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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 Ca2+, Ba2+, Sr2+, and silica – the principal scale-forming precursors – were attained via ICD 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.

The integration of membrane RO desalting and accelerated precipitation softening (APS) was evaluated for achieving high product water recovery (>95%) from desalting of mildly brackish surface water. A systematic approach that included laboratory RO demonstrations, membrane scaling diagnostics, and mineral solubility analysis was undertaken to develop an effective physical/chemical APS strategy for reducing the concentration of scale-forming ions in the primary RO (PRO) concentrate. APS demineralization, as an inter-stage process, between the PRO and secondary RO (SRO) desalting, involved alkaline pH adjustment and calcite crystal seeding of the PRO concentrate, followed by microfiltration and pH reduction by acid dosing to avoid calcite scaling in the SRO stage. Colorado River water desalting case study demonstrated that, at 90% PRO recovery, the PRO concentrate stream contained barite, calcite, gypsum, and silica above their solubility limits by factors of 122, 47, 1.1, and 0.8, respectively. Further desalting was not feasible with traditional scale-control strategies (e.g., pH reduction and antiscalant addition). However, APS demineralization enabled significant concentration reduction of calcium (>90%), barium (>95%), and strontium (∼78%) and moderate reduction (10–20%) of magnesium and silica. APS kinetics were favorable even in the presence of antiscalant carryover from the PRO stage. The study demonstrated that high recovery desalination of up to 98% is feasible with the PRO-APS-SRO sequence, which reduces the limitations imposed by membrane scaling.

Reverse osmosis testing using monochloramine (NH 2 Cl) and free chlorine (HOCl) in the presence of ferrous iron [Fe(II)] resulted in accelerated chlorination of polyamide membranes. Similar effects were not observed when using ferric iron. Membrane damage was detected through irreversible increases in salt passage and the presence of chloride on the membrane surface using energy-dispersive spectroscopy. A mechanistic study suggested that the formation of an amidogen radical ( NH 2 ) during NH 2 Cl decomposition with Fe(II) led to the reduction of the activation energy for the chlorination reaction to proceed using NH 2 Cl. Enhanced oxidation using HOCl and Fe(II) may have resulted from the formation of OH radicals, which also lowered the chlorination-activation energy. Both pH suppression (from 8.0 to 6.0) and dechlorination (7.0 mg/L sodium sulfite) successfully halted the chlorination reaction.