With the development of China's social economy and urbanization, the scale of urban sewage treatment plant is expanding, and the treatment level is improving. So far, nearly 300 municipal sewage treatment plants have been built or under construction in China, with a sewage treatment capacity of 10 million m3 / d.
1. The concept of urban comprehensive sewage treatment
In recent years, with the continuous improvement of science and technology, sewage treatment technology has a great development. Generally speaking, urban comprehensive sewage is treated by primary treatment or secondary biochemical treatment. After treatment, the main pollutants of urban sewage are nitrogen, phosphorus and other eutrophic substances, and then the land treatment system is used for advanced treatment. The primary treatment is mainly to remove the suspended solid pollutants in the sewage. The methods include grid, sedimentation, grit, oil separation, air flotation, etc. The purpose of secondary treatment is to greatly remove colloidal and dissolved organic pollutants in sewage. At present, the commonly used treatment methods are activated sludge process and their improved type. The process can be mixed in the first and second stages, and some parts have reached the third level mixed treatment. In order to better remove nitrogen and phosphorus in sewage, phosphorus and nitrogen removal technology is further developed. It is characterized by using the principle that dominant bacteria (mainly phosphorus accumulating bacteria) need a large amount of oxygen for growth in the process of anoxic anaerobic aerobic treatment (especially in the aerobic process), so as to reduce the phosphorus and nitrogen content in the sewage and make the sewage reach the third level treatment in this process.
2. Classification of urban comprehensive sewage treatment
Due to the variety and nature of sewage, the sewage treatment strategies are also very different.
2.1 domestic sewage
Usually, taking urban domestic sewage as an example, sewage treatment only needs one-stage treatment and simple two-stage treatment to meet the water quality requirements of urban reclaimed water, which may meet the requirements of industrial circulating water cooling and household toilet, so as to achieve the purpose of reclaimed water reuse. Membrane bioreactor (MBR) refers to the combination of ultrafiltration technology in membrane separation technology and traditional activated sludge process (CAS) in wastewater treatment. Membrane module is used to replace the secondary sedimentation tank of activated sludge process for solid-liquid separation, so as to remove suspended solids, bacteria and macromolecular organic matters. MBR has the advantages of good effluent quality, high volume load, small occupied area, low excess sludge production, convenient operation and management.
2.2 hospital sewage
Hospital sewage is a kind of wastewater produced in the process of diagnosis, treatment and prevention of diseases in hospitals or other medical institutions, which has potential infectivity and acute infectivity. It contains a variety of microorganisms and infectious pathogens, such as AIDS, hepatitis B, hepatitis C, typhoid, dysentery, tuberculosis and other viruses, which is listed as the national hw01. Dangerous pollutant. If it is discharged directly without treatment, the bacteria will spread through water, soil and atmosphere, causing threat to human body. After secondary treatment, the water quality has been improved and the bacterial content has been greatly reduced, but the absolute number of bacteria is still considerable.
2.2.1 chlorine disinfection
Chlorine disinfection commonly used are liquid chlorine method and sodium hypochlorite method. Liquid chlorine method is a popular method in China in 1980s, with mature technology and low price. The equipment failure rate and cost are significantly lower than that of ozone method. However, chlorine leakage occurs frequently, and it is easy to produce secondary pollutants. Sodium hypochlorite method is widely used in the 1990s, with less equipment investment, low operating cost, safety, reliability, easy management, and is suitable for areas with reliable drug sources It is widely used in sewage disinfection, but its disinfection effect is not as strong as chlorine gas.
2.2.2 peroxide disinfection
Chlorine dioxide and ozone are commonly used as peroxide disinfectants.
Chlorine dioxide has different characteristics from chlorine disinfection. Its bactericidal mechanism is mainly that chlorine dioxide molecules penetrate into cells and oxidize nucleic acid (DNA or RNA), so as to rapidly inhibit the synthesis of microbial protein and destroy the survival of microorganisms. Chlorine dioxide is a highly effective and powerful bactericide recognized by the World Health Organization (who). It is an ideal replacement product for chlorine series disinfection internationally. It is recommended by the State Environmental Protection Administration. Its advantages lie in that it reacts with humus or organic matter, hardly produces volatile organic halogen, does not generate and inhibit the formation of carcinogen trihalomethane, and does not combine with ammonia and amino Chemical reaction.
In ozone disinfection, ozone is a strong oxidant with good bactericidal effect and short contact time. The germicidal effect of chlorine is gradual, while that of ozone is rapid and instantaneous. In addition, ozone not only does not produce carcinogenic trihalomethanes, but also can effectively degrade and eliminate the color, odor, taste, phenol, cyanogen and other pollutants in the sewage. The effect of deodorization and decolorization is good, and the water dissolves more. Now it is widely used in Western European countries, but the investment is large, the equipment failure rate is high, the power consumption is large, and the cost is higher than that of chlorination disinfection The disadvantages of ozone disinfection are that it can not be stored and transported easily. After ozone disinfection, the residual ozone in water can not be maintained, and it is difficult to control the growth of bacteria.
2.2.3 ultraviolet radiation disinfection
As a physical disinfection method, ultraviolet radiation disinfection is non-toxic, cumulative toxicity and corroding crops. It is easy to operate, without chemical agents, and does not produce secondary pollution. After disinfection, there is no residual radiation. The disadvantage is that the service life of ultraviolet light source is short. If the light intensity is low, the sterilization efficiency is not high It has no follow-up effect and can not be applied on a large scale.