RO plant experiences with high silica waters in the Canary Islands

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 undesirab le levels of silica or to restrict recovery rates. High silica brines can c ause severe and irreversible membrane fouling. Restricting recovery rates i s never a popular choice with the owners. A prolonged drought and high dema nd for ground waters in some areas of Gran Canaria (Canary Islands), Spain, has led many owners to conserve feed-water supplies by running their syste ms with high recovery rates and consequential high silica levels in the con centrate. The lowering of the water table has caused silica levers far high er than those anticipated at the system design stage. The membrane manufact urers and original equipment manufacturers (OEMs) have urged caution, advic e which is often at variance with the needs of the owners, who want to maxi mize output by using as little feedwater 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 350m(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 o perating 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 irreversi ble problems with silica fouling. This paper considers some of the design f eatures used and concludes that operation with silica levels significantly higher than accepted values is practicable, provided safeguards are taken.