Redox environment modulation via microaeration for intensified las removal in uasb reactors: from cometabolism to alternative electron acceptors

Abstract

The biodegradation of linear alkylbenzene sulfonate (LAS) in wastewater treatment systems poses a significant technical challenge, especially in anaerobic environments, where the structural complexity of the benzene ring and the surfactant's toxicity limit biological efficiency. Although aerobic processes are well-established for their high efficiency in surfactant removal, the anaerobic route offers greater economic viability through lower energy consumption, reduced sludge production, and potential for energy recovery through methane. However, the persistent nature of LAS in strictly anaerobic systems limits its practical application. In this scenario, controlled microaeration and the use of redox mediators emerge as metabolic intensification strategies that activate oxidative pathways for aromatic ring cleavage and accelerate interspecies electron transfer, overcoming the thermodynamic barriers of degradation without compromising energy efficiency and the robustness of the anaerobic system. This research investigated, in an integrated manner, the effect of microaeration associated with the selection of cosubstrates (Step I), the supplementation of the redox mediator anthraquinone-2-sulfonate (AQS) (Step II), and the optimization of hydraulic retention time (Step III) on LAS removal, in the performance of upflow anaerobic sludge blanket (UASB) reactors, and the selection of microbial consortia specialized in the destabilization of the aromatic ring. In the first step, the results indicated that the combination of microaeration with sucrose was crucial for process optimization, increasing LAS removal efficiency by 34.3% compared to strictly anaerobic systems. Microbiological analysis revealed that this configuration selected a specialized consortium, highlighting the genera Pleomorphomonas and Desulfovibrio, which acted synergistically in the partial oxidation of the benzene ring and in the desulfonation of the surfactant, without compromising biogas production. Biogas production increased with microaeration under ethanol feeding (2.7 vs. 1.8 L·day⁻¹), while both reactors performed similarly under sucrose feeding (~2.7 L·day⁻¹). In the second stage, the addition of 50 µM AQS alone stabilized LAS removal at 65%, whereas the anaerobic control showed fluctuating performance around 32%. Maximum synergy was achieved with the coexistence of AQS and microaeration, increasing removal to 77% and promoting the enrichment of hydrogenotrophic methanogenic archaea (Methanobacterium), suggesting that AQS acts as an electron bridge to facilitate interspecies transfer and mitigate the inhibitory effects of LAS. The third stage demonstrated that increasing the HRT to 16h under microaeration created an essential "kinetic window" for the activity of oxygenase enzymes, enabling ω-oxidation to proceed more completely. This configuration not only maximized LAS biotransformation (64%) but also doubled energy recovery efficiency, achieving a specific methane yield of 0.55 LCH4.gCODapl, with the stable coexistence of oxidative bacteria (Smithella, Thauera) and acetoclastic methanogens. In an integrated manner, the results demonstrate that managing the redox environment through the strategic combination of microaeration and mediators (AQS) constitutes a robust tool for transforming UASB reactors into high-performance systems for treating effluents containing surfactants, ensuring functional stability and energy sustainability.