Effect of pretreatment on reverse osmosis membrane performance and an evaluation of the electrosorption of inorganic salts onto carbon aerogels

In order to offset broad, societal costs associated with using mildly brackish Colorado River water (CRW) in coastal Southern California, reverse osmosis (RO)—the industry standard for surface water desalting—and capacitive deionization (CDI) with carbon aerogels—a bench-level prototype technology—were evaluated to reduce the cost of desalinating CRW. The project objectives were to (1) evaluate the effects of pretreatment type (conventional treatment with and without ozone and biological filtration and microfiltration) on RO membrane performance, (2) evaluate the effects of aluminum sulfate (alum) and ferric chloride residuals on membrane flux, and (3) evaluate CDI with carbon aerogel electrodes on the bench-scale to determine its efficacy as a desalting technology. Each pretreatment process was operated in tandem with a RO unit using ultra-low-pressure polyamide membranes. The RO unit was operated at 85 percent water recovery and constant flux (median 14 gal/ft2/day) using both antiscalant and acid addition (pH 7.0) to control for scaling. Additional testing was conducted using effluent from a full-scale drinking water filtration plant using both alum and ferric chloride coagulation prior to RO desalting.; A 2-cell bench-scale CDI unit was used to determine the effects of voltage, flow rate, and pH on ion sorption and desorption to carbon aerogel electrodes. In addition, the affect of ion radii, charge, and mass on carbon aerogel sorption dynamics were evaluated.; Conventional treatment alone leaves the RO unit susceptible to biological fouling, while microfiltration and conventional treatment with ozone/biofiltration allowed for trouble-free RO operation. While testing with 6 to 8 mg/L of alum severely fouled the membrane surface with coagulant residuals within 1,500 hrs of operation, testing with ferric chloride (4 to 6 mg/L) resulting in membrane degradation over the same time period when chloramines were present.; CDI with carbon aerogels may not be a suitable technology for desalination or water softening due to its ability to preferentially sorb monovalent ions (e.g., sodium or chloride) over divalent ions (e.g., calcium or sulfate). Additionally, NOM may foul the aerogel surface; thereby further reducing CDI's applicability to natural water.