Feasibility Evaluations for St. Johns River Membrane Water Plant Demineralization Concentrate Management

The St. Johns River Water Management District (SJRWMD) initiated focused studies on the environmental and regulatory feasibility of returning the concentrate from conceptual demineralization facilities in east-central Florida back to the river through new surface water discharges. A work plan was developed in conjunction with the Florida Department of Environmental Protection (FDEP) for an initial planning-level analysis of key environmental and regulatory feasibility criteria that were identified. The work included: 1. Review of the literature 2. Compilation of key hydrologic, water quality, and biological data 3. Preliminary field site reconnaissance; focused on characterization of river channel physical conditions (depths and widths and sediment characteristics) 4. Mass balance analysis for each of the potential demineralization water treatment plant (WTP) locations 5. Evaluation of mixing zone feasibility for the potential outfalls These activities were developed to: 1) provide the basis for an initial fatal flaws evaluation of a set of candidate water supply project study zones located along a portion of the St. Johns River (SJR) and 2) support dilution modeling analyses to directly address the feasibility of gaining regulatory approval of mixing zones that are anticipated to be necessary as part of the NPDES permitting process. Preliminary results indicated that an inland discharge might be feasible, but care is needed in selecting a location for the outfall. Very high recovery of freshwater can yield high concentrations of some parameters that may be of concern. However, there was no fatal flaw found that excluded this management option, pending further development of design concepts. This paper focuses on describing the nature of the concentrate, preliminary modeling results, and the main conclusions of the overall study.