Last update:
10/28/2004
Accelerated Precipitation Softening as Pretreatment for Secondary RO Desalination
Low pressure reverse osmosis processes offer a unique opportunity for membrane desalting of high salinity agricultural drainage (AD) water, surface water and waste water. For example, salinity of AD water in the San Joaquin Valley (California) is in the range of about 3000-15,000 mg TDS/L and Colorado River water salinity is expected to soon exceed 1000 mg TDS/L. At present, water recovery is usually kept at 75%-85% (even with the use of water soluble polymeric antiscalants) to avoid the formation of mineral salt scale on the membrane surface. In the above range of water recovery, the brine volume is significant and thus represents a loss of valuable water resource and a major disposal challenge. At higher levels of water recovery the concentration of ions on the membrane feed-side may exceed the solubility limits of calcium sulfate, calcium carbonate, barium sulfate and possibly other salts. As a result, crystallization of these sparingly soluble salts may take place, leading to permeate flux decline and shortening of membrane life.
Our research is focused on evaluating the feasibility of increasing RO water recovery by combining accelerated seeded precipitation removal of divalent cations with the added use of polymeric antiscalants. In this approach brine from primary RO desalting is treated by an accelerated precipitation (ACP) process, via calcium carbonate seeding coupled with pH adjustment to about pH~9-10.5, to achieve 80%-90% removal of the major divalent cations (i.e., calcium, barium and magnesium). The treated brine is subsequently filtered and desalted to achieve a total product water recovery of 95%-98%. Precipitation studies, in both a small crystallization vessel (600 ml) and in a 25 L crystallizer, demonstrated optimal conditions (residence time in the crystallizer, pH level and loading and size of seed crystals), with respect to the removal of divalent cations. At the optimal conditions, desupersaturation reduced the concentration of calcium toward saturation with respect to its mineral salts with sulfate and carbonate anions. Precipitation induction times for the treated solutions were sufficiently long such that mineral salt scaling of the membrane in the secondary RO desalting was avoided. The effectiveness of ACP and antiscalant treatment has been evaluated via membrane scaling tests, with brine produced from both primary and secondary RO desalting stages, using a plate-and-frame diagnostic RO unit operated in a total recycle mode. The impact of scale formation was quantified in by flux decline measurements. Scaled membranes were analyzed by elemental surface analysis and by optical surface imaging to determine the surface density and axial distribution of surface scale. In addition, high resolution scanning electron microscope (SEM) images were obtained to identify and quantify surface crystal morphology.
Achievement of high product water recovery may also require the use of polymeric antiscalants to further retard the onset mineral salts crystallization (e.g., calcium sulfate, calcium carbonate and barium sulfate). The current study revealed that antiscalants effectiveness varies depending on the membrane types, the presence of colloidal silica, and the presence of aluminum cation. Aluminum, which is often used as a coagulant (i.e., alum) in conventional water treatment, can reduce the effectiveness of antiscalants, especially with respect to mitigating calcium sulfate scaling. Therefore, it is important to screen antiscalants with respect to their effectiveness in the presence of aluminum. TO date, our studies have shown that it is feasible to reach 95%- 98% overall product water recovery. Limited pilot-scale studies have suggested that optimization of the desalination process will require careful membrane selection, and optimization of both water pretreatment and process conditions so as to minimize the potential for membrane surface scaling.
