Pilot plant evaluation of advanced vs. conventional scale inhibitors for RO desalination

Abstract

An advanced anti-scalant, consisting of a polyacrylate and hydroxyethylidene diphosphonate (HEDP), was tested against the conventional H 2SO 4 and sodium hexa-meta-phosphate (SHMP) inhibitors in a pilot plant which had parallel RO units. The two units were fitted with identical brackish hollow fine fiber (HFF) permeators arranged in a two-stage mode which received the same brackish feed water, and were operated at the same feed pressure (27.6 bar/400 psig) and product water recovery (70%). Unit #1 was treated 9 ppm of as-received advanced anti-scalant, and unit #2 was treated with 6 ppm of SHMP and about 130 ppm of 98% H 2SO 4. The pilot plant trial lasted for 3,000 h. The techno-economic evaluation of the two scale control treatments was carried out in terms of pumping energy, the anti-scalants consumed, and the quality and total output of the product water produced in 3,000 h. The pumping energy and scale control treatment costs (as $/m 3 of water produced) were same for the two treatments; but in terms of salt rejection, product TDS and output, performance of the advanced anti-scalant was inferior; after 3,000 h the salt rejection of unit #1 (which operated on the advanced anti-scalant) was about 81% vs. 94% for unit #2 (H 2SO 4+SHMP). Similarly, in 3,000 h unit #1 produced 20% less water of unacceptable quality (620 ppm) whereas unit #2 produced more water of excellent quality (180 ppm), providing a clear testimony to the superior performance of the conventional H 2SO 4+SHMP treatment over the advanced anti-scalant.