AWWA WQTC64106

The Membrane Concentrate Pilot Wetland Project was conducted by the City of Oxnard, California between 2003-2005 to investigate use of membrane concentrate as a water source to brackish or salt marsh wetlands. Twelve 1 m³ tanks were constructed of six wetland types, including five flow-through mesocosms (surface flow [SF] high marsh, SF low marsh, horizontal subsurface flow [SSF], peatbased vertical upflow [VF], and submerged aquatic vegetation [SAV]), and a saltgrass evaporation system. The water source was RO membrane concentrate trucked weekly from the Port Hueneme Water Authority's Brackish Water Research Desalination Facility. This paper presents previously unpublished results of supplemental testing of bins re-plumbed as two sequential treatment trains of three cells each to test the hypothesis that a sequence of varied types of wetland treatment cells will provide higher mass removal rates of trace metals than individual cells. Train 1 consisted of VF, SSF, and SAV cells in series, designed as a pretreatment system with the least potential exposure of the concentrate to sensitive ecological receptors. Train 2 consisted of VF, SFHM, and SFLM cells in series, designed to approximate a natural salt marsh gradient of high to low marsh types. Average inflow rates averaged 51.1 gpd (16.3 cm/d HLR) to Train 1 and 56.5 gpd (18.1 cm/d HLR) to Train 2. Train 1 showed varying concentration changes in TDS, SO4, alkalinity, and Cl through the system, from a 16% alkalinity reduction to a 19% Cl increase. Through water loss by evapotranspiration, all four salts showed a net mass removal through the series. Train 2 showed salt concentration increases of 37% for Cl, but mass removals for all salts ranged from 33% for Cl to 59% for alkalinity. Metals (Hg, Cu, Mn, Be, As, Zn) were near or below reporting limits in the concentrate influent and showed little change through the wetland cells. Al typically increased slightly through the wetland. Fe and Se decreased measurably. NO₃ decreased measurably in all systems. TP decreased and COD increased in all systems. Tests on mysid shrimp demonstrated that effluent from both treatment trains was less toxic than raw concentrate water. Chronic toxicity testing results indicated that only raw membrane concentrate samples affected survival of both mysid shrimp and topsmelt. These data suggest that a pretreatment train of wetland systems could be constructed for treatment of concentrate to safely discharge to a natural system. Includes 11 references, tables, figure.