Long-term performance evaluation of an anoxic sulfur oxidizing moving bed biofilm reactor under nitrate limited conditions

Abstract

An anoxic sulfur-oxidizing moving bed biofilm reactor (MBBR) treating sulfur and nitrate-contaminated synthetic wastewater was monitored for 306 days under feed nitrogen-to-sulfur (N/S) molar ratios of 0.5, 0.3 and 0.1. Thiosulfate (S₂O₃²⁻) removal efficiencies (RE) exceeding 98% were observed at a N/S ratio of 0.5 and a S₂O₃²⁻ loading rate of 0.9 g S₂O₃²⁻–S L⁻¹ d⁻¹, whereas a RE of 82.3 (±2.6)% and 37.7 (±3.4)% were observed at N/S ratios of 0.3 and 0.1, respectively. Complete nitrate (NO₃⁻) removal was obtained at all tested N/S ratios. A comparison of the kinetic parameters of the MBBR biomass under the same stoichiometric conditions (N/S ratio of 0.5) revealed a 1.3-fold increase of the maximum specific rate of S₂O₃²⁻ oxidation (r_max) and a 30-fold increase of the affinity constant for S₂O₃²⁻ (K_s) compared to those observed after long-term NO₃⁻ limitation (N/S ratio of 0.1). The MBBR showed optimal resilience to NO₃⁻ limitation as the S₂O₃²⁻ RE recovered from 37.3% to 94.1% within two days after increasing the N/S ratio from 0.1 to 0.5. Based on PCR-DGGE analysis, sulfur-oxidizing nitrate-reducing bacteria, i.e. Thiobacillus sp. and Sulfuritalea sp., dominated in the MBBR biofilm during the entire study.