COMPACT ACCELERATED PRECIPITATION SOFTENING (CAPS) AS PRETREATMENT FOR MEMBRANE DESALINATION .2. LIME SOFTENING WITH CONCOMITANT REMOVAL OFSILICA AND HEAVY-METALS
A. Masarwa et al., COMPACT ACCELERATED PRECIPITATION SOFTENING (CAPS) AS PRETREATMENT FOR MEMBRANE DESALINATION .2. LIME SOFTENING WITH CONCOMITANT REMOVAL OFSILICA AND HEAVY-METALS, Desalination, 113(1), 1997, pp. 73-84
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.