High-salt, high-COD, high-ammonia nitrogen wastewater, due to its complex characteristics and serious hazards, has become a pressing challenge in the field of water treatment. This article comprehensively analyzes the sources, characteristics, and hazards of this type of wastewater, details the current mainstream treatment technologies, including physical, chemical, and biological treatment methods, discusses the combined use strategies of treatment technologies, analyzes existing challenges and prospects for future development, aiming to provide a theoretical basis and technical reference for the efficient and economical treatment of this type of wastewater.

1. Sources

In the chemical industry, the use of a large amount of inorganic salts and nitrogen-containing compounds in organic synthesis and pesticide production processes will produce this type of wastewater; the fermentation and drug synthesis processes in the pharmaceutical industry, the pickling and fermentation processes in the food processing industry, and the pretreatment and dyeing processes in the printing and dyeing industry are also the main sources of this type of wastewater.

2. Characteristics

High-salt, high-COD, high-ammonia nitrogen wastewater has complex components. In addition to containing a large amount of salts such as sodium chloride and sodium sulfate, and high concentrations of organic matter characterized by high chemical oxygen demand (COD), there is also a high concentration of ammonia nitrogen, including free ammonia and ammonium ions. The high salinity in the wastewater leads to high osmotic pressure, which has a toxic effect on microorganisms and inhibits their growth and metabolism; high concentrations of ammonia nitrogen are not only toxic to aquatic organisms but also consume dissolved oxygen in the water, exacerbating water pollution; complex organic matter makes the wastewater poorly biodegradable, and conventional biological treatment is difficult to effectively degrade.

3. Hazards

Direct discharge will increase the salinity of the receiving water body, affecting the survival and reproduction of aquatic organisms, and leading to a decline in biodiversity. High ammonia nitrogen will cause eutrophication of the water body, causing excessive algae growth, consuming dissolved oxygen in the water, causing the water body to turn black and smelly, and destroying the aquatic ecosystem. In addition, high-salt, high-COD, high-ammonia nitrogen wastewater can also seep into the soil, causing soil salinization, affecting soil fertility and crop growth.

1. Physical Treatment Technologies

Stripping: Utilizing the fact that ammonia nitrogen in wastewater exists in the form of free ammonia under alkaline conditions, air or steam is introduced into the wastewater to transfer free ammonia from the liquid phase to the gas phase, thereby achieving the purpose of removing ammonia nitrogen. At a pH of around 11 and with increased water temperature, stripping efficiency can be significantly improved. However, the stripping process will produce a large amount of ammonia-containing waste gas, which needs to be treated later, otherwise, it will cause air pollution.

Evaporation and Concentration: Heating causes the wastewater to evaporate, and after the water vaporizes, the salts, organic matter, and ammonia nitrogen are concentrated. Multiple-effect evaporation and mechanical vapor recompression evaporation (MVR) are commonly used evaporation technologies. Multiple-effect evaporation uses multiple evaporators in series to achieve cascade utilization of energy and reduce energy consumption; MVR compresses secondary steam through a compressor to improve its thermal energy utilization rate. Evaporation and concentration can achieve preliminary separation of water and pollutants, but the equipment investment is large, the operating cost is high, and scaling and blockage problems are prone to occur.

2. Chemical Treatment Technologies

Breakpoint Chlorination: Adding an excess of chlorine gas or sodium hypochlorite to the wastewater to oxidize ammonia nitrogen into nitrogen gas. In the reaction process, when the amount of chlorine gas added reaches a certain value, the residual chlorine content in the water is the lowest, and the ammonia nitrogen concentration is reduced to zero, and this point is the breakpoint. Breakpoint chlorination has a fast reaction speed and high removal efficiency, but it will produce secondary pollutants such as chlorinated organic compounds, and the use of chlorine gas has safety risks.

Chemical Precipitation (MAP method): Adding magnesium salts and phosphates to the wastewater to react with ammonia nitrogen to form magnesium ammonium phosphate (MAP) precipitate. At a pH of 9-11, when the molar ratio of magnesium ions, phosphate ions, and ammonia nitrogen is 1.2:1.2:1, the ammonia nitrogen removal rate can reach more than 90%. The chemical precipitation method is simple to operate and can recover phosphorus and ammonia resources, but the amount of flocculant added is large, the cost is high, and the resulting sludge needs further treatment.

3. Biological Treatment Technologies

Short-cut nitrification and denitrification: In the traditional nitrification and denitrification process, ammonia nitrogen is first oxidized to nitrite, then further oxidized to nitrate, and then reduced to nitrogen gas through denitrification. Short-cut nitrification and denitrification control the nitrification process to the nitrite stage and directly perform denitrification. Under high-salt conditions, by controlling dissolved oxygen, pH, and temperature, short-cut nitrification and denitrification bacteria can be enriched to improve ammonia nitrogen removal efficiency, reduce energy consumption, and carbon source consumption. However, salt fluctuations and water quality changes have a greater impact on the activity of short-cut nitrification and denitrification bacteria.

Anaerobic ammonium oxidation: Under anaerobic conditions, anaerobic ammonium oxidizing bacteria use nitrite as an electron acceptor to directly oxidize ammonia nitrogen to nitrogen gas. The anaerobic ammonium oxidation process does not require the addition of an external carbon source, has low energy consumption, and produces less sludge. However, anaerobic ammonium oxidizing bacteria grow slowly and have strict requirements for environmental conditions, such as temperature, pH, and dissolved oxygen, and high salinity will inhibit their activity.

A single treatment technology is difficult to meet the treatment requirements of high-salt, high-COD, high-ammonia nitrogen wastewater, so the combined use of multiple technologies has become a research hotspot.

"Stripping - biological treatment" combination: First, part of the ammonia nitrogen is removed by stripping to reduce the load of subsequent biological treatment, and then salt-tolerant microorganisms are used for biological treatment to degrade organic matter and remaining ammonia nitrogen;

"Chemical precipitation - advanced oxidation" combination: First, most of the ammonia nitrogen is removed by chemical precipitation, and then advanced oxidation technologies such as Fenton oxidation and ozone oxidation are used to degrade refractory organic matter;

"Evaporation and concentration - biological treatment" combination: First, evaporation and concentration are used to achieve preliminary separation of water and pollutants, reduce salinity and pollutant concentration, and then biological treatment is used to further purify the wastewater. The combination of technologies can fully utilize the advantages of each technology to improve treatment efficiency and economy.

Current technologies for treating high-salt, high-COD, and high-ammonia nitrogen wastewater face challenges such as high treatment costs, low treatment efficiency, and secondary pollution. The high investment and operating costs of treatment equipment limit the promotion and application of some technologies; some technologies are poorly adaptable to changes in wastewater quality and quantity, resulting in unstable treatment efficiency; and some chemical treatment methods produce secondary pollutants that require further treatment.

In the future, research and development of new and efficient treatment technologies should be strengthened, such as developing new adsorbents, catalysts, and salt-tolerant microbial strains; optimizing combined treatment processes to achieve synergistic effects of different technologies; and exploring new pathways for resource recovery and utilization, such as recovering salts, phosphorus, and ammonia from wastewater, to achieve reduced volume, harmlessness, and resource-oriented treatment of wastewater, promoting sustainable industrial development.