Boron rejection in SWRO at high pH conditions versus cascade design

Orthoboric acid dissociation, which affects boron rejection in RO membranes, depends on salinity, temperature and pH. Lately, increasing demand of permeate low boron concentration raises questions regarding the optimal method for boron removal in both technical and economical aspects. Several methods were presented in previous papers for boron removal, using RO membranes (SW & BW RO), at elevated pH and/or ion exchangers. In this paper, tests results of a full scale plant operation will be presented, at elevated pH of sea water feed to the RO membranes. Further, a techno-economical comparison will be presented, comparing two options for boron removal: SWRO operation at elevated pH versus Cascade design (IDE’s patented design).

• Short period field tests of high pH operation in a full scale plant have shown no evidence of scaling. However, additional tests for prolonged period of time and at higher sea water temperatures are required to assure scale free operation at such conditions. Optimization of antiscalant dosage is required. • Once scaling free operation is assured, then both designs, cascade system and two pass design can operate at high pH conditions of pH 8.1–8.3. Even then, the chemicals cost increase the overall operational costs of the plant. • Cascade design assures highly reliable process, better than the two pass design with recycle. It is capable of adjusting operational conditions as per plant temporal needs, and decrease the water cost, especially for sites with warm temperature conditions.