Pre-treatment of seawater feeding reverse osmosis (RO) membranes is a key step in designing desalination plants, especially when treating surface seawater with highly variable quality. The objective of the study was to assess the potential of ultrafiltration (UF) pre-treatment prior to RO for desalting seawater with high-fouling tendency. A UF pilot plant equipped with an Aquasource membrane was directly operated on Gibraltar surface seawater in dead-end mode. The competitiveness of UF pre-treatment towards conventional pre-treatment was assessed by looking at the impact on RO hydraulic performances. The study showed that UF provided permeate water with higher quality than with a conventional pre-treatment. The main seawater compounds responsible for UF fouling were organic matter released by phytoplanktonic organisms. The combination of UF with a pre-coagulation at low dose helped in controlling this fouling and providing water in steady state conditions. The performance of RO membranes downstream of UF exceeded the usual operating conditions encountered in seawater desalination. The combined effect of higher recovery and higher flux rate promises to significantly reduce the RO plant costs.

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