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Performance evaluation of integrated closed-circuit RO/NF desalination and simulated solar energy for the Navajo Nation brackish groundwater
Footracer, Cheyenne
Footracer, Cheyenne
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2021
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Exacerbated by anthropogenic climate change, the water-energy nexus has become a major challenge for rural and remote communities that experience low water supplies, low drinking water quality, and underdeveloped electrical grid, such as in the Navajo Nation reservation in the southern U.S. As a result, desalination systems powered with renewable energy can becomes a viable and attractive option for treating brackish groundwater. This study focuses on removing total dissolved solids and specific contaminants from brackish groundwater using the newly developed closed-circuit desalination (CCD) reverse osmosis (RO) and nanofiltration (NF) processes powered by simulated solar energy for the Navajo Nation. Bench-scale and pilot-scale investigations and computer simulations were conducted to compare RO and NF desalination based on water productivity, product water quality, and energy demand.Desalination of two different synthetic feed solutions, with and without pretreatment, with the BW30 and NF90 membranes (DuPont), were compared at 80%, 90%, and 95% water recoveries. Multiple 50-hour long bench scale experiments were conducted under variable conditions. The NF90 membrane produced water with higher permeate concentrations (~80-400 mg/L) at all recoveries compared to the BW30 (50-100 mg/L), and specific water energy consumption (SWEC) for the NF90 was 0.2-0.9 kWh/m3 compared to 0.2-0.5 kWh/m3 for the BW30. CCD-RO system performance under two simulated solar energy profiles (5-days in winter and summer) were evaluated at 90% recovery with one BW30-4040 membrane on a pilot-scale system. System inefficiencies were determined from SWEC calculations (9-18 kWh/m3) due to small number of membranes installed in the system. However, salt rejections of 85-98% over the course of a day resulted in high quality, drinkable water. Computer simulations with the WAVE and ROSA design platforms determined optimal system design and performance by evaluating different membrane configurations with the intent of producing 3,000 gallons of purified water per day. Results revealed that a CCD system with one BW30-400 or 6-8 BW30-4040 membranes in series will be required to produce good quality drinking water at high recovery (80-94%) and low SWEC (<1 kWh/m3).
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