Application of cell immobilization technology to promote nitritation: A review

Nitritation, the oxidation of ammonia to nitrite without subsequent oxidation to nitrate, is a starting step for nitrite-based nitrogen removal approaches. This process can be induced by maintaining specific operating conditions that facilitate ammonia oxidation but deteriorate nitrite oxidation. In recent years, a number of publications have demonstrated the ability of cell immobilization to maintain nitritation and an oxygen-limiting strategy was suggested to be a key to the success of the approach. However, several aspects related to the success and failure of such systems remains unclear and requires further in-depth clarification. This review provides current information on the utilization of cell immobilization in nitritation reactors. Common operating strategies that promote nitritation by controlling environmental conditions are summarized in the first part of the review. The application of cell immobilization, including cell attachment, cell granulation, and cell entrapment systems, as well as microenvironments, and microbial distributions within cell immobilization matrices, are elaborated in the middle part of the review. Problems encountered in the operation of nitritation reactors using cell immobilization are discussed as opportunities for further research at the end of the review.