Exogenous addition of cellular extract N-acyl-homoserine-lactones accelerated the granulation of autotrophic nitrifying sludge

Granulation of nitrifying bacteria could achieve high biomass concentration in treatment system and result in the higher rate of ammonia oxidation and removal even under short hydraulic retention time. However, it is a major challenge to achieve granular sludge in nitrification stage. The addition of chemically synthetic N-acyl-homoserine lactones (AHLs) into bioreactors could enhance the attached-growth of nitrifying microorganisms from sludge and followed by sludge granulation, but it is not suitable for full-scale operations due to the high cost. In this study, cellular extract of AHLs from nitrifying granular sludge (NGS) was added into the nitrifying activated sludge to investigate the effect of the NGS cellular extract on the adhesion and granulation of autotrophic nitrifying sludge. The results indicated that NGS contained AHL molecules and the exogenous addition of the NGS cellular extract could improve the cell adhesion and aggregation. Moreover, addition of the NGS cellular extract increased the microbial activity, the concentration of extracellular proteins (PN) and the biomass of autotrophic nitrifying bacteria, which eventually contributed to the nitrifying sludge granulation. Addition of exogenous cellular-based AHL from NGS in the early start-up stage is a potentially promising approach for full-scale application of nitrifying sludge granulation with reduced cost.