Indicative impacts of sludge properties and biological metabolic characteristics on high-rate contact stabilization process performance

Regarding curtailing carbon emissions in wastewater treatment, the high-rate contact stabilization (HiCS) process outperforms others in removing dissolved organic matter (DOM) but struggles with poor settling performance. To boost operation performance and clarify the correlation between process parameters, DOM variations, effluent quality, and microbial metabolism within the HiCS system, the impacts of sludge properties on sludge settlement and organic matter (OM) capture efficiency were explored, and soluble fluorescent components in the DOM and extracellular polymeric substances (EPS) were identified and scrutinized. Results unveil that the feast/famine (F/F) regime in the HiCS process predominantly governs sludge activation in the stabilization phase, influencing sludge properties such as morphology characteristics, biological activity, and EPS secretion. At the same hydraulic retention time, reducing the sludge retention time (SRT) led to looser and smaller activated sludge flocs, increased microbial activity, and higher EPS production, particularly protein content in loosely bound EPS (LB-PN), which adversely impacted settling performance. High-throughput sequencing revealed that richness and diversity of the microbial community decreased with SRT. Acidobacteriota and Patescibacteria, associated with nitrifying and denitrifying bacteria, significantly decreased. EPS-producing Firmicutes increased, enhancing EPS secretion, while filamentous Chloroflexi decreased, aligning with a reduced organic mineralization rate. Settlement and biological activity emerged as key factors affecting OM recovery, peaking at 43.97% with a 4-day SRT. The ratio of the sum of tryptophan-like and tyrosine-like components to fulvic-like components ((C1+C2)/C3) was proposed as a fluorescence indicator, serving as a hub to connect operational parameters, F/F regime, sludge status and process performance. When this ratio falls within the range of 1.04–1.36 during the stabilization phase, HiCS sludge achieves optimal status for OM capture with low aeration energy consumption.