Efeito da estratégia de alimentação do AnSBBR utilizado no tratamento de drenagem ácida sintética de mina

Abstract

The aim of this work was to evaluate the feeding strategy effect on anaerobic sequencing batch reactor with recirculation of liquid phase and immobilized biomass on polyurethane foam (AnSBBR) used for treating synthetic acid mine drainage (AMD). Three affluent sulfate concentrations related to three applied sulfate loading rate were tested: 1,5 g.L-1.d-1 in condition A, 3,0 g.L-1.d-1 in condition B and 4,5 g.L-1.d-1 in condition C. For each condition, feeding strategy transition was made after system stabilization and time profile data collection. Thus, strategies were applied always in this order: batch, fed-batch (2h) and fed-batch (4h). In all situations, synthetic wastewater affluent pH varied between 3,0 and 3,3 and dissolved iron, cooper and zinc affluent concentrations were kept constant at 100 mg.L-1, 20 mg.L-1 e 5 mg.L-1, respectively. Whey was used as carbon source and added to synthetic AMD to reach COD/SO42- ratio equal to 2,0. System was always fully discharged and able to treat 2,8 L of wastewater in 8h cycles at 30±1 ºC. Reactor was innoculated with anaerobic biomass from a bench scale UASB used for same wastewater treatment. Before starting AnSBBR operation, abiotic hydrodynamics assays were assessed proving liquid phase recirculation provided system complete homogenization in an irrelevant time interval when compared to total cycle time and flow through reactor bed approaches plug flow. During batch phase in condition A, metabolic pathway of sulfetogenesis predominated in the system, providing average sulfate removal efficiency of (97±1)%, average iron removal efficiency of (95±1)% and average soluble COD removal efficiency of (61±1)%. By increasing filling time, AnSBBR performance gradually decreased in that condition. Sulfate average removal efficiency was equal to (90±3)% and (77±3)% for 2h fed batch and 4h fed batch, respectively. In contrast, gradual filling strategy was satisfactory for the system in condition B. With higher applied organic loading rate, just (40±8)% of SO42- influent molecules were reduced in batch mode because hydrolytic and acidogenic bacteria were more adapted to wastewater conditions and outcompeted sulfate reducers bacteria (SRB). However, 4h fed-batch caused higher organic matter availability, decreased substrate competition and enhanced SBR activity, reaching average sulfate removal efficiency of (61±1)%. Average iron removal efficiency decayed with filling time increase and organic matter removal was statistically the same for all phases, ensuring a higher organic molecules destination to sulfetogenesis metabolic pathway in 4h gradual feeding. In condition C, ANOVA tests pointed average COD and SO42- removal efficiencies were the same for all feeding strategies studied. Nevertheless, maximum SO42- removal efficiencies, (62±8)%, and higher operational stability were achieved on 4h fed batch mode. The results of this study show batch strategy is the best option for this synthetic AMD treatment on AnSBBR when applied organic loading rate reach values approaching 3,0 gDQO.L-1.d-1, and gradual feeding strategy of 4h ensures higher SO42- reduction for applied organic loading rates varying between 6,0 gDQO.L-1.d-1 and 9,0 gDQO.L-1.d-1.