First, the background
Urban domestic sewage is the product of urban development. With the acceleration of urbanization and industrialization, the generation of urban sewage is increasing and the pollution is becoming more and more serious, which has severely restricted the sustainable development of urban social economy. In today's rapid global economic development, environmental issues, especially urban sewage treatment has become a hot spot in all countries. The treatment of urban sewage plays a key role in improving urban water environment and ensuring urban economic development. The economic development of developed countries in the 1950s in the 1950s led to serious environmental pollution in the 1960s. By the late 1970s, more than 18,000 urban sewage treatment plants were constructed by the United States and trillions of dollars were invested. Britain, France and Germany each spent a huge sum of money to build 7,000 to 8,000 urban sewage treatment plants. Sewage treatment in China started in the 1970s. According to statistics, by the end of 2000, 427 sewage treatment plants have been built in the country and the total investment for the construction of urban sewage treatment projects is about 15 billion yuan.
Second, an overview of urban sewage treatment process
A typical urban sewage treatment process includes mechanical treatment, biochemical treatment, sludge treatment and other sections. The system composed of mechanical treatment and biochemical treatment belongs to the secondary treatment system, in which the removal rate of BOD5 and SS can reach 90% -98%. The treatment effect is between the first and the second treatment, generally referred to as strengthening and processing, a semi-treatment or incomplete secondary treatment, there are two main categories of high-load biological treatment and chemical treatment, BOD5 removal rate of 45 % -75%. Secondary biological treatment system with biological phosphorus removal and denitrification is usually called deep secondary treatment. In order to remove specific substances, the treatment system provided after the secondary treatment belongs to tertiary treatment such as chemical phosphorus removal and activated carbon adsorption.
Classification of pollutants
From the perspective of sewage treatment, pollutants can be divided into suspended solid pollutants, organic pollutants, toxic substances, contaminated organisms and contaminated nutrients. The large amount of organic matter contained in the urban sewage is discharged into the water body, which will reduce the dissolved oxygen content in the water body and even reach the state of hypoxia, seriously pollute the water body and make the fish in the water unable to survive. The concentration of organic matter in wastewater is generally expressed as BOD5, COD, TOD and TOC. Nutrients mainly nitrogen, phosphorus, which can make algae and plankton breeding, the formation of "bloom" and "red tide."
Sewage treatment methods
Sewage treatment methods can be divided into physical treatment according to the type of water quality, biological treatment, sewage treatment sludge disposal and chemical treatment, but also according to the degree of treatment is divided into the first-level treatment, secondary treatment and tertiary treatment processes . The physical treatment of urban sewage is the use of physical separation and removal of pollutants in sewage methods. Common methods are screening retention, gravity separation, centrifugal separation, the corresponding treatment equipment are mainly grille, grit chamber, sedimentation tank and centrifuge oxygen sedimentation tank with the town water treatment in the sedimentation tank. Biological treatment is the use of microbial metabolism, removal of organic matter in sewage. Commonly used activated sludge method, biofilm method, as well as oxidation ponds and sewage land treatment. Chemical treatment methods are less commonly used in municipal wastewater treatment, and generally involve other chemical methods in urban water treatment such as neutralization and redox, ion exchange. Electrolysis is mainly used for industrial wastewater treatment and seldom used in urban sewage treatment. Sludge to be treated to prevent secondary pollution, its disposal methods often concentrated, anaerobic digestion, dehydration and heat treatment. A primary treatment mainly for suspended solids in water, often using physical methods, after a treatment, the removal of suspended solids in sewage up to about 40%, attached to the suspended solids can be removed about 30%; secondary treatment of the main removal of sewage In colloidal and dissolved organic pollutants. The commonly used method is the microbial treatment method, the specific method of activated sludge and biofilm method. Biological treatment is the use of microbial decomposition of organic matter oxidation of this function, and take some artificial measures to create conducive to microbial growth and reproduction of the environment, so that the proliferation of microorganisms in order to improve its decomposition of organic matter oxidation efficiency. Sewage after a treatment, has been removed floats and some suspended solids, BOD5 removal rate of about 25% to 30%. After secondary treatment, BOD5 removal rate of up to 90%, secondary sedimentation tank effluent discharge standards. Activated sludge treatment system, in the field of sewage treatment, is one of the most widely used treatment technology, aeration tank is its reactor. Sewage and sludge are mixed in the aeration tank. The microorganisms in the sludge will degrade the complicated organic matters in the sewage and use the released energy to realize the reproduction and movement of the microorganisms themselves.
Third, the sewage treatment technology
The current popular sewage treatment process are: AB method, SBR method, oxidation ditch method, the general aeration method, membrane separation machines, each with its own characteristics.
AB method
The process of aeration tank according to high and low load is divided into two oxygen. Class A load is high, aeration time is short, the amount of sludge is large, the sludge load is above 2.5 kg BOD / (kg MLSS? D), and the pool volume load is above 6 kg BOD / (m3? D) , Sludge age is longer. A-level and B-level construction can be phased, A level and B-level intermediate sedimentation tank. The F / M (the ratio of the amount of pollutants to the amount of micro-organisms) of two stages of ponds is different and forms different microbial populations. AB method despite the advantages of energy saving, but not suitable for low concentrations of water quality.
SBR method
Foaming Agent,Sodium Bicarbonate Foaming Agent,Amino Acid Foaming Agent,Blowing Agent In Foam
GuangDong DuBa New Material Technology Co.,LTD , https://www.dubachem.com