Total nitrogen includes nitrate nitrogen (NO3-), nitrite nitrogen (NO2-), ammonia nitrogen (NH4+), and organic nitrogen. At present, biological treatment of wastewater can stably remove nitrogen from wastewater, which is a relatively economical and effective method for removing total nitrogen. How to achieve the discharge of total nitrogen in compliance with the standard?
1. Removal of ammonia nitrogen
The technology for treating ammonia nitrogen-containing wastewater in the market is very mature, and many sewage treatment plants can ensure the stable removal of ammonia nitrogen.
(1) Breakpoint chlorination oxidation method, by adding sodium hypochlorite or bleaching powder for oxidation, ammonia nitrogen is converted into nitrogen gas for release. The common ammonia nitrogen removers on the market are mainly bleaching powder. The reaction equation is as follows:
2NH2Cl + HClO →N2↑+3H++3Cl- +H2O
(2) Use microbial nitrification and denitrification to remove ammonia nitrogen from wastewater. The principle is the combined action of nitrifying bacteria and denitrifying bacteria to convert ammonia nitrogen in water into nitrogen gas to achieve the purpose of denitrification. First, ammonia nitrogen is converted into nitrite and nitrate by nitrifying bacteria and nitrite bacteria, and then denitrification is carried out to convert nitrate into nitrogen gas. The reaction process is as follows:
2NH3+3O2→HNO2+H2O+energy (nitrification)
2HNO2+O2→ 2HNO3+energy (nitrification)
HNO3+CH3OH→N2 + CO2+H2O+energy (denitrification)
(3) The acidic group of sulfonic acid (-SO3H) contained in the ammonia nitrogen removal resin is easily ionized into H+ ions in water. The NH4+ ions contained in the water exchange ions with the H+ ionized by the ammonia nitrogen removal resin, so that the cation NH4+ in the solution is transferred to the resin, and the H+ on the resin is exchanged into the water. When ammonia nitrogen exists in the form of NH4+ cations in wastewater, the ammonia nitrogen removal resin containing sulfonic acid groups (-S03H) has the following reaction for the separation of NH4+ in water:
RS03H + NH4+ → RS03NH4 + H+
The regeneration principle of ammonia nitrogen removal resin after adsorption saturation
With the continuous exchange of H+ on the resin functional group with NH4+ in water, when the resin is adsorbed saturated, 5% HCL solution is used for regeneration; at this time, the H+ in the regeneration solution exchanges ions with the NH4+ adsorbed on the resin functional group. The resin restores its exchange capacity. The reaction is as follows:
RS03NH4 + H+→ RS03H + NH4+
The exchange potential increases with the increase of ion concentration. High concentrations of H+ ions can even replace NH4+ ions, which is the regeneration principle of ammonia nitrogen removal resin.
2. Removal of organic nitrogen
The process in which nitrogen-containing organic matter in sewage is oxidized and decomposed into ammonia nitrogen by aerobic or anaerobic heterotrophic microorganisms during biological treatment;
Biological method, nitrogen compounds can be converted into nitrogen gas under biological action;
Chemical method, nitrogen compounds are directly converted from organic nitrogen and ammonia nitrogen into nitrogen gas through oxidation;
The biological method has low cost and stable effect, but the process is complex, the operation is difficult, and the floor space is large and the operation time is long; the chemical method omits the intermediate conversion step, which is faster and more direct, but the cost is high, the breakpoint chlorination method is difficult to control, and the effect is unstable.
3. Removal of nitrate nitrogen
Nitrate nitrogen mainly refers to nitrate ions. Currently, there are methods using ion exchange, membrane permeation, adsorption and biological denitrification.
The principle of ion exchange method for removing nitrate is: NO3- in the solution is removed by exchanging with Cl- or HCO3- on the ion exchange resin. After the resin is saturated, it is regenerated with NaCl or NaHCO3 solution. Generally, the selectivity of anion exchange resins for several anions is in the following order: HCO3- < Cl- < NO3- < SO42- Therefore, it is difficult to treat nitrates in sulfate-containing water with conventional ion exchange resins. This is because the resin exchanges almost all sulfates in the water before exchanging with nitrates in the water. In other words, the presence of sulfates will reduce the resin's ability to remove nitrates. The use of resins with preferential selectivity for nitrates can better solve this problem. This resin preferentially exchanges nitrates, and its exchange capacity for nitrates is not affected by sulfates in the water. Increasing the number of carbon sources around the N atom in the resin functional group NR3+ can increase the selectivity of the resin for nitrates. The selectivity of this type of resin for nitrates is in the following order: HCO3- < Cl- < SO42- < NO3-
When the methyl group around the nitrogen atom in NR3+ on the resin is changed to ethyl, the selectivity coefficient KSN of the resin for nitrates and sulfates increases from 100 to 1000. The results of Clifford et al. show that increasing the distance between ion exchange sites can reduce the selectivity for sulfate, and increasing the hydrophobicity of the resin base and functional groups can increase the selectivity for nitrate. The increase in the selectivity of this resin for nitrate can be attributed to the fact that as the number of alkyl carbon sources increases, their volume increases and they need to occupy more space, which causes the steric tension of the resin to increase. NO3- has a stronger ability to reduce this steric tension than SO42-.
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