Treatment methods for high-ammonia-nitrogen wastewater

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This post was last edited by O.K. on 2016-4-19 at 22:38. With industrial development and the increase in the number of factories, the amount of wastewater with high ammonia nitrogen levels discharged across the country is growing. Such wastewater has a complex composition and is highly toxic, thus posing a serious threat to the environment. Excessive ammonia nitrogen can lead to eutrophication of water bodies, and it also has strong carcinogenic properties. Wastewater with high ammonia nitrogen levels has drawn attention in the field of environmental protection as well as on a global scale. In response to this situation, many methods for treating wastewater with high ammonia nitrogen concentrations have been developed both domestically and internationally, and these methods encompass various approaches such as physical, chemical, and biological ones. For the treatment of wastewater with high ammonia nitrogen levels, no single treatment process can ensure that the effluent meets all the required standards. Generally, a pretreatment process is combined with one or more advanced treatment processes. There are various pretreatment methods such as stripping (steaming) method, membrane absorption technology, catalytic wet oxidation method, etc. As for advanced treatment techniques, there are many options including chemical precipitation method, biological nitrogen removal method, chemical oxidation-reduction method, breakpoint chlorination method, and electrolysis method. The methods that are widely used generally include the stripping (stripping) method + biological nitrogen removal method, the stripping (stripping) method + chemical oxidation-reduction method; or, if the liquid film method is well controlled and there are no issues with the membrane, a single process can meet the requirements. Stripping (stripping method): Stripping involves bringing water into contact with air as a discontinuous phase; by taking advantage of the difference between the actual concentration and the equilibrium concentration of the components in water, ammonia nitrogen is transferred to the gas phase, thereby removing it from the wastewater. Ammonia nitrogen usually exists in equilibrium in the form of ammonium ions (NH4+) and free ammonia (NH3). When the pH of the wastewater is adjusted to an alkaline level, ionic ammonium is converted into molecular ammonia, which is then stripped out by introducing air. It is better to use steam for the air used in stripping, as it can accelerate the separation of free ammonia (NH3) from wastewater at high temperatures; however, the cost of using steam is too high, so air stripping is generally used instead. However, in practical operation, there are issues such as low processing efficiency, scale formation that hinders operations, high energy consumption and maintenance requirements, as well as the potential for secondary pollution. This method is commonly used as a pretreatment for high-ammonia-nitrogen wastewater. Chemical redox method: This method involves the use of a new type of agent for removing ammonia nitrogen. This agent possesses strong redox properties; during the denitrification process, the nitrogen in nitrogen-containing organic compounds, as well as in nitrogen-containing inorganic substances such as sulfides and phosphate salts, is first converted into NH3, NH2, and NH4+, before being transformed into NO and NO2, and finally reduced to N2. Of course, this method is generally used for the secondary advanced treatment of high-concentration ammonia nitrogen wastewater, as well as for the treatment of medium and low-concentration ammonia nitrogen wastewater. During the denitrification process, if dosing is properly controlled, ammonia nitrogen in the water can be completely removed, achieving zero emissions. At the same time, no waste (solid waste) harmful to the environment is generated during the reaction process. There is no need to consider the issue of handling solid waste at all. Liquid film method: The principle of using membrane absorption to treat ammonia-containing wastewater is as follows: A hydrophobic microporous membrane (polypropylene, polytetrafluoroethylene, polyvinylidene fluoride) separates the ammonia-containing wastewater and H2SO4, as well as the absorption solution, on opposite sides of the membrane. By adjusting the pH value of the wastewater, the ionic NH4+ in it is converted into molecular, volatile NH3. Driven by the concentration difference of NH3 on both sides of the membrane, NH3 in the wastewater vaporizes at the interface between the wastewater and the microporous membrane. Gaseous NH3 diffuses across the membrane pores to the other side of the membrane, where it is absorbed by H2SO4 at the interface between the absorbent solution and the membrane, reacting to form the non-volatile (NH4)2SO4, which is then recovered. Due to the different forms in which ammonia exists in wastewater and the absorption solution, ammonia in the wastewater can be continuously transferred to the absorption solution through changes in its form, until all of the H2SO4 in the absorption solution is neutralized by ammonia. Theoretically, the ammonia nitrogen concentration in the treated wastewater can reach zero. The current challenge in membrane absorption processes is preventing membrane leakage. To ensure a high flux, the membrane thickness of conventional microporous membranes is usually relatively thin, which makes it easy for the aqueous phases on both sides of the membrane to leak under the effect of pressure differences. Biological nitrogen removal: The biological method for removing ammonia nitrogen involves the conversion of ammonia nitrogen in wastewater into nitrogen gas through a series of reactions such as nitrification and denitrification, thereby achieving the removal of ammonia nitrogen. At present, this method is still widely used and is quite effective for wastewater containing medium to low concentrations of ammonia nitrogen. There are many related processes available, such as A/O, A2/O, SBR, IBR, AB, etc. The disadvantages of the biological method are its large space requirement, lower microbial activity at low temperatures, and low efficiency in removing ammonia nitrogen.