Nanofiltration and reverse osmosis evaluation in gold acid mine drainage treatment aiming water reuse

Abstract

Acid mine drainage (AMD) is an effluent characterized by low pH and high concentrations of sulfate, metals, and metalloids. AMD treatment by membrane separation processes (MSP), specifically nanofiltration (NF) and reverse osmosis (RO) is particularly interesting as theseprocesses can retain divalent ions efficiently to produce high quality permeate for industrial reuse. The literature suggests the technical viability of these processes; however, further study is needed, in special on operational parameters optimization, on economic viabilityevaluation, on chemical cleaning strategies, and on membrane ageing. Therefore, the aim of this study was to evaluate the main operational conditions of the AMD treatment by MSP, optimize these parameters, and conduct a preliminary capital and operational cost evaluation (Chapter 2). Then, it aimed to evaluate the membrane fouling formed during AMD treatment, evaluate the best chemical cleaning procedure in this conditions, and the membrane ageing after prolonged exposure to AMD retentate and to the chemical cleaning solution (Chapter 3).The results showed that although effluent pretreatment had no considerable effect on the membrane-fouling tendency, it improved the final NF permeate quality. The NF showed higher potential for AMD treatment than RO, as it had higher permeate flux and satisfactorysolutes retention efficiency. Among the NF membranes, the NF90 had the highest solutes retention efficiency, while the NF270 had the highest permeate flux. Effluent pH affected both the retention efficiency and the membrane-fouling tendency. The best combination of membrane type and feed pH was the NF270 at pH 5.5. The maximum water recovery rate at this condition was 60%, when a sharper decrease in retention efficiency and permeate flux was observed. The estimated capital and operational costs of the UF-NF system were US$ 131,250.00, and 0.257 US$/m3 of effluent. The AMD is mainly comprised by dissolvedinorganic compounds, which resulted in an inorganic fouling layer rich in aluminum, arsenic, calcium, chromium, nickel, potassium, and sodium. Among the evaluated cleaning solutions, the best cleaning agent was hydrochloric acid (HCl) at a concentration of 0.20% w/w; this concentration also provided the lowest membrane exposure to the acid. Membrane ageing caused by AMD retentate, and AMD retentate plus periodic HCl solution decreased the NF270 water permeability after 270 days by 49 and 45%, respectively. However, the membrane rejection to magnesium sulfate and glucose decreased less than 10% in bothconditions. These results suggest that the NF270 membrane is stable during AMD treatment.