High-recovery reverse osmosis desalination using intermediate chemical demineralization

A two-stage reverse osmosis (RO) process for high water recovery (up to 95%) desalination of Colorado River water was evaluated and demonstrated at the pilot scale. The two-stage process involved intermediate chemical demineralization (ICD) of the concentrate stream from a primary RO (PRO) process followed by secondary RO (SRO) desalting. Using alkaline-induced precipitation in a pilot-scale solids contact reactor (SCR), the membrane scaling propensity of the PRO desalting step was reduced, allowing for further recovery of product water via SRO of the demineralized PRO concentrate stream. When operating the SCR effluent above pH 10, the removal of Ca 21, Ba 21, Sr 21, and silica - the principal scale-forming precursors - were attained via lCD at levels upwards of 94%, 97%, 88%, and 67%, respectively. Despite significant process variations. 95% overall water recovery desalination was demonstrated to be feasible in the pilot scale via the PRO-ICD-SRO approach, with SRO desalting operated at specific permeate flux varying by <= 7% and at stable normalized salt passage of 2.9 0.7% over extended periods of time. This Study suggests that long-term implementation of the PRO-ICD-SRO process to achieve overall water recovery >= 95% would require robust on-line pH control strategy to attain consistent SCR effluent pH, along with appropriate selection and use of make-up antiscalants for the secondary RO desalting step. primarily to control membrane scaling by silicates and gypsum. (c) 2007 Elsevier B.V. All rights reserved.