Pilot-Scale Testing of the Precipitative Softening-Secondary RO Process to Reduce RO Concentrate Volume

To comply with the Santa Clara River Chloride Total Maximum Daily Load, the Sanitation Districts of Los Angeles County plan to implement an advanced water treatment facility at the Valencia Water Reclamation Plant (WRP) consisting of microfiltration (MF) followed by reverse osmosis (RO) for chloride removal. Due to the inland location of the Valencia WRP, RO concentrate disposal represents a significant fraction of the overall project costs. To address this issue, the precipitative softening - secondary RO process was evaluated to determine its viability for minimizing RO concentrate disposal volume.Bench-scale precipitative softening tests were initially conducted to evaluate the removal of scale-forming constituents from RO concentrate. The tests illustrated the effectives of the process for the removal of calcium, barium, strontium, and silica. The results also illustrated the effective removal of magnesium. Although not a concern for RO membrane fouling, magnesium removal is important to the removal of silica.Pilot-scale testing was subsequently conducted to verify the results from bench-scale testing and to evaluate the operation and performance of the secondary RO process. A pilot-scale precipitative softening system was operated for approximately four months, treating concentrate from a pilot-scale primary RO system that was operated at 85% recovery. Average removals for calcium, silica, barium, strontium, total phosphate, fluoride, iron, and aluminum were 91.7%, 68.0%, 87.8%, 83.2%, 80.7%, 26.7%, 71.3%, and 82.2%, respectively. Based on this performance and preliminary scaling projections, a pilot-scale secondary RO system was operated at 75% recovery. The secondary RO process was operated for approximately two weeks before a membrane clean-in-place (CIP) was required. The CIP was conducted using generic high pH and low pH cleaning solutions and restored the membranes to within 10% of their initial condition as measured by feed pressure requirements. Although long term performance of the secondary RO system was not determined during the study, scaling projections of the concentrate indicated that fouling was likely due to organics or biofouling. Both types of fouling can be cleaned using generic cleaning solutions, as was demonstrated in the study. The results of this study demonstrate that precipitative softening - secondary RO is a viable concentrate volume minimization process and may be used at the Valencia WRP to reduce disposal volume by up to 75%.