BIOLOGICAL PRE-TREATMENT OF BRACKISH GROUNDWATER USING SHALLOW, UNIT PROCESS OPEN WATER WETLANDS

Due to changes in climate and hydrologic cycles, arid regions may become increasingly vulnerable to decreasing water supply and require additional sources of potable water. Brackish groundwater offers the potential to supplement existing water resources, particularly in inland arid states such as Colorado. At present, desalination using Brackish Water Reverse Osmosis (BWRO) presents a unique set of challenges that include membrane fouling due to elevated concentrations of inorganic precipitates and the production of brine concentrate that can harbor contaminants such as heavy metals. A possible mechanism for increasing BWRO efficacy is by integrating a pretreatment step that uses diatom-driven photosynthesis within shallow, unit process open water (UPOW) wetlands. The photosynthetic diel (day/night) cycling in the wetlands, which passively increases pH during the day as carbonate is consumed, results in alkaline conditions that are favorable for cation precipitation without exogenous chemical additions. Past work in freshwater systems has established that through processes such as sorption, precipitation, and biotransformation, the microbial mat (biomat) that naturally forms within UPOW wetlands has the potential to reduce scalants (calcium, magnesium, and sulfates) and heavy metals, as well as oxidize challenging constituents such as arsenite, from the water prior to membrane treatment. Therefore, laboratory-scale UPOW wetlands were created using biomat harvested from an operational field-scale freshwater constructed wetland and challenged with brackish water. Preliminary results show 30% – 40% reductions in scalant concentrations, which, along with heavy metal oxidation, can prevent excess membrane fouling and enhance RO rejection. This approach promotes environmental sustainability by eliminating the need for chemical additions, reducing brine concentrate volumes, and by decreasing energy and labor requirements. This integration of nature-based solutions into membrane-based treatment offers an opportunity to harness geomicrobial processes as a component of engineered systems to mitigate technoeconomic and environmental challenges associated with BWRO.