1. background
urban sewage is the product of urban development. with the acceleration of urbanization and industrialization, the amount of production is increasing and pollution is becoming more and more serious, which has seriously restricted the sustainable development of urban social and economic. with the rapid development of the global economy, environmental protection, especially urban sewage treatment, has become a hot topic in various countries. the treatment of urban sewage plays a key role in improving urban water environment and ensuring urban economic development. the economic development of western developed countries in 1950s has caused serious environmental pollution in 60s. by the end of 1970s, more than 18000 municipal wastewater treatment plants were built in the united states, and billions of dollars were invested; the uk, france and germany have spent a huge amount of money to build 7000~8000 sewerage treatment plants. the sewage treatment in our country began in 1970s. according to statistics, by the end of 2015, 1943 sewage treatment plants have been built in china, and the daily handling capacity of sewage plants is 140 million 280 thousand cubic meters. the total amount of sewage treatment is 41 billion 30 million cubic meters in the year, and the national urban sewage treatment rate of 91.97% in 2015.
two. general description of municipal wastewater treatment process
typical urban sewage treatment processes include mechanical treatment, biochemical treatment, sludge treatment and so on. the system consisting of mechanical treatment and biochemical treatment is a two level treatment system, where the removal rate of bod5 and ss can reach 90%-98%. the treatment effect between the first class and the two stage treatment is generally called the strengthening first order treatment, the first order semi processing or the incomplete two stage treatment. there are two major classes of high load biological treatment and chemical treatment, and the removal rate of bod5 is up to 45%-75%. the two level processing system with biological phosphorus and nitrogen removal functions is usually called depth two level processing. in order to remove specific substances, the treatment system set up after the two stage treatment belongs to three stages, such as chemical phosphorus removal, activated carbon adsorption and so on.
classification of pollutants
from the perspective of sewage treatment, pollutants can be classified as suspended solid pollutants, organic pollutants, toxic substances, pollution organisms and pollution nutrients. a large number of organic substances in urban sewage are discharged into the water body, which will reduce the content of dissolved oxygen in the water, even reach the state of oxygen, which seriously pollute the water body, so that the fish in the water can not survive. the concentration of organic matter in sewage is generally expressed by biochemical oxygen demand (bod5), chemical oxygen demand (cod), total oxygen demand (tod) and total organic carbon (toc). nutrients mainly refer to nitrogen and phosphorus, which can make algae and plankton reproduce and form "bloom" and "red tide".
sewage treatment method
the sewage treatment method can be divided into physical treatment, biological treatment, sewage disposal and chemical treatment according to the water quality. it can also be divided into first grade treatment, two stage treatment and three grade treatment process according to the treatment degree. the physical treatment of municipal sewage is to separate and remove pollutants from sewage by physical action. the commonly used methods include sieve filtration, gravity separation, centrifuge separation and so on. the corresponding treatment equipment mainly includes grille, sanding pool, sedimentation tank and sedimentation tank of centrifuge oxygen, and sedimentation tank in water treatment of town. biological treatment is a method of removing organic matter from sewage by using microbial metabolism. activated sludge and biofilm are commonly used, as well as oxidation pond and sewage land treatment. chemical treatment is less used in urban sewage treatment, which generally involves other chemical methods in urban water treatment, such as neutralization redox, ion exchange and electrolysis, which are mainly used in industrial wastewater treatment, and are rarely used in urban sewage treatment. the sludge must be treated in order to prevent the two pollution. the disposal methods often include concentration, anaerobic digestion, dehydration and heat treatment. the primary treatment is mainly aimed at the suspended matter in the water, and the physical method is often used. after the first stage treatment, the removal of the suspended matter can be about 40%, and the organic matter attached to the suspended matter can be removed about 30%. the two grade treatment mainly removes the organic contaminants in the colloid and dissolving state of the sewage. the commonly used method is microbial treatment, in particular, activated sludge and biofilm methods. biological treatment is the function of decomposition and oxidation of organic matter by microorganism, and some artificial measures are taken to create environment conducive to the growth and reproduction of microorganisms, so that microorganisms can be propagated in large quantities to improve the efficiency of decomposition and oxidation of organic matter. after the first level treatment, the floater and some suspended solids have been removed. the removal rate of bod5 is about 25% ~ 30%. after two stage treatment, the removal rate of bod5 can reach more than 90%, and the effluent of two sedimentation tank can meet the standard discharge. activated sludge treatment system is one of the most widely used technologies in the field of sewage treatment, and the aeration tank is the reactor. sewage and sludge are mixed in the aeration pool, and the microorganisms in the sludge degrade the complex organic matter in the sewage and use the released energy to achieve the reproduction and movement of the microorganism itself.
three. the technological technology of sewage treatment
the popular sewage treatment processes include ab method, sbr method, oxidation ditch method, ordinary aeration method, membrane separator and so on, each of which has its own characteristics.
adsorption biodegradation
the process divides the aeration tank into two levels of oxygen supply at high and low loads. high grade a load, short aeration time, large amount of sludge, sludge load 2.5kgbod / (kgmlss d), pool volume load of 6kgbod / (m3 / d), b class load low, sludge age longer. class a and b can also be built in stages, with intermediate sedimentation tanks between class a and b. the level of f / m (the ratio of pollutant to microbial biomass) in the two level ponds is different, forming different microbial communities. although the ab method has the advantages of energy saving
