AWWA WQTC58994

Ultra-low pressure reverse osmosis (RO) membranes were used for groundwater softening during a 3.5-month pilot evaluation conducted by a Northern California water district. Completed in January 2002, this predesign-focused pilot study was performed as one component of the District's comprehensive hardness reduction program. RO membranes were chosen as the most appropriate hardness reduction technology for a number of reasons including the benefit of pathogen protection provided by membrane separation. The primary objectives of the RO pilot testing program included assessment of the following issues: ability of RO membranes to remove hardness in order to meet the District's production water hardness goals; rejection of heavy metal species in the RO concentrate (brine) regulated by the Regional Water Quality Control Board (RWQCB) under the San Francisco Bay Basin Water Quality Control Plan (Basin Plan) and the California Toxics Rule; toxicity of RO concentrate to aquatic species in freshwater and marine environments; and, treatability of local groundwater to establish full-scale plant design criteria and scale formation potential by sparingly soluble salts. The focus of this discussion will be: the determination of the maximum viable system recovery that the RO membranes in this study could achieve without evidence of scale formation; and, the evaluation of the need for pH adjustment via acid addition for control of calcium carbonate scaling and anti-scalant addition for control of barium sulfate scale. Testing was conducted over a range of RO system recoveries in order to challenge the limits of conventional RO design. Specifically, the pilot study system recoveries and experimental conditions were adjusted to evaluate the edge of failure with respect to anti-scalant addition, pH depression, and precipitation of scaling salts. Includes tables, figures.