Redox mediators and microaeration boosting the linear alkylbenzene sulfonate removal

Linear alkylbenzene sulfonate (LAS) is one of the most widely used anionic surfactants and is frequently detected in wastewater, while its removal under anaerobic conditions remains limited and unstable. This study evaluated the independent and combined effects of the redox mediator anthraquinone-2-sulfonate (AQS) and controlled microaeration on LAS biodegradation in upflow anaerobic sludge blanket (UASB) reactors. Two reactors treating synthetic wastewater containing 15 mg L^-1 LAS were operated for 90 days: R1, a strictly anaerobic UASB, and R2, a micro-aerated UASB. Each reactor was assessed without (Period I) and with AQS addition (50 μM, Period II). Under strictly anaerobic conditions without AQS, LAS removal was low and highly unstable (32 ± 16%), with pronounced performance fluctuations. The addition of AQS alone significantly improved LAS removal to 65 ± 4% without affecting methane production, indicating enhanced LAS transformation via electron shuttling rather than changes in methanogenic activity. Microaeration combined with AQS yielded the best overall performance, increasing LAS removal to 77 ± 1%, COD removal to 92.3 ± 3.8%, total methane production to 5.6 ± 0.7 L/day, and specific methane yield to 0.50 ± 0.11 LCH₄·gCOD_app^-1. Microaeration without AQS improved process stability but did not achieve LAS removal comparable to AQS. Microbial analyses showed that AQS promoted the enrichment of hydrogenotrophic methanogens, particularly Methanobacterium, and syntrophic taxa such as midas_g_2229, while microaeration increased microbial richness. The combined condition exhibited the highest archaeal enrichment and microbial richness. Results demonstrate that AQS and microaeration act through complementary mechanisms, and that their combination effectively overcomes limitations of anaerobic LAS removal.