Seasonal weather changes cause southern high-speed winds in the northern. Red Sea coast, mixing the seawater column and releasing nutrients from the bottom of the sea. Turbidity level instantly rises and micro-algae of the Synechococcus type start blooming, fouling the RO membranes, and eventually causing shut-off of the plant. Our objective was to evaluate the design and operational parameters of deep-bed filters for pretreatment of the seawater, primarily, under such extreme conditions in order to reduce membrane fouling. The main parameters of study were filtration rate, filter bed depth, sand grain size (fine vs. coarse), iron chloride dose and polymer aid. Membrane feed, water quality criteria were turbidity less than 0.2 NTU and SDI less than 3. Flocculation, simulated contact and direct filtration of artificial and natural seawater with various clay-algae-pa combinations were studied in the laboratory. A dual-column pilot filtration System was later installed at the Sabha RO plant in Eilat. When seawater was not turbid, the specific algae (grown in situ) and clay were injected into the feed stream to simulate storm conditions up to 10(5) cells/ml and 5 NTU, respectively. Residual turbidity, iron, SDI, particle size distribution (PSD), pH, temperature, microbial count and head loss across the filter columns were monitored. Modified Jar Tests with clay/algae mixtures resulted in 95-99% removal at natural pH at wide range of flocculant dose (0.5-20 mg/l). Column pilot experiments showed good results with deeper (1.2 m) and coarser (1.0 mm effective grain size) than usually applied. A filter run period under storm conditions reached 35 hours with satisfactory filtrate quality. An iron chloride dose of 0.3 mg/l during normal conditions and 0.5 mg/l for stormy condition should be injected, mixed well before the filters, while maintaining 10 m/h filtration rate and pH 6.8.