Modern sewage treatment technology can be divided into primary, secondary and tertiary treatment according to the degree of treatment.
Primary treatment mainly removes suspended solid pollutants from sewage, and most physical treatment methods can only meet the requirements of primary treatment. After primary treatment, BOD can generally be removed by about 30%, which can not meet the discharge standard. Primary treatment belongs to the pretreatment of secondary treatment.
The secondary treatment mainly removes colloid and dissolved organic pollutants (BOD, COD) in sewage, and the removal rate can reach over 90%, so that the organic pollutants can meet the discharge standards.
Tertiary treatment, further treatment of refractory organic matter, soluble inorganic matter such as nitrogen and phosphorus that can lead to eutrophication of water body. The main methods include biological nitrogen and phosphorus removal, coagulation and sedimentation, sand ratio, activated carbon adsorption, ion exchange, electrodialysis and so on.
The whole process is as follows: the raw sewage after coarse grid removal is lifted by the sewage lifting pump, and then enters the grit chamber, and the sewage after sand-water separation enters the primary sedimentation chamber, which is the primary treatment (that is, physical treatment). The effluent from the primary sedimentation tank enters biological treatment equipment, including activated sludge method and biofilm method. (The reactors of activated sludge process include aeration tank and oxidation ditch, etc. ), biofilm method includes biological filter, biological turntable, biological contact oxidation method and biological fluidized bed). The effluent from biological treatment equipment enters the secondary sedimentation tank, and the effluent from the secondary sedimentation tank is discharged after disinfection or enters the tertiary treatment. The primary treatment ends here as secondary treatment, and the tertiary treatment includes biological nitrogen and phosphorus removal, coagulation and precipitation, sand filtration, activated carbon adsorption, ion exchange and electrodialysis. Part of the sludge in the secondary sedimentation tank flows back to the primary sedimentation tank or biological treatment equipment, and part of it enters the sludge concentration tank and then enters the sludge digestion tank. After dehydration and drying equipment, the sludge is finally utilized.
Energy consumption analysis of various treatment structures
1. Sewage lifting pump house
The sewage entering the sewage treatment plant enters the sewage lifting pump house through the coarse grid, and then is lifted to the front of the grit chamber by the sewage pump. Pump operation consumes a lot of energy, accounting for a considerable proportion of the total energy consumption of sewage plant operation, which is related to the need to improve sewage flow and lift.
2. grit chamber
The function of grit chamber is to remove inorganic particles with high specific gravity. The grit chamber is generally located in front of the pumping station and inverted siphon to reduce the abrasion of pumps and pipelines by inorganic particles. It can also be installed in front of the primary sedimentation tank to reduce the load of sedimentation tank and improve the treatment conditions of sludge treatment structures. Commonly used grit chambers are advection grit chamber, aeration grit chamber, Dole grit chamber and Bell grit chamber.
The main energy sources of grit chamber are sand-water separator and sand suction machine, as well as the aeration system of aerated grit chamber, the power system of Dor grit chamber and Bell grit chamber.
3. Primary sedimentation tank
The primary sedimentation tank is the theme treatment structure of the first-class sewage treatment plant, or it is set in front of the biological treatment structure as the pretreatment structure of the second-class sewage treatment plant. The treatment targets are SS and part of BOD5, which can improve the operating conditions of biological treatment structures and reduce their BOD5 load. The primary sedimentation tank includes advection sedimentation tank, radial sedimentation tank and vertical sedimentation tank.
The main energy-consuming equipment of the primary sedimentation tank is the sludge discharge device, such as chain scraper, sludge scraper and sludge suction pump. However, due to the influence of sludge discharge cycle, the energy consumption of the primary sedimentation tank is relatively low.
4. Biological treatment structure
The process energy consumption of sewage biological treatment unit accounts for a considerable proportion of the direct energy consumption of sewage plants, and the sum of the process energy consumption of sewage biological treatment unit and sludge treatment unit accounts for more than 60% of the direct energy consumption of sewage plants. The aeration of activated sludge aeration system consumes a lot of electric energy, which is basically linked and runs at high power, otherwise it will not achieve good aeration effect and poor treatment effect. The aerator installed in oxidation ditch treatment process is also a kind of high energy consumption equipment. Compared with activated sludge process, biofilm process has lower energy consumption, but it is less used at present and needs to be popularized in the future.
5. Secondary sedimentation tank
The volume consumption of the secondary sedimentation tank is mainly in the pumping of sludge and the removal of floating objects indicated by sewage, and the energy consumption is relatively low.
6. Sludge treatment
In the process of sludge treatment, the concentration tank, sludge dewatering and drying consume a lot of electricity, and the energy consumption of sludge treatment unit is quite large, so the power consumption of these equipment is very large.
Energy-saving ways of various treatment structures
1. Sewage lifting pump house
In order to save energy consumption, the sewage lifting pump room mainly considers how to save electricity. It is an effective means to select the pump correctly and scientifically to make the pump work in the high efficiency area. Reasonable use of terrain and reduction of sewage lifting height are also effective ways to reduce pump shaft power n. Regular maintenance of the pump and reducing friction can also reduce power consumption.
2. grit chamber
Use advection grit chamber to avoid grit chambers that need power equipment, such as advection grit chamber. Gravity sand discharge is adopted to avoid mechanical sand discharge, which can greatly save energy consumption.
3. Primary sedimentation tank
The energy consumption of the primary sedimentation tank is low, and the main energy is spent on sludge discharge equipment. Hydrostatic pressure method will undoubtedly greatly reduce energy consumption.
4. Biological treatment structure
Foreign scholars have compared biological treatment processes through energy consumption and cost-benefit analysis. They believe that most of the energy consumption of treatment facilities occurs on single equipment such as motors, so energy saving should start with improving the power factor of the whole plant, selecting efficient electromechanical equipment and reducing peak power consumption requirements. Their energy-saving measures include not only improving the electrical performance of the motor, but also solving technical problems in operation and recovering energy from the products of the sewage plant.
Resume).
The energy consumption of aeration system is quite large, and the research on energy consumption and energy efficiency of aeration system always involves the transformation and innovation of aeration equipment. Although new aeration equipment emerges one after another, it can still be divided into two types at present: 1 is a method of transferring oxygen into aqueous solution by using submerged porous diffuser or air nozzle, and the second is a method of stirring sewage mechanically to dissolve oxygen in the air. Microporous aeration, the arrangement of aeration diffuser and the adjustment of aeration system are all effective measures to save energy. An energy-saving biological phosphorus removal scheme is proposed, that is, a pre-anaerobic zone is set in the aeration tank of a traditional activated sludge treatment plant and stirred by a submerged agitator. This simple transformation can save nearly 20% of aeration energy consumption, and if the mixed energy consumption is included, the energy saving can also reach 12%. The application of automatic control system in sewage treatment can save energy. The aeration system carries out sectional aeration, and there is a concentration gradient of dissolved oxygen, which not only reduces energy consumption, but also improves the treatment effect and reduces the amount of sludge.
Using anaerobic treatment in biofilm process can obviously reduce energy consumption.
5. Secondary sedimentation tank
It is an effective way to reduce energy consumption to study the sludge discharge equipment and improve the sludge discharge mode of the secondary sedimentation tank.
6. Sludge treatment
The energy-saving research of sludge treatment system mainly focuses on the energy recovery of sludge treatment. Recovering energy from organic pollutants in sewage sludge for treatment was put into practice as early as the beginning of last century, but it was not paid attention to before the energy crisis. At present, there are two recycling ways: one is the utilization of anaerobic digestion gas of sludge, and the other is the utilization of waste heat from sludge incineration.
Digestive gas has stable properties and is easy to store. It can be converted into mechanical energy or electric energy by internal combustion engine or fuel cell, and can also recover waste heat for heating and digesting sludge. Therefore, the utilization of digested gas can solve the energy self-sufficiency problem of sewage plants to varying degrees. Lin et al. compared biogas generator and fuel cell, and thought that fuel cell has high energy utilization rate and good development prospect. Maximizing the utilization of digested gas is the main way to improve energy efficiency. The research and application of grid-connected biogas generator set has been applied in China, which is a feasible way for comprehensive utilization of biogas in large sewage treatment plants.
Another way of energy recovery is to build a municipal solid waste incineration plant next to the sewage treatment plant, and incinerate the solid waste together with sewage sludge, and the obtained electric energy is used for the operation of the treatment plant.
The energy consumption analysis and research of urban sewage treatment are often out of step with the development of energy-saving technologies and means. Due to the lack of research on the energy balance analysis method of sewage treatment, the formulation and implementation of energy-saving measures are often ahead of schedule. Most energy-saving ways and means are often put forward by the operation and management personnel of the treatment plant in combination with the actual situation of each treatment facility, which is empirical and individual and may not be applicable to other sewage plants or even sewage plants with similar processes; On the other hand, in a broad sense, technological innovation, the use of new materials and new equipment in the field of sewage treatment all contain the potential of energy saving and efficiency improvement, so the ways and means of energy saving are often very extensive.
General situation of urban sewage treatment plants at home and abroad
Water is an important factor of economic development and social sustainable development. With the continuous expansion of city scale and the increase of population, water pollution has become a major problem. Urban sewage is an important cause of water pollution in rivers and lakes at present, and it is one of the main reasons that restrict the sustainable development of many cities. "Environmental protection" is the basic national policy of China. China's sustainable development strategy and countermeasures in 2000 require that the centralized treatment rate of urban sewage should reach 20%. At present, China is in a period of great development of urban sewage treatment, especially with the implementation of the national strategy of developing the western region, the environmental and ecological protection in the central and western regions has been put on the agenda.
Since the industrial revolution 200 years ago, people have paid more and more attention to the treatment of urban domestic sewage. Urban sewage treatment rate has become an important symbol of a region's civilization or not. In the past 200 years, urban sewage treatment has developed from the initial natural treatment and simple primary treatment to the advanced treatment and reuse of sewage by various advanced technologies. The treatment process has also developed from the traditional activated sludge process and oxidation ditch process to A/O, A2/O, AB and SBR (including CCAS process) to meet different effluent requirements. Compared with foreign developed countries, China's urban sewage treatment started late, and the current urban sewage treatment rate is only 6.7%. While vigorously attracting foreign advanced technology, equipment and experience, we must combine the development of China, especially the local actual situation, and explore an urban sewage treatment system suitable for the actual situation in China.
Combined with the actual situation in China and drawing lessons from foreign advanced technology and experience, the construction of urban sewage treatment plants should meet the following development directions:
(1) province of total investment. As a developing country, China needs huge funds for its economic development, so strictly controlling the total investment is of great benefit to the national economy.
(2) Low operating cost. Operating cost is an important factor for the normal operation of sewage treatment plants, and it is also one of the main indicators to judge the quality of a set of processes.
(3) It occupied a province. China has a large population, and per capita land resources are extremely scarce. Land resources are an important factor in the development and planning of many cities in China.
(4) Effect of nitrogen and phosphorus removal. With the eutrophication of large-scale water environment in China, nitrogen and phosphorus removal from sewage has become an urgent problem. China's latest national integrated wastewater discharge standard (GB8978- 1996) also clearly stipulates that it is applicable to all pollutant discharge units, and very strictly stipulates the phosphate discharge standard and ammonia nitrogen discharge standard. This means that most urban sewage treatment plants should consider the problem of nitrogen and phosphorus removal in the future.
(5) Organic combination of modern advanced technology and environmental protection engineering. The appearance and perfection of modern advanced technology, especially computer technology and automatic control system equipment, has provided strong support for the development of environmental protection engineering. At present, most sewage treatment plants in developed countries abroad adopt advanced computer management and automatic control systems to ensure the normal operation of sewage treatment plants and stable and qualified effluent, but China is still relatively backward in this respect. Computer control and management will also be the development direction of urban sewage treatment plants in China.
conclusion
Sewage treatment is an energy-intensive comprehensive technology. For some time, high energy consumption and high operating cost have hindered the construction of urban sewage treatment plants in China to some extent, and some completed treatment plants are also in a state of shutdown or semi-shutdown due to energy consumption. For a long time in the future, energy consumption will become the bottleneck of urban sewage treatment. Whether it can solve the energy consumption problem of sewage treatment plant and allocate energy reasonably has become the key factor to determine the operation efficiency of sewage treatment plant. Whether the energy consumption is low is also a decisive factor in the feasibility analysis of new sewage treatment plants in the future. Developing energy-efficient sewage treatment technology and designing and operating sewage treatment plants rationally will be the only way for the design and operation of sewage treatment plants in the future.
At present, the difficulty of sewage treatment lies in reducing the high content of chloride ions and fluoride ions in water.