Comparison between Compact Accelerated Precipitation Softening (CAPS) and conventional pretreatment in operation of brackish water reverse osmosis (BWRO)

Preliminary tests have previously been conducted on compact accelerated precipitation softening (CAPS) as a pretreatment for membrane processes operating on various natural and industrial waters. In the present study, we compare CAPS efficacy for desalting brackish groundwater from the Negev region of Israel (original pH = 7.8) to that for conventional pretreatment, namely, dual media filtration (DMF) with acid or antiscalant. Samples of groundwater (400 L) were pretreated alternately by CAPS and conventional means and concentrated > 8-fold in a batch desalination experiment with 2.5 in. × 40 in. spiral low-pressure reverse osmosis (RO) elements. The pure water flux of the membrane element was preserved in an experiment reaching nearly 90% recovery from CAPS-treated water adjusted to pH 7.6, whereas it declined to 30% of its original value after desalting the brackish groundwater treated by dual media filtration and adjusted to the same pH. In runs comparing CAPS pretreatment and conventional pretreatment with the supplemental use of antiscalant or acid, membrane processes operated with a CAPS-treated feed stream required less antiscalant and acid than those operated with conventionally pretreated water. In addition, the membrane modules required no cleaning at the end of the experiments, whereas membranes operated with conventionally pretreated water required significant cleaning to restore the pure water flux obtained at the beginning of each run. A comparison is made of the chemical requirements and brine disposal cost associated with each type of operation.