COMPACT ACCELERATED PRECIPITATION SOFTENING (CAPS) AS PRETREATMENT FOR MEMBRANE DESALINATION .2. LIME SOFTENING WITH CONCOMITANT REMOVAL OFSILICA AND HEAVY-METALS

In a previous paper the CAPS softening process is suggested as a pretr eatment step for RO: in addition to removal of calcium it is an effect ive filtration and may thus replace part of the customary pretreatment . In the present study the removal of silica and heavy metals in addit ion to water softening was investigated. Precipitation with both sodiu m hydroxide and with lime was studied. With lime precipitation in CAPS , it was found that magnesium is necessary for silica removal, if no o ther additives such as Al and Zn are used. With conventional methods o f precipitation, the well-known reagents for silica removal, aluminium salts, lead to very slow precipitation or difficult filtration. In co ntrast, aluminium chloride used as an additive in CAPS can remove a ma jor part of the silica without interfering with filtration, if the pre cipitation conditions are chosen correctly; optimal conditions were in keeping with the regime of softening plants. It was found that zinc c hloride, which is environmentally more acceptable than the aluminium s alt, could also be used for silica removal, but in larger amounts than aluminium chloride. With these two salts, a substantial fraction of t he silica can be removed, but the precipitation of silica results in d ecreased calcium removal. To achieve simultaneous softening and silica removal, it is necessary to add carbonate. Heavy metals are coprecipi tated with the calcium carbonate.