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A Continuous Process for RO Concentrate Desupersaturation

  • Author(s): Lei, Jack
  • Advisor(s): Cohen, Yoram
  • et al.
Abstract

Reverse osmosis (RO) desalination of inland brackish water can replenish dwindling water supplies in various regions around the world. However, successful implementation of RO technology requires high product water recovery (>85%) in order to minimize the volume of generated concentrate brine. Therefore, brine management is a critical aspect of inland water desalination. At high water recovery, dissolved mineral salts (e.g. CaSO4, CaCO3) may concentrate above their solubility limits and may crystallize, potentially blocking or damaging RO membrane surfaces, reduce water permeate flux, and shorten membrane life. Therefore, it is essential to reduce the propensity for mineral scaling in order to increase the potential for high product water recovery. Attaining high recovery for inland water desalination, while avoiding membrane mineral scaling, can be achieved via an intermediate concentrate demineralization (ICD) method that utilizes two-step chemically-enhanced seeded precipitation (CESP) process. In the CESP approach, primary RO concentrate is first treated via partial lime softening in which residual antiscalant in the PRO concentrate is scavenged by precipitating calcium carbonate (CaCO3). The filtered lime treated PRO concentrate is then treated in a seeded gypsum (CaSO4•2H2O) precipitation step whereby, gypsum crystal seeds promote rapid crystal growth. As a consequence, the treated PRO stream is desupersaturated with respect to gypsum and upon filtration step; a secondary RO desalting step is carried out to increase the overall product water recovery.

Development of the ICD approach as a continuous process suitable for deployment in RO desalting is the focus of the present study. Accordingly, a novel system for continuous chemically enhanced seeded precipitation (CCESP) pilot was developed and constructed consisting of an alkaline chemical softening flocculation tank followed by a vertical static mixing bed reactor for seeded precipitation. The overall feasible feed slow rate for the pilot CCESP system was 0.026 – 0.25 gpm. Evaluation of the continuous ICD process performance was undertaken with a range of solutions that mimic PRO concentrate produced from desalination of San Joaquin Valley brackish water at a recovery of 63%. The major salts in the PRO concentrate feed to the CCESP included CaCl2 (30.7 mM), Na2SO4 (145.4 mM), MgSO4 (31.2 mM), NaHCO3 (11.4 mM), and NaCl (20.3 mM). Antiscalant (Flocon 260, 5 mg/L) was introduced to the PRO concentrate in order to assess the feasibility for residual antiscalant (typically present in PRO concentrate) removal so as to avoid retardation of the subsequent gypsum desupersaturation step. The CCESP system enabled continuous gypsum desupersaturation by purging spent gypsum seeds and recycling a portion of the seeds or introducing fresh seeds to the fluidized bed. Various gypsum seeds were tested, with a focus on industrial sources for gypsum (e.g. mining, drywall, food, agriculture) due to their availability and low cost. The purity of the gypsum seeds was found to be a key factor, where gypsum seeds with >98% purity were found to be most effective. Using the synthetic PRO concentrate, each of the two steps of the process were first evaluated individually to determine the optimal operating conditions and subsequently combined to evaluate the complete continuous operation. In the CCESP, lime softening occurs in a flocculation tank with recirculation, solids removal from the lime treated stream is via an inline centrifugal separator, and the gypsum seeded precipitation takes place in a fluidized bed. It was found that CCESP treatment of the PRO concentrate with 5.75 mM lime enabled up to 68% removal of the residual antiscalant. Subsequent gypsum seeded precipitation (initial seed loading of 240 g/L gypsum) reduced the PRO concentrate gypsum supersaturation index (SIg) level from 2.36 to nearly unity. The above level of gypsum desupersaturation was assessed to be sufficient for carrying out a secondary RO desalting that would enable increased recovery from 63% at the PRO step to an overall recovery of about 85% and possibly higher.

The present study successfully developed a continuous ICD process and demonstrated its technical feasibility. The present results are encouraging and support the merit of evaluating the process under field conditions. Overall, it is expected that deployment of the CCESP process will enable high recovery desalting of challenging inland water of high mineral scaling propensity.

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