(606b) Technical and Economic Feasibility of Reverse Osmosis Reclamation of Agricultural Drainage Water in the San Joaquin Valley

The rising salinity level of agricultural drainage water in the San Joaquin (SJ) Valley is a problem of growing concern because of the threat it poses to California's most productive agricultural region. In order to reduce further buildup of salt in the soil and groundwater, surface and subsurface drains have been installed to collect agricultural drainage (AD) water runoff. However, as environmental concerns grow over disposal of high salinity AD water and declining water quality, membrane RO desalting has been proposed as a possible solution to the salinity/drainage problem. In order to evaluate the viability of inland RO desalination it is imperative that the product water recovery limits are established for the SJ Valley. Recovery limits are imposed by mineral salt scaling, biofouling and osmotic pressure.

One of the complicating factors in determining recovery limits is the significant variation of feed water quality throughout the SJ valley. Accordingly, in order to evaluate the feasibility of RO desalination and associated recovery limits, historical water quality data, provided by the California Department of Water Resources (DWR), were first analyzed by multi-electrolyte thermodynamic solubility calculations. The main mineral salts scalants were found to be calcite (CaCO₃), gypsum (CaSO₄•2H₂O), and barite (BaSO4) with a number of locations in which gypsum and barite were either at or above their saturation levels. Diagnostic laboratory test with field water samples from selected locations in the SJ Valley were performed to evaluate the relative scaling propensity of the different water sources, the biofouling propensity of the source water and feed filtration requirements. The results of the above analyses revealed that RO water recoveries without chemical demineralization would typically be limited to the range of 50% to 80% for AD waters found in the San Joaquin Valley. However, if chemical demineralization by accelerated chemical precipitation (ACP) is employed, recoveries can be attained in excess of 90%. A preliminary economic analysis suggests that even when ACP must be used to achieve reasonable recoveries (greater that 80%), product water can be produced at a competitive cost for brackish water desalination.