Physical treatment is to separate and recover insoluble and suspended pollutants (including oil film and oil beads) in sewage through physical action, and its chemical properties will not change during the treatment. Commonly used methods are filtration, precipitation and flotation.
(1) filtration method: suspended solids in sewage are intercepted by filter media. The filter media are screens, gauze and particles, and the commonly used filter devices are grids, screens and microfilters.
1) Grid and screen. In the drainage project, when wastewater flows into the water treatment plant through the sewer, it must first pass through a group of metal frames (grids), perforated plates or filter screens (screens) obliquely placed in the channel, so that floating objects or suspended substances cannot pass through and are blocked on the grids, fine nets or filter materials.
This step belongs to the pretreatment of wastewater, and its purpose is to recover useful substances; The initial overflow of wastewater is beneficial to the later treatment and reduces the load of sedimentation tank or other treatment equipment; Protect the pumping machinery from being blocked by particulate matter. Protect pumps and other processing equipment. The effect of grid interception mainly depends on the quality of sewage and the size of grid gap. There are two methods for slag removal: manual and mechanical. Slag should be cleaned and treated in time.
Screen is mainly used to intercept fine suspended impurities, such as fiber, pulp, algae and so on, with particle size ranging from several millimeters to tens of millimeters. Generally, it is woven from metal wire and chemical fiber, or perforated steel plate, and the aperture is generally less than 5mm, and the minimum can reach 0.2 mm Screen filtering devices include rotary drum, rotary, rotary disc and fixed vibrating inclined screen. No matter what kind of structure, it is necessary to intercept the dirt and facilitate the removal and cleaning of the screen surface.
2) filtration of granular media (also called filtration, filtration and surprise filtration). When wastewater passes through a bed of granular filter materials (such as quartz sand), tiny suspended solids and limbs are trapped in the surface and internal gaps of the filter materials. Commonly used filter media are quartz sand, anthracite and garnet. In the process of filtration, the filter material simultaneously carries out physical interception, precipitation and adsorption of suspended solids. The effect of filtration depends on the pore size of filter material, the thickness of filter material layer, the filtration speed and the nature of sewage.
When the wastewater flows through the granular filter material layer from top to bottom, the suspended particles with larger diameter are first trapped in the gaps of the surface filter material, which makes the gaps of this filter material smaller and smaller, and gradually forms a filter membrane mainly composed of trapped particles, which plays a major filtering role. This effect belongs to resistance interception or screening.
When wastewater passes through the filter media layer, many filter media surfaces provide a huge effective area for the sedimentation of suspended matter, forming numerous small "sedimentation tanks" in which suspended matter is easily precipitated. This effect belongs to gravity settlement.
Due to the huge surface area of filter material, it has obvious physical adsorption effect on suspended solids. In addition, sand particles are often negatively charged in water, which can adsorb positively charged limbs such as iron and aluminum, thus forming a positively charged film on the surface of filter material, and then adsorbing negatively charged colloids such as clay and various organic substances, and causing contact flocculation on sand particles.
(2) precipitation method. Sedimentation is to separate suspended matter from water by gravity sedimentation based on the principle that the relative densities of suspended matter and water in sewage are different. According to the concentration and flocculation characteristics of suspended particles in water (that is, the ability to stick together), it can be divided into four types:
1) Separate settlement (or free settlement). In the process of precipitation, the particles do not polymerize with each other, but precipitate separately. The position of particles in water is only affected by their own gravity and water flow resistance, and their shape, size and mass remain unchanged, and their falling speed remains unchanged.
2) Coagulation precipitation (or flocculation precipitation). Coagulation and sedimentation means that colloids and fine suspended solids in wastewater condense into separable flocs under the action of coagulant, and then are separated and removed by gravity sedimentation. The characteristic of coagulation sedimentation is that in the process of sedimentation, particles contact and collide with each other to form larger flocs, so the size and quality of particles will increase with the increase of depth, and its sedimentation speed will also increase with the increase of depth.
Commonly used inorganic coagulants are aluminum sulfate, ferrous sulfate, ferric chloride and polyaluminum; Commonly used organic flocculants include polypropylene gel and so on. Coagulation AIDS, such as water glass and lime, can also be used.
3) Regional settlement (also called crowded settlement and layered settlement). When the content of suspended solids in wastewater is high, the distance between particles is small, and the polymerization force between them can make them gather into a whole and sink together, while the position of particles remains unchanged, so there is an obvious interface between clarified water and mixed water, which gradually moves down. This kind of settlement is called regional settlement. Most of the sedimentation tanks and secondary sedimentation tanks with high turbidity water fall into this category.
4) Pressurized precipitation. When the concentration of suspended solids in the suspension is high, the particles contact each other and squeeze. Under the gravity of the upper particles, the water in the gap between the lower particles is squeezed out and the particle group is compressed. Compressive sedimentation occurs in sludge hopper or sludge concentration tank at the bottom of sedimentation tank, and its progress is very slow. According to the different properties of suspended solids in water, there are two kinds of equipment: grit chamber and sedimentation tank.
The grit chamber is used to remove inorganic particles with high density in water, such as sand and cinder. The grit chamber is generally located in front of the sewage treatment device to prevent abrasion of other machinery and equipment for sewage treatment.
The sedimentation tank uses gravity to separate the suspended impurities in the water. Can it separate the diameter of 20 ~ 100? Particles larger than m can be divided into horizontal flow, radial flow and vertical flow according to the direction of water flow in the sedimentation tank.
① advection sedimentation tank. Wastewater flows out from one end of the pond and flows horizontally in the pond. Suspended substances in the water gradually sink to the bottom of the pond, and clarified water overflows from the other end.
② Radial sedimentation tank. The water tanks are mostly round with large diameter, generally above 20 ~ 30m, and are suitable for large water treatment plants. After the raw water enters the central cylinder through the water inlet pipe, it flows radially to the periphery of the sedimentation tank through the small holes in the cylinder wall and the annular perforated baffle at the periphery. As the cross-section of water is getting larger and larger, the flow velocity gradually decreases, the particles settle, and the clarified water overflows from its surroundings and is discharged into the water collection tank.
③ Vertical flow sedimentation tank. The cross section is mostly round, but there are also squares and polygons. Water flows into the pool from the lower mouth of the central pipe, and is distributed around the whole horizontal section through the baffle, and slowly flows upward. Particles whose settling velocity exceeds the rising velocity will sink into the sludge hopper, and clarified water will overflow from the surrounding buried holes.
Among the methods of sewage treatment and utilization, sedimentation (or air flotation) is often used as pretreatment before other treatment methods. For example, when biological treatment is used to treat sewage, it is generally necessary to remove most of the suspended solids in advance through the pre-sedimentation tank to reduce the load of biochemical treatment, while the effluent after biological treatment still needs to be separated from mud through the secondary sedimentation tank to ensure the effluent quality.
(3) Flotation method. Air is introduced into sewage and separated from water as a carrier in the form of tiny bubbles. Micro-particle pollutants (such as emulsified oil) with relative density close to water in sewage attach to bubbles, rise to the water surface with the bubbles, and then skim them off by mechanical means, so that pollutants in sewage can be separated from sewage. Hydrophobic substances are easy to float, while hydrophilic substances are not easy to float. Therefore, in order to improve the efficiency of air flotation, it is sometimes necessary to add flotation agent to sewage to change the surface characteristics of pollutants, so that some hydrophilic substances can be 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 number of bubbles, which is beneficial to improve the air flotation effect. The stability of foam layer should be appropriate, which not only facilitates the stability of scum on the water surface, but also does not affect the transportation and dehydration of scum. There are two ways to generate bubbles:
1) mechanical method. Make air pass through microporous tubes, microplates, perforated turntables, etc. Produce tiny bubbles.
2) pressure dissolved gas method. Dissolve air in water under a certain pressure and reach saturation, then suddenly decompress, and the supersaturated air will escape from the water in the form of tiny bubbles. At present, the air flotation process in sewage treatment mostly adopts pressure dissolved air method.
The main advantages of air flotation are: the running capacity of equipment is better than that of sedimentation tank, and solid-liquid separation can be completed in 15 ~ 20min generally, with less land occupation and higher efficiency; The sludge produced by air flotation is dry, not easy to rot, scraped on the surface and easy to operate; The whole work is to introduce air into the water, which increases the dissolved oxygen in the water and has obvious effect on removing organic matter, algae surfactant and odor in the water. The effluent quality provides favorable conditions for subsequent treatment and utilization.
The main disadvantages of air flotation are: high power consumption; The workload of equipment maintenance and management increases, and the running part is often blocked; Scum is exposed to water and is easily affected by weather factors such as wind and rain.
In addition to the above two air flotation methods, electrolytic air flotation is commonly used at present.
(4) centrifugal separation. When sewage containing suspended pollutants rotates at high speed, the centrifugal force of suspended particles (such as emulsified oil) is different from that of sewage, thus achieving the purpose of separation. Commonly used centrifugal equipment includes cyclone separator and centrifugal separator.
2. Chemical treatment methods
Chemical reagents are added to the sewage, and the pollutants in the sewage are separated and recovered through chemical reaction, or the pollutants are converted into harmless substances. This method can not only separate pollutants from water and recover some useful substances, but also change the properties of pollutants, such as lowering the pH value of wastewater, removing metal ions and oxidizing some toxic and harmful substances, so it can achieve a higher degree of purification than physical methods. The commonly used chemical methods include chemical precipitation, neutralization, redox and coagulation.
The limitations of chemical treatment are as follows:
Because chemical agents (or materials) are often used in the chemical treatment of wastewater, the treatment cost is generally high and the requirements for operation and management are strict.
Chemical methods should also be combined with physical methods. Before chemical treatment, precipitation and filtration are usually used as pretreatment; In some cases, physical means such as precipitation and filtration are needed as post-treatment of chemical treatment.
(1) chemical precipitation method.
Chemical precipitation method refers to adding some chemicals into wastewater to react with pollutants dissolved in wastewater to form insoluble salts (precipitates) to precipitate out of water, thus reducing or removing pollutants in water. Chemical precipitation method is often used to remove calcium ions, mirror ions and heavy metal ions in wastewater, such as plum, pot, lead and bowl. According to the different precipitants used, precipitation methods can be divided into lime method (also known as hydroxide precipitation method), sulfide method and silver salt method.
The total content of Ca 2+ and Mg2+ in water is called total hardness, which can be divided into carbonate hardness and noncarbonate hardness. Adding lime can precipitate CaC03 and Mg (OH) 2 from Ca 2+ and Mg2+ in water, which can reduce the hardness of carbonate. If it is necessary to remove non-carbonate hardness at the same time, lime-soda softening method can be used to make Ca 2+ and Mg2+ form CaC03 spear llMg (OH) 2 precipitation and remove it. Therefore, when the hardness or alkalinity of raw water is high, chemical precipitation can be used as the pretreatment of ion exchange softening to save the operating cost of ion exchange.
When removing heavy metal ions from wastewater, the method of adding carbonate is generally adopted, and the solubility product of the generated metal ions and carbonate is very small, which is easy to recover. For example, sodium bicarbonate is used to treat pounds of wastewater.
ZnS 04+Na 2c 03→znc 03↓+naz 04。
This method has the advantages of economy, simplicity and wide sources of chemicals, and is the most widely used method for treating heavy metal wastewater at present. The existing problems are poor labor hygiene conditions and easy scaling, blockage and corrosion of pipelines; The precipitation is large, and dehydration is difficult.
(2) neutralization.
Neutralization treatment is a treatment method to adjust wastewater from acidic or alkaline to neutral by using the chemical principle that acid-base interaction generates salt and water. For wastewater with acid or alkali concentration greater than 3%, acid-base recovery should be carried out first. For low-concentration acid-base wastewater, neutralization method can be used for treatment.
Acidic sewage is usually treated by adding lime, caustic boiler and carbonic acid boiler or using limestone and marble as cleaning agents and materials to neutralize acidic sewage. Alkaline sewage is usually treated by adding nitric acid and hydrochloric acid or neutralizing alkaline sewage with carbon dioxide gas. In addition, acidic and alkaline sewage can also be treated by mutual neutralization.
(3) redox method.
Redox method is a method to convert toxic and harmful pollutants in sewage into non-toxic or slightly toxic substances through the redox reaction between chemicals and pollutants in water. This method mainly deals with inorganic pollutants such as heavy metals and oxides. Using strong oxidants such as strong acid, liquid chlorine and ozone or anode reaction of electrodes, the harmful substances in wastewater are oxidized and decomposed into partial harmful substances; The harmful substances in wastewater are reduced to harmless substances by using reducing agents such as iron powder or cathode reaction of electrodes; Ozone oxidation is used for decolorization, sterilization and deodorization of sewage; Treatment of sulfur-containing wastewater by air oxidation: Reduction treatment of gold-containing electroplating wastewater is an example of oxidation-reduction treatment of wastewater.
Commonly used oxidants for water treatment are oxygen, ozone, chlorine and hypochlorous acid. Commonly used reducing agents are ferrous sulfate, sulfite, iron filings, casting powder, etc.
(4) coagulation.
Coagulation method is to add electrolyte into wastewater containing fine particles and colloidal particles that are not easy to settle, so as to destroy the stability of limbs and make them agglomerate. Commonly used coagulants include aluminum sulfate, ferrous sulfate, ferric chloride, polyethylene wire or polypropylene gel. In order to accelerate coagulation, coagulant AIDS such as lime, activated silica gel and bone glue are often added.
3. Physical and chemical treatment methods
Physico-chemical method (physico-chemical method for short) is a method to treat or recover industrial wastewater by using physical and chemical principles such as extraction, adsorption, ion exchange, membrane separation technology and gas stripping. It mainly separates inorganic or organic (difficult to biodegrade) dissolved or colloidal pollutants in wastewater, recovers useful components, and deeply purifies wastewater. Therefore, it is suitable for treating wastewater with high impurity concentration (as a recycling method) or wastewater with low concentration (as advanced treatment of wastewater). Before using physical and chemical methods to treat industrial wastewater, pretreatment is generally needed to reduce impurities such as suspended solids, oils and harmful gases in wastewater, or to adjust the pH value of wastewater to improve recovery efficiency and reduce losses. At the same time, the concentrated residue is post-treated to avoid secondary pollution. Commonly used methods include extraction, adsorption, ion exchange and membrane separation (including dialysis, electrodialysis, reverse osmosis and ultrafiltration).
(1) extraction method.
Extraction is a method of adding water-insoluble organic solvent with lower density to sewage, fully mixing and contacting, redistributing pollutants, transferring them from water phase to solvent phase, and separating solvents by using the density difference between solvent and water, thus purifying sewage. Then the solute is recovered by using the boiling point difference between the solute and the solvent, and the regenerated solvent can be recycled. The solvent used is called extractant, and the substance proposed is called extract. Extraction is a liquid-liquid mass transfer process, which uses the solubility difference of pollutants (solutes) in water and organic solvents for separation.
When selecting extractant, we should pay attention to the selectivity of extractant to the extracted substance (pollutant), that is, solubility. Generally, the greater the solubility, the better the extraction effect. The greater the density difference between extractant and water, the easier it is to separate from water after extraction. Commonly used extractants include oxygen-containing extractant, phosphorus-containing extractant and nitrogen-containing extractant. Commonly used extraction equipment includes pulsed sieve plate tower, centrifugal extractor and so on.
(2) Adsorption method.
Adsorption method uses the surface of porous solid materials (adsorbents) to adsorb one or more dissolved pollutants and organic pollutants. (called dissolved substances or adsorbents) in water, so as to recover or remove them and purify wastewater. For example, activated carbon can adsorb chlorine, chlorine, oxygen and other toxic substances in waste white water, and has the functions of decoloration and deodorization. At present, the advanced treatment of sewage mostly adopts adsorption method, which can be divided into static adsorption and dynamic adsorption, that is, adsorption treatment is carried out when the sewage is static and dynamic respectively. Commonly used adsorption equipment includes fixed bed, moving bed and flowing bed.
Commonly used adsorbents in wastewater treatment include activated carbon, sulfonated coal, charcoal, coke, diatomite, sawdust and adsorption resin. Activated carbon and adsorption resin are widely used. Generally, adsorbents are porous structures with huge specific surface area. Its adsorption force can be divided into three types: molecular attraction (van der Waals force), chemical bond force and electrostatic attraction. Most of the adsorption in water treatment is the result of the joint action of the above three adsorption forces.
After the adsorbent is saturated, it must be regenerated to remove adsorbate from the pores of the adsorbent and restore its adsorption capacity. Regeneration methods include heating regeneration, stripping and chemical oxidation regeneration (wet oxidation, electrolytic oxidation and ozone oxidation, etc.). ), solvent regeneration and biological regeneration.
Because the adsorbent is expensive and the adsorption method requires high pretreatment of influent, it is often used in feed water treatment.
(3) ion exchange method.
Ion exchange method is a method to replace ionic pollutants in sewage by ion exchange of ion exchanger. With the production of ion exchange resin and the development of ion exchange technology, it has been applied to the recovery and treatment of toxic substances in industrial sewage in recent years because of its good effect and convenient operation. For example, cation exchangers are used to remove (recycle) heavy metals such as copper, nickel, cadmium, zinc, mercury, gold, silver and platinum from sewage.
Ion exchange method is mostly used for softening and desalting in industrial water treatment, mainly to remove metal ions from wastewater. Sodium ion exchange resin is used in ion exchange softening method.
(4) Membrane separation method.
1) electrodialysis Electro-doping method is a water treatment method, which makes use of the selective permeability of anion exchange membrane to anion in solution under the action of DC electric field (that is, cation membrane only allows cation to pass through, anion membrane only allows anion quotient to pass through), so that some ions in the solution migrate to another part of the solution, and the electrolyte in the solution is separated from water, thus achieving the purposes of concentration, purification and separation. Electrodialysis is a new method developed on the basis of ion exchange technology, which can be used not only for sewage treatment, but also for seawater desalination and preparation of deionized water (pure water).
2) Reverse osmosis.
Reverse osmosis has been used in the treatment of wastewater containing heavy metals, advanced treatment of sewage and seawater desalination. Today, when the world's fresh water supply crisis is serious, the seawater desalination technology combining reverse osmosis and steam method has broad prospects. Another important use of ion exchange system is to prepare deionized ultra-pure water as a pretreatment method of ion exchange. In wastewater treatment, reverse osmosis is mainly used to remove and recover heavy metal ions and remove salts, organic matter, chromaticity and radioactive elements.
At present, semi-permeable membranes widely used in the field of water treatment include cellulose acetate membrane and poly-cool membrane sulfonated polystyrene vinegar and other polymers. Commonly used reverse osmosis devices are tubular, spiral, hollow fiber and plate-frame type. Permeable water can be reused.
4, biological treatment method
Biological treatment is a method of purifying wastewater by using the biochemical action of microorganisms in the natural environment to oxidize and decompose organic pollutants and some inorganic poisons (such as fluoride and sulfide) dissolved in sewage or limbs into stable and harmless inorganic substances. This method has the advantages of less investment, good effect and low operating cost, and is widely used in the treatment of urban sewage and industrial wastewater.
Modern biological treatment methods are divided into aerobic biological treatment and anaerobic biological treatment according to whether microorganisms need oxygen in biochemical reactions.
(1) aerobic biological treatment.
Under aerobic conditions, the process of wastewater treatment depends on the biochemical action of aerobic bacteria and facultative bacteria, which 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.
Activated sludge process is the most widely used biological treatment method at present. In this method, air is continuously introduced into the wastewater rich in organic pollutants and bacteria in the aeration tank, and after a certain period of time, suspended flocculent sludge particles will appear, which are actually an aggregate of organic substances adsorbed by aerobic bacteria (and facultative aerobic bacteria) and aerobic bacteria metabolites, and have strong ability to decompose organic substances, which is called "activated sludge". The mixed liquid of sewage and activated sludge flowing out of the aeration tank is precipitated and separated by the sedimentation tank, and then the clear water is discharged, and the sludge flows back to the aeration tank as seed sludge to continue running. This biological treatment method with activated sludge as the main body is called activated sludge method. "When the wastewater stays in the aeration tank for 4 ~ 6h, the organic matter (BOD6) in the wastewater can be removed by about 90%. There are many types of activated sludge process and its operation modes, such as ordinary activated sludge process, fully mixed surface aeration process, adsorption regeneration process and so on.
2) Biofilm method is a method in which sewage continuously flows through solid filler (gravel, cinder or plastic filler), and microorganisms multiply on the filler to form an attached film similar to sludge, which is called biofilm method, and sewage is treated by biofilm method. Biofilm is mainly composed of a large number of bacterial micelles, fungi, algae and protozoa. Microorganisms on biofilm have the same purification effect as activated sludge, absorbing and degrading organic pollutants in water. The aged biofilm falling off the filler flows into the sedimentation tank with the treated sewage, and the sewage is purified after precipitation and separation in the sedimentation tank. Commonly used biofilm methods include biological filter, biological contact oxidation tank, biological turntable and so on.
(2) Anaerobic biological treatment.
Under anaerobic conditions, the method of decomposing and purifying sewage by anaerobic microorganisms is called anaerobic biological treatment. In recent years, the worldwide energy shortage makes sewage treatment develop in the direction of energy saving and energy utilization, thus promoting the development of anaerobic microbial treatment methods. A large number of high-efficiency new anaerobic bioreactors have appeared one after another, including anaerobic biofilter, upflow anaerobic sludge bed, anaerobic sulfide bed and so on. Their common characteristics are that the concentration of biological groups in the reactor is very high and the municipal sludge is very old, so the treatment capacity is greatly improved, which fully embodies the advantages of anaerobic biological treatment, such as low energy consumption, energy recovery, less excess sludge, stable and easy treatment of generated sludge and high treatment efficiency of high-concentration organic wastewater. After years of development, anaerobic biological treatment has become one of the main methods of sewage treatment.
5. Phosphorus and nitrogen removal
(1) Phosphorus removal. The main sources of phosphorus in urban sewage are feces, detergents and some industrial wastewater, which are dissolved in water in the form of orthophosphate, polyphosphate and organophosphorus. The commonly used methods of phosphorus removal are chemical method and biological method.
1) chemical dephosphorization. Phosphate reacts with iron salt, lime, aluminum salt, etc. Precipitates such as iron phosphate, calcium phosphate and aluminum phosphate are generated, and phosphorus is removed from wastewater. Chemical method has the characteristics of high phosphorus removal efficiency and stable treatment results. In the process of sludge treatment and disposal, phosphorus will not be released again to cause secondary pollution, but the sludge output is relatively large.
2) Biological phosphorus removal. Biological phosphorus removal is to remove phosphorus through excessive absorption, precipitation and separation of dissolved phosphate in wastewater by microorganisms under aerobic conditions. The whole treatment process is divided into two stages: anaerobic phosphorus release and aerobic phosphorus absorption.
After the wastewater containing excessive phosphorus and activated sludge containing phosphorus enter anaerobic state, polyphosphate in activated sludge decomposes accumulated polyphosphate into inorganic phosphorus, which is released back into wastewater in anaerobic state. This is "anaerobic phosphorus release". Part of the energy generated by phosphorus-accumulating bacteria when decomposing phosphorus is for their own survival, and the rest is for phosphorus-accumulating bacteria to absorb the organic matter in wastewater, which is converted back to acetic acid under the action of anaerobic fermentation acid-producing bacteria, and then further converted into PHB (poly-short alkali butyric acid) for storage in the body.
After the phosphate accumulating bacteria enter the aerobic state, the PHB stored in the body is decomposed aerobically, and a large amount of energy is released, one part of which is for self-proliferation, and the other part is for self-absorption of phosphate in wastewater, which is accumulated in the body in the form of polyphosphate. This is "aerobic phosphorus absorption". At this stage, activated sludge proliferates continuously. Except a part of phosphorus-containing activated sludge flows back to the anaerobic tank, the rest is used as the residual sludge discharge system to achieve the purpose of phosphorus removal.
(2) Denitrification.
The proportion of various forms of nitrogen in domestic sewage is relatively constant: organic nitrogen accounts for 50%~60%, ammonia nitrogen accounts for 40% ~ 50%, and nitrogen in nitrite and nitrate accounts for 0 ~ 5%. All come from protein in people's food. There are two methods to remove nitrogen: chemical method and biological method.
1) chemical denitrification. Include ammonia absorption method and chlorination method.
① Ammonia absorption method. Firstly, the pH value of wastewater is adjusted to above 10, and then ammonia is desorbed in the desorption tower.
② Chlorination method. Chlorine is added to wastewater containing ammonia nitrogen. Ammonia nitrogen in water can be completely removed by properly controlling the dosage of chlorine. In order to reduce the amount of chlorine, this method is often combined with biological nitrification, that is, nitrification first, and then removal of trace residual ammonia nitrogen.
2) Biological denitrification. Biological nitrogen removal is a process of transforming organic nitrogen and ammonia nitrogen into nitrogen under the action of microorganisms, including nitrification and denitrification.
Nitrification reaction is that ammonia nitrogen in wastewater is transformed into nitrite and nitrate by nitrifying bacteria (nitrite bacteria and nitrate bacteria) under aerobic conditions. Denitrification means that nitrate nitrogen (NO3-) and nitrite nitrogen (NH2-) are reduced to nitrogen by denitrifying bacteria under anaerobic conditions. Therefore, the whole denitrification process needs to go through two stages: aerobic and anoxic.