Small and medium-sized city sewage treatment plants should pay attention to the design problems

In order to protect the water environment, the state has listed urban water supply and drainage as an industry supported by the capital construction sector, and proposed that the overall sewage treatment rate will reach 25% in 2000 and the sewage treatment rate will reach 40% in 2010. Urban sewage treatment plants should be built in river basins, “three major lakes” and key coastal cities and cities with coastal waters and non-agricultural population of more than 500,000. According to relevant information, in 2005 China's urban sewage discharge amounted to 420 × 108 m3, the state's investment in urban drainage facilities during the "10th Five-Year Plan" period will reach 120 billion yuan, while sewage treatment projects in small and medium-sized cities will occupy a relatively high proportion. In order to achieve good benefits for limited investment, combined with the status quo and construction characteristics of small and medium-sized city drainage projects, some views on the design of sewage treatment plants in small and medium-sized cities.

1. Drainage pipe network 1.1 Drainage system The drainage system is the primary problem faced by the design of sewage treatment plants. It not only involves engineering investment, environmental protection, and the difficulty of project implementation, but also directly affects the process selection of sewage treatment plants. In the long-term development process of China's small and medium-sized cities, due to the constraints of investment factors and the impact of development models, the existing built-up areas are mostly rainwater and sewage combined systems. The combined flow area accounts for at least 80% of the built-up area, and the built-up area before the 1980s is densely built, and various underground pipelines are crowded. It is necessary to transform into a diversion system. It is necessary to add a sewage pipe network system, which is very difficult. . In combination with the construction of domestic urban sewage treatment plants in recent years, I believe that a mixed drainage system should be adopted, that is, the urban new area and the built-up area with conditional transformation adopt the diversion system. Most of the existing built-up areas adopt the interception confluence system, and it is not appropriate to blindly pursue the diversion. system.
1.2 Overflow problem of confluent sewage The traditional concept of drainage believes that the confluence pipe is drained by heavy rain, and when the diameter-to-pollution ratio reaches 5-7 times, it will not cause harm to the water body. This is not the case. The interception combined pipe network system should consider the impact of confluent sewage overflow on the water environment during heavy rain. For the design of the confluence pipe network system, the following issues should be noted:
1.2.1 Interception rate According to the environmental capacity of the receiving water body and the composition of the drainage system, a reasonable interception multiple is determined to reduce the overflow frequency and overflow flow of the combined sewage. Generally, the intercepting main pipe takes n0=1-2, and the branch pipe system is appropriately higher than the intercepting multiple of the main pipe, taking n0=1-3.
1.2.2 Rainwater storage and storage combined with urban water body water environment planning and park green space construction, using the existing pit and depression, construct a regulating pool near the intercepting main pipe to store and regulate the overflow sewage, and then send the overflow sewage to the storm after the storm The sewage treatment plant is processed. The engineering measures can be gradually implemented in conjunction with the overall urban development plan.
1.2.3 Environmental Assessment When conducting preliminary research work such as environmental impact assessment of the project, the amount of pollutants overflowing during the rainy season is included, and the tail water discharge standard of the sewage treatment plant is revised.
1.2.4 Connection of the diversion pipe network to the drainage system For the pipe network system of the mixed drainage system, the diversion system shall be connected downstream of the final overflow well of the confluence system, and it is strictly forbidden to confluent and then overflow.

2. Design scale of sewage treatment plant 2.1 Determination of design scale In actual projects, the phenomenon that the actual water quality of the incoming plant deviates from the design scale often occurs. In some engineering projects, the amount of sewage in the preliminary design stage is estimated from 0.5×104 to 0.8×104 m3/d per square kilometer, or the water consumption of the design period is multiplied by a reduction factor of 0.8, regardless of the perfection of the existing pipe network. Estimated, that is, the amount of water designed as a sewage treatment plant. For the design of influent water quality, it is simply analogous to data from other municipal wastewater treatment plants. The weak work in the early stage and the lack of basic research are the main reasons for the actual water quality of the plant entering the factory deviating from the design scale.
The design scale of the sewage treatment plant includes the design water quantity and the designed influent water quality concentration. It is the basic data for the process selection and design of the sewage treatment plant. Especially when the sewage treatment plant has the requirements for phosphorus and nitrogen removal, in addition to determining the concentration of conventional pollutants, Water quality characteristics such as nutrient concentration and alkalinity should be determined and should be paid enough attention by designers.
2.1.1 Designing the amount of water The sewage volume of the sewage treatment plant shall be based on the annual water supply and water saving statistics of the city, based on the current annual water consumption, and the annual growth rate method to predict the water demand for the design period within the sewage treatment plant's water collection range. The actual amount of water in the urban sewage outlet is measured, and the scale of the water is determined by the measured sewage reduction factor. For the intercepting combined pipe network system, the water network design water volume should also be calculated in combination with the pipe network design as the process selection and calculation basis for each structure.
2.1.2 Designing the influent water quality of the influent water quality sewage treatment plant, selecting several representative sewage outlets in the urban area, periodically measuring the water quality and quantity, and using the weighted average to determine the current water quality concentration, based on this, combined Other monitoring data and consider a certain amount of space to determine the design influent water quality. Due to the differences in industrial structure between different cities, it is forbidden to use simple analogies.
2.2 The recent construction scale The design of the sewage treatment plant should be carried out in the near-term and long-term scale to determine the project stage. The long-term scale is used as the basis for the site selection of sewage treatment plants. The site selection conditions should meet the needs of long-term treatment land to facilitate the expansion of the project. For small and medium-sized city sewage treatment plants, the recent construction scale should not be too large. The reasons are as follows:
1 The penetration rate of sewage pipe network in small and medium-sized cities is relatively low, and the transformation of sewage pipe network takes a long time, at least 3-5 years.
2 The lack of basic data in small and medium-sized cities, there are many uncertain factors, such as industrial restructuring, the periodic operation of some key polluting enterprises, etc., the water quality and quantity are difficult to accurately predict. If the construction scale is too large in the near future, the sewage treatment plant will not reach the design scale for a long time, resulting in a large amount of equipment idle and reduced investment efficiency.

3, treatment process 3.1 treatment process type and selection With the pressure of the environment and regulations, urban sewage treatment plants generally use secondary biological treatment process, in the biological method, there are two major categories of activated sludge method and biofilm method, active pollution Due to its high processing efficiency, the mud method is widely used in urban sewage treatment plants.
There are many types of activated sludge processes. There are three main types of activated sludge: 1 traditional activated sludge process and its improved A/0, A2/0, AB process, 2 oxidation ditch process, and 3SBR process.
The traditional activated sludge and its improved A/0, A2/0, and AB processes have many processing units and complicated operation management, especially the sludge anaerobic digestion process, which requires high management level. The anaerobic digestion of sludge can recover a part of energy. According to the practical experience of sewage treatment in China, the design scale of sewage treatment plant is above 20×104m3/d, which is economical.
The design scale of small and medium-sized city sewage treatment plants is generally below 10×104m3/d. Due to its relatively weak technical strength, the use of oxidation ditch and SBR process has obvious advantages. The advantages are as follows:
(1) Oxidation ditch method and SBR method have strong anti-impact load capacity, 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 method, usually without primary sedimentation tank and sludge digestion system, the process flow is simple, suitable for small and medium-sized cities with relatively low management level.
(3) The infrastructure cost of the oxidation ditch and SBR method is low.
3.2 Rainwater impact load and sludge loss Under the condition that the drainage pipe network is combined, the sewage flow into the sewage treatment plant is 2-4 times that of the sunny day. When the rainwater impact load occurs, a large amount of activated sludge from the aeration tank Transfer to the secondary settling tank and cause sludge loss. In the conventional treatment system, the overrunning pipe is usually set, overflowing in front of the biological aeration tank, or shunting part of the impact load to the secondary settling tank, and the sludge is allowed to settle in the aeration tank by stopping the aerator 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.
3.2.1 Improved Orabl Oxidation Ditch Process
The Orabl oxidation ditch is composed of three elliptical concentric ditches. The sewage enters the middle ditch and the inner ditch sequentially from the outer ditch. The concentration of organic matter and the dissolved oxygen concentration in each ditch are different. When the organic matter is removed, the dephosphorization and denitrification can be realized. purpose. The modified Orabl oxidation ditch is designed as a rainwater flow mode. When the peak flow of rain occurs, the influent can be switched to the intermediate channel, and the return sludge is continuously sent to the outer channel to be stored in the outer channel. The aeration can effectively prevent the loss of activated sludge and at the same time degrade the organic matter. When the rainwater impact load stops, the system switches to normal operation.
3.2.2 SBR process
SBR is a batch type active mud system. Its basic feature is to complete the biochemical reaction, solid-liquid separation, drainage and sludge discharge of sewage in a reaction tank. Continuous inlet and outlet water can be realized by a combination of double or multiple pools. SBR achieves different processing goals by controlling the aeration volume and dissolved oxygen in the reaction tank, and has great flexibility.
The SBR pool usually runs 4-6 hours per cycle. When there is a peak flow of rainwater, the SBR system automatically switches from the normal cycle to the rainwater operation mode, and adjusts its cycle time to adapt to the change of the incoming water volume. SBR systems are typically able to withstand 3-5 times the impact load of dry flow.

4. Wastewater reuse China is one of the 12 poor countries in the world. The per capita water resources are only 2400m3, especially the per capita water resources in the northern region is only 200-400m3. The shortage of water resources limits industrial and agricultural to some extent. In terms of production and urban development, many cities have to open up water sources dozens of kilometers away. The investment is above 1,000 yuan/m3, and the water production cost is as high as 1.0 yuan/m3. In contrast, the secondary treatment of tailwater in 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. Its investment is about 200. -300 yuan / m3, the cost of water production is about 0.30 yuan /. The engineering practice of wastewater reuse in Tianjin, Dalian, Taiyuan, Qingdao, Tai'an and other places in China fully proves the economical efficiency of urban sewage reuse.
The industrial water consumption in small and medium-sized cities accounts for about 50%-70% of the total water consumption, of which the water consumption such as cooling and washing is large but the water quality requirements are not high. In the design of sewage treatment plants in small and medium-sized cities, the possibility of reuse of sewage should be studied, the object of reuse and water quality requirements should be investigated, and the selection of sewage treatment process should be carried out in combination with the requirements of water quality for reuse. When the general layout of the plant is to be carried out, sewage should be considered. Processing land used.