Compact accelerated precipitation softening (CAPS) as pretreatment for membrane desalination II. Lime softening with concomitant removal of silica and heavy metals

In a previous paper the CAPS softening process is suggested as a pretreatment step for RO: in addition to removal of calcium it is an effective filtration and may thus replace part of the customary pretreatment. In the present study the removal of silica and heavy metals in addition to water softening was investigated. Precipitation with both sodium hydroxide and with lime was studied. With lime precipitation in CAPS, it was found that magnesium is necessary for silica removal, if no other additives such as Al and Zn are used. With conventional methods of precipitation, the well-known reagents for silica removal, aluminium salts, lead to very slow precipitation or difficult filtration. In contrast, aluminium chloride used as an additive in CAPS can remove a major part of the silica without interfering with filtration, if the precipitation conditions are chosen correctly; optimal conditions were in keeping with the regime of softening plants. It was found that zinc chloride, which is environmentally more acceptable than the aluminium salt, could also be used for silica removal, but in larger amounts than aluminium chloride. With these two salts, a substantial fraction of the silica can be removed, but the precipitation of silica results in decreased calcium removal. To achieve simultaneous softening and silica removal, it is necessary to add carbonate. Heavy metals are coprecipitated with the calcium carbonate.