Effects of sludge morphology on the anammox process: Analysis from the perspectives of performance, structure, and microbial community

The nitrogen removal characteristics, physicochemical properties, and microbial community composition of four different anaerobic ammonium oxidation (anammox) sludge morphologies were investigated. The morphologies considered in this study, namely suspended sludge (Rs), biofilm (Rm), granular sludge (Rg), and encapsulated biomass (Re), were prepared from floc sludge. The results show that Re exhibited the maximum anammox activity, followed by Rg, Rm, and Rs. Additionally, the anammox contribution rate was higher in Rg and Re. The higher extracellular polymer content in Rg promoted sludge accumulation, and tryptophan was observed in Rm and Rg, which was replaced by humic acids in Rs. Re showed the largest specific surface area, hydrophobicity and strength, and its good structure ensured enrichment of anammox bacteria (AnAOB). In terms of the microbial community, the functional bacterium Candidatus Kuenenia accounted for the highest proportion in Rm (39.27%), but the presence of both anaerobic and aerobic regions led to increased community complexity with more nitrifying bacteria. In contrast, Rg and Re had a more specific microbial community. In addition, denitrifying bacteria tended to grow in Rs, while nitrifying bacteria were retained in Rm. The AnAOB were more likely to be enriched in sludge aggregates (both Rm and Rg) and carriers (Re). Through correlation analysis, the potential relationship involving bacterial flora evolution of each sample was clarified. Finally, the structural models of different morphologies of sludge were proposed. This study deepens the understanding of various anammox sludge morphologies as well as provides useful information for the cultivation of AnAOB and further application of anammox.