1. What is the form of nitrogen in water? What are the influencing factors?

Natural nitrogen reserves are abundant and exist in three forms: molecular nitrogen N2, which accounts for 78% of the atmosphere; organic nitrogen compounds; inorganic nitrogen compounds. Among them, the nitrogen in the water body mainly includes organic nitrogen and inorganic nitrogen, and the total amount is called total nitrogen (abbreviated as TN in English).

Organic nitrogen refers to nitrogen in the form of organic compounds, such as proteins, amino acids, peptides, urea, organic amines, nitro compounds, and diazonium compounds. The organic nitrogen present in agricultural wastes and municipal sewage is mainly protein and its decomposition products—polypeptides and amino acids. However, some industrial wastewater may contain other nitrogen-containing organic compounds. Inorganic nitrogen refers to ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen. Some of them are produced by the decomposition and transformation of organic nitrogen by microorganisms, and some are directly derived from the application of chemical fertilizers and industrial drainage.

Nitrogen will be transformed in water bodies. As time goes by, organic nitrogen is very unstable and is easily decomposed into inorganic nitrogen under the action of microorganisms (under anaerobic conditions, it is decomposed into ammonia nitrogen; under aerobic conditions, it is first decomposed into ammonia nitrogen and then decomposed It is nitrite nitrogen and nitrate nitrogen), and keep decreasing.

Ammonia nitrogen exists in sewage in the form of free ammonia (NH3) and ionic ammonium salt (NH4+). The concentration of free ammonia mainly depends on the concentration of ammonia nitrogen, but also increases with the pH and temperature of the water. And increase. In addition, ionic strength also has an effect on the concentration of free ammonia.

Nitrate in water is a decomposition product of nitrogen-containing organic matter in the final stage of inorganicization. Nitrate can be reduced to nitrite under anoxic and acidic conditions. Nitrite nitrogen is an intermediate product of the nitrogen cycle and is unstable. Depending on the water environment, it can be oxidized to nitrate nitrogen or reduced to nitrogen.

2. What is Kjeldahl nitrogen?

Kjeldahl nitrogen is the sum of organic nitrogen and ammonia nitrogen. The Kjeldahl nitrogen index can be used as a basis for judging whether nitrogen nutrition is sufficient when sewage is treated by biological methods. The Kjeldahl nitrogen content in domestic sewage is about 40mg/L (of which organic nitrogen is about 15mg/L, ammonia nitrogen is about 25mg/L), the difference between total nitrogen and Kjeldahl nitrogen is approximately equal to the sum of nitrite nitrogen and nitrate nitrogen; Kjeldahl The difference between Nitrogen nitrogen and ammonia nitrogen is approximately equal to organic nitrogen.

3. What is the harm of nitrogen?

Sewage from domestic sewage, chemical fertilizers, food and other industries, and farmland drainage all contain a lot of nitrogen. After natural water bodies receive these wastewater, eutrophication of the water bodies will occur. Water eutrophication means that under the influence of human activities, nutrients such as nitrogen and phosphorus required by organisms enter slow-flowing water bodies such as lakes, estuaries, bays, etc., causing rapid growth of algae and other plankton, and a decrease in dissolved oxygen in the water. Deterioration of water quality and massive death of fish and other organisms. Under natural conditions, lakes will also transition from oligotrophic state to eutrophic state, but this natural process is very slow. The eutrophication of water bodies caused by the artificial discharge of nutrient-containing industrial wastewater and domestic sewage can appear in a short time. When eutrophication occurs in the water body, planktonic algae multiply and form blooms. Due to the different colors of the dominant planktonic algae, the water surface often appears blue, red, brown, and milky white. This phenomenon is called red tide or red tide in the ocean.

Nitrate in water is a decomposition product of nitrogen-containing organic matter in the final stage of inorganicization. After the human body ingests nitrate, it is transformed into nitrite by the action of microorganisms in the intestinal tract, resulting in toxic effects. Nitrite can oxidize the normal hemoglobin of the human body to methemoglobin, causing methemoglobinemia, losing its ability to transport oxygen, leading to tissue hypoxia.

When sewage is subjected to biological treatment, ammonia nitrogen not only provides nutrients for microorganisms, but also buffers the pH value of sewage. But when the ammonia helium is too high, especially when the free ammonia concentration is high, it has an inhibitory effect on the life activities of microorganisms.

4. What is the source of nitrogen? What is the form of ammonia?

The nitrogen in sewage comes from chemical fertilizers and agricultural waste on the one hand. On the other hand, it comes from urban domestic sewage and certain industrial wastewater. Urban domestic sewage is rich in nitrogen, and manure is the main source of nitrogen in domestic sewage. The main sources of ammonia nitrogen are industrial wastewater such as tannery wastewater and pickling wastewater. The effluent of certain biochemical treatment facilities and farmland drainage may contain a large amount of nitrate nitrogen.

5. How is nitrogen converted?

The conversion of nitrogen-containing compounds in water bodies can be divided into three stages: the first stage is that nitrogen-containing organic matter is gradually decomposed into simpler compounds by microorganisms in the water body, and finally inorganic ammonia nitrogen is generated, which is called the ammoniating process; The second stage is the conversion of ammonia nitrogen into nitrite and nitrate under aerobic conditions, which is called the nitrification process; the third stage is the reduction and conversion of nitrite and nitrate by denitrifying bacteria under hypoxic or anaerobic conditions It is nitrogen, which is called denitrification process. Ammonification can be carried out under aerobic or anaerobic conditions, while nitrification can only be carried out under aerobic conditions. If the water body is hypoxic, the nitrification reaction cannot proceed.

6. What is the concept of nitrification?

The traditional biological denitrification theory believes that ammonia nitrogen is removed by converting the ammonia nitrogen in the water into nitrogen by means of two different types of bacteria (nitrifying bacteria and denitrifying bacteria). First, under aerobic conditions, nitrite bacteria use oxygen as an electron acceptor to convert ammonia nitrogen into nitrite, and then nitrite bacteria convert nitrite into nitrate. This reaction is called nitrification.

7. What is the concept of denitrification?

After the nitrification reaction is completed, denitrifying bacteria use various organic substrates as electron donors, and use nitrate or nitrite as electron acceptors to perform hypoxic respiration and convert nitrate or nitrite into nitrogen. This The process is called denitrification.

8 What are the commonly used biological denitrification processes?

(1) Traditional denitrification process

The traditional activated sludge denitrification process initiated by Barth is a three-stage activated sludge process, which is established on the basis of the process of ammonification, nitrification, denitrification, and biochemical reactions.

Traditional activated sludge denitrification process (three-stage activated sludge method)

The process flow will remove BOD5 and ammoniation, nitrification and decontamination in three reaction tanks, and each has its own independent sludge return system. The first-stage aeration tank is a general two-stage aeration tank. Its main function is to remove BOD and COD and convert organic nitrogen into NH3-N, which is to complete the oxidation of organic carbon and the ammoniating of organic nitrogen. After the mixed liquid of the first-stage aeration tank is precipitated, the effluent enters the second-stage aeration tank, called nitrification aeration tank. The BOD5 value of the sewage that enters the tank has dropped to a low of 15-20mg/L Level, the nitrification reaction is carried out in the nitrification aeration tank to oxidize NH3-N to NO3-N, while the organic matter is further decomposed, and the BOD5 in the sewage is further reduced. The nitrification reaction consumes alkalinity, so alkali needs to be added to prevent the pH value from dropping. The mixed liquid of the nitrification aeration tank enters the sedimentation tank, and the effluent after precipitation enters the third-stage activated sludge system, which is called the denitrification reaction tank. Under anoxic conditions, NO3--N is reduced to gaseous N2 and discharged into the atmosphere. Because the BOD5 value in the sewage entering this stage is very low, in order to make the denitrification reaction proceed normally, it is necessary to add methanol as an external carbon source, but in order to save operating costs, raw sewage can also be introduced as a carbon source.

Behind this system, in order to remove the BOD value caused by the addition of methanol, a post-aeration tank can be set up to discharge the water body after treatment.

The advantage of this system is that organic matter degrading bacteria, nitrifying bacteria, and denitrifying bacteria grow and proliferate in their respective reactors, and the environmental conditions are suitable, and they have their own sludge return system. The removal of BOD and nitrification are fast, and More thorough. However, there are also shortcomings such as a large number of processing equipment, high cost, high processing cost, and inconvenient management.

In order to reduce the treatment equipment, the first-stage aeration tank and the second-stage nitrification aeration tank for BOD removal in the three-stage activated sludge denitrification process can be combined to combine BOD removal and nitrification. The reaction process is carried out in a unified reactor, so a two-stage biological denitrification system is produced.

Two-stage biological nitrogen removal system process

Although the two-stage biological denitrification traditional process has been improved, it still has the disadvantages of more processing equipment, inconvenient management, high cost, and high processing cost. Therefore, the above-mentioned traditional biological deoxygenation process has been rarely applied.

2. A/O process

In order to overcome the shortcomings of the traditional biological denitrification process, based on the principle of biological denitrification, an anoxic/aerobic activated sludge denitrification system (A/O) was created in the early 1980s, as shown in Figure 1. Shown.

The biological denitrification process places the denitrification reactor before the system, so it is also called the pre-denitrification biological denitrification system. In the denitrification anoxic tank, the denitrifying bacteria in the return sludge use the organic matter in the raw sewage as a carbon source to reduce the large amount of nitrate nitrogen in the return mixture to N2 to achieve the purpose of denitrification. Then biochemical reactions such as biological oxidation of organic matter, ammoniation of organic nitrogen and nitrification of ammonia nitrogen are carried out in the subsequent aerobic tank.

A/O process has the following advantages:

1) The process is simple, the structure is few, there is only one sludge return system and mixed liquid return system, and the capital construction cost can be greatly saved.

2) No external carbon source is required for the denitrification tank, which reduces operating costs.

3) After the anoxic tank of the A/0 process, the aerobic tank can further remove the residual organic pollutants from denitrification and improve the effluent quality.

4) Before the anoxic tank, the organic carbon in the sewage is used by denitrifying bacteria, which can reduce the organic load of the subsequent aerobic tank. At the same time, the alkalinity generated by the denitrification reaction in the anoxic tank can compensate for about half of the alkalinity requirement for the nitrification reaction in the aerobic tank.

The main disadvantage of the A/0 process is that the denitrification efficiency is not high, generally 70% to 80%. In addition, if the sedimentation tank is not operated properly, a denitrification reaction will occur in the sedimentation tank, causing the sludge to float up and deteriorating the quality of the treated water. In spite of this, the A/O process is still valued for its outstanding characteristics, and this process is currently a relatively widely used denitrification process. In this process, an anoxic tank and an aerobic tank can be built into a combined aeration tank, with baffles in the middle, anoxic denitrification in the front section, and aerobic nitrification in the back section. This form is particularly convenient to modify the existing push-flow aeration tank.

9. What is the concept and principle of short-cut nitrification and denitrification?

Short-cut nitrification and denitrification is to control the nitrification process at the NO2- stage, prevent further oxidation to NO3-, and directly use it as the final electron acceptor for denitrification.

Compared with the traditional biological denitrification process, the short-cut nitrification and denitrification biological denitrification process can save about 25% of the oxygen supply, save about 40% of the carbon source required for denitrification, reduce the amount of sludge generated, and reduce the nitrification process The amount of alkali dosing shortens the reaction time and correspondingly reduces the reactor volume by 30% to 40%.

10. What is the concept and principle of simultaneous nitrification and denitrification?

Traditional denitrification theory believes that denitrification requires two different processes, nitrification and denitrification. Denitrification is a hetero-oxygen facultative anaerobe. Only when there is no molecular oxygen and nitric acid and nitrite ions are present at the same time, they can use the oxygen in these ions to breathe and reduce nitrate. However, studies in recent years have shown that nitrification and denitrification can occur simultaneously in the same reactor, and total nitrogen loss is often found in many aerobic nitrification tanks in actual operation. This phenomenon is called simultaneous nitrification and denitrification ( SND). Simultaneous nitrification and denitrification have the advantages of reducing carbon sources and saving aeration. Currently, simultaneous nitrification and denitrification are rarely used in engineering, and they are basically in the laboratory research stage.

11. What is the concept and principle of anammox?

Anammox (Anammox) is the biological reaction process in which anammox bacteria use nitrite as electron acceptor to oxidize ammonia nitrogen to nitrogen under anaerobic conditions. The anammox reaction is a chemical autotrophic archaea (Anammox) reaction. The archaea are autotrophic, require only inorganic carbon sources, and play an important role in the global carbon cycle. It is widely optimistic about the current ammonia nitrogen treatment of sewage. However, since the nitrite content is not significant enough in most sewage, this technology should be used in combination with other technologies.

12. How to remove nitrogen by blowing off method?

Ammonia nitrogen in wastewater usually exists in a balanced state between ammonium ions and free nitrogen. When the pH is neutral, the ammonia nitrogen mainly exists in the form of ammonium ions. When the pH is alkaline, the ammonia nitrogen mainly exists in the state of free ammonia.

Blowing off is to adjust the pH of the wastewater to alkaline, and then blow off the free ammonia in the wastewater to the atmosphere through gas-liquid contact. When using the stripping method to treat ammonia nitrogen, it is necessary to consider that the total amount of free ammonia discharged should meet the atmospheric emission standard of ammonia, or the gas phase ammonia should be subjected to catalytic oxidation treatment. So as not to cause secondary pollution.

13. What is the principle of nitrogen removal by chemical precipitation?

Dosing wastewater containing Mg2+ and PO3-4 and chemicals into the wastewater containing ammonia and helium to form a composite salt MgNH4PO4 (struvite) with the wastewater, thereby removing ammonia nitrogen from the wastewater. This method can simultaneously treat ammonia nitrogen, phosphorus and magnesium-containing wastewater. The general formula of the chemical reaction is

The reaction formula shows that the formation of MgNH4PO4 is closely related to the ratio of NH4+, Mg2+, and PO34- ions, and when [NH4+] [Mg2+] [PO34-] is greater than the concentration product Ksp, the reaction proceeds to the right, the ammonia nitrogen in the solution Can be removed. The opposite is not true. At the same time, other reactions also exist. The appropriate pH value should be between 9-11. Because H3PO4 is mainly dissociated into H+ and HPO24-that is, at this time, Mg2+ and H3PO4 mainly produce MgHPO4. This is the most favorable pH range for ammonia removal. In an acidic environment, Mg(H2PO4) 2 is mainly produced, which is not conducive to the production of MgNH4PO4, and it is not conducive to the removal of ammonia nitrogen. Under strong alkaline conditions, Mg(H3PO4) 2 is formed, and the concentration product is the smallest, only 9.8X10-25. At this time, there is almost no Mg2+ and PO3-4 in the solution, which is the most unfavorable for the reaction. conduct.

14. What is the principle of breaking point chlorination to remove ammonia? What is the actual application effect?

The breakpoint chlorination method is a chemical denitrification process in which chlorine or sodium hypochlorite is passed into the wastewater to oxidize the ammonia nitrogen in the wastewater to nitrogen. When the chlorine gas in the wastewater reaches a certain point, the free chlorine content in the water is the lowest, and the ammonia concentration drops to zero. When the amount of chlorine inflow exceeds this point, the free chlorine in the water will increase, so this point is called the breaking point, and the chlorination in this state is called breaking point chlorination. There are many influencing factors for the actual ammonia nitrogen wastewater treatment effect, which mainly depend on temperature, pH value and ammonia nitrogen concentration. The optimal reaction conditions are pH 6-7 and contact time 0.5-2h.

The reaction equation of the mechanism of ammonia removal by the breaking point chlorination method is:

The main advantage of the breaking point chlorination ammonia removal method is that it can reduce the total ammonia nitrogen in the wastewater to zero by correctly controlling the amount of chlorination, and at the same time achieve the purpose of disinfection. The treatment rate of the chlorination method is 90%-100%, the treatment effect is stable, and it is not affected by the water temperature. This method is particularly attractive in cold regions.

The breaking point chlorination method for ammonia removal is less investment, but the operating cost is high. The by-product chloramine and chlorinated organic matter will cause secondary pollution. The chlorination method is only suitable for the treatment of low-concentration (less than 50mg/L) ammonia nitrogen wastewater .

15. What is the principle and application of ammonia removal by zeolite ion exchange method?

Ion exchange refers to the process of ion exchange that occurs at the interface between solid particles and liquid. In the zeolite ion exchange method, zeolite with strong selectivity to NH4+ ions is selected as the exchanger to achieve the purpose of removing ammonia nitrogen. Zeolite has the effect of adsorbing non-ionic ammonia and ion exchange with ionic ammonia. It is a kind of siliceous cation exchanger, with low cost and strong selectivity to NH4+. Phthalo can not only be used as an ion exchange material to separate ammonia nitrogen from waste water as a splitter; it can also be used as a carrier for nitrifying bacteria by organically combining zeolite with a biochemical treatment system; as a process for treating ammonia nitrogen, it has more advantages. High removal rate and stability.

The ion exchange of zeolite has a lot to do with the choice of pH value. The range of pH value from 4 to 8 is the best area for zeolite ion exchange. When pH<4, H+ competes with NH4+; when pH>8, NH4+ becomes NH3 and loses ion exchange performance.

The ion exchange method treats urban sewage containing 10-20mg/L of ammonia nitrogen, and the effluent concentration can reach less than 1mg/L. The ion exchange method has the characteristics of simple process, low investment, and high removal rate. It is suitable for medium and low-concentration ammonia nitrogen wastewater. For high-concentration ammonia nitrogen wastewater, it will be difficult to operate due to frequent resin regeneration. However, the regenerated liquid is high-concentration ammonia nitrogen wastewater, which still needs further treatment.

16. What is the principle and application effect of ammonia removal by membrane separation?

Ammonia removal by membrane separation is a method of removing ammonia and nitrogen using the selective permeability of the membrane. This method is easy to operate, has a high ammonia nitrogen recovery rate, and has no secondary pollution. The gas-water separation membrane is an ideal method to remove ammonia nitrogen.

Ammonia nitrogen has a dissociation equilibrium in water. As the pH value increases, the proportion of ammonia in the NH3 form in the water increases. At a certain temperature and pressure, the gaseous and liquid NH3 balances. According to the principle of chemical balance, all balances in nature are relative and temporary. Chemical equilibrium can only be maintained under certain conditions. "If one of the conditions of the equilibrium system is changed, such as concentration, pressure, or temperature, the equilibrium will move in a direction that can weaken this change." The degassing membrane is designed to remove ammonia from wastewater according to this principle. One side of the membrane is high-concentration ammonia nitrogen wastewater, and the other side is the absorption liquid (such as water, acidic water, etc.). When the temperature on the left is greater than 20 degrees , PH value is greater than 9, when the left side gas partial pressure is greater than the right side gas partial pressure, and a certain pressure difference is maintained, then the free ammonia NH4+ in the wastewater becomes ammonia molecule NH3 and diffuses to the membrane surface through the raw material liquid side interface Under the effect of the partial pressure difference on the membrane surface, it passes through the membrane pores and enters the absorption solution, which quickly reacts with the H+ in the acidic solution to form ammonium salt.

The essence of the process is a continuous process of diffusion and absorption, and both desorption and absorption are completed at the same time in the membrane. The mass concentration of the by-product ammonium salt can reach 20%-30%, which becomes a clean industrial raw material, and the ammonia nitrogen in the wastewater can be reduced to less than 1mg/L, which is suitable for the treatment of high-concentration ammonia nitrogen wastewater in coal chemical, pharmaceutical, metallurgical and other industries.

Advantages of degassing membrane for wastewater deamination:

(1) The ammonia removal rate is high, and the ammonia content in the wastewater can be reduced to less than 5mg/L;

(2) The operating cost is low, only less than 5% of the traditional process;

(3) The equipment occupies a small area, less than 1/3 of the traditional craft;

(4) There is no ammonia leakage and clean production is realized.