Water purification information

Sewage treatment process and analysis summary of advantages and disadvantages

Update time : 2021-10-21 16:14:09

1. Sewage treatment level and technology

1. Sewage treatment level

The sewage treatment level includes primary treatment (including primary enhanced treatment), secondary treatment (including secondary enhanced treatment) and advanced treatment.

2. Composition of sewage treatment process

(1) Physical processing section

(2) Biochemical treatment section

Second, the principle of selection of sewage treatment process

1. The main technical and economic indicators for process selection include:

①Treatment unit water investment;

②Reduce unit pollution investment;

③Processing unit water and electricity consumption and cost;

④Reduce the power consumption and cost of unit pollutants;

⑤Occupation area;

⑥ Reliability of operational performance;

⑦The difficulty of management and maintenance;

⑧ Overall environmental benefits, etc.

2. The urban sewage treatment process should be optimized after comprehensive technical and economic comparisons based on the scale of treatment, water quality characteristics, environmental functions of the receiving water body, and local actual conditions and requirements.
3. The quality of sewage influent should be determined realistically, and the process design parameters should be optimized. The current water quality characteristics of sewage and the composition of pollutants must be investigated or measured in detail, and reasonable analysis and prediction should be made. When the water quality composition is complicated or special, it should be carried out. For dynamic tests of sewage treatment processes, pilot studies should be carried out when necessary.
4. Actively and prudently adopt new technology. For the first application of new technology in China, it must go through pilot and production tests and provide reliable design parameters before applying it.
5. When the same sewage treatment plant is constructed in stages, the same process should be used as much as possible in each stage, and the construction scale of each stage should be as the same as possible.

3. Sewage treatment method

Modern sewage treatment methods are mainly divided into four categories: physical treatment, chemical treatment, physical and chemical treatment and biological treatment.
1. Physical treatment
The physical treatment method is to separate and recover the insoluble and suspended pollutants (including oil film and oil droplets) in the sewage through physical action, without changing its chemical properties during the treatment process. Commonly used are filtration, precipitation, flotation and so on.
(1) Filtration method: Use filter media to intercept suspended solids in sewage. Filter media include screens, gauze, and particles. Commonly used filter equipment include grilles, screens, and microfilters.

1) Grid and screen: In drainage engineering, wastewater flows into the water treatment plant through the sewer. The filter mesh (screen mesh) prevents floating or suspended matter from passing and is blocked on the grid, fine sieve or filter material.

Grille
This step is wastewater pretreatment and its purpose is to recover useful substances; initially flood the wastewater to facilitate subsequent treatment and reduce the load on the sedimentation tank or other treatment equipment; protect the pumping machinery from blockage by particulates and malfunction.
To protect water pumps and other processing equipment, the effect of grille interception mainly depends on the sewage water quality and the size of the grille gap. There are two methods for cleaning slag: manual and mechanical. The grid slag should be cleaned and treated in time.
The screen is mainly used to intercept fine suspended debris with a particle size of several millimeters to tens of millimeters, such as fibers, pulp, algae, etc. It is usually woven with metal wires, chemical fibers, or perforated steel plates. The aperture is generally less than 5mm, the smallest It can be 0.2mm.
Screen filter devices include drum type, rotary type, turntable type, fixed vibrating inclined screen and so on. Regardless of the structure, it must be able to trap dirt, but also facilitate unloading and cleaning the screen surface.

2) Granular media filtration (also known as red, filter, surprise filter): When wastewater passes through the bed of granular filter material (such as quartz sand), the fine suspended matter and limbs are trapped on the surface and internal voids of the filter material. .
Commonly used filter media include quartz sand, anthracite and garnet. In the filtration process, the filter material simultaneously performs physical interception, sedimentation and adsorption of suspended solids. The effect of filtration depends on factors such as the pore size of the filter material, the thickness of the filter material layer, the filtering speed and the nature of the sewage.
When the wastewater flows through the granular filter material layer from top to bottom, the suspended particles with larger diameters are first trapped in the voids of the surface filter material.
The gap of this layer of filter material is getting smaller and smaller, and a layer of filter membrane mainly composed of the trapped group particles is gradually formed, and it plays the main role of filtration. This effect belongs to the role of resistance interception or sieving.
When the wastewater passes through the filter layer, the numerous filter surfaces provide a huge effective area for the sedimentation of suspended solids, forming countless small "sedimentation tanks" where the suspended solids can easily settle down. This effect belongs to gravity settlement.
Because the filter material has a huge surface area, it has obvious physical adsorption between it and the suspended matter. In addition, sand particles often have a negative surface charge in the water, which can adsorb positively charged iron, aluminum and other limbs, thereby forming a positively charged film on the surface of the filter material, and then adsorbing negatively charged colloidal clay and various organic substances, etc. Colloid, contact flocculation occurs on sand grains.

(2) Sedimentation method: Sedimentation method uses the principle of different relative densities of suspended matter in sewage and water, and separates suspended matter from water by means of gravity sedimentation. According to the concentration of suspended particles in the water and the flocculation characteristics (that is, the ability of the other side to agglomerate), it can be divided into four types:
1) Separation and sedimentation (or free sedimentation): In the sedimentation process, the particles do not aggregate with each other, and settle separately. The particle position is only affected by its own gravity and water flow resistance in the water, its shape, size, and mass do not change, and its descending speed does not change.
2) Coagulation sedimentation (or called flocculation sedimentation): Coagulation sedimentation refers to the coagulation of colloids and fine suspended matter in wastewater into separable flocs under the action of coagulants, and then gravity sedimentation Separation and removal. The characteristic of coagulation sedimentation is that during the sedimentation process, particles contact and collide with each other to form larger flocs. Therefore, the size and mass of the particles will increase with the increase of depth, and the sedimentation speed will also increase with the depth.
Commonly used inorganic coagulants are aluminum sulfate, ferrous sulfate, ferric chloride and polyaluminum; commonly used organic flocculants include polypropylene resin, etc., and coagulant aids such as water glass, lime, etc. can also be used.
3) Regional settlement (also known as crowded settlement, stratified settlement): When the content of suspended solids in the wastewater is high, the distance between particles is small, and the cohesive force between them can make it gather into a whole and sink together, and The position of the particles does not change, so there is a clear interface between the clarified water and the mixed water, which gradually moves downwards. This type of settlement is called regional settlement. Sedimentation in sedimentation tanks and secondary sedimentation tanks with increased turbidity water (in the middle and late stages of sedimentation) are mostly of this type.
4) Compression precipitation: When the concentration of suspended solids in the suspension is high, the particles contact and squeeze each other. Under the gravity of the upper particles, the water in the gap between the particles in the lower layer is squeezed out, and the particle group is compressed. Compression sedimentation occurs in the sludge bucket or sludge thickening tank at the bottom of the sedimentation tank, and proceeds very slowly. According to the different nature of suspended substances in the water, there are two equipments: grit tank and sedimentation tank.

Sedimentation tank
The grit tank is used to remove particles from the water with relatively high density such as sand and coal slag. The grit chamber is generally set up in front of the sewage treatment plant to prevent other mechanical equipment for sewage treatment from being worn out.
The sedimentation tank uses gravity to separate suspended impurities from water. It can separate particles with a diameter of 20-100µ, m or more. According to the direction of water flow in the sedimentation tank, it can be divided into three types: advection, radial flow and vertical flow.
①Advection sedimentation tank: Waste water flows from one end of the tank and flows in the tank in a horizontal direction. The suspended solids in the water gradually sink to the bottom of the tank, and the clarified water overflows from the other end.
②The radial flow sedimentation tank: the tanks are mostly circular, with a large diameter, generally above 20-30m, suitable for large-scale water treatment plants. After the raw water enters the central cylinder through the water inlet pipe, it flows radially toward the periphery of the sedimentation tank through the orifice on the cylinder wall and the peripheral annular perforated baffle. As the cross-section of the water continues to increase, the flow rate gradually decreases, the particles settle down, and the clarified water overflows from its surroundings into the sump and is discharged.
③Vertical flow sedimentation tank: Most of the cross-sections are round, but also square and polygonal. The water flows into the pool from the lower port of the central pipe, is distributed around the entire horizontal section through the obstruction of the reflector, and flows slowly upwards. The particles whose settling speed exceeds the rising flow rate sink to the sludge bucket, and the clarified water overflows from the surrounding burrows to the outside of the pool.
Among the methods of sewage treatment and utilization, the sedimentation (or floating) method is often used as a pretreatment before other treatment methods. For example, when using biological treatment method to treat sewage, it generally needs to go through a pre-sedimentation tank to remove most of the suspended matter in advance to reduce the load during biochemical treatment, and the effluent after biological treatment still has to be treated in a secondary sedimentation tank for muddy water Separation to ensure the quality of the effluent.

(3) Flotation method: the air is passed into the sewage and precipitated from the water as a carrier in the form of tiny bubbles. The tiny particulate pollutants (such as emulsified oil, etc.) in the sewage with a relative density close to water are attached to the bubbles. And as the bubbles rise to the surface of the water, they are then removed mechanically, so that the pollutants in the sewage can be separated from the sewage. Hydrophobic materials are easy to float, while hydrophilic materials are not easy to float. Therefore, sometimes in order to improve the efficiency of air flotation, it is necessary to add a flotation agent to the sewage to change the surface characteristics of the pollutants, so that some hydrophilic substances are transformed into hydrophobic substances, and then removed by air flotation. This method is called "flotation" .
Air flotation requires a high degree of dispersion and a large amount of bubbles, which is beneficial to improve the effect of air flotation. The stability of the foam layer should be appropriate to facilitate the stability of the scum on the water surface without affecting the transportation and dehydration of the scum. There are two ways to generate bubbles:
1) Mechanical method: Make air pass through microporous tubes, microporous plates, perforated turntables, etc. to generate tiny bubbles.
2) Pressure-dissolved air method: Dissolve air in water under a certain pressure and reach a saturated state, and then suddenly depressurize, and the supersaturated air escapes from the water in the form of tiny bubbles. At present, the air flotation process in wastewater treatment mostly adopts the pressure dissolved air method.
The main advantages of the air flotation method are: the equipment operating capacity is better than that of the sedimentation tank. Generally, it only takes 15 to 20 minutes to complete the solid-liquid separation, so it occupies less land and has higher efficiency; the sludge produced by the air flotation method is relatively dry, It is not perishable, and the surface is scraped, and the operation is more convenient; the whole work is to pass air into the water, which increases the amount of latent oxygen in the water, and has obvious effects on removing organic matter, algae surfactants and odors in the water. Water quality provides favorable conditions for subsequent treatment and utilization.
The main disadvantages of the air flotation method are: high power consumption; increased equipment maintenance and management workload, and the possibility of blockage in the operation part; scum is exposed to the surface of the water and is easily affected by wind, rain and other climatic factors.
In addition to the above two air flotation methods, the more commonly used method is electrolytic air flotation.

(4) Centrifugal separation method: When the sewage containing suspended pollutants rotates at high speed, the centrifugal force of the suspended particles (such as emulsified oil) and the sewage are different to achieve the purpose of separation. Commonly used centrifugal equipment includes cyclone separator and centrifugal separator.

2. Chemical treatment method

Adding chemical reagents to the sewage, using chemical reactions to separate and recover pollutants in the sewage, or convert pollutants into harmless substances. This method can not only separate pollutants from water, recover some useful substances, but also change the nature of pollutants, such as reducing the pH of wastewater, removing metal ions, and oxidizing certain toxic and harmful substances, etc., so it can be compared to physical methods. Higher degree of purification. Commonly used chemical methods include chemical precipitation, neutralization, redox and coagulation.

The limitations of chemical treatment are as follows:
Since chemical agents (or materials) are often used for chemical treatment of wastewater, the treatment costs are generally high, and the requirements for operation and management are also strict.
The chemical method also needs to be used in conjunction with the physical method. Before chemical treatment, it is often necessary to use precipitation and filtration as pretreatment; in some cases, it is necessary to use precipitation and filtration as the post-treatment of chemical treatment.

(1) Chemical precipitation method
The chemical precipitation method refers to the addition of certain chemical agents to the waste water to cause it to react with the soluble pollutants in the waste water, and form the salt (precipitate) of the difficult ficus in the water to precipitate out of the water, thereby reducing or removing Pollutants in the water. The chemical precipitation method is mostly used to remove calcium ions, mirror ions and heavy metal ions in wastewater, such as Li, pot, lead, bowl, etc. in water treatment. According to the different precipitants used, the precipitation method can be divided into the lime method (also known as the hydroxide precipitation method), the sulfide method and the silver salt method.
The sum of Ca 2+ and Mg2+ content in water is called total hardness, which can be divided into carbonate hardness and non-carbonate hardness. Carbonate hardness can be reduced by adding lime to make Ca 2+ and Mg2+ in the water form CaC03 and Mg(OH)2 precipitation. If the hardness of non-carbonate needs to be removed at the same time, lime-soda softening method can be used to make Ca2+ and Mg2+ The formation of CaCO3 spears 11Mg(OH)2 precipitated and removed. Therefore, when the hardness or alkalinity of the raw water is high, the chemical precipitation method can be used as the pretreatment of ion exchange softening to save the operating cost of ion exchange.
When removing heavy metal ions in waste water, the method of adding carbonate is generally adopted, and the solubility product of the produced metal ions and carbonate is small, which is convenient for recycling. Such as the use of carbonic acid pin to treat wastewater containing pounds.
ZnS04 + Na 2C03 one by one→ZnC03 ↓+ NazS04
The advantages of this method are economy, simplicity, and a wide range of sources of agents, so it is the most widely used in the treatment of heavy metal wastewater. The existing problems are poor labor and sanitation, the pipelines are prone to fouling, clogging and corrosion; the sedimentation volume is large and dehydration is difficult.

(2) Neutralization method
Neutralization treatment is a treatment method that uses the chemical principle of acid-base interaction to generate salt and water to adjust wastewater from acidic or alkaline to near neutral. For wastewater with acid or alkali concentration greater than 3%, acid and alkali recovery should be carried out first. For low-concentration acid-base wastewater, neutralization can be used for treatment.
The treatment of acid sewage usually adopts adding lime, caustic pot, carbonated pot or using limestone and marble as cleaning materials to neutralize the acid sewage. The alkaline sewage treatment usually adopts adding nitric acid, hydrochloric acid or using carbon dioxide gas to neutralize the alkaline sewage. In addition, acid and alkaline sewage can also be treated by neutralizing the two.

(3) Redox method
The oxidation-reduction method is a method of converting toxic and harmful pollutants in sewage into non-toxic or slightly toxic substances through the oxidation-reduction reaction between chemical agents and pollutants in the water. This method mainly deals with inorganic pollutants, such as heavy metals and oxide pollution. The use of strong oxidants such as high-health acid, liquid chlorine, ozone, or the anode reaction of the electrode to oxidize and decompose harmful substances in the wastewater into harmful substances; the use of iron powder and other reducing agents or the cathode reaction of the electrode to remove the harmful substances in the wastewater Reduction to harmless substances; ozone oxidation method to decolorize, sterilize and deodorize sewage; air oxidation method to treat sulfur-containing wastewater; reduction method to treat brocade-containing electroplating wastewater are all examples of redox treatment of wastewater.
Commonly used oxidants for water treatment are oxygen, ozone, chlorine, hypochlorous acid, etc. Commonly used reducing agents are ferrous sulfate, sulfite, iron filings, casting powder and so on.

(4) Coagulation
The coagulation method is to add electrolyte to the wastewater containing fine particles and colloidal particles that are not easy to settle to destroy the stability of the limbs and make them coagulate. Commonly used coagulants are aluminum sulfate, ferrous sulfate, ferric chloride, polyethylene sub-strand or polypropylene resin. To accelerate coagulation, lime, active silica gel, bone glue, etc. are often added as coagulant aids.

3. Physical and chemical treatment
Physical and chemical methods (abbreviated as physical and chemical methods) are the use of physical and chemical principles such as extraction, adsorption, ion exchange, membrane separation technology, and gas stripping to treatOr the method of recycling industrial waste water. It is mainly used to separate inorganic or organic (difficult to biodegradable) dissolved or colloidal pollutants in waste water, recover useful components, and deepen the purification of waste water. Therefore, it is suitable for the treatment of waste water with a high concentration of impurities (used as a method of recycling), or waste water with a very low concentration (used for advanced treatment of wastewater). Before using physical and chemical methods to treat industrial wastewater, it is generally pretreated to reduce suspended solids, oils, harmful gases and other impurities in the wastewater, or to adjust the pH value of the wastewater to improve recycling efficiency and reduce losses. At the same time, the concentrated residue must undergo post-treatment to avoid secondary pollution. Commonly used methods include extraction, adsorption, ion exchange, membrane analysis (including dialysis, electrodialysis, reverse osmosis, ultrafiltration, etc.)

(1) Extraction method
The extraction method is to add an organic solvent that is incompatible with water and has a density less than water into the sewage. After thorough mixing and contact, the pollutants are redistributed and transferred from the water phase to the solvent phase, using the density difference between the solvent and water. The method of separating the solvent to purify the sewage. The boiling point difference between the solute and the solvent is reused to recover the solute vaporizer, and the regenerated solvent can be recycled. The solvent used is called the extractant, and the substance proposed is called the extract. Extraction is a liquid-liquid mass transfer process, which uses the difference in solubility of pollutants (solutes) in water and organic solvents for separation.
When choosing an extractant, pay attention to the selectivity of the extractant to the extracted substance (pollutant), that is, the size of the dissolving power. Generally, the greater the dissolving power, the better the extraction effect; the greater the difference between the density of the extractant and water, The easier it is to separate from water after extraction. Commonly used extractants include oxygen-containing extractants, phosphorus-containing extractants, and nitrogen-containing extractants. Commonly used extraction equipment includes pulse sieve tray, centrifugal extractor, etc.

(2) Adsorption method
The adsorption method uses the surface of a kind of porous solid material (adsorbent) to absorb one or more dissolved pollutants, organic pollutants, etc. (called melt or adsorbate) in the water to recover or remove They enable the wastewater to be purified. For example, the use of activated carbon can absorb highly toxic substances such as hope, trioxide, aluminum, and oxygen in the waste white water, and has the functions of decolorization and deodorization. At present, the adsorption method is mostly used for the advanced treatment of sewage. It can be divided into two methods: static adsorption and dynamic adsorption, that is, adsorption treatment is carried out when the sewage is in a static state and a flow state respectively. Commonly used adsorption equipment includes fixed bed, moving bed and fluid bed.
Commonly used adsorbents in wastewater treatment include activated carbon, sulfonated coal, charcoal, coke, diatomaceous earth, wood chips and adsorption resins. The application of activated carbon and adsorption resin is more common. Generally, adsorbents have a loose porous structure and a huge specific surface area. Its adsorption force can be divided into three types: molecular gravity (van der Waals force), chemical bond force and electrostatic attraction. Most of the adsorption in water treatment is the result of the above three adsorption forces.
After the adsorbent is saturated, it must be regenerated to remove the adsorbate from the pores of the adsorbent and restore its adsorption capacity. The regeneration methods include heating regeneration method, steam stripping method, chemical oxidation regeneration method (wet oxidation, electrolytic oxidation and ozone oxidation, etc.), solvent regeneration method and biological regeneration method.
Because the adsorbent is more expensive, and the adsorption method requires high pretreatment of influent water, it is mostly used in water supply treatment.

(3) Ion exchange method
The ion exchange method is a method that uses the ion exchange effect of ion exchangers to replace ionic pollutants in sewage. With the production of ion exchange resins and the development of ion exchange technology, due to good effects and convenient operation, in recent years, certain applications have been obtained in the recovery and treatment of toxic substances in industrial sewage. For example, cation exchangers are used to remove (recycle) heavy metals such as copper, nickel, cadmium, zinc, mercury, gold, silver, and platinum in sewage.
The ion exchange method is mostly used for softening and desalination in industrial water treatment, mainly to remove metal ions in wastewater. The ion exchange softening method uses Na+ exchange resin.

(4) Membrane analysis method
1) Electrodialysis method. The electro doping method uses the selective permeability of anion and cation exchange membranes to anions and cations in the solution under the action of a direct current electric field (that is, the cation membrane only allows cations to pass, and the anion membrane only allows anion quotients to pass) to make a part of the solution The ions in the solution migrate to another part of the solution, so that the electrolyte in the solution is separated from the water, so as to achieve a water treatment method of concentration, purification, and separation. Electrodialysis is a new method developed on the basis of ion exchange technology. In addition to sewage treatment, it can also be used for seawater desalination and preparation of deionized water (pure water).

2) Reverse osmosis method
The reverse osmosis method has been used in the treatment of heavy metal-containing wastewater, advanced treatment of sewage, and seawater desalination. Today, when the world's fresh water supply crisis is serious, the prospect of seawater desalination technology combining reverse osmosis and steaming methods is broad. Another important use of it is to be used in conjunction with ion exchange systems as a pretreatment method for ion exchange to prepare deionized ultrapure water. In wastewater treatment, reverse osmosis is mainly used to remove and recover heavy metal ions, to remove salt, organic matter, color, and radioactive elements.
At present, semipermeable membranes widely used in the field of water treatment include cellulose acetate membranes and polyurea membranes, sulfonated polyphenylene vinegar and other polymers. Commonly used reverse osmosis devices include tube type, spiral type, hollow fiber type and plate and frame type. The permeated water can be reused.

4. Biological treatment method
Biological treatment method uses the biochemical action of microorganisms in the natural environment to oxidatively decompose organic pollutants and certain inorganic poisons (such as fluoride and sulfide) dissolved in sewage or limbs, and convert them into stable and harmless Inorganic substances, so that waste water can be purified. This method has the advantages of low investment, good effect, low operating cost, etc., and is the most widely used in the treatment of urban wastewater and industrial wastewater.
Modern biological treatment methods are divided into two types: aerobic biological treatment and anaerobic biological treatment according to whether microorganisms need oxygen in the biochemical reaction.

(1) Aerobic biological treatment method
Under aerobic conditions, the process of relying on the biochemical action of aerobic bacteria and facultative bacteria to complete wastewater treatment is called aerobic biological treatment. This method requires aerobic supply. According to the state of aerobic microorganisms in the treatment system, it can be divided into activated sludge method and biofilm method.

1) The activated sludge method is currently the most widely used biological treatment method. This method is to continuously pass air (aeration) into the wastewater rich in organic pollutants and bacteria in the aeration tank. After a certain period of time, suspended flocculent mud particles will appear. This is actually caused by The aerobic bacteria (and facultative aerobic bacteria) adsorbed organic matter and aerobic bacteria metabolic activity of the product composed of aggregates, has a strong ability to decompose organic matter, called "activated sludge". After the mixed liquid of sewage and activated sludge flowing out of the aeration tank is settled and separated in the sedimentation tank, the clarified water is discharged, and the sludge is returned to the aeration tank as seed sludge to continue operation. This biological treatment method with activated sludge as the main body is called the activated sludge method. The wastewater stays in the aeration tank for 4-6 hours, which can remove about 90% of the organic matter in the wastewater. There are many types of activated sludge methods. And the operation mode, usually there are ordinary activated sludge method, completely mixed surface aeration method, adsorption regeneration method and so on.

2) The biofilm method is to make the sewage flow continuously through the solid filler (gravel, coal cinder or plastic filler), and the microorganisms multiply on the filler to form a sludge-like film called biofilm. The biofilm is used to treat sewage. Known as the biofilm method. The biofilm is mainly composed of a large number of bacterial micelles, fungi, algae and protozoa. The microorganisms on the biofilm have the same purification function as the activated sludge, adsorbing and degrading organic pollutants in the water, and the aging biofilm shed from the filler flows into the sedimentation tank with the treated sewage, and after sedimentation and separation in the sedimentation tank, So that the sewage can be purified. Commonly used biofilm methods include biological filter, biological contact oxidation pond, biological turntable and so on.

(2) Anaerobic biological treatment method
Under anaerobic conditions, the method of using anaerobic microorganisms to decompose and decompose organic matter in sewage to purify sewage is called anaerobic biological treatment. In recent years, the world's energy shortage has caused sewage treatment to develop in the direction of energy conservation and realization of energy, thereby promoting the development of anaerobic microbial treatment methods. A large number of high-efficiency new anaerobic bioreactors have appeared one after another, including anaerobic biological filter, upflow anaerobic sludge bed, anaerobic sulfurized bed, etc. Their common feature is that the concentration of biological groups in the reactor is very high, and the age of the sludge is very long, so the treatment capacity is greatly improved, so that the anaerobic biological treatment method has low energy consumption, can recover energy, and has a small amount of remaining sludge. , The generated sludge is stable and easy to handle, and the advantages of high-concentration organic wastewater treatment efficiency are fully reflected. After years of development, anaerobic biological treatment has become one of the main methods of sewage treatment.

5. Phosphorus removal, nitrogen removal
(1) Phosphorus removal: The main sources of phosphorus in urban wastewater are feces, detergents and certain industrial wastewater, which are dissolved in water in the form of orthophosphate, polyphosphate and organic phosphorus. Commonly used methods of phosphorus removal are chemical and biological methods.

1) Chemical removal of phosphorus: the use of phosphate and iron salt, lime, aluminum salt, etc. to generate precipitation of iron phosphate, calcium phosphate, aluminum phosphate, etc., to remove phosphorus from wastewater. The chemical method is characterized by high phosphorus removal efficiency and stable treatment results. The sludge will not be re-released in the process of treatment and disposal to cause secondary pollution, but the output of sludge is relatively large.
Comparison of the advantages and disadvantages of three phosphorus removal processes combining chemical phosphorus removal methods and secondary treatment processes

2) Biological Phosphorus Removal: Biological Phosphorus Removal is the use of microorganisms under aerobic conditions to absorb excessive amounts of soluble phosphate in wastewater, precipitate and separate to remove phosphorus. The whole treatment process is divided into two stages: anaerobic phosphorus release and aerobic phosphorus absorption.
After the waste water containing excessive phosphorus and the activated sludge containing phosphorus enter the anaerobic state, the polyphosphorus quotient in the activated sludge decomposes the polyphosphate accumulated in the body into inorganic phosphorus and releases it back into the wastewater under the anaerobic state. This is the "anaerobic release of phosphorus".

Except for a part of the energy produced by the phosphorous accumulating bacteria for their own survival, the rest is used by the phosphorous accumulating bacteria to absorb the organic matter in the wastewater and convert it into acetic acid under the action of anaerobic fermentation acid-producing bacteria, and then further into PHB (Poly from-short butyric acid) is stored in the body.

After entering the aerobic state, the phosphorus accumulating bacteria decompose the PHB stored in the body aerobicly and release a large amount of energy. One part is used for self-proliferation, and the other part is used for it to absorb the phosphate in the wastewater and accumulate in the body in the form of polyphosphate. . This is "aerobic phosphorus uptake". At this stage, the activated sludge continues to proliferate. Except for a part of the active active sludge containing phosphorous returned to the anaerobic tank, the rest is used as a residual sludge discharge system to achieve the purpose of phosphorus removal.

(2) Denitrification
The proportion of various forms of nitrogen in domestic wastewater is relatively constant: organic nitrogen is 50% to 60%, ammonia nitrogen is 40% to 50%, and nitrogen in nitrite and nitrate accounts for 0 to 5%. They are all derived from protein in people's food. There are two types of denitrification methods: chemical method and biological method.

1) Chemical denitrification: including ammonia absorption method and chlorination method.
①Ammonia absorption method: first adjust the pH value of the wastewater to above 10, and then desorb the ammonia in the desorption tower
②Chlorination method: Chlorine is added to wastewater containing ammonia nitrogen. By appropriately controlling the amount of chlorination, the ammonia nitrogen in the water can be completely removed. In order to reduce the amount of chlorine added, this method is often combined with biological nitrification, first nitrification and then remove traces of residual ammonia nitrogen.

2) Biological denitrification: Biological denitrification is the process of converting organic nitrogen and ammonia nitrogen into nitrogen under the action of microorganisms, including two reaction processes of nitrification and denitrification.
In the nitrification reaction, under aerobic conditions, the ammonia nitrogen in the wastewater is converted into nitrite and nitrate by nitrifying bacteria (nitrite and nitrate bacteria). In the denitrification reaction, under anaerobic conditions, denitrifying bacteria reduce nitrate nitrogen (N03-) and nitrite nitrogen (NH2-) to nitrogen. Therefore, the entire denitrification process needs to go through two stages: aerobic and anoxic.

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