Comparative analysis of direct contact and air–gap membrane distillation techniques for water recovery from gold mining wastewater

• Published in: Separation and purification technology 2024-09, Vol.344, p.127300, Article 127300
• Main Authors: Moreira, Victor R., Castro, Livia M.C., Amaral, Míriam C.S.
• Format: Article
• Language: English
• Subjects: Distillation; Economic viability; Effluent water recovery; Polymeric membranes; Sustainability
• ISSN: 1383-5866; 1873-3794
• DOI: 10.1016/j.seppur.2024.127300

[Display omitted] •DCMD and AGMD were empirically compared for water reclamation and energy efficiency.•MD proved effective in treating complex mining wastewater.•Energy efficiency was influenced by the morphology and material of the membrane.•Reuse of waste heat makes MD economically competitive with other membrane processes. This study conducted a comparative analysis of air–gap membrane distillation (AGMD) and direct contact membrane distillation (DCMD) for water recovery from gold mining wastewater. The wastewater is characterized by high concentrations of sulfuric acid and metals, and it is typically produced at elevated temperatures (60 – 80 °C). Distillation tests were performed in both configurations, with commercial membranes made of poly(tetrafluoroethylene) (PFTE) and poly(vinylidenefluoride) (PVDF). It was monitored and compared the fluxes, energy efficiency, and economic viability. AGMD (PTFE: 5.42 L/m2h and PVDF: 1.86 L/m2h) exhibited lower fluxes compared to DCMD (PTFE: 7.16 L/m2h and PVDF: 3.17 L/m2h) due to its higher resistance to mass transfer. However, AGMD achieved a recovery rate of 70 % with high-quality distillate, while DCMD reached a maximum of 30 % due to membrane wetting. From an energy perspective DCMD-PVDF configuration (τDCMD-PVDF: 0.851) had the higher temperature polarization. Consequently, AGMD-PTFE demonstrated superior energy efficiency, yielding greater gain output ratio (GORAGMD-PTFE:> 0.6) and thermic efficiency (TEAGMD-PTFE: ∼ 0.7) and smaller specific energy consumption (SECAGMD-PTFE: < 4 KW/m3). The results highlight that solely increasing distillate flux, as achieved by DCMD when the membrane was changed from PVDF to PTFE, is not the only important variable to consider when designing these systems. Instead, there must be a trade-off between the configuration and the membranes used, finding an equilibrium point for distillate flux and energy efficiency. Economic analyses, considering the use of heat exchangers to maintain desired temperatures, showed that 98 % of membrane distillation costs were associated with energy expenditure. In scenarios with reuse of waste heat and process water as coolant solution, costs could be reduced to values below the currently practiced for fresh water (DCMD: 0.18 US$/m3 and AGMD: 0.14 425 US$/m3), with AGMD emerging as the most cost-effective alternative. In conclusion, this study suggests that the use of AGMD for water recovery is viable, offering a sustainable alternative that avo