Continuous-mode acclimation and operation of lignocellulosic sulfate-reducing bioreactors for enhanced metal immobilization from acidic mining-influenced water

Lignocellulosic sulfate-reducing bioreactors are an inexpensive passive approach for treatment of mining-influenced water (MIW). Typically, microbial community acclimation to MIW involves bioreactor batch-mode operation to initiate lignocellulose hydrolysis and fermentation and provide electron donors for sulfate-reducing bacteria. However, batch-mode operation could significantly prolong bioreactor start-up times (up to several months) and select for slow-growing microorganisms. In this study we assessed the feasibility of bioreactor continuous-mode acclimation to MIW (pH 2.5, 6.5 mM SO42−, 18 metal(loid)s) as an alternate start-up method. Results showed that bioreactors with spent brewing grains and sugarcane bagasse achieved acclimation in continuous mode at hydraulic retention times (HRTs) of 7–12-d within 16–22 days. During continuous-mode acclimation, extensive SO42− reduction (80 ± 20% –91 ± 3%) and > 98% metal(loid) removal was observed. Operation at a 3-d HRT further yielded a metal(loid) removal of 97.5 ± 1.3 –98.8 ± 0.9% until the end of operation. Sulfate-reducing microorganisms were detected closer to the influent in the spent brewing grains bioreactors, and closer to the effluent in the sugarcane bagasse bioreactors, giving insight as to where SO42− reduction was occurring. Results strongly support that a careful selection of lignocellulose and bioreactor operating parameters can bypass typical batch-mode acclimation, shortening bioreactor start-up times and promoting effective MIW metal(loid) immobilization and treatment. [Display omitted] •Lignocellulosic sulfate-reducing bioreactor acclimation was achieved in continuous mode using an acidic mining-influenced water.•Substantial sulfate-reduction and metal(loid) immobilization occurred during continuous-mode acclimation.•Metal(loid) removal (≥ 97.5%) was maintained during operation at a 3-d hydraulic retention time.•Sulfate-reducing bacterial ASVs were most abundant at the influent of spent brewing grains and the effluent of sugarcane bagasse bioreactors.