Diatomite Precoat Filtration for Pretreatment of Seawater Prior to Reverse Osmosis

摘要

Investigation established that diatomaceous earth (DE) filtration could more consistently produce acceptable reverse osmosis feed water (based on 15-minute plugging index (PI-15) test data) than an existing central pretreatment (CPT) system over a substantial assessment period. The DE product was at or below 45 or 85% of all cycles; the CPT system was at or below 45 for only 42% of the same test days. Further, the DE product met its target PI-15 value of 40 or less, 53% of the time. Using DE to filter CPT system product gave PI values at/below 40 for 90% of 73 readings made, despite some substantial CPT system operating problems. Installed DE capital costs would be only about 40% of that for current CPT replication. Estimated operating and maintenance costs slightly favor DE over the CPT system by 14.4 to 16.3 cents/1000 gallons, respectively; addition of amortization decreases the total DE cost to only 58% that of the CPT system product. Of the DE processes evaluated, two have the capability of producing low PI-15 water. One process uses aluminum-hydrate coated DE; the other uses a separate feed of a cationic polymer, followed by DE body feed. While these products have similar PI-15 values, their other characteristics have notable differences requiring testing.