Air-gap membrane distillation to desalinate seawater and hypersaline-produced water

Abstract

Air gap membrane distillation (AGMD) offers a promising approach to tackling global freshwater shortages. It achieves this by effectively desalinating seawater and treating complex-produced water from the oil and gas sector. AGMD utilizes a thermal gradient to transport water vapor across a hydrophobic membrane, while an air gap improves separation efficiency by minimizing conductive heat loss. Salinity levels in brackish water, seawater, and produced water vary, with produced water from the Permian Basin in Texas having the maximum salinity and brackish water having the lowest. Treating hypersaline-produced water is challenging. This study systematically assesses the performance of commercial polytetrafluoroethylene (PTFE) membranes under a variety of operational conditions, including feed temperature (40–60°C), flow rate (1–3 L min⁻¹), and pore size (0.1 µm vs. 0.45 µm) to optimize the desalination of brackish water, seawater, laboratory-filtered PW, and pretreated PW. This research tracked the real-time flux reduction with three distinct saline-level water matrices and assessed the permeate water quality. The average flux reduced by 1–7 %, 23–28 %, 45–50 %, and 48–53 %, respectively, for saline water, seawater, produced water (A) and (B) compared to deionized water at a flow rate of 2 L min⁻¹, a 50°C hot feed temperature, and a 27.3°C coolant temperature. The results of all water matrices demonstrated extraordinary salt rejection (>99 %), resulting in a permeate with a total dissolved solids (TDS) concentration of less than 500 mg L⁻¹, which is applicable for agriculture and other beneficial reuse applications. The energy consumptions were ∼6.4, ∼8.2, ∼11.0, and ∼11.4 kWh L⁻¹, respectively, for saline water, seawater, produced water (A), and (B) with a 0.45 μm pore size membrane. Post-membrane characteristics revealed minimal structural degradation, underscoring the durability of PTFE even after exposure to high-strength produced waters. AGMD offers a promising alternative by addressing the drawbacks of traditional desalination techniques, such as the high energy consumption associated with thermal methods and the intense pressure demands of reverse osmosis when dealing with hypersaline sources. It is suitable for scalable and energy-efficient applications. This research proved AGMD to be a sustainable global water security technology, balancing high performance in challenging environments and operational feasibility.