Bacterial and Microfauna Mechanisms for Sludge Reduction in Carrier-Enhanced Anaerobic Side-Stream Reactors Revealed by Metagenomic Sequencing Analysis

Packing carriers into the anaerobic side-stream reactor (ASSR) can enhance sludge reduction and save footprint by investigating ASSR-coupled membrane bioreactors (AP-MBRs) under different hydraulic residence times of the ASSR (HRT_SR). Three AP-MBRs and an anoxic-aerobic MBR (AO-MBR) showed efficient chemical oxygen demand (>94.2%) and ammonium nitrogen removal (>99.3%). AP-MBRs have higher (p < 0.05) total nitrogen (61.4–67.7%) and total phosphorus (57.5–63.8%) removal than AO-MBRs (47.8 and 47.7%). AP-MBRs achieved sludge reduction efficiencies of 11.8, 31.8, and 36.2% at HRT_SR values of 2.5, 5.0, and 6.7 h. Packing carriers greatly improved sludge reduction under low HRT_SR and is promising for accelerating sludge reduction in compact space. Metagenomic sequencing analysis showed that genes responsible for metabolism were enriched in AO-MBRs, while genes related to cellular motility and cell signaling were more abundant in the AP-MBRs. A longevity-regulating pathway showed that long lifespan provided more opportunities for worms to prey bacteria. Microscopic examination revealed that some specific protozoa (Arcella, Clathrulina, Aspidisca, Litonotus, Chiloclonella, and Vorticella) and metazoa (Rotaria and Aeolosoma hemprichi) were enriched in ASSRs. Aeolosoma hemprichi was only detected in ASSRs, and unique Cylops appeared on carriers. These results contribute to growing understanding of micrometabolic mechanisms including functional genes and microfauna-driving sludge reduction.