AWWA MTC95008

Reverse osmosis (RO) concentrate produced by the Island Water Association (IWA) water treatment plant (WTP) is disposed of by discharge, at an approximate rate of 1.05 million gallons per day (MGD), at an ocean outfall located approximately 600 feet offshore of Sanibel Island, Florida. RO concentrate is classified by the Florida Department of Environmental Protection (FDEP) and the USEPA as an industrial waste, therefore requiring a discharge permit. In early 1993, the IWA, through routine sampling of the concentrate at the WTP found that Class III marine water quality standards were being exceeded for the parameters of dissolved fluoride and radium 226. It was concluded by the IWA that a hydrographic study should be undertaken for the purpose of providing site specific discharge information to support an application for a modification of the discharge permit. The modification would consist of the establishment of mixing zones for those water quality parameters in noncompliance. The hydrographic study was undertaken at the site during "worst case" tidal flushing conditions. These conditions occur approximately 20 times per year at the site during periods of low low tides. The hydrographic study characterized site specific hydraulic (water depth, current velocities) and water quality conditions during such a period of low low tide in late September, 1993. Relatively strong longshore current velocities were found to occur at the site even during "worst case" conditions. Site specific hydraulic data, including quantification of the outfall characteristics, were input, along with the concentrate water quality data, into a commercially available EPA expert-type computer model (CORMIX) to generate cross-sectional graphs of concentrations for the three water quality parameters versus distance along the azimuth direction of flow of the concentrate plume. The hydrographic study established that all regulatory criteria for permitting of mixing zones for the parameters of dissolved fluoride, radium 226, and acute toxicity, could be met at the site. Based upon the results of the study, the FDEP issued the discharge permit modification to include the requested mixing zones for those three parameters.