Treatment Processes for High Ammonia Nitrogen Wastewater

1. What is High Ammonia Nitrogen Wastewater?

If you ask a fish, it will tell you that when the concentration of non-ionic ammonia in water exceeds 0.5mg/L, it will cause death in a short time. Long-term exposure to concentrations above 0.1mg/L will also lead to chronic poisoning and eventually death. 

The discharge standard for general sewage treatment plants is typically below 5–15 mg/L of ammonia nitrogen. When the discharged water exceeds 15 mg/L and the pH is alkaline, it can cause toxicity to fish and other aquatic life.

In my personal opinion, wastewater with an ammonia nitrogen concentration higher than 300mg/L can be regarded as high ammonia nitrogen wastewater. That is, ordinary biochemical treatment systems cannot bear it, which will inhibit bacterial activity and even cause the system to collapse.

2. Sources of High Ammonia Nitrogen Wastewater

- **Agricultural field**: It is mainly concentrated in livestock and poultry excreta, with ammonia nitrogen content ranging from 500 to 8000mg/L. At present, the mainstream treatment method is anaerobic treatment followed by aerobic biochemical treatment for returning to the field. At this time, the ammonia nitrogen may still be several hundred mg/L. However, returning manure to the field has been a practice since ancient times, which is also legally permitted. Only a few farms can meet the discharge standard of natural water bodies.

- **Industrial field**: It is an important source of high ammonia nitrogen wastewater, mainly concentrated in chemical fertilizer production, coal chemical industry, pharmaceutical industry, hydrometallurgy, slaughtering, brewing and many other fields, with complex and diverse water quality. Industrial high ammonia nitrogen wastewater generally needs to be treated in the factory to meet the pipe connection standard before being discharged to the corresponding receiving sewage treatment plant for further treatment, or directly treated to meet the discharge standard before being discharged into natural water bodies.

- **Municipal domestic field**: Landfill leachate is the most important source of high ammonia nitrogen wastewater in the municipal field. By the end of 2022, there were more than 1,400 landfills in mainland China, and the ammonia nitrogen content in leachate ranged from 500 to 4,000mg/L. Generally speaking, the ammonia nitrogen content in leachate will gradually increase with time in the first 10 years or so after landfilling, and then gradually decrease.

3. Treatment Processes for High Ammonia Nitrogen Wastewater

As mentioned above, the basic treatment route in the agricultural field is biochemical treatment followed by returning to the field. The mainstream treatment for landfill leachate is biochemical treatment, and the non-mainstream method is membrane concentration. However, due to the complex and diverse situation of high ammonia nitrogen wastewater in the industrial field, it has become an arena for various processes.

3.1 Chemical Agent Method

That is, using sodium hypochlorite, breakpoint chlorination, ammonia nitrogen removers, etc. It should be said that this scheme is not suitable for high ammonia nitrogen wastewater treatment, and it is not easy to master in operation. Except for a few enterprises using it as an emergency, it is rarely used now.

The ideal scenario for the use of the chemical addition method is: small water volume, irregular wastewater generation, and low ammonia nitrogen content.

3.2 Steam Stripping for Ammonia Removal

The principle of ammonia stripping in a gas tower (also known as ammonia steam stripping) is: under alkaline conditions, by introducing steam into ammonia nitrogen-containing wastewater, using the volatility of ammonia, the ammonium ions in the wastewater are converted into free ammonia (NH₃), which is then carried by the steam to escape from the top of the tower and then cooled to obtain ammonia water. The main advantage of this method is that it is suitable for treating wastewater with very high ammonia nitrogen content. The higher the ammonia nitrogen content in the wastewater, the higher its ammonia removal efficiency, and the greater the amount of ammonia nitrogen that can be removed from the wastewater per unit of steam, and more concentrated ammonia water can be obtained, which can be recycled by many enterprises. Its disadvantage is that it requires a large amount of steam, so when the ammonia nitrogen content in the wastewater is low, the efficiency is somewhat low, the concentration of the obtained ammonia water is also affected, and the overall investment is large, and the operation and maintenance are relatively complex.

The ideal environment for the use of steam stripping for ammonia removal is: availability of steam, large amount of ammonia nitrogen wastewater, high ammonia nitrogen content, and need for ammonia water reuse.

3.3 Air Stripping

The principle of air stripping for ammonia nitrogen removal is to utilize the dissolution equilibrium and volatility of ammonia nitrogen in water. By introducing air into ammonia nitrogen wastewater with pH adjusted to a certain alkaline level, the free ammonia (NH₃) in the water is transferred to the gas phase and discharged from the top of the stripping tower. In the past, many manufacturers directly discharged ammonia nitrogen into the air after stripping, but now it is basically not allowed. Therefore, supporting absorption towers are required, using dilute sulfuric acid as the absorption liquid to produce ammonium sulfate as a by-product. The investment of the stripping tower is cheaper than that of the ammonia distillation tower, and the operating cost is also lower, with less investment, but its efficiency is not as good as that of the ammonia distillation tower, and the ammonia nitrogen removal rate is often not very high.

The ideal environment for air stripping is: large amount of ammonia nitrogen wastewater, relatively high ammonia nitrogen content, not very high requirements for the ammonia nitrogen content of the effluent, and the ammonium sulfate solution can be reused or there is a supporting evaporation and crystallization device.

3.4 Ammonia Removal by Membrane

Although membrane ammonia removal is a relatively new technology for treating high ammonia nitrogen wastewater, it has a history of decades, so it is also a relatively mature technology. Specifically, its principle is: in ammonia nitrogen wastewater, by adjusting the pH value to alkaline, ammonium ions (NH₄⁺) are converted into free ammonia (NH₃). At this time, free ammonia can pass through the pores of the hydrophobic hollow fiber membrane or flat membrane and diffuse from the wastewater side (feed liquid side) to the other side of the membrane (absorption side). The absorption side is usually equipped with an acidic solution (such as sulfuric acid), and the free ammonia will react with the acid to form non-volatile ammonium salts (such as ammonium sulfate), which are fixed in the absorption liquid, and finally realize the removal and recovery of ammonia nitrogen in the wastewater. Its advantage is that the membrane contactor has a large specific surface area, so the ammonia removal efficiency is high, and the effluent can reach a very low ammonia nitrogen content. It is more energy-efficient than the ammonia distillation and stripping methods, occupies less space, and is easier to operate. The investment is often between that of the ammonia distillation tower and the stripping tower, but it has relatively high requirements for the water quality of the wastewater, and when the water volume is large, its scale effect is not as good as that of the ammonia distillation tower and the stripping tower.

The ideal environment for ammonia removal by membrane is: wastewater with ammonia nitrogen content between 200-10000mg/L, small or medium water volume, high requirements for the ammonia nitrogen content of the effluent, limited space, and the ammonium sulfate solution can be reused or there is a supporting evaporation and crystallization device.

3.5 Evaporation and Crystallization

For wastewater with very high salt content, such as more than tens of thousands of mg/L, high ammonia nitrogen content, and small water volume, which is difficult to treat by the above conventional physical and chemical methods, evaporation and crystallization is a more practical method. The investment and operating cost of this scheme are often the highest, and it is a fallback scheme when other processes cannot be used.

4. Comparison and Conclusion

On the premise that various schemes or processes are mature and reliable, enterprises need to comprehensively evaluate their own specific situations when choosing to treat high ammonia nitrogen wastewater. The primary goal of enterprises is to make profits. Usually, our selection criterion is to use a more economical scheme on the premise of meeting the treatment requirements. Therefore, purchasers need to fully understand the characteristics of various schemes and conduct a comprehensive comparison to finally determine the most suitable scheme.