Successful antiscalant field trial — Optimization at higher pH and seawater temperature — Larnaca Desalination Plant

Abstract

A B S T R A C TLarnaca Desalination Plant (LDP) has lead the way in operating for a number of years at elevated pH both at the first and second Reverse Osmosis (RO) stages at higher feed sea water temperature up to 30 C. The main reason for the higher pH was to enhance the boron rejection capability of existing membranes and thus reduce the need for a second stage at lower sea water temperatures and subsequently produce more water at less energy. However, higher pH in conjunction with high sea water temperatures create conditions for membrane scaling. Therefore an appropriate cost effective antiscalant has to be used with minimum dosing rate. This article describes field trials of choosing and applying an appropriate antiscalant and dosing optimisation as a function of feed sea water temperature and pH.

The impact of sulphuric acid replacement by a phosphonate-based antiscalant on operational costs of seawater desalination

“…For high TDS seawater, boron rejection by the membrane is better at lower temperatures [4]. The lower temperatures allow the plant not to require the second pass to be operated in the winter mode.…”

“…In a two-pass boron removal (BR) system (SWRO design studied in this article), the calcium carbonate scale can occur at both pass of the process, while the magnesium hydroxide normally represents a scaling risk only on the second pass. One reason for the high scaling tendency in the second pass is that the feed water pH is above 9 for efficient boron rejection, which is ideal for the formation of magnesium hydroxide and calcium carbonate precipitates [4]. Calcium carbonate scaling in the first pass is typically prevented by an acidification of the feed water, the injection of a scale inhibitor or a combination of the two.…”