What are the methods of sewage treatment?

To treat sewage, we must first understand the types, quality, quantity and treatment requirements of sewage.

In view of the sewage treatment at this stage, the following methods are summarized.

1, physical method

Physical sewage treatment is to separate pollutants mainly in suspension from sewage through physical action, and the chemical properties of water are not changed during the treatment.

(1) precipitation (gravity separation)

As the flow rate of sewage into the tank decreases, the solid matter in the sewage precipitates under the neutral action, thus separating the solid matter from the water.

This process has good separation effect, simple operation and wide application, such as grit chamber and sedimentation tank in sewage treatment plant. The grit chamber is mainly used to remove solid particles with high density in sewage, and the sedimentation tank is mainly used to remove a large number of granular suspended solids in sewage.

(2) Screening (river closure)

The screen intercepts suspended solids in sewage. The equipment belonging to sand filtration treatment includes grid, microfilter, sand filter, vacuum filter and filter press (the latter two are mainly used for sludge dehydration).

(3) Air floatation (floating)

For some fine particles whose relative density is close to that of water, because their own weight is difficult to sink or float in water, air flotation devices can be used. In this method, air is pumped into sewage and separated from water in the form of tiny bubbles. Micro-particle pollution impurities (such as emulsified oil) whose density is close to that of water in sewage are attached to bubbles, which rise to the water surface to form foam scum and are removed. According to different gas injection methods, air flotation equipment includes pressurized dissolved air flotation, impeller air flotation and jet air flotation. In order to improve the effect of air flotation, it is sometimes necessary to add coagulant to sewage.

(4) Centrifugation and cyclone separation

Sewage containing suspended solids or emulsified oil is subjected to different centrifugal forces due to different qualities of suspended solids and sewage, and the suspended solids with large mass are thrown out of the sewage, so that the suspended solids and sewage are discharged from the equipment through their respective discharge ports, thereby purifying the sewage.

2. Chemical methods

The chemical treatment method of sewage is to add chemicals to sewage, and use chemical reaction to separate and recover pollutants in sewage, or convert them into harmless substances. There are the following chemical treatment methods.

(1) coagulation method

Coagulation method is to add a certain amount of chemicals to sewage, and make pollutants in sewage coagulate and settle through the reaction processes such as destabilization and bridging. Colloidal pollutants in water are usually negatively charged, and colloidal particles repel each other to form a stable mixed solution. If the electrolyte (coagulant) with opposite charge in water can turn colloidal particles in sewage into electric neutrality and condense into large particles under the action of molecular attraction.

⑵ neutralization method

The process of eliminating excess acid and alkali in sewage by chemical method and making its pH neutral is called neutralization method. Use alkali as neutralizer to treat acid-containing sewage, use acid as neutralizer to treat alkali-containing sewage, or blow CO2-containing flue gas for neutralization. Both acids and bases refer to inorganic acids and bases. Generally speaking, according to the principle of "using waste to make waste", they can also be neutralized with chemicals, continuously or intermittently.

(3) redox method

After adding oxidant and reductant, organic and inorganic substances dissolved in sewage are oxidized and reduced to harmless substances due to electron migration. Commonly used oxidants include oxygen in the air, pure oxygen, bleaching powder, ozone, chlorine and so on, and oxidation method is mostly used to treat cyanide-containing phenol-containing wastewater. Commonly used reducing agents are iron filings, ferrous sulfate, sodium bisulfite, etc. Reduction method is mainly used to treat wastewater containing chromium and mercury.

④ electrolysis

When the electrode is inserted into the wastewater and a current passes through it, electrons are received on the cathode plate. In the process of electrolyzing water, the anode produces oxygen and the cathode produces hydrogen. The above comprehensive process leads to anodic oxidation and cathodic reduction. At present, electrolysis is mainly used to treat wastewater containing chromium and cyanide.

5] adsorption method

Sewage adsorption treatment mainly uses the surface of solid substances to adsorb pollutants in sewage, and adsorption can be divided into physical adsorption and biological adsorption. Physical adsorption is produced by molecular force between adsorbent and adsorbate, and there is no chemical change, while chemical adsorption law is that adsorbent and adsorbate adsorb under the action of chemical bond force, so chemical adsorption selectivity is strong. In addition, biosorption can also occur under biological action. Commonly used adsorbents in sewage treatment include activated carbon, sulfonated coal, diatomite, coke and so on.

[6] Chemical precipitation method

A chemical reagent is added to the sewage to react with some dissolved substances to form insoluble salts and precipitate them. It is mainly used to treat industrial wastewater containing heavy metal ions.

Primary ion exchange method

Ion exchange method is widely used in sewage treatment. The ion exchangers used are divided into inorganic ion exchange methods (natural zeolite and synthetic zeolite) and organic ion exchange resins (strong acid cation resin, weak acid cation resin, strong base anion resin, weak base anion resin, chelating resin, etc. ). When using ion exchange method to treat sewage, the selectivity of resin must be considered. The exchange ability of resin to various ions is different, which mainly depends on the affinity of various ions to resin, also known as selectivity, and the regeneration method of resin is also considered.

Being membrane separation method

Dialysis, electrodialysis, ultrafiltration, microfiltration, reverse osmosis and other technologies separate ions and molecules in water through a special semi-permeable membrane, which is collectively called membrane separation method. Electrodialysis is mainly used for desalting water and recovering some metal ions. Reverse osmosis is mainly the function of membrane surface chemistry. Solute separated by reverse osmosis has small particle size, high desalination rate and high working pressure. Ultrafiltration uses the same materials as reverse osmosis, but ultrafiltration is a screening function, which has the advantages of large particle size, high water permeability, low desalination rate and low working pressure.

3. Biological methods

Sewage biofilm method is to take some artificial measures to create an environment conducive to the growth and reproduction of microorganisms, so that microorganisms can multiply in large numbers, thereby improving the oxidation ability of microorganisms, decomposing organic pollutants and transforming them into harmless substances, and purifying sewage.

Biological treatment can be divided into aerobic treatment and anaerobic treatment. The former is the main method of biological treatment with high treatment efficiency, good effect and wide application range. There are the following processes that belong to biological treatment.

(1) activated sludge process

It is the most widely used biological treatment technology. Air is continuously blown into the sewage containing a lot of dissolved organic pollutants, and after a period of time, activated sludge, that is, flocs containing a lot of aerobic microorganisms, is formed in the water.

Activated sludge can adsorb organic matter in water. Microorganisms in domestic sewage on activated sludge feed on organic matter, gain energy, continue to proliferate, the organic matter is decomposed and removed, and the sewage is purified. Generally speaking, the effluent treated by aeration tank is a sewage mixture containing a lot of activated sludge. The water after precipitation and separation is purified and discharged, and the sludge after precipitation and separation is used as seed mud, and part of it flows back to the aeration tank. Since the emergence of activated sludge process, after more than 80 years of evolution, there have been various

The activated sludge process has changed, but its principle and process have not changed fundamentally.

(2) Ordinary activated sludge process

This method has been widely used and is a common process in many sewage treatment plants. In the traditional activated sludge process, sewage and reflux sludge are introduced from the first stage of the aeration tank, and then flow out by pushing at the end of the aeration tank. This method is suitable for treating sewage with high requirements and stable water quality, but its adaptability to load change is weak, and some improved forms have been produced on this basis.

(3) Multi-point water inflow method

In order to make the organic load in the pool close to a certain value, the sewage is divided into several points, which is beneficial to solve the overload problem.

⑷ Adsorption regeneration method

The activated activated sludge in the contact tank adsorbs pollutants. After the sludge is separated from water, the adsorbed pollutants are oxidized in the aeration tank. This method is beneficial to improve the sewage treatment capacity and has certain impact load resistance.

5] Extended aeration method.

Prolonging the aeration time of sewage in the aeration tank is beneficial to complete oxidation and reduce sludge. This method is suitable for small sewage treatment plants.

[6] Anaerobic-anoxic

-Aerobic Activated Sludge Process In order to effectively remove nutrients such as nitrogen and phosphorus while removing organic pollutants by the conventional activated sludge process, people combine anaerobic, anoxic and aerobic conditions into the activated sludge process, so that anaerobic-anoxic-aerobic conditions can coexist or be realized repeatedly in the reaction aeration tank, forming the anaerobic-anoxic-aerobic activated sludge process. Some processes also use anaerobic-aerobic activated sludge process.

(7) intermittent activated sludge process

Sewage flows into a single reaction tank, and each process is controlled by program according to time. In a working cycle of the reaction tank, the running procedures are water inlet, reaction, precipitation, water outlet and standby in turn. This method is suitable for occasions with small and medium water quantity and high effluent quality, which is beneficial to automatic control. Through the adjustment of operation, this method can also carry out phosphorus and nitrogen removal and chemical treatment, which is beneficial to sewage reuse.

In recent years, SBR process has developed rapidly, especially with the development of instrumentation and automatic control technology and equipment, new processes of intermittent activated sludge process have emerged, such as CASS process, CAST process, IDEA process, MSBR and so on.

Process and UNITANK process, etc.

Become ab method

This method is the abbreviation of adsorption degradation method and belongs to ultra-high load activated sludge method. This is a series system consisting of two activated sludge processes, and each process has its own independent secondary sedimentation tank. This method has strong impact load resistance, is beneficial to phosphorus and nitrogen removal and chemical treatment, and is especially suitable for treating sewage with high concentration and great changes in water quality and quantity.

Levies oxidation ditch

Oxidation ditch is a continuous annular aeration tank, which is long and shallow. Oxidation ditch system is a treatment technology with low cost, simple structure and easy maintenance and management. Its effluent quality is good, which can be used for denitrification and is beneficial to prolong aeration.

4. Biofilm method

When sewage continuously flows through the solid packing, a biofilm similar to sludge can be formed on the packing, and a large number of microorganisms multiply on the biofilm, adsorbing and degrading organic pollutants in water, which can play the same role as activated sludge in purifying sewage. The dead biofilm falling off the filler flows into the sedimentation tank with the sewage and is clarified and purified by the sedimentation tank. Biofilm has a variety of treatment structures, such as biological filter material, biological turntable, biological contact oxidation and biological fluidized bed.

(1) biological filter

Biological filter is developed on the basis of soil self-purification. There is a fixed filler in the filter. When sewage flows, it contacts the filter material, and microorganisms form biofilm on the surface of the filter material.

Sewage purification device consists of filter bed, water distribution system and drainage system, which provides habitat for microbial growth. Biological filter is simple in operation and low in cost, and is suitable for small and medium-sized towns and remote areas. Biological filter is divided into ordinary biological filter, high-load biological filter, tower biological filter and aerated biological filter.

(2) Biological turntable

Driven by the transmission device, the biological turntable rotates at a certain speed in the contact reaction tank, alternately contacts with air and sewage, and completes the process of adsorption-oxygen absorption-oxidation decomposition in each cycle, and the pollutants in the sewage are continuously decomposed and oxidized in the continuous rotation. In addition to the biological turntable, the biological turntable process also has a primary sedimentation tank and a secondary sedimentation tank. Biological turntable is widely used, which can be applied to domestic sewage and various industrial wastewater. At the same time, the biological turntable has low power consumption, strong impact load resistance and simple management and maintenance.

③ Biological contact oxidation

A filler is arranged in the tank, so that the oxygenated sewage is immersed in all fillers, and a biofilm is covered on the filler, so that the sewage contacts with the biofilm, and the organic matter in the water is adsorbed by microorganisms, oxidized and decomposed, and transformed into a new biofilm. The biofilm falling off the filler is removed after flowing to the secondary sedimentation tank with the water, and the sewage is purified. Biological contact oxidation method has strong adaptability to impact load, small sludge output and can ensure the effluent quality.

(4) Biological fluidized bed

Fine inert particles with relative density greater than 1, such as sand, coke, activated carbon, ceramsite, etc. As a carrier, microorganisms attach and grow on the surface of the carrier to form a biofilm. The oxygenated sewage flows from top to bottom, so that the carrier is in a fluidized state and the biofilm fully contacts with the sewage. The biological fluidized bed has high treatment efficiency, can adapt to large impact load and occupies a small area.

5. Natural biological treatment methods

Using microorganisms that grow and reproduce under natural conditions to treat sewage and form an ecosystem composed of water, microorganisms and plants, and carrying out a series of physical, chemical and biological purification of pollutants can make full use of nutrients in sewage, which is conducive to the growth of green plants and realize the resource, harmlessness and stability of sewage. The method has the advantages of simple process, low construction and operation cost and high efficiency. It is an ecological sewage treatment method, but it is easily affected by natural conditions and occupies a large area. There are mainly aquatic plant ponds, aquatic animal ponds, land treatment systems and the above-mentioned process combination systems. The stabilization pond uses microorganisms naturally growing in the pond water to treat sewage, while the photosynthesis of algae growing in the pond and oxygen in the atmosphere provide oxygen for xiang tang. Sewage stays in the stabilization pond for a long time, and its biochemical process is similar to that of natural water purification. Stabilization ponds are divided into aerobic ponds, facultative ponds, anaerobic ponds and aerated pond according to their microbial reaction types. Land treatment takes land purification as the core, and uses soil filtration, adsorption, chemical reaction and precipitation and microbial decomposition to treat pollutants in sewage. Crops growing on land can make full use of water and nutrients in sewage. For example, sewage farmland irrigation is a land treatment method.

6. Anaerobic biological treatment

Using facultative anaerobic bacteria to degrade organic pollutants under anaerobic conditions is mainly used to treat high concentration and refractory organic industrial wastewater and organic sludge. The main structure is digester, which has made great progress in this field in recent years. A series of new efficient anaerobic treatment structures have been created, such as anaerobic filter, anaerobic turntable, upflow anaerobic sludge bed, anaerobic fluidized bed and other efficient reaction devices. The method has the advantages of low energy consumption, energy generation and less sludge.