Natural Treatment of Membrane Concentrate for Beneficial Use in Oxnard, California

The Membrane Concentrate Pilot Wetlands Project (Project) was conducted between 2003 and 2006 by CH2MHill and the City of Oxnard as part of a program to desalinate brackish local groundwater for potable water service. The Project assessed the feasibility of using reverse osmosis membrane concentrate to restore local coastal wetlands and investigated water quality effects of different wetland types on contaminants present in the concentrate.The Project consisted of twelve 1 m2 wetland tanks with two replicates of six wetland types. These included five flow-through tanks (surface flow (SF) high marsh (SFHM), SF low marsh (SFLM), subsurface flow (SSF), peat-based vertical flow (VF), submerged aquatic vegetation (SAV)), and a no-discharge saltgrass evaporation tank (SE).Data from the pilot study indicated the following conclusions:• Membrane concentrate supported healthy plant communities for over three years; deleterious effects on plants were not observed.• Both metals and nutrients were effectively removed in many of the wetland system types.• Concentrations of key constituents such as nitrate and selenium decreased to levels at or below toxicological thresholds in some systems.• Due to significant ET losses through all systems, significant mass removals were observed for virtually all chemical constituents including salts.• Discharge from the systems was acutely and chronically less toxic to mysid shrimp and acutely less toxic to topsmelt than raw concentrate.• Salts and other contaminants did not accumulate in plant tissues and sediments outside of ranges found in natural systems and it is expected that such a system would have a lifespan comparable to other common natural treatment systems.Based upon successful results from the pilot systems, the city of Oxnard is currently undergoing design of a large scale demonstration membrane concentrate treatment wetlands facility.