Can microaeration and hydraulic retention time steer anaerobic microbiomes toward efficient linear alkylbenzene sulfonate degradation?

The sustainable treatment of surfactant-rich wastewater in anaerobic systems is often hindered by the biochemical recalcitrance of linear alkylbenzene sulfonate (LAS) and its inhibitory effects on methanogenesis. This study investigated controlled microaeration under two hydraulic retention time (HRT) and organic loading rate (OLR) regimes as a strategy to intensify LAS degradation and energy recovery in upflow anaerobic sludge blanket (UASB) reactors. Four configurations were evaluated: R1 (anaerobic, 8-h HRT), R2 (microaerated, 8-h HRT), R3 (anaerobic, 16-h HRT), and R4 (microaerated, 16-h HRT). Results demonstrated that under strictly anaerobic conditions, operating at a 16-h HRT and the corresponding lower OLR resulted in higher chemical oxygen demand (COD) removal than at 8-h HRT (87.4% vs. 65.7%). In contrast, microaeration played an important role in enhancing surfactant biotransformation. R4 achieved the highest LAS removal (64.0 ± 6.2%), significantly outperforming the strictly anaerobic R3. The 16-h HRT/lower-OLR configuration provided operational conditions that may have favored the progression of successive LAS biotransformation steps, including oxygenase-mediated ω-oxidation and the transformation of aromatic compounds under microaerated conditions. Notably, R4 exhibited a remarkable specific methane yield of 0.28 ± 0.01 LCH4/gCODapp, doubling the efficiency of the other reactors. Microbial analysis indicated that microaeration was associated with a more diverse consortium in which potentially oxidative bacteria (Smithella and Thauera) coexisted with acetoclastic methanogens (Methanosaeta). These findings indicate that microaeration, when combined with the longer HRT tested, enhanced LAS degradation and process performance compared with the 8-h condition, highlighting the potential of this strategy for process intensification in treatment plants.