Synergy of N-(3-oxohexanoyl)-L-homoserine lactone and tryptophan-like outer extracellular substances in granular sludge dominated by aerobic ammonia-oxidizing bacteria

Nitrogen removal via nitrite is an energy-saving method for high-strength ammonia wastewater treatment. A better understanding of the formation of granular sludge dominated by aerobic ammonia-oxidizing bacteria (AerAOB) could facilitate the improved use of rapid sludge granulation for nitritation. In this study, AerAOB-dominated activated sludge (NAS) and granular sludge (NGS) produced different N-scyl-homoserine lactones (AHLs). N-(3-oxohexanoyl)-L-homoserinelactone (OHHL), only released from NGS, was shown to accelerate sludge aggregation by increasing the biomass growth rate, microbial activity, extracellular protein, and AerAOB biomass. For both NAS and NGS, sludge cells were glued together by inner extracellular polymeric substances (EPSs) with similar components to form microcolony. Different from the characterized negative effect of NAS's outer-EPS on cell adhesion, the outer-EPS of NGS played a positive role in the attached growth of AerAOB-dominated sludge and contained more tryptophan-like substances. More interesting, OHHL enhanced the yields of tryptophan-like substances after mixing with the outer-EPS of NGS, enhancing cell adhesion. In a word, OHHL and more tryptophan-like substances were produced in the process of granulation under the selective sludge discharge condition, which was proved to be able to accelerate NAS granulation. Therefore, the sludge granulation process for nitritation can be improved by increasing the levels of OHHL and tryptophan in the initial startup stage. The appropriate engineering strategy should be further studied to facilitate the actual application of granular sludge for nitrogen removal on a large scale.