Field evaluation of membrane distillation technologies for desalination of highly saline brines

• Two MD pilot units were tested side-by-side to treat seawater and thermal brines.
• MD was able to consistently produce high quality distillate (< 10 μs/cm).
• Operating conditions were optimized; unit A operated at 6.2 LMH and 52% recovery.
• Pretreatment is critical to avoid wetting and ensure proper system performance.
• Antifoam wets the MD membranes and activated carbon was an effective pretreatment.

Membrane distillation (MD) is a hybrid thermal-membrane desalination process that uses low-grade waste heat and hydrophobic membrane to produce high quality distillate. The MD process can treat highly saline brines that other conventional desalination processes cannot treat. These unique features of the MD process make it an ideal candidate to desalinate concentrated brines from thermal desalination plants to augment fresh water production from existing facilities.A consortium consisting of ConocoPhillips Global Water Sustainability Center, Qatar University, and Qatar Electricity & Water Company was formed to evaluate the application of MD for the desalination of concentrated brines from thermal plants. Five different MD technologies were evaluated and the two most suitable technologies were selected for field-testing. The pilot units A & B are based on multi-effect vacuum and air gap MD technologies, respectively. These units were tested side-by-side at a full-scale thermal desalination plant in Qatar. Pilot unit A showed a stable flux of 6.2 LMH under optimized conditions with excellent salt rejection (> 99.9%). Pilot unit B achieved a distillate flux of 2.5 LMH and salt rejection greater than 98.9%. Overall, MD was shown to be a feasible technology to produce potable quality water from the brines discharged from thermal desalination plants.