Many wastewater treatment plants in the world do not remove reactive nitrogen from wastewater prior to release into the environment. Excess reactive nitrogen not only has a negative impact on human health, it also contributes to air and water pollution, and can cause complex ecosystems to collapse. In order to avoid the deleterious effects of excess reactive nitrogen in the environment, tertiary wastewater treatment practices that ensure the removal of reactive nitrogen species need to be implemented. Many wastewater treatment facilities rely on chemicals for tertiary treatment, however, biological nitrogen removal practices are much more environmentally friendly and cost effective. Therefore, interest in biological treatment is increasing. Biological approaches take advantage of specific groups of microorganisms involved in nitrogen cycling to remove reactive nitrogen from reactor systems by converting ammonia to nitrogen gas. Organisms known to be involved in this process include autotrophic ammonia-oxidizing bacteria, heterotrophic ammonia-oxidizing bacteria, ammonia-oxidizing archaea, anaerobic ammonia oxidizing bacteria (anammox), nitrite-oxidizing bacteria, complete ammonia oxidizers, and dissimilatory nitrate reducing microorganisms. For example, in nitrifying–denitrifying reactors, ammonia- and nitrite-oxidizing bacteria convert ammonia to nitrate and then denitrifying microorganisms reduce nitrate to nonreactive dinitrogen gas. Other nitrogen removal systems (anammox reactors) take advantage of anammox bacteria to convert ammonia to nitrogen gas using NO as an oxidant. A number of promising new biological treatment technologies are emerging and it is hoped that as the cost of these practices goes down more wastewater treatment plants will start to include a tertiary treatment step.

世界上许多废水处理厂在释放到环境中之前并没有从废水中去除反应性氮。过量的反应性氮不仅对人体健康产生负面影响，还会造成空气和水的污染，并可能导致复杂的生态系统崩溃。为了避免过量的反应性氮对环境的有害影响，需要实施确保去除反应性氮物质的三次废水处理实践。许多废水处理设施都依赖化学品进行第三级处理，但是生物脱氮方法对环境更加友好且具有成本效益。因此，对生物治疗的兴趣正在增加。生物方法利用参与氮循环的特定微生物群的优势，通过将氨转化为氮气，从反应器系统中去除反应性氮。已知与该过程有关的生物包括自养氨氧化细菌，异养氨氧化细菌，氨氧化古细菌，厌氧氨氧化细菌（anammox），亚硝酸盐氧化细菌，完全氨氧化剂和异化硝酸盐还原微生物。例如，在硝化-硝化反应器中，氨和亚硝酸盐氧化细菌将氨转化为硝酸盐，然后反硝化微生物将硝酸盐还原为非反应性氮气。其他脱氮系统（厌氧氨氧化反应器）利用厌氧氨氧化细菌利用NO作为氧化剂将氨转化为氮气。