Seawater filtration for fouling prevention under stormy conditions

Seasonal weather changes cause southern high-speed winds in the northern. R ed Sea coast, mixing the seawater column and releasing nutrients from the b ottom of the sea. Turbidity level instantly rises and micro-algae of the Sy nechococcus type start blooming, fouling the RO membranes, and eventually c ausing 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 foul ing. The main parameters of study were filtration rate, filter bed depth, s and grain size (fine vs. coarse), iron chloride dose and polymer aid. Membr ane feed, water quality criteria were turbidity less than 0.2 NTU and SDI l ess than 3. Flocculation, simulated contact and direct filtration of artifi cial and natural seawater with various clay-algae-pa combinations were stud ied in the laboratory. A dual-column pilot filtration System was later inst alled at the Sabha RO plant in Eilat. When seawater was not turbid, the spe cific 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. Res idual turbidity, iron, SDI, particle size distribution (PSD), pH, temperatu re, 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 exp eriments showed good results with deeper (1.2 m) and coarser (1.0 mm effect ive grain size) than usually applied. A filter run period under storm condi tions 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 conditio n should be injected, mixed well before the filters, while maintaining 10 m /h filtration rate and pH 6.8.