Insight into dead space effects in granular anammox process with organic stress.

In this study, the dead space was demonstrated to enhance the robustness of anammox nitrogen (N)-removal under organic stress. Different from the "yellow aggregates" that inhabit in mixing space were assembled by anammox and heterotrophic micro-colonies, the "red granules" that inhabit in dead space were formed by initial anammox aggregates that growing outward with higher anammox-activity, settleability and sludge stability, which endowed the dead space the role of "anammox-stabilizer" with prominent anammox N-removal contribution (63.8%) especially under high organic stress. The extracellular polymeric substances (EPS) dynamic balance test revealed that the high and stable EPS contents in dead space were attributed to the low EPS degradation rate and low proportion of heterotrophic bacteria (HB)-produced EPS, respectively. The weak hydrodynamic forces were the key to less HB-colonization and high granular stability in dead space. Retaining a certain dead space is necessary to prevent anammox bacteria (AnAOB) loss under organic stress.