A metabolomic view of how the anaerobic side-stream reactors achieves in-situ sludge reduction

Anaerobic side-stream reactors (ASR), which were inserted in the sludge recirculation line of activated sludge process, can achieve in-situ sludge reduction. The lysis-cryptic growth occurred in ASR was accompanied with the release and reuse of dissolved organic matter (DOM), resulting in changes in DOM characteristics and affecting the microbial metabolic strategies, ultimately causing lower sludge yield in ASR. However, the variations in metabolic strategies of microorganisms in response to DOM changes in ASR systems remained unclear. Here, the sludge morphological feature, molecular properties of DOM and microbial metabolomics were analyzed to investigate the phenotype discrepancies and intrinsic causes of low biomass yield. Results showed that ASR systems achieved 22.9% sludge reduction efficiency. In the ASR system, the decreased microbial activity (adenosine triphosphate decreased by 10.3%) and unstable flocs structure implied the sludge destruction, causing the accumulation of proteins in extracellular polymeric substances (increased by 58.3%). The micro-organisms in stressed ASR tended to utilize some tough aromatic carbon sources in DOM, finally affecting biomass growth. At the metabolite level, decreased lipid and purine/pyrimidine metabolism pathways activities provided less nucleic acids for synthesis of cellular structure and bacterial proliferation in the mainstream aer-obic tank of ASR system, respectively. Microbes facilitate the metabolism of cheaper amino acids rather than costly ones due to the weaker cell activity. Notably, this change of amino acid utilization strategy would lead to the low level of nucleic metabolism, thus restraining the biomass yield. This study unraveled the low biomass yield mechanisms on sludge-reduction bacteria in response to the change of DOM molecular characteristics, which will help provide a deep theoretical foundation for promoting its full-scale applications.