Anammox sludge granulation and synergy with comammox: A critical review

Abstract

Anaerobic ammonia oxidation (anammox) is regarded as a particularly promising process in the field of biological nitrogen removal. However, when treating municipal wastewater, whose water quality is inherently low in ammonia and nitrogen, which is not conducive to nitrite accumulation, the low dissolved oxygen operation tends to result in a deterioration of sludge settling performance in the activated sludge system. This can even lead to problems such as sludge swelling and sludge loss. In contrast, anammox with sludge granulation has the potential to address this issue. Furthermore, complete ammonia oxidation (comammox) bacteria are well adapted to flourish in environments with low ammonia nitrogen and low dissolved oxygen levels. It is anticipated that they will coexist in a mutually beneficial relationship with short-range denitrifying microorganisms and anammox microorganisms. This synergy has the effect of inhibiting the growth of nitrite-oxidising bacteria (NOB), which in turn leads to the accumulation of nitrite. Consequently, this approach offers a novel method for the conventional removal of nitrogen in municipal wastewater treatment. This paper will focus on the granulation of anammox sludge, reviewing the research progress on its bacterial characteristics and formation mechanism and extracellular polymer properties. It will then proceed to provide a systematic description of the distribution and physiological properties of comammox bacteria, before finally analysing the feasibility of the coupling of anammox and comammox, which provides a theoretical guarantee for the granulation of anammox sludge and the coupling of the comammox process.