How to remove total nitrogen from sewage?

Author：

Huanke

In water treatment, several terms related to nitrogen are often mentioned, including total nitrogen (TN), Kjeldahl nitrogen (TKN), organic nitrogen, inorganic nitrogen, and ammonia nitrogen. There is a certain relationship between these terms:
Total nitrogen (TN)=organic nitrogen+inorganic nitrogen=Kjeldahl nitrogen (TKN)+NOx N;
Inorganic nitrogen=ammonia nitrogen (NH3-N, NH4-N)+nitrate nitrogen (NO3-- N)+nitrite nitrogen (NO2-N);
Kjeldahl nitrogen (TKN)=organic nitrogen+ammonia nitrogen (NH3-N, NH4-N).

In the sewage discharge standards, the total nitrogen index has become a key subject of supervision by the environmental inspection team in just six months. Especially in 2018, this trend became more apparent, and more regions will be included in the key regulatory scope. In this urgent situation, many enterprises have begun to seek ways to improve nitrogen treatment technology, while traditional activated sludge methods have received more attention due to their inability to meet new emission requirements due to their nitrogen removal efficiency.
Biological methods are widely used in wastewater denitrification technology. Biological denitrification is the process of converting different forms of nitrogen into nitrogen through the physiological metabolism of microorganisms in water bodies. This process includes the following steps: the difficult to degrade organic nitrogen in the wastewater is decomposed into ammonia nitrogen (NH3-- N, NH4-N) through hydrolysis and ammonification. Ammonia nitrogen is converted into nitrate nitrogen (NOX-N) through nitrite and nitrification, and finally into nitrogen through denitrification.

At present, biochemical method is one of the commonly used methods for treating total nitrogen, which has an early origin, mature technology, and low cost. In the decades of sewage treatment in China, biochemical methods have always played a dominant role. However, with the improvement of emission standards, the shortcomings of traditional activated sludge methods are gradually becoming apparent. For example, when the concentration of nitrogen and phosphorus in wastewater is high, relying solely on traditional activated sludge methods often cannot achieve the expected results.

Although biochemical methods have many deformation processes in practice, such as membrane bioreactors, biofilter technology, and biological turntables, these methods have high costs and are not mature enough, so many companies are unwilling to easily try them. This leads to very few high-quality cases as models, and very few companies are willing to jointly seek practical improvements in technology, making it difficult for these technologies to achieve breakthrough progress in a short period of time.
In actual production, different stages of biochemical denitrification should be strengthened according to different water quality requirements. For example, wastewater generated in pesticide production plants usually contains a large amount of organic nitrogen, so large-scale hydrolysis processes are needed to convert difficult to degrade organic nitrogen into easily convertible small molecule organic nitrogen, thereby converting it into ammonia nitrogen. For example, some electroplating plants require a large amount of ammonia water as a buffer, so the wastewater contains a large amount of ammonia nitrogen. In such a situation, if the ammonia nitrogen is not treated separately, it will still cause the ammonia nitrogen in the biochemical effluent to exceed the standard. At present, there are better methods such as blowing off and point chlorination; Some industries also have high levels of nitrate in wastewater, and only the biochemical method is more mature in the removal of nitrate nitrogen. However, the denitrification efficiency of existing biochemical technologies is relatively low. When facing high concentrations of nitrate nitrogen, it is necessary to build a larger scale anaerobic tank, which has high infrastructure costs and occupies a large area, greatly increasing the overall investment cost and making it difficult to achieve.

In order to solve the problem of a large area occupied by biochemical methods, the following improvement methods can be considered:
1. Selection and domestication of bacterial strains: Long term domestication can improve the tolerance and metabolic ability of denitrifying bacteria, thereby improving denitrification efficiency.
2. Improving the reactor structure: For example, changing the problem of nitrogen gas generated after the denitrification process in traditional biochemical methods that cannot be quickly discharged, increasing the nitrogen emission rate, and thereby improving the reactor efficiency.
3. Adopting new microbial enrichment modes, such as using biological contact oxidation, biological moving bed, and biological fixed bed, can improve bacterial adsorption efficiency and reduce the production of suspended sludge, thereby improving effluent quality and reducing the loss of effective sludge utilization rate.