CONCENTRATE CHEMICAL SOFTENING TO IMPROVE THE RECOVERY OF BRACKISH WATER REVERSE OSMOSIS DESALINATION

摘要

Access to affordable water has become the limiting factor in growth for arid (and even semi-arid) regions of the world. Reverse osmosis (RO) of seawater and brackish groundwaters is one solution to this problem. However, the cost of concentrate disposal is high (up to 70% of operational costs) for inland brackish groundwater RO (BWRO) processes. Antiscalants are used to prevent precipitation on the RO membrane surface and improve recovery, but their range is limited. This research focused on a side-stream process that takes the RO concentrate stream and inactivates the antiscalants with "peroxone" (ozone plus hydrogen peroxide). The concentrate is then exposed to conditions favorable for precipitation (elevated pH) before undergoing a solid/liquid separation process. The permeate of the solid/liquid separation is then sent to a final RO membrane to increase the overall recovery of the system. Ozonation, in combination with a small increase in precipitation pH, was found to be ineffective at achieving optimal calcium precipitation for two synthetic water recipes (representative of Maricopa County, Arizona and Martin County, Florida) and concentrate from an actual BWRO plant in Cameron County, TX. A dramatic increase in the precipitation pH (and therefore the calcium carbonate and dolomite saturation ratios) achieved the greatest improvements in the resulting water quality; ozonation made little difference at the high pH precipitation conditions. BWRO plants that implement this side-stream process may achieve significant recovery improvements to existing operations.