High concentrations of nitrate (NO₃⁻-N) and nitrite (NO₂⁻-N) are commonly detected in industrial effluents from feed production, meat processing, electronics manufacturing and steel production sectors. Notably, these substances may not exist in raw wastewater, and can be generated as secondary metabolites after aerobic biological treatment. Nitrite acts as a temporary intermediate in the nitrogen cycle. Due to its unstable property in aqueous environments, it may be reduced back to ammonia nitrogen (NH₄⁺-N).
From a limnological perspective, clean water bodies usually contain low nitrite content, while nitrate is the end product formed from organic nitrogen mineralization in the later decomposition stage. When nitrate nitrogen becomes the main inorganic nitrogen form, it reflects low organic nitrogen content and shows that the water body is in a relatively stable self-purification state. In contrast, increased nitrate concentration indicates that the self-purification process is still in progress, or the water body receives industrial wastewater containing nitrate.
To fully evaluate water pollution status and nitrogen cycling changes, it is necessary to test three inorganic nitrogen forms, namely ammonia nitrogen (NH₃-N), nitrite nitrogen (NO₂⁻-N) and nitrate nitrogen (NO₃⁻-N), together with total nitrogen (TN) and organic nitrogen (ON). These indexes can be used to assess wastewater treatment performance and adjust denitrification schemes. Combined with wastewater monitoring, these indicators help support the operation of wastewater treatment systems. Toxicologically, nitrite carries health hazards. It can react with secondary amines in the gastrointestinal tract to generate N-nitrosamines, a class of carcinogenic substances, so nitrite is an important index for public health monitoring.
After nitrate intake by human bodies, endogenous reduction to nitrite may occur, which may affect physical and mental health. Children are more vulnerable. Excessive nitrate intake through diet may oxidize hemoglobin into methemoglobin and trigger methemoglobinemia (blue baby syndrome). Therefore, nitrate limits are clearly specified in Chinese national standards. According to Environmental Quality Standards for Surface Water (GB 3838-2002), the limit value of nitrate for conventional surface water is 20 mg/L, and centralized drinking water sources adopt a stricter limit of 10 mg/L.
Common treatment methods for industrial wastewater containing nitrate and nitrite include micro-electrolysis coupled with biological denitrification with dedicated packing materials. Other physical and chemical technologies such as electrodialysis (ED), reverse osmosis (RO) and ion exchange (IX) can also remove such pollutants.
Post-treatment monitoring is an important part of environmental compliance management. Continuous sample analysis helps verify that discharged effluent meets relevant standard limits of key water quality parameters, and judges the stable operation of the treatment system.
