How to remove ammonia nitrogen from water

Based on the different concentrations of ammonia nitrogen in wastewater, wastewater can be divided into three categories:

High-concentration ammonia nitrogen wastewater (NH3-N>500mg/l);

Medium-concentration ammonia nitrogen wastewater (NH3-N: 50-500mg/l);

Low-concentration ammonia nitrogen wastewater (NH3-N<50mg/l).

However, high concentration of ammonia nitrogen wastewater has an inhibitory effect on the activity of microorganisms, which restricts the application and effect of biochemical method on its treatment, and at the same time, it will reduce the degradation efficiency of biochemical system on organic pollutants, which will lead to difficulties in treating the effluent to meet the requirements.

The main methods to remove ammonia nitrogen are: physical, chemical and biological methods. Physical method has reverse osmosis, distillation, soil irrigation and other treatment technologies; chemical method has ion exchange, ammonia blowing off, folding point chlorination, incineration, chemical precipitation, catalytic cracking, electrodialysis, electrochemistry and other treatment technologies; biological method has algae farming, biological nitrification, immobilization of biotechnology and other treatment technologies.

Currently the more practical methods are: folding point chlorination, selective ion exchange, ammonia blowing off method, biological methods and chemical precipitation.

1. Folding-point chlorination to remove ammonia and nitrogen

Folding-point chlorination is chlorine or sodium hypochlorite into the wastewater will be oxidized in the wastewater in the NH3-N N2 chemical nitrogen removal process. When the chlorine gas into the wastewater reaches a certain point when the water free chlorine content is the lowest, the concentration of ammonia to zero. When the chlorine flux exceeds this point, the free chlorine in the water increases. Therefore this point is called the folding point and chlorination in this state is called folding point chlorination. The actual amount of chlorine required to treat ammonia-nitrogen wastewater depends on temperature, pH, and ammonia-nitrogen concentration. To oxidize each gram of ammonia nitrogen requires 9 to 10mg of chlorine. pH 6 to 7 is the optimal reaction zone, and the contact time is 0.5 to 2 hours.

Folding-point chlorination of treated effluent in the discharge before the general need for activated carbon or sulfur dioxide for reverse chlorination to remove residual chlorine in the water. 1mg of residual chlorine requires about 0.9 to 1.0mg of sulfur dioxide. Hydrogen ions are generated during the counter-chlorination process, but the resulting drop in pH is generally negligible, so that only about 2mg of residual chlorine is consumed to remove 1mg of residual chlorine (in terms of CaCO3). The mechanism of ammonia removal by fold point chlorination is as follows:

Cl2+H2O→HOCl+H++Cl-

NH4++HOCl→NH2Cl+H++H2O

NHCl2+H2O→NOH+2H+2Cl-

NHCl2+NaOH→N2+HOCl+H++Cl-

NHCl2+NaOH→N2+HOCl+H+Cl-

NHCl2+NaOH→NH2+HOCl+H+Cl-

The most outstanding advantage of the folding point chlorination method is that it can reduce all the ammonia nitrogen in the wastewater to zero by correctly controlling the amount of chlorination and homogenizing the flow rate, and at the same time make the wastewater achieve the purpose of disinfection. For wastewater with low ammonia concentration (less than 50mg/L), this method is more economical. In order to overcome the disadvantages of using the folding point chlorination method alone to treat ammonia wastewater requires a large amount of chlorine, often this method is used in conjunction with biological nitrification, the first nitrification and then in addition to trace residual ammonia nitrogen. Chlorination method of the treatment rate of 90% to 100%, the treatment effect is stable, not affected by the water temperature, in cold areas this method is particularly attractive. Less investment, but high operating costs, by-products of chloramines and chlorinated organic matter will cause secondary pollution, chlorination is only suitable for the treatment of low concentration of ammonia wastewater.

2. Selective ion exchange to remove ammonia nitrogen

Ion exchange refers to the solid particles and liquids at the interface of the ion exchange process. Ion exchange method uses zeolite, which has strong selectivity for NH4+ ions, as the exchange resin, so as to achieve the purpose of removing ammonia nitrogen. Zeolite has adsorption effect on non-ionic ammonia and ion exchange effect with ionic ammonia, it is a class of silica cation exchanger, low cost, strong selectivity to NH4+, can successfully remove ammonia nitrogen in raw water and secondary effluent.

Zeolite ion exchange and pH selection has a great relationship, pH in the range of 4 to 8 is the best zeolite ion exchange area. When pH <4, H+ and NH4+ competition; when pH>8, NH4+ into NH3 and lose ion exchange performance. With the ion exchange method to treat municipal wastewater containing ammonia nitrogen 10-20mg/L, the concentration of effluent can be up to 1mg/L or less. Ion exchange method is characterized by simple process, low investment and high removal rate, and it is suitable for low and medium concentration of ammonia wastewater (<500mg/L), but for high concentration of ammonia wastewater, it will be difficult to operate due to frequent resin regeneration. But the regeneration liquid is high concentration of ammonia wastewater, still need further treatment.

3. Air blowing and vapor extraction method to remove ammonia nitrogen

Air blowing is the wastewater and gas contact, the ammonia nitrogen from the liquid phase to the gas transfer method. This method is suitable for the treatment of high concentration of ammonia nitrogen wastewater. Blowing off is to make water as a discontinuous phase in contact with air, using the difference between the actual concentration of the components in the water and the equilibrium concentration, so that the ammonia nitrogen is transferred to the gas phase and removed Ammonia nitrogen in wastewater is usually in the state of ammonium ions (NH4+) and free ammonia (NH3) to maintain the equilibrium and exists. When the pH of the wastewater is adjusted to alkaline, the ionized ammonium is converted to molecular ammonia, and then the ammonia is blown off by passing air. Blow off method in addition to ammonia nitrogen, the removal rate of up to 60% ~ 95%, the process is simple, the treatment effect is stable, blow off the ammonia with hydrochloric acid absorption to generate ammonium chloride can be used back to the production of soda ash as mother liquor, but also according to the market demand, the production of ammonia absorption with water or with sulfuric acid absorption production of ammonium sulfate by-products, the end of the gas back to the blow off tower. However, the efficiency of blowing off is low when the water temperature is low, and it is not suitable for use in the cold winter. When treating ammonia nitrogen with this method, it is necessary to consider that the total amount of free ammonia discharged should be in accordance with the atmospheric emission standards for ammonia, so as not to cause secondary pollution. Low-concentration wastewater is usually blown off with air at room temperature, while steam is commonly used to blow off high-concentration wastewater from steelmaking, petrochemicals, fertilizers, organic chemicals, non-ferrous metal smelting and other industries. The method is more suitable for the treatment of high concentration of ammonia and nitrogen wastewater, but blowing off the efficiency of the influence of many factors, not easy to control, especially the temperature effect is relatively large, in the northern cold season efficiency will be greatly reduced, and now many blowing off the device taking into account the economy, there is no recycling of ammonia, and discharged directly into the atmosphere, resulting in atmospheric pollution.

The vapor extraction method is to use steam to transform the free ammonia in wastewater into ammonia gas to escape, the treatment mechanism is the same as the blow-off method is a mass transfer process, that is, at high pH, so that the wastewater is in close contact with the gas, so as to reduce the concentration of ammonia in the wastewater process. The driving force of the mass transfer process is the difference between the partial pressure of ammonia in the gas and the equilibrium partial pressure of the equivalent concentration of ammonia in the wastewater. Extending the contact time between gas and water and the tightness of the contact can improve the efficiency of ammonia nitrogen treatment, with packed tower can meet this requirement. Tower packing or filling can be increased by increasing the surface area of infiltration and the formation of small droplets throughout the tower or to generate a film to increase the contact time between gas and water vapor stripping method is suitable for the treatment of continuous discharge of high concentrations of ammonia and nitrogen wastewater, the operating conditions are similar to the blowing method, the removal rate of ammonia and nitrogen can reach more than 97%. However, the content of the vaporization tower is easy to generate scale, so that the operation can not be normal.

The ammonia gas escaping from the wastewater treatment of blowing off and stripping can be recycled: absorbed with sulfuric acid and used as fertilizer; condensed into a 1% ammonia solution.

4. Biological method to remove ammonia and nitrogen

Biological method to remove ammonia and nitrogen refers to the ammonia and nitrogen in wastewater in the role of a variety of microorganisms, through nitrification and denitrification and a series of reactions, and ultimately the formation of nitrogen, so as to achieve the purpose of removing ammonia and nitrogen. There are many kinds of biological nitrogen removal processes, but the mechanism is basically the same. All need to go through two stages of nitrification and denitrification.

Nitrification reaction is under aerobic conditions through the role of aerobic nitrifying bacteria in wastewater ammonia nitrogen oxidation to nitrite or nitrate, including two basic reaction steps: the nitrite bacteria involved in the conversion of ammonia nitrogen to nitrite reaction. By the nitrate bacteria involved in the conversion of nitrite to nitrate reaction. Nitrite bacteria and nitrate bacteria are autotrophic bacteria, they use the carbon source in the wastewater, through the redox reaction with NH3-N to obtain energy. The reaction equation is as follows:

Nitrosation:2NH4++3O2→2NO2-+2H2O+4H+

Nitrification:2NO2-+O2→2NO3-

Nitrifying bacteria's suitable pH value is 8.0～8.4, the optimal temperature is 35℃, the temperature has a great influence on the nitrifying bacteria, and the nitrifying rate will be reduced by half when the temperature drops 10℃; DO concentration: 2~3mg/L; BOD5 loading: 0.06-0.1kgBOD5/(kgMLS?d); Mud age is more than 3~5 days.

Under anoxic conditions, the use of denitrifying bacteria (denitrifying bacteria) will nitrite and nitrate reduction to nitrogen and escape from the wastewater due to parthenogenetic denitrifying bacteria (denitrifying bacteria), nitrate or nitrite produced in the nitrification process is reduced to N2 process, known as denitrification. The electron donor in the denitrification process is a variety of organic substrates (carbon source). Taking methanol as the carbon source as an example, the reaction formula is:

6NO3-+2CH3OH→6NO2-+2CO2+4H2O

6NO2-+3CH3OH→3N2+3CO2+3H2O+6OH-

The suitable pH value of denitrifying bacteria is 6.5-8.0; the optimal temperature is 30℃, when the temperature is lower than 10 The optimum temperature is 30℃, when the temperature is lower than 10℃, the denitrification rate decreases significantly, and when the temperature is as low as 3℃, the denitrification will stop; DO concentration＜0.5mg/L; BOD5/TN＞3～5. Biodenitrification can remove many kinds of nitrogenous compounds, the total nitrogen removal rate can reach 70%～95%, the secondary pollution is small and relatively economic, so it is used the most at home and abroad. The disadvantage is that it occupies a large area and has low efficiency at low temperatures.

The common biological denitrification process can be divided into 3 categories:

1) Multi-stage sludge system

Multi-stage sludge system is often referred to as the traditional biological denitrification process. This process can get quite good BOD5 removal effect and nitrogen removal effect, its disadvantage is the process is long, more structures, high capital costs, the need for additional carbon source, high operating costs, a certain amount of methanol residual in the effluent water;

(2) single-stage sludge system

Single-stage sludge system in the form of the system includes a pre-denitrification system, a post-denitrification system and an alternating work system. The biological denitrification process of pre-denitrification is usually called A/O process. Compared with the traditional biological denitrification process, the process is characterized by: simple process, fewer structures, only a sludge return system and mixed liquid return system, infrastructure costs can be greatly saved; the denitrification pool is set in the anoxic pool, reducing operating costs; aerobic pool in the anoxic pool after the denitrification of organic pollutants left behind can be further removed to improve the quality of the effluent water; anoxic pool in the front, the sewage in the organic carbon is denitrifying bacteria, can be reduced by the denitrifying bacteria. The organic carbon in the wastewater is utilized by denitrifying bacteria before the anoxic pool, which can reduce the organic load of the aerobic pool after it. In addition, the rear denitrification system, because the mixture lacks organic matter, generally also need to manually add carbon source, but the effect of nitrogen removal is higher than the front type, theoretically close to 100% nitrogen removal effect. The biological denitrification process that works alternately is mainly composed of two tandem pools, and by changing the direction of the inlet and outlet water, the two pools are operated alternately under anoxic and aerobic conditions. It is still essentially an A/O system, but by utilizing the alternating work, the reflux of the mixed liquid is avoided, and its nitrogen removal effect is better than that of the general A/O process. Its disadvantage is that the operation and management costs are high, must be configured with computer-controlled automatic operating system;

(3) Biofilm system

The anoxic and aerobic pools in the above A/O system are changed to fixed biofilm reactors, i.e., biofilm denitrification system is formed. This system should have mixed liquid return, but no sludge return, in the anoxic aerobic reactor is preserved adapted to denitrification and aerobic oxidation and nitrification reaction of the two sludge systems.

Conventional biological treatment of high concentration of ammonia and nitrogen wastewater is to exist the following conditions:

In order to be able to make the microbial normal growth, it is necessary to increase the reflux ratio to dilute the original wastewater;

Nitrification process not only requires a large amount of oxygen, but also denitrification requires a large amount of carbon source, it is generally believed that the COD/TKN is at least 9.

5.Chem. Precipitation method to remove ammonia nitrogen

Chemical precipitation method is based on the nature of the pollutants in the wastewater, if necessary, add a chemical raw material, in certain process conditions (temperature, catalyst, pH, pressure, mixing conditions, reaction time, the proportion of ingredients, etc.) to carry out a chemical reaction, so that the pollutants in the wastewater to produce a very small solubility of the precipitates or polymers, or the generation of insoluble gas products, so that the wastewater to produce a very small amount of water, or to produce the water. The chemical reaction causes the pollutants in the wastewater to generate precipitates or polymers with little solubility, or to generate gaseous products that are insoluble in water, thus purifying the wastewater or achieving a certain removal rate.

The main principle of chemical precipitation for NH3-N treatment is that NH4+, Mg2+, PO43- generate precipitate in alkaline aqueous solution. In the ammonia nitrogen wastewater injection of chemical precipitant Mg (OH) 2, H3PO4 and NH4 + reaction to generate MgNH4PO4?6H2O (guano stone) precipitation, the precipitate after granulation and other processes, can be developed as a composite fertilizer use. The pH value of the whole reaction is in the range of 9 to 11. When the pH value is <9, the PO43- concentration in the solution is very low, which is not favorable for the generation of MgNH4PO4?6H2O precipitation, and mainly generates Mg(H2PO4)2; if the pH value is >11, this reaction will generate in the strong alkaline solution is more difficult to dissolve Mg3(H2PO4)3(Mg3) and Mg3(H2PO4)4(Mg3) in a water-soluble Mg3(H2PO4)6(Mg3) solution. precipitate of Mg3(PO4)2 that is soluble in water. At the same time, NH4+ in the solution will volatilize into free ammonia, which is not conducive to the removal of ammonia and nitrogen from the wastewater. Using the chemical precipitation method, the ammonia nitrogen in wastewater can be recovered as fertilizer.