RO plant experiences with high silica waters in the Canary Islands

Abstract

In many arid regions of the world, the shortage of good quality feed-water is forcing plant owners either to run their membrane systems with undesirable levels of silica or to restrict recovery rates. High silica brines can cause severe and irreversible membrane fouling. Restricting recovery rates is never a popular choice with the owners. A prolonged drought and high demand for ground waters in some areas of Gran Canaria (Canary Islands), Spain, has led many owners to conserve feed-water supplies by running their systems with high recovery rates and consequential high silica levels in the concentrate. The lowering of the water table has caused silica levels far higher than those anticipated at the system design stage. The membrane manufacturers and original equipment manufacturers (OEMs) have urged caution, advice which is often at variance with the needs of the owners, who want to maximize output by using as little feed-water as possible. This paper describes 8 y of experience in three systems operating with up to 320 mg/L silica in the concentrate. Two of the systems are producing 780 m 3/d and 350 m 3/d, respectively, for agricultural purposes, and the third is making 1,200 m 3/d for potable use. In two systems the recovery rates were reduced, safe operating practices were agreed and only then were the recovery rates slowly increased. In the third case there was no reduction in output. Each system has produced the desired water quantity and quality, and although routine cleaning has been necessary, none of the systems have experienced irreversible problems with silica fouling. This paper considers some of the design features used and concludes that operation with silica levels significantly higher than accepted values is practicable, provided safeguards are taken.