Nitrate contamination of groundwater sources is a prominent issue in remote small communities that are residing in proximity of agricultural activities. Nitrate exposure through potable water consumption poses multiple human health risks and thus impaired community groundwater sources must be treated to ensure the availability of safe drinking water. In this regard, reverse osmosis (RO) water treatment can be integrated into existing community small water systems for effective nitrate removal and salinity reduction.
RO membrane treatment offers a broad range of protection against multiple different contaminants, but high recovery operation is essential in order to reduce the challenge of managing the discharge from RO treatment. Accordingly, the present research provides a detailed investigation of the technical feasibility of high recovery RO treatment utilizing steady state RO with partial concentrate recycle. Through extensive process simulations, and based on water use patterns in three small communities in the California Salinas Valley, it was concluded that nitrate removal can be achieved to produce treated water at a nitrate level that is significantly below the regulatory maximum contamination level (MCL). The above treatment performance was possible via single and two pass RO treatment for the small remote communities considered in the present study, while enabling sufficient high recovery that would generate a residual stream that can be accommodated in the communities’ septic systems. The process configuration was optimized with respect to the number of RO elements and number of treatment passes. Detailed RO system design specifications were then developed, along with the design of treatment stages (pretreatment, RO module, and post-treatment) to meet the above-mentioned specifications. In addition, the correlation between nitrate passage and salt passage were explored for RO treatment of the source water in the study communities demonstrating that is may be feasible to predict the nitrate concentration in the permeate stream based on measurement of permeate salinity.
RO process design specifications were derived on the basis of optimizing high recovery operation for permeate production capacity for each of the study sites ranging from 1,966 to 5,600 gallons per day. System design was based on treatment of well water of nitrate level of 45 – 389.7 mg/L as NO3-, and salinity in the range of 564 – 1,927 mg/L as total dissolved solids (TDS). RO operation specifications under the production capacity were not to exceed average element recovery of 20%, and single-pass recovery of 15% per element. The daily concentrate stream discharge from the RO systems constituted about 4.9% - 12.5% of the community septic tank capacity for system treatment at 90% recovery operating at recycle ratios ranging from 0.67 – 2.05. Post-treatment of the produced permeate was also considered using a limestone contactor to remineralize and pH stabilize the product water.
