2 is used to intercept the main pipe. The interception multiple of the branch pipe system is appropriately higher than that of the main pipe, so n0=1-3. 2.2.2 Rainwater regulation and storage should be combined with urban water environment planning and park green space construction, using existing pits, ponds and depressions to build regulating pools near interception trunk pipes to store and regulate overflow sewage. After heavy rains, the overflow sewage will be sent to the sewage treatment plant for treatment. This engineering measure can be implemented gradually in conjunction with the city's overall development plan. 2.2.3 Environmental Assessment When conducting preliminary research work such as environmental impact assessment of the project, the amount of pollutants overflowing during the rainy season will be taken into account, and the tail water discharge standards of the sewage treatment plant will be revised. 2.2.4 Connection between the diversion pipe network and the drainage system. For the pipe network system of the mixed drainage system, the diversion sewage main pipe should be connected downstream of the final overflow well of the combined interception main pipe. It is strictly prohibited to merge first and then overflow. 3 Design scale of sewage treatment plant 3.1 Determination of design scale In actual projects, the actual water volume and quality of the water entering the plant often deviates from the design scale. For some engineering projects, in the preliminary design stage, the sewage volume is estimated at 0.5×104 to 0.8×104m3/d per square kilometer, or the water consumption in the design period is estimated by multiplying the water consumption by a reduction factor of 0.8, regardless of the perfection of the current pipe network, which is used as the design water volume of the sewage treatment plant. For the design inlet water quality, the data of other urban sewage treatment plants are simply compared. Weak preliminary work and lack of basic research are the main reasons why the actual water quantity and quality entering the plant deviate from the designed scale. The design scale of a sewage treatment plant includes the design water volume and the design inlet water quality concentration, which is the basic data for process selection and design of the sewage treatment plant. Especially when the sewage treatment plant has requirements for phosphorus and denitrification, in addition to determining the concentration of conventional pollutants, water quality characteristics such as nutrient concentration and alkalinity should also be determined, which should attract sufficient attention from designers. 3.1.1 Design water volume The water volume scale of the sewage treatment plant should be based on the city's historical water supply and water conservation statistics, based on the water consumption in the current year, and use the annual growth rate method to predict the water demand within the design years of the sewage treatment plant's water collection range. The water volume of the main sewage outlets in the urban area should be measured, and the water volume scale should be determined by the measured sewage reduction coefficient. For the interception-type combined pipe network system, the design water volume in rainy season should also be calculated in conjunction with the pipe network design, as a basis for process selection and calculation of various structures. 3.1.2 Design inlet water quality For the design inlet water quality of the sewage treatment plant, select several representative sewage outlets in the urban area, measure their water quality and quantity regularly, and use a weighted average to determine the current water quality concentration. Based on this, combined with other monitoring data and considering a certain margin, determine the design inlet water quality. Due to the differences in industrial structures in different cities, simple analogies should be avoided. 3.2 The design of the sewage treatment plant for recent construction scale should conduct research on the short-term and long-term scale to reasonably determine the project phases. The long-term scale is used as the basis for site selection of sewage treatment plants, and the site conditions should meet the needs of long-term treatment land to facilitate the expansion of the project. For sewage treatment plants in small and medium-sized cities, the scale of recent construction should not be too large. The reasons are as follows: ① The penetration rate of sewage pipe networks in small and medium-sized cities is low, and the transformation of sewage pipe networks takes a long time, at least 3-5 years. ② There is a lack of basic data in small and medium-sized cities and there are many uncertain factors, such as industrial structural adjustment and the periodic operation of some key polluting enterprises. It is difficult to accurately predict the water quality and quantity. If the recent construction scale is too large, the sewage treatment plant will not reach the design scale in the long term, resulting in a large number of idle equipment and reduced investment benefits. 4 Treatment process 4.1 Treatment process type and selection With the pressure of the environment and regulations, urban sewage treatment plants generally adopt secondary biological treatment processes. Among biological methods, there are two categories: activated sludge method and biofilm method. The activated sludge method is widely used in urban sewage treatment plants because of its high treatment efficiency. There are many types of activated sludge process, and the following three types are widely used:: ①Traditional activated sludge process and its improved A/0, A2/0, AB process, ② oxidation ditch process, ③ SBR process. Traditional activated sludge and its improved A/0, A2/0, and AB processes have many treatment units and complex operation and management. Especially the sludge anaerobic digestion process requires higher management levels. Anaerobic digestion of sludge can recover part of the energy. According to the practical experience of my country's sewage treatment, the design scale of the sewage treatment plant must reach more than 20×104m3/d to be economical. The design scale of sewage treatment plants in small and medium-sized cities is generally below 10×104m3/d. Due to their relatively weak technical strength, the use of oxidation ditch and SBR processes has obvious advantages. Its advantages are as follows: (1) The oxidation ditch method and the SBR method have strong impact load resistance and can adapt to the characteristics of large changes in water quality and quantity in small and medium-sized cities. (2) Oxidation ditch and SBR methods usually do not have primary sedimentation tanks and sludge digestion systems. The process flow is simple and suitable for small and medium-sized cities with relatively low management levels. (3) The infrastructure cost of oxidation ditch and SBR method is low. 4.2 Rainwater impact load and sludge loss When the drainage pipe network is a combined flow system, the sewage flow entering the sewage treatment plant on rainy days is 2-4 times that on sunny days. When rainwater impact load occurs, a large amount of activated sludge is transferred from the aeration tank to the secondary sedimentation tank, causing sludge loss. In conventional treatment systems, an override pipe is usually set up to overflow in front of the biological aeration tank, or to divert part of the impact load to the secondary sedimentation tank. The aerator is stopped to allow the sludge to settle in the aeration tank to prevent sludge loss. However, none of the above methods can effectively degrade organic matter. The improved Orbal oxidation ditch process and SBR process can solve the above problems. 4.2.1 Improved Orabl oxidation ditch process Orabl oxidation ditch consists of three oval concentric ditches. The sewage flows from the outer ditch into the middle ditch and the inner ditch in sequence. The concentration of organic matter and dissolved oxygen in each ditch are different. While removing organic matter, the purpose of phosphorus and denitrification can be achieved. The improved Orabl oxidation ditch is designed to operate in a rainwater diversion mode. When the peak flow of rainwater occurs, the incoming water can be switched to the middle channel, while the return sludge is still continuously sent to the outer channel so that it can be stored and aerated in the outer channel. This can effectively prevent the loss of activated sludge and at the same time degrade organic matter. When the rainwater impact load stops, the system switches to normal operation. 4.2.2 SBR process SBR is an intermittent activated sludge system. Its basic feature is to complete the biochemical reaction of sewage, solid-liquid separation, drainage, and sludge discharge in a reaction tank. Continuous water inflow and outflow can be achieved through combined operation of double pools or multiple pools. SBR achieves different treatment goals by controlling the aeration volume and dissolved oxygen of the reaction tank, and has great flexibility. The SBR pool usually runs for 4-6 hours per cycle. When the peak flow of rainwater occurs, the SBR system automatically switches from the normal cycle to the rainwater operation mode and adjusts its cycle period to adapt to changes in incoming water volume. SBR systems are usually able to withstand impact loads of 3-5 times dry flow. 5. Sewage reuse. my country is one of the 12 countries with poor water resources in the world. * * First, the per capita water resources are only 2,400m3, especially in the northern region, where the per capita water resources are only 200-400m3. The shortage of water resources has restricted industrial and agricultural production and urban development to a certain extent. Many cities have to go dozens of kilometers away to develop water sources. The investment is more than 1,000 yuan/m3, and the water production cost is as high as more than 1.0 yuan/m3. In contrast, the tail water from the secondary treatment of urban sewage treatment plants is a stable water resource. After treatment, it can be used as industrial cooling and washing water, municipal miscellaneous water, and landscape water for urban rivers and lakes. The investment is about 200-300 yuan/m3, and the water production cost is about 0.30 yuan/m3. The engineering practice of sewage reuse in Tianjin, Dalian, Taiyuan, Qingdao, Tai'an and other places in my country has fully proved the economics of urban sewage reuse. Industrial water consumption in small and medium-sized cities accounts for about 50% to more than 70% of the total water consumption. Among them, cooling, washing, etc. use a large amount of water but the water quality requirements are not high. In the design of sewage treatment plants in small and medium-sized cities, the possibility of sewage reuse should be studied, the reuse objects and water quality requirements should be investigated, and the sewage treatment process should be selected based on the water quality requirements of reused water. When making the overall layout of the plant area, the treatment land for sewage reuse should be considered. References 1 Research on the selection of interception multiples for Wang Wenyuan's sewage interception project, Water Supply and Drainage, 1997, 10 2 Research on the engineering application performance of Yan Xiuqin and Aubert's oxidation ditch, China Water Supply and Drainage, 1999, 7 3 Zhou Hail's optimized technology for medium and small urban sewage treatment plants, China Water Supply and Drainage, 2000, 10