BTEX removal under salinity stress in a microaerobic reactor: Performance and microbial dynamics

Abstract

This study evaluated the impact of increasing NaCl concentrations on the microaerobic removal of BTEX compounds (benzene, toluene, ethylbenzene, and xylenes) from synthetic brackish water using an upflow microaerobic sludge blanket (UMSB) reactor. The reactor operated with a hydraulic retention time of 24 h and was fed with BTEX (∼4.5 mg·L⁻¹ each) and ethanol (1 g COD·L⁻¹). Initially, no salt was added (Stage I). NaCl was then gradually introduced to reach salinities of 1.03 % (Stage II), 1.91 % (Stage III), and 3.42 % (Stage IV). In Stage I, BTEX removal efficiencies ranged from 83 % to 96 %, with benzene being the most recalcitrant compound. As salinity increased from Stage I to IV, removal efficiency declined for all compounds, with toluene showing the smallest decrease (7.5 %) and benzene the largest (∼18 %). Despite these reductions compared to Stage I, the reactor still achieved removal efficiencies of 65.8 % (benzene), 88.7 % (toluene), 78.7 % (ethylbenzene), 72.1 % (m,p-xylenes), and 69.0 % (o-xylene) in Stage IV. Microbial community analysis revealed that salinity markedly restructured the microbial community. Nevertheless, the persistence of key taxa (e.g., Methanobacterium) and the enrichment of halotolerant and halophilic genera (e.g., Brachymonas, Carnobacterium, and Mycobacterium) suggested a degree of functional resilience, as indicated by the maintained albeit reduced BTEX removal, although no direct functional metrics (e.g., enzyme activity) were measured.