By analyzing the operation of sewage treatment plants in Shanghai Bailonggang, Guangzhou Xinhua, Baoji High-tech Zone, Tongliao City, Taihu Lake area and Three Gorges reservoir area, it is found that there are some problems in their operation, such as low operating load rate, large fluctuation of inlet water quality and quantity, and difficulty in reaching the standard of outlet water quality. Through in-depth analysis of the reasons, the targeted solutions are put forward scientifically in order to provide reference for the stable operation of urban sewage treatment plants in China and contribute to the comprehensive management of water environment.
Urban sewage treatment plants play an important role in ensuring the safety of water environment, and the construction of sewage treatment plants is an important means to solve urban water pollution.
In the "Thirteenth Five-Year Plan" national urban sewage treatment and recycling facilities construction plan, it is proposed that by the end of 2020, urban sewage treatment facilities will be fully covered, and the urban sewage treatment rate will reach 95%, and the county level will not be less than 85%. During the "Ninth Five-Year Plan" period, China's key river basin water pollution prevention and control planning began to be implemented, and the construction and operation of urban sewage treatment facilities began to become the main way for all localities to implement the responsibility goal of reducing water pollutants.
With the strong support of the central financial funds and relevant policies, after the development of the "Eleventh Five-Year Plan" and "Twelfth Five-Year Plan", the construction of sewage treatment plants in China has made great progress, the number and scale of urban sewage treatment plants have increased rapidly, and the urban sewage treatment capacity has been continuously improved.
Statistics show that by the end of 20 16, the urban sewage treatment rate reached 93.44%, of which the centralized sewage treatment rate was 89.8%. By the end of 20 10, there were 2496 urban sewage treatment plants in China, an increase of 140% over 2006. By the end of 20 16, there were 3,552 urban sewage treatment plants, an increase of 29% over 20 10.
However, the contradiction between the continuous improvement of sewage treatment rate and capacity and water environmental pollution is still outstanding. According to "20 16 Bulletin on Environmental Status of China" published by the Ministry of Environmental Protection, 32% of the surface water 1940 monitoring sections in China are still Grade IV or below. By the end of 20 17, there were 2 100 urban black and odorous water bodies recognized by the Ministry of Housing and Urban-Rural Development and the Ministry of Environmental Protection.
At the same time, the discharge standards of sewage treatment plants have been continuously improved. The Action Plan for Water Pollution Prevention issued on 20 15 clearly states that the existing urban sewage treatment facilities should be reformed according to local conditions, so as to meet the corresponding discharge standards or reuse requirements by the end of 2020; Urban sewage treatment facilities in sensitive areas (key lakes, key reservoirs, and coastal water catchment areas) should fully meet the first-class A discharge standard before the end of 20 17. In cities where the water quality in the built-up area cannot meet the Class IV standard of surface water, the newly-built urban sewage treatment facilities shall implement the Class A discharge standard; By 2030, we will strive to improve the overall quality of the water environment in the country and restore the functions of the water ecosystem initially.
There are many outstanding problems in urban sewage in China, such as great changes in water quality and quantity, low ratio of carbon to nitrogen, high content of inorganic suspended solids, low water temperature in winter, and the influence of industrial toxic and harmful pollutants, which obviously affect the normal operation of sewage treatment facilities and make it difficult for the effluent to reach the standard stably. Even under the discharge standard, the water quality meeting Class A/B standard is still close to Class V water (Table 1), which is an important pollution source of water environment.
Table 1 Comparison of Main Pollutant Indexes in Discharge Standard of Urban Sewage Treatment Plants in China: mg/L
Some suburbs have also brought serious harm to the water supply environment because residents litter and discharge domestic sewage. Therefore, the author of this paper deeply analyzes the problems existing in the representative sewage treatment plants in the north and south of China and their causes, and puts forward some countermeasures in order to provide reference for the stable operation of urban sewage treatment plants in China and make contributions to the comprehensive management of water environment.
1 Problems and Causes Analysis
The operating load rate of 1. 1 is generally low and partially overloaded.
According to the Technical Specification for Operation, Maintenance and Safety of Urban Sewage Treatment Plants issued by the Ministry of Housing and Urban-Rural Development in 2065438+02 (CJJ 60-2065438+065438+0), the annual treatment water volume of urban sewage treatment plants should reach more than 95% of the planned indicators. The operating load rate of sewage treatment plants in most areas of China is low, which makes it difficult to meet the requirements of the Ministry of Housing and Urban-Rural Development.
The average monthly load of sewage treatment plants in Liaoning Province is above 80%, accounting for only 32% of all sewage treatment plants. At present, the sewage treatment plant in Tongliao Economic and Technological Development Zone has not reached the design value, and nearly half of the treatment facilities are idle. In 20 10, the load rate of urban sewage treatment plants in Guangxi reached over 60%, accounting for 65% of the total. The average operating load of the three Gorges reservoir area 176 sewage treatment plants in 20 14 years is only 56.5%.
The average operating load rate of sewage treatment plants built in China is only 65% ~ 70%, which is far lower than the average operating load rate of sewage treatment plants in Germany in 2008, which is 95%. However, in some cities, due to the rapid economic development and population growth, sewage treatment plants have been overloaded and the treatment pressure is high.
The main reasons for the low load rate of sewage treatment facilities are that the plant network construction is not matched, and the coverage rate and collection rate of sewage pipe network are low. The sewage treatment plant can only play a role in pollution control if it is used in conjunction with the sewage pipe network.
Because the construction of sewage plant is relatively simple, centralized and short, and the construction of pipe network is relatively complex, involving a wide range and taking a long time, urban managers in China generally attach importance to the plant, neglect the pipe network and neglect management.
The data show that by 20 16, there were 3,552 urban sewage treatment plants in China, an increase of 29% compared with 20 10, and the length of drainage pipes only increased by 17%. The construction of supporting pipe network and sewage treatment plant is not synchronized, which leads to the embarrassing situation that some sewage treatment plants face no sewage treatment after completion.
In some cities, only the main sewage pipes were built in advance, but the construction of branch pipe networks was slow due to insufficient funds. There are many problems in the new pipe network in some cities, such as the design elevation is inconsistent with the existing main pipe, and the existing main pipe is blocked by water.
As a result, the built pipe network is not "woven into blocks" and the sewage collection rate is low. Another reason is that the design scale of the sewage plant does not match the actual situation. Because some cities do not pay enough attention to the preliminary work of sewage treatment plant construction, lack of detailed planning and full demonstration of sewage source and collection, the construction of supporting facilities such as pipe network and pumping station is relatively backward, and the design scale is often based on theoretical design and calculation. The actual per capita water consumption and pollutant discharge in economically backward areas are relatively low, which leads to a large design scale and insufficient actual sewage discharge.
In some fast-developing cities, with the rapid development of economy and the continuous improvement of residents' living standards, the amount of sewage is increasing. The design scale of sewage plant lags behind the growth rate of population economy, and the treatment capacity of sewage plant is insufficient, resulting in overload operation.
1.2 The inflow water quality and quantity fluctuate greatly, which is inconsistent with the design value.
The quality and quantity of raw water in sewage plant are important factors affecting the operation stability of sewage treatment process. The water quality and quantity of urban sewage plants in China fluctuate greatly, and the fluctuation range of water load of some sewage plants can reach -47% ~ 4%.
The daily average concentration of BOD5 in Bailonggang Sewage Treatment Plant in Shanghai fluctuated within the range of 14 ~ 382 mg/L, and the CODCr fluctuated within the range of 96 ~ 824 mg/L. The influent of sewage treatment plant in the confluence area of Kunming was affected by the rainy season, and the suspended solids fluctuated greatly. In addition to the fluctuation of water quality, the concentration of organic matter in some sewage plants is different from the design value, which seriously affects the sewage treatment effect.
Except NH3-N and TN, the actual influent quality of Baoji High-tech Zone Sewage Treatment Plant is higher than the design value. The influent TP of Baoji Shilipu Sewage Treatment Plant is higher than the design value, and other indicators are lower than the design value.
The main reasons are that the rainwater and sewage diversion of drainage pipe network is not complete, the pipe network leaks and the sewage interception along the river affects the sewage treatment system. The drainage system of old cities in our country is generally the combined system of rain and sewage, and later some cities changed to the combined system of interception.
Under the combined drainage system, the influent quality of sewage treatment plant is affected by many factors. In rainy season, the drainage pipe network collects domestic sewage and a lot of rainwater at the same time, which causes the fluctuation of water quantity in sewage plant.
Among them, the concentration of pollutants in the initial rainwater is high, and the pollution is serious. Some pollutants are higher than the concentration of sewage in the dry season, causing water quality fluctuations. In China, due to the untimely maintenance of the pipe network, the leakage of the old pipe network is serious, and the mixing of groundwater, river water and rainwater directly affects the quantity and quality of water entering the sewage treatment plant.
In some parts of the south, due to the imperfect construction of the front-end pipe network, the water quantity of sewage plants is small in dry season, so it is necessary to extract river water; However, in the rainy season, the water volume of the rainwater and sewage confluence pipe network far exceeds the treatment scale of the sewage plant, which leads to great fluctuation of water quality in dry season and rainy season.
The influence of sewage interception system along the river on sewage treatment system is another reason for the fluctuation of water quality and quantity. As a transitional product of combined system reform, sewage interception system along the river is more common in some southern cities.
The system can greatly improve the long-term black and smelly situation of rivers, but there are also some problems. The combined water collected by the rainy season system contains a lot of rainwater, which leads to the sewage treatment capacity of sewage treatment plants increasing year by year in drought and rainy season, and the load of sewage treatment plants is generally large in rainy season.
However, most of the sewage interception box culvert systems are not equipped with corresponding terminal treatment facilities. In the early days, light rain brought a lot of pollutants directly into the sewage plant, which caused water quality fluctuations and the treatment effect was difficult to guarantee.
In addition, China is in the stage of rapid economic development, and the regional economic differences are obvious. Cities in relatively developed and densely populated areas continue to expand, but the actual expansion speed is often inconsistent with the plan, which leads to the inconsistency between sewage growth and the design scale of sewage treatment plants.
When the sewage volume exceeds the design scale, the sewage treatment plant is in a state of "not enough to eat", and when the design scale exceeds the actual treatment demand, it causes the phenomenon of "big horse-drawn cart".
The sewage treatment facilities in northwest China are in a state of "not enough to eat" due to insufficient service and poor pipe network, which have affected the water quality and quantity of sewage treatment plants.
1.3 The effluent quality is difficult to reach the standard stably, and NH3-N and TN exceed the standard.
Most of the existing sewage treatment plants in China implement the first-class standard of Pollutant Discharge Standard for Urban Sewage Treatment Plants (GB 189 18-2002), of which 29.3% implement the first-class A standard and nearly 60% implement the first-class B standard. By the end of 20 16, only 30% of the tail water of sewage treatment plants in China has reached Grade A standard, and as high as 70% of the tail water of sewage treatment plants has reached or fallen below Grade A standard.
Ammonia nitrogen and total nitrogen are the main pollutants exceeding the standard in most sewage treatment plants. Shanghai Bailonggang Sewage Treatment Plant adopts A2/O process, and the qualified rate of NH3-N Grade I B is only 46%, while that of TN Grade I B is 68%.
The compliance rate of Grade B in the Three Gorges Reservoir Area 176 Sewage Treatment Plant is 60.7%, that of Grade A in Tongliao Sewage Treatment Plant is less than 50%, and that of NH3-N and TN in Baoji Shilipu Sewage Treatment Plant is 42.4% and 42.5% respectively.
The effluent TN and NH3-N of Guangzhou Xinhua Sewage Treatment Plant occasionally exceeded the standard in June 5438+10-March, and could not reach the first-class A standard stably. The effluent quality of sewage treatment plant is not up to standard, and its efficiency cannot be fully exerted, which not only reduces the investment benefit of sewage treatment plant, but also brings difficulties to the operation and management of sewage treatment plant, which should be paid full attention by operation managers.
Process is the key factor to ensure the treatment effect of sewage treatment plant. The main technologies adopted by urban sewage plants in China are ordinary activated sludge process, oxidation ditch and its improvement process, A2/O and its improvement process, SBR and its improvement process, A/O and its improvement process and biological aerated filter (BAF) process. These six processes cover more than 90% of the main process types of urban sewage treatment plants in China.
The above process has the function of nitrogen removal, but in actual operation, it is difficult to discharge the effluent up to the standard due to the fluctuation of influent quality and quantity or the inconsistency with the design value, the overload operation of biological treatment facilities, the shortage of carbon source and the shortage of carbon-nitrogen ratio.
When the influent concentration of BOD5, TN and TP in sewage treatment plant is lower than the designed influent concentration, it will seriously affect the sewage treatment effect in many ways. On the one hand, the low concentration of BOD5 in sewage leads to insufficient organic matter needed by microorganisms in the biological treatment unit, which affects the nitrogen removal effect.
On the other hand, when the concentration of influent pollutants is low, the aeration amount in the biological reaction tank is higher than the microbial demand. If the aeration amount of the aeration tank cannot be adjusted in time, it will easily lead to excessive aeration, resulting in poor separation effect of activated sludge precipitation.
In addition, the lack of heat preservation measures in winter in southern China leads to low inlet temperature, which is not conducive to the growth of nitrifying and denitrifying bacteria, and the concentration of NH3-N and TN in outlet water cannot be guaranteed. In addition to technical reasons, the operation and management level of sewage plants also has an important impact on the effluent quality.
The operation of sewage plant is a complicated process. Operators should make full use of the flexibility of various processes, make appropriate adjustments in the case of changes in water quality and environmental conditions, and find and solve problems in time.
In addition to certain knowledge of physics, chemistry and microbiology, operators also need to know the basic knowledge of sewage treatment, the function of buildings in the factory, the significance and application of laboratory indicators, etc.
In foreign countries, the operation of sewage treatment plants is usually completed by doctors. In China, due to the limitation of salary level and other reasons, most employees of sewage treatment plants generally have low academic level and insufficient technical literacy, and often operate the process facilities of sewage treatment plants by experience, which seriously restricts and affects the overall operation level of sewage treatment plants.
1.4 Other issues
With the advancement of industrialization and urbanization, the ecological environment problems in suburban areas have become increasingly prominent. This "combination" is the combination of urban and rural areas, agriculture and industry, farmers and citizens, and it is full of an uncertain and dynamic transition and transformation.
Construction waste and domestic waste in suburban villages are piled up everywhere, and residents discharge domestic sewage at will. The river flowing through it is black in color, with garbage floating and serious pollution.
If it cannot be effectively controlled, it will always threaten the health of local residents. Due to the imperfection of institutional measures and inadequate management, such problems appear in the suburban fringe. The development of industrial parks has played a more and more significant role in promoting economic development, but environmental pollution has also become increasingly serious.
Large centralized industrial parks generally have sewage treatment plants. A large amount of wastewater from small and medium-sized industrial enterprises is combined with domestic sewage in the park after pretreatment, and there are also many problems in actual operation.
First, the actual water quantity does not match the design. In the design stage of the sewage treatment plant in the park, the actual sewage volume exceeded the treatment capacity of the sewage treatment plant due to insufficient prediction of the development scale. Due to the unstable production situation of some enterprises, the treatment capacity of sewage treatment plants is insufficient.
Second, the actual water quality does not match the design. The actual types of enterprises entering the park do not conform to the plan, which leads to great changes in sewage characteristics and makes it difficult for sewage plants to meet the discharge standards.
2 Countermeasures and suggestions
2. 1 The government made overall planning, and the construction of sewage treatment plants and pipe networks was promoted simultaneously.
Government departments should combine their respective functions, scientifically coordinate and organize, and realize the long-term management of sewage treatment plants [1 1]. The housing and construction department, together with the environmental protection, development and reform commission and other departments, closely followed the pace of urban development, took the lead in compiling the master plan of sewage treatment plants and sewage pipe networks, and planned and designed sewage treatment plants with forward-looking thinking.
Local governments should formulate policies to promote the standardized operation of sewage treatment plants, and work with departments such as price, housing construction and finance to study and formulate a sewage treatment charging system that is suitable for the local economic and social development level.
The financial department should increase the capital investment in sewage treatment plants, innovate the investment and construction operation mode, improve the salary level of operators of sewage treatment plants, and attract high-level and high-quality talents to operate and manage. Environmental protection departments should strengthen the inspection and supervision of the effluent quality of sewage treatment plants, and severely punish those who fail to rectify.
2.2 Improve the sewage collection system and realize the "double increase" of water concentration.
In order to give full play to the efficiency of the sewage plant, we must adhere to the simultaneous development of the plant and the network, and integrate the drainage pipe network with the sewage plant. First of all, speed up the construction of new sewage pipe network, and build a sewage collection system with complete path, smooth connection and efficient operation from "user-branch pipe-main pipe-sewage treatment plant" to improve the operating load of existing sewage treatment plants.
Secondly, it is necessary to strengthen the transformation of old pipe networks, repair pipe networks, sewage outlets and inspection wells with serious leakage, reduce pipeline siltation, and ensure the stability of the quality and quantity of collected sewage. Furthermore, it is necessary to completely transform the confluence pipe network, speed up the construction of interception, storage and other facilities in areas with difficult transformation, and reduce the impact of rainy season rain on the water quantity and quality of sewage treatment plants.
2.3 The source disperses the initial rainwater, reducing the variation range of incoming sewage.
In view of the problem that the initial rain affects the quality and quantity of incoming water, it should be treated at the source. Judging from the characteristics of initial rainwater and the experience of initial rainwater treatment at home and abroad, initial rainwater should be collected at the source and treated separately. It is very difficult to collect the initial rainwater centrally. The main reason is that if a centralized collection system is set up, when the initial rainwater in the upper reaches arrives, it is already clean rainwater in the lower reaches, so it is difficult to ensure that the initial rainwater can be collected 20 ~ 30 minutes ago.
Cities that have built early rainwater collection systems should add corresponding terminal treatment facilities to reduce the impact of early rainwater on the water quality of sewage treatment plants. Conditional access to the sewage treatment plant, it should be demonstrated that the sewage treatment plant has the receiving conditions before access.
2.4 Strengthen the fine management of pipe network to nip in the bud.
Pay attention to the daily management and maintenance of sewage pipe network and strengthen the fine management of pipe network [12]. First of all, it is necessary to strengthen daily inspections, "repair and test" and "regular physical examination" to repair the existing pipe network.
Closed-circuit television and QV are used to detect the inside of the pipeline, so as to grasp the distribution and degree of pipeline diseases and provide basis for pipeline repair. Secondly, it is necessary to implement a regular dredging system to ensure the normal and smooth flow of sewage pipes.
At present, most urban pipelines still use manual dredging, which not only has a bad working environment, but also has low efficiency and cannot meet the demand. High-tech dredging methods such as dredging robots can be introduced to realize automatic and efficient dredging.
Furthermore, the information processing of drainage pipe network data, the establishment of online monitoring system of sewage pipe network water quality, etc. , grasp the water quality situation in real time. When the water quality is abnormal, the problems existing in the pipeline section can be found in time to remind the sewage treatment plant to take effective measures [34].
2.5 Optimize the service scope of sewage treatment plant, improve standards and expand capacity.
Sewage treatment plants are generally located in urban construction areas. With the development of urban construction and urban renewal, the contradiction between the rapid growth of urban sewage and the difficulty of expanding sewage treatment plants or sewage systems has become increasingly prominent.
It is of great significance to solve the problem of sewage increment in sewage treatment plants through the adjustment of service scope in areas where sewage treatment plants are overloaded. At the same time, it is considered to improve the treatment capacity of sewage treatment plants and expand sewage treatment plants.
According to the requirements of GB18918-2015 "Pollutant Discharge Standard for Urban Sewage Treatment Plants (Draft for Comment)", since July 16, both newly-built sewage treatment plants and existing urban sewage treatment plants in sensitive areas have implemented Class A standard.
For the transformation of sewage treatment plants with low emission standards, reasonable transformation measures should be chosen according to local conditions to improve the effluent quality. The upgrading path generally includes hydraulic transformation, equipment transformation and process upgrading, in which the sewage treatment process transformation is the key to improve the effluent quality.
The treatment of biochemical system mainly removes TN and NH3-N, which are the main target pollutants in the transformation of biochemical system. The removal efficiency of TN is limited by the ratio of carbon to nitrogen in influent. Because the influent carbon and nitrogen of most sewage treatment plants in China are relatively low, denitrification capacity can be improved by improving operation mode, rationally utilizing internal carbon sources or adding carbon sources.
When NH3-N is not up to standard, biological aerated filter can be added after secondary biological treatment. When specific projects are involved, appropriate process schemes should be formulated according to the principle of "one factory, one policy and classification".
2.6 Integrated water distribution and water control
The domestic sewage in the main urban area of the city should be treated centrally. Through the construction and improvement of sewage pipe network, the sewage will be collected to the sewage treatment plant for centralized treatment. In the suburbs, cities that have the conditions to build a pipe network should gradually improve the pipe network system and carry out centralized treatment of sewage. If it is impossible to build a pipe network system in the short term, measures of decentralized treatment should be taken.
The decentralized integrated sewage treatment device has the characteristics of flexible movement, high degree of automatic control and good treatment effect. It has been widely used in decentralized sewage treatment, such as villages in cities, and is an effective measure to solve water pollution in suburbs.
The problems existing in sewage plants in industrial parks are not the problems of an enterprise, but need to be solved by reform and development. The control of pollution sources should be strengthened, and industrial enterprises should strictly implement relevant laws and regulations to ensure the discharge of wastewater up to standard.
3 Conclusion
Urban sewage treatment and regeneration facilities are indispensable infrastructure for urban development and an important means to reduce external pollution of water bodies. Ensuring their safe, stable and efficient operation is of great significance to water environment management.
At present, there are still some problems in the operation of sewage treatment plants in China. A targeted summary of the existing problems can lay a foundation for the scientific management of sewage treatment plants in the future. Only when the government departments make overall plans, strengthen the top-level design, constantly improve the sewage collection system, strengthen the fine management of the pipe network, and upgrade and expand the construction, can we give full play to the environmental benefits of sewage treatment plants, improve the quality of urban water environment, and promote the long-term maintenance of the effectiveness of water environment management.
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