Sewage treatment

sewage treatment (wastewater treatment) : The process of purifying sewage so that it can be discharged into a body of water or reused. Sewage treatment is widely used in construction, agriculture, transportation, energy, petrochemical, environmental protection, urban landscape, medical, catering and other fields, but also more and more into the daily life of ordinary people.

Development:

The topic of water pollution is constantly raised, especially the problem of groundwater pollution, Hangzhou, Wenzhou and other places in Zhejiang province, farmers or entrepreneurs invited the director of environmental protection to swim in the river, in order to attract everyone's attention to the severity of water pollution, although each director of environmental protection has chosen to silence or refuse, but the public's awareness of environmental protection, the level of concern about water pollution has reached an unprecedented level.

Surface water pollution is obvious, but groundwater pollution is alarming. 70% of China's 1.3 billion people drink groundwater, and more than 400 of China's 660 cities use groundwater as a source of drinking water. However, it is reported that 90% of the country's urban groundwater has been polluted.

Another set of data also shows that groundwater is facing serious challenges. In 2011, Beijing, Shanghai and other nine provinces and cities in the area of 857 monitoring Wells assessed water quality is class I, Class II monitoring Wells accounted for 2%, and class IV, class V monitoring Wells up to 76.8%.

Among the nine provinces and cities, the best water quality is Hainan Province, mainly Class II, followed by Shanghai and Beijing, mostly Class III, Heilongjiang and Jiangsu have the highest proportion of Class IV water, while Jilin, Liaoning, Guangdong and Ningxia generally only reach the level of Class V.

The situation of water pollution continues to worsen, making the sewage treatment and recycling industry has received unprecedented attention, the gross profit margin in the past two years has remained at about 70%, and even more than 100% in some areas, and the development potential of the industry is very large.

Categories:

According to the classification of sewage sources, sewage treatment is generally divided into production sewage treatment and domestic sewage treatment. Production sewage includes industrial sewage, agricultural sewage and medical sewage, etc., and domestic sewage is the sewage generated by daily life, refers to various forms of inorganic substances and organic compounds of complex mixture, including: (1) floating and suspended solid particles; ② Colloidal and gel-like diffusers; ③ Pure solution.

According to the quality of water pollution, there are two types of water pollution: one is natural pollution; The other is man-made pollution, which is more harmful to the water body at present. Water pollution can be divided into chemical pollution, physical pollution and biological pollution according to the difference of pollution impurities. The main pollutants are: (1) industrial wastewater discharged without treatment; (2) Domestic sewage discharged without treatment; (3) Farmland sewage with large amounts of chemical fertilizers, pesticides and herbicides; (4) Industrial waste and domestic waste piled up in the river; (5) Water and soil erosion; (6) Mine sewage.

There are many ways to treat sewage, which can be generally summarized as physical method, chemical method and biological method.

Sewage treatment plant: Some people investigate more than 100 large treatment plants, there are more serious problems, such as insufficient funds, high costs, low efficiency, the general efficiency of less than 70%, only 40% of the low.

Sewage treatment cost energy consumption: basically high energy consumption and low efficiency.

In 2013, urban domestic sewage discharge has been the main source of urban water pollution in China, and domestic sewage treatment is the top priority of current and future urban water saving and urban water environmental protection work, which requires us to treat the construction of domestic sewage facilities as an important part of urban infrastructure to grasp, and it is an urgent thing.

With the development of science and technology, the direct utilization of sewage has become possible, and the sewage source heat pump system is used to utilize urban raw sewage.

The so-called raw sewage is the city directly discharged untreated domestic and industrial wastewater, the current utilization method is the raw sewage directly into the sewage source heat pump system for heat exchange, in the case of consumption of a small amount of electricity for indoor cooling and heating of urban buildings. Sewage reuse has several technical difficulties to overcome: blockage, corrosion, heat transfer efficiency.

Sewage source heat pump system is composed of sewage heat exchanger and sewage source heat pump two parts. After the urban raw sewage directly enters the sewage heat exchanger for heat exchange, the heat exchanged is transferred to the room by the heat pump inside the sewage source heat pump.

For the reuse of urban raw sewage, the advantages are: energy saving and environmental protection, no pollution.

Processing method:

Village sewage is mainly composed of domestic sewage and agricultural wastewater. The composition of domestic sewage is relatively fixed, mainly containing carbohydrates, proteins, amino acids, fats and other organic matter, which is more suitable for the growth of bacteria, and become a place for bacteria and viruses to survive and reproduce; However, domestic sewage generally does not contain toxicity, and has a certain fertilizer effect, which can be used to irrigate farmland. The composition of agricultural wastewater is diverse, different seasons, different places, different development goals of the villages and towns, the wastewater needs to use different treatment methods. In the treatment of sewage, in order to reduce the discharge of sewage and its complexity, it should be combined with the construction of biogas digester that is being vigorously promoted by the country, and the flushing water (black water) in domestic water and other domestic water (gray water) should be separated. Gray water is treated with natural purification system, black water and human and animal feces are treated by anaerobic biogas digester, which can not only reduce the discharge, complexity and treatment cost of sewage, but also have important significance for the development of rural clean new energy, protect the living environment, and promote the sustainable development of rural economy and society.

The function of the sewage treatment station is to treat the production and domestic sewage, meet the prescribed discharge standards, and is an important facility to protect the environment. Sewage treatment stations in industrialized countries have been very common, and there are few sewage treatment stations in villages and towns in China, but they will gradually increase in the future. In order to make these sewage treatment stations really play a role, it also needs to rely on strict discharge system, organization and management system to ensure.

Where conditions exist, sewage treatment stations should be built in villages or single villages. And shall comply with the following provisions:

① When rain and pollution are divided, the sewage is transported to the sewage treatment station for treatment;

(2) When the rain and pollution are combined, the combined sewage is transported to the sewage treatment station for treatment; In front of the sewage treatment station, it is advisable to set up interceptor Wells to remove the combined sewage in the rainy season.

③ The sewage treatment station can use biochemical treatment technologies such as constructed wetlands, biological filters or stabilization ponds, or other treatment technologies with engineering examples or mature experience can be used according to local conditions.

Constructed wetland is suitable for treating pure domestic sewage or rain-polluted combined sewage, which covers a large area and should adopt two-stage series; The plane shape of the biological filter should be circular or rectangular. The filler should be solid, corrosion resistant, high strength, large specific surface area, high porosity, and should use inorganic filter materials such as gravel, pebbles, slag, coke, etc. When the geographical environment is suitable and the technical conditions allow, the village sewage can be considered to use wasteland, waste land, pits, depressions and other stable pond treatment system. The stabilization pond system used for secondary treatment should not be larger than 5000m3/d.

The site selection of the station should be arranged in the summer under the dominant wind direction, the downstream of the village water body, the lower terrain, to facilitate the sewage to flow into the sewage treatment station, do not pollute the village water, and facilitate the downstream discharge after treatment. It has a protective distance from the residential area of the village to reduce the pollution to the residential area. If the sewage is considered for farmland irrigation and sludge fertilization, its location should be correspondingly close to the farmland irrigation area and easy to transport. The sewage of medical institutions must be strictly disinfected and discharged into the sewage network after meeting the prescribed discharge standards, and should comply with the relevant provisions of the current national standard "Hospital sewage treatment design Code" (CECS07:2004). When using reclaimed water, the water quality should conform to the relevant provisions of the current national standards "Code for the Design of Water in Buildings" (GB50336-2002) and "Code for the Design of Sewage recycling Engineering" (GB503352002), and the opening and closing device should be set up to stop the use of public health emergencies.

The effluent from the sewage treatment station shall comply with the relevant provisions of the current national standard "Pollutant Discharge Standard for Urban Sewage Treatment Plants" (GB18918-2002); When the effluent from the sewage treatment station is used for farmland irrigation, it should comply with the relevant provisions of the current national standard "Water Quality Standard for Farmland Irrigation" (GB5084-2005). There are many methods for sewage treatment and utilization, and the following factors should be considered in the selection of the scheme:

① Environmental protection requirements for sewage treatment degree;

(2) The amount and quality of sewage;

(3) Investment capacity. Sewage treatment technology is to use various methods to separate the pollutants contained in the sewage, or convert the pollutants into harmless substances, so that the sewage can be purified.

Treatment method

According to the function;

Sewage treatment can be divided into physical method, biological method and chemical method according to its role.

Physical method: The main use of physical action to separate insoluble substances in sewage, in the treatment process does not change the chemical properties. Commonly used are gravity separation, centrifugal separation, reverse osmosis, air flotation and so on. The physical method is simple and economical to treat structures, which is used for the situation of large water capacity, strong self-purification ability and low requirement of sewage treatment degree in villages and towns.

② Biological method: Using the metabolic function of microorganisms, the organic matter in the sewage is dissolved or colloidal state is decomposed and oxidized into stable inorganic substances, so that the sewage is purified. Activated sludge method and biofilm method are commonly used. The degree of treatment by biological method is higher than that by physical method.

Chemical method: It is a method of using chemical reaction to treat or recycle dissolved substances or colloidal substances of sewage, which is mostly used in industrial wastewater. Commonly used are coagulation method, neutralization method, REDOX method, ion exchange method, etc. The chemical treatment method has good treatment effect and high cost, and is mostly used as the effluent after biochemical treatment for further treatment to improve the effluent quality.

According to the degree of treatment

Sewage treatment can be divided into primary treatment, secondary treatment and tertiary treatment according to the degree of treatment.

Primary treatment is mainly to remove solid substances in the suspended state of sewage, and physical methods are commonly used. The removal rate of BOD of wastewater after primary treatment is only 20%, which is still not suitable for discharge, and secondary treatment is also required. The main task of secondary treatment is to greatly remove colloidal and dissolved organic matter in sewage, and the BOD removal rate is 80% to 90%. Generally, after secondary treatment, the sewage can meet the discharge standard, and the activated sludge method and biofilm treatment are commonly used. The purpose of tertiary treatment is to further remove a special pollutant, such as fluorine removal, phosphorus removal, etc., which is an advanced treatment and commonly used chemical method.

1. The transformation of the existing primary treatment process to strengthen the treatment effect should be based on the actual situation, make full use of the existing treatment facilities, and transform the structure or operation mode of the septic tank and contact tank that are widely used in the existing hospital, and add some facilities if necessary to improve the treatment effect as much as possible. To meet the discharge standards of hospital sewage treatment.

Primary enhanced treatment

1. Process description

For sewage treatment in general hospitals (non-infectious wards), the process of "pretreatment → primary intensive treatment → disinfection" can be adopted. Through coagulation precipitation (filtration) to remove the particles carrying viruses and germs, improve the disinfection effect and reduce the amount of disinfectants, so as to avoid the adverse impact of excessive disinfectants on the environment. The hospital sewage enters the regulating tank through the septic tank. The regulating tank is equipped with an automatic grille in front of the regulating tank and a lifting pump in the regulating tank. After the sewage is lifted, it enters the coagulation sedimentation tank for coagulation and precipitation. The effluent from the sedimentation tank enters the contact tank for disinfection, and the effluent from the contact tank is discharged according to the standard.

The waste generated in the sewage treatment station, such as sludge and grid slag in the regulating tank, coagulation settling tank, contact tank, etc., is centrally disinfected and transported. Disinfection can be done by pasteurizing steam or adding lime.

2. Process characteristics

The first-level intensive treatment can improve the treatment effect, remove the particles carrying viruses and germs, improve the effect of further disinfection and reduce the amount of disinfectant. The transformation of the existing primary treatment process can make full use of the existing facilities and reduce the investment cost.

3. Scope of application

The primary intensive treatment is suitable for general hospitals where the treated effluent ends up in the secondary municipal sewage treatment plant.

Secondary treatment technology

1. Process description

The secondary treatment process is "regulation tank → biological oxidation → contact disinfection". Hospital sewage enters the regulating tank through the septic tank. An automatic grille is provided at the front of the regulating pool. The lifting pump is set in the regulating tank. After the sewage is lifted, it enters the aerobic tank for biological treatment. The effluent from the aerobic tank enters the contact tank for disinfection.

The waste generated in the sewage treatment station, such as sludge and grid slag in the regulating tank, biochemical treatment tank and contact tank, is centrally disinfected and transported for incineration. Disinfection can be done by pasteurizing steam or adding lime.

Secondary treatment process Flow (NCD and infectious wastewater)

Sewage and feces from infectious disease hospitals are collected separately. The domestic sewage goes directly into the pre-disinfection tank for disinfection treatment and then into the regulating tank. The feces of the patient should be independently disinfected first and then enter the septic tank through the sewer or be treated separately. Each structure shall be operated in a closed environment, ventilated by a unified ventilation system, and the exhaust gas shall be discharged after disinfection, which may be carried out by a ultraviolet disinfection system.

2. Process characteristics

Aerobic biochemical treatment unit to remove CODcr, BOD5 and other organic pollutants, aerobic biochemical treatment can choose contact oxidation, activated sludge and efficient aerobic treatment processes, such as membrane bioreactor, biological aerated filter and other processes. The high efficiency aerobic treatment process with filtration function can reduce the concentration of suspended solids and is conducive to subsequent disinfection.

3. Scope of application

It is suitable for infectious disease hospitals (including general hospitals with infectious wards) and general hospital sewage treatment that is discharged into natural water bodies.

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Modern sewage treatment technology, according to the degree of treatment, can be divided into one, two and three levels of treatment, generally according to the status of water quality and the destination of the treated water to determine the degree of sewage treatment.

Primary treatment

It mainly removes solid pollutants in the suspended state of sewage, and most of the physical treatment methods can only complete the requirements of primary treatment. After primary treatment of sewage, BOD can generally be removed about 30%, can not meet the discharge standards. Primary treatment belongs to the pre-treatment of secondary treatment.

Secondary treatment

The main removal of colloidal and dissolved organic pollutants in sewage (BOD, COD substances), the removal rate can reach more than 90%, so that organic pollutants meet the discharge standard, suspended matter removal rate of 95% of the effluent effect is good.

Tertiary treatment

Further treatment of refractory organic matter, nitrogen, phosphorus and other soluble inorganic substances that can lead to eutrophication of water bodies. The main methods include biological nitrogen and phosphorus removal, coagulation and precipitation, sand filtration, activated carbon adsorption, ion exchange and electrodialysis.

The whole process is that the raw sewage through the coarse grid is lifted by the sewage lifting pump, passes through the grid or sieve rate, and then enters the sedimentation tank, the sewage separated by sand water enters the primary sedimentation tank, the above is the primary treatment (that is, physical treatment), the effluent of the primary sedimentation tank enters the biological treatment equipment, there are activated sludge method and biofilm method, (the activated sludge method has an aeration tank in the reactor, The effluent from the biological treatment equipment enters the secondary sedimentation tank, and the effluent from the secondary sedimentation tank is disinfected and discharged or enters the tertiary treatment. The tertiary treatment ends here as the secondary treatment. The tertiary treatment includes biological nitrogen and phosphorus removal method, coagulation sedimentation method, sand filtration method, activated carbon adsorption method, etc. Ion exchange and electrodialysis. Part of the sludge in the secondary sedimentation tank is returned to the primary sedimentation tank or biological treatment equipment, part of it goes into the sludge thickening tank, and then into the sludge digestion tank. After dehydration and drying equipment, the sludge is finally used.

Electron beam technique

On September 21, 2022, the China Atomic Energy Authority announced ten major events in the field of nuclear technology application in Beijing, including the industrialization of electron beam technology in the field of sewage treatment [1].