1 Overview of wastewater treatment? 1. 1 pollutants in wastewater and water quality control methods for wastewater treatment? According to the pollutants that cause pollution harm to the environment in different wastewater, there are roughly the following categories: solid pollutants, organic pollutants, toxic pollutants, nutritional pollutants, biological pollutants, sensory pollutants, acid-base pollutants, thermal pollutants and other pollutants. In order to reuse, safely discharge and recover useful substances in wastewater, water quality control methods such as separation treatment, conversion treatment and dilution treatment can be used to treat wastewater. ? 1.2 wastewater treatment process? In the process of wastewater generation and treatment, people gradually discovered and explored many practical and effective treatment methods. Traditional methods include filtration, separation, biotransformation, neutralization, chemical precipitation, stripping, extraction, redox and adsorption. In recent years, wastewater treatment technology has developed rapidly and many new methods have appeared. ? 1.3 wastewater treatment system? The composition of wastewater is very complex, so it is often necessary to combine several unit operations into an organic whole, rationally configure the primary and secondary relations and sequences, and form a complete wastewater treatment system to effectively complete the treatment task. Figure 1 shows a typical urban sewage treatment system.
? According to the different treatment tasks, the system can be divided into three levels: the main object of the first level treatment is the larger suspended solids, and the separation equipment used is grid, grit chamber and sedimentation tank in turn. The sludge intercepted by the sedimentation tank can be digested or treated in other ways, and the effluent can be discharged into water or used for sewage irrigation; Secondary treatment is a biochemical treatment method, which mainly treats colloid and dissolved organic matter in wastewater. Its typical treatment equipment includes biological aeration tank and secondary sedimentation tank. The purpose of tertiary treatment is sometimes not discharge but direct reuse, so it sometimes belongs to the category of advanced treatment. The objects of tertiary treatment are nutrients (nitrogen and phosphorus) and other soluble substances, and the methods adopted are chemical flocculation and filtration, and adsorption, ion exchange, reverse osmosis and disinfection can also be used in places with high requirements for effluent quality. ? 2 thermal power plant wastewater treatment? 2. 1 Overview of water system in thermal power plant? The water system of thermal power plants is huge. Taking 1 000 MW power plant as an example, if the open circulation mode is adopted, the circulating cooling water quantity is about 120 000 t/h, and if the water replenishment rate is calculated at 3%, the water consumption for chemical water treatment is about 320 t/h, and the water consumption for thermal power plants is generally divided into two categories: one is production water; The other is domestic and fire water. Production water mainly includes circulating cooling water, ash flushing water, machine cooling water and boiler make-up water. ? 2.2 What are the main waste water sources of modern thermal power plants? The main sources of wastewater from modern coal-fired power plants are: ash flushing water; Chemical cleaning of thermal equipment and shutdown protection of discharged wastewater and boiler sewage; Sewage discharge of main workshop, auxiliary equipment and mechanical cooling water; Drainage of condensate purification equipment; Cooling drainage of condenser or sewage from cooling equipment; Sewage from water treatment plant; Domestic sewage; Cleaning water of coal conveying system, drainage of coal storage yard and rainwater drainage of plant area. ? 2.3 thermal power plant wastewater treatment process? According to the composition of wastewater and the content of pollutants, thermal power plants generally adopt the following wastewater treatment processes:
2.3. 1 filtration method? Separation and filtration methods are divided into fence method, screen filtration method and filtration method. Both the fence method and the screen filtration method intercept the coarse suspended solids in the wastewater by plugging, and the filtration method has the functions of adsorption, flocculation and precipitation besides interception, so that suspended particles finer than pores can also be separated from the wastewater. ? 2.3.2 activated sludge process? Activated sludge process is a typical aerobic biotransformation treatment method, especially suitable for domestic sewage treatment. It uses organic pollutants contained in wastewater as culture medium, continuously cultivates activated sludge in the presence of dissolved oxygen, and then purifies the organic pollutants in wastewater by its adsorption, coagulation and oxidative decomposition. Fig. 2 is the treatment process of ordinary activated sludge process.
The primary sedimentation tank is used to remove primary suspended solids from wastewater; The aeration tank makes the organic pollutants in wastewater fully contact with activated sludge, and absorbs and oxidizes the organic pollutants; The aeration system provides oxygen for biological reaction in the aeration tank and plays the role of mixing and stirring; The secondary sedimentation tank is used to separate the activated sludge from the effluent of the aeration tank; The sludge return system returns part of the precipitated sludge in the secondary sedimentation tank to the aeration tank to ensure that the aeration tank has sufficient microbial concentration; The excess sludge discharge system discharges the ever-increasing sludge in the aeration tank through the discharge system to ensure the effective reaction volume of the aeration tank. ? 2.3.3 neutralization? Neutralization treatment is a simple method to effectively adjust the pH value according to the principle of acid-base neutralization, so that the wastewater can meet the discharge requirements. Generally, chemical cleaning drainage and water treatment regeneration drainage in thermal power plants need neutralization. At present, most power plants design and use neutralization tanks to treat waste acid solution and waste alkali solution discharged during water regeneration. Of course, due to the large discharge of acid-base waste liquid in thermal power plants, the neutralization process should be adapted to local conditions and be economical and effective. In addition to direct treatment with acid-base solution, acidic wastewater can also be neutralized by adding alkaline chemicals, using alkaline waste residue, natural water, carbonate and bicarbonate in soil through alkaline filter materials. Alkaline wastewater can be treated with waste inorganic acid, acidic waste gas (such as CO2 and flue gas) and acidic wastewater. ? In addition, some thermal power plants also use coagulation, adsorption, ion exchange, electrodialysis, reverse osmosis and other methods to treat sewage. ? 2.4 Prevention and control measures for waste water in thermal power plants? The measures to control the pH value of grey water exceeding the national emission standard in power plants in China include: closed circulation of grey water, treatment of furnace smoke (using CO2 or SO2 in furnace smoke), treatment with acid, use of circulating water and dilution, and use of wet dust collector. Separator and precipitation method of grey water suspended matter; Most oily wastewater from power plants is treated by oil separation first, and then by air flotation or oil-water separation device and biological treatment. Domestic sewage is generally treated with activated sludge; Production wastewater is neutralized, clarified, disinfected and filtered; Washing wastewater from coal conveying system is recovered after being treated by precipitation or water purification device. ? 3 wastewater reuse technology? With the increasing scarcity of water resources, wastewater reuse technology has become more and more important. In order to solve the water source problem of thermal power plants, the technology of using urban sewage as circulating cooling water of thermal power plants has been implemented and applied in some newly-built thermal power plants in recent years. ? 3. 1 Introduction of sewage reuse abroad? Since 1960s, sewage has been widely used for irrigation and recharge. From 65438 to 0968, after the first sewage reuse plant was built in Lake Nantaho, California, USA, the research on sewage reuse in some water-deficient countries began to develop rapidly. In Texas and California, which are short of water in the United States, the annual reuse of water reaches 2.5 billion m3 (equivalent to 70% of the total sewage here). The treated sewage is used for cooling water, boiler water, industrial water and fire water. The secondary effluent of Sanhe Island Sewage Treatment Plant in Tokyo, Japan reaches138,000m3 per day. After treatment, these sewage are supplied to more than 400 factories in Jiangdong area. 23.2% of the sewage in Pretoria-Witwatersrand-Fehonihin area of South Africa has been treated for power plant cooling and industrial reuse. ? 3.2 The development of sewage reuse in China? Reuse of wastewater after treatment is an effective way to solve the water shortage. China is one of the countries that lack water in the world. After the founding of the People's Republic of China, the party and the state attached great importance to the construction of sewage treatment projects. 1960, the first temporary sewage treatment plant for agricultural irrigation-Gaobeidian sewage treatment plant was built in China. Wastewater reuse and industrial production in China began in 1970s. 1973 The former Dongfanghong Refinery sent the sewage after secondary treatment back to the circulating cooling water as its supplementary water. At that time, the reuse rate was 20%, and in five years, * * * was reused for 530 days. Although there were many problems and difficulties in the later implementation of this project, some achievements were made at that time, which accumulated valuable experience for the development and research of sewage reuse technology in China. In the 1990s, China's sewage reuse project gradually became formal, and a large number of sewage treatment plants were completed and put into operation. Many factories began to study the water balance of the whole factory. However, due to the late start, the reuse rate of industrial water in cities in China is very low, generally around 30%, with only 50% in Shanghai, 64% in Germany and 60% in Japan. ? 3.3 Sewage utilization of thermal power plants in China? The serious shortage of water resources and the increase of water price make coal-fired power plants with large water consumption have to seek new water sources. In China, municipal sewage was introduced into the circulating cooling water of thermal power plants in the 1990s. Because the composition of urban sewage is complex and the time, season and regional environment are different, many influencing factors must be considered in the design of sewage treatment process. At present, many treatment methods have been established in the treatment of urban sewage (recycling of circulating cooling water in thermal power plants) in China, among which lime treatment is the most. The typical processing flow is shown in Figure 3.
? Of course, the above process flow is based on the secondary treatment of sewage by the sewage treatment plant (see figure 1), and various operating parameters and chemicals need to be adjusted according to the different water quality. If the reasonable process sequence and scale inhibitor are selected, utilized and controlled, the concentration ratio of circulating water can reach about 3.0. ? It is worth noting that after treatment, the contents of BOD, COD and bacteria are still high. Therefore, when urban sewage is used as circulating cooling water in thermal power plants, strict experimental research must be carried out and sufficient safety factor must be left. We should carefully choose condenser materials; The dosage, monitoring and control of bactericide, scale inhibitor and corrosion inhibitor should be strictly controlled. At the same time, in the design of the whole circulating water system, remedial measures should also be considered in the case of water quality deterioration. ? 4 Water-saving project of newly-built thermal power plants-Zero emission project In order to save water and protect the environment more effectively, in recent years, major power plants (especially water-deficient areas) have successively implemented zero emission projects. The zero-emission system not only protects the environment but also saves water resources, and will certainly receive good economic and social benefits. Take a factory in Hebei southern power grid as an example. The installed capacity of this plant is 4×300 MW, and the circulating cooling water of 1 and No.2 units is treated by adding acid and water quality stabilizer, and the concentration ratio is controlled below 2.3. Therefore, in addition to ash flushing, this device also has a large amount of residual sewage. In order to make full use of water resources and solve the problem of water balance in the whole plant, in the design of circulating cooling water of units 3 and 4, the plant skillfully partially treated the sewage of circulating cooling water of units 1 and 2 (such as weak acid cation exchange) and directly used it as supplementary water of circulating water of units 3 and 4. However, part of the sewage from the circulating water of Units 3 and 4 is used for ash flushing, and the other part of the water is sent to the water treatment workshop for recycling after certain treatment (such as reverse osmosis). The design idea is novel, which achieves the purpose of improving concentration ratio, reducing pollution discharge and saving water, and is worth learning from water-saving projects and sewage reuse technology. ? 5 conclusion? Wastewater treatment and reuse in thermal power plants is an important way to save water and protect the environment. But in this respect, China is still in its infancy, and there is a big gap compared with foreign countries. According to the present situation of our country, we should also increase the research and scientific and technological investment in sewage treatment and reuse technology, focus on improving the concentration ratio of circulating water, improve the ash flushing method, break through the thick slurry conveying technology, solve the problem of ash water reuse and the problem of pollutants exceeding the standard in direct discharge ash water.