I'd usually recommend lime precipitation and removal by gravity settlement if sulphate removal is needed. Other removal techniques can be considered if space is limited. It's fairly simple to design, and likely to be reliable and cheap to run. The use of milk of lime may well be best for small operations, or where existing neutralisation dosing equipment is to be reused, as it can be bought in grades which physically act like a solution. Larger installation will require lime slurry production, or even quicklime slaking plant for best economics.
It will be far cheaper to build and run than the main alternatives, evaporation, membranes, or ion exchange.
Electrolysis and air stripping, which have on occasions been suggested within these forums do not work.
For removal down to low concentrations, more advanced technology is needed.
Calcium sulphate exhibits an inverse temperature / solubility relationship; i.e. solubility decreases with increasing temperature. By subjecting calcium sulphate bearing waters to conditions of elevated temperature and pressure in an agitated, seeded reactor, it is possible to reduce the sulphate (SO4) concentration in the water to below 200 mg/l and the calcium (Ca) concentration to below 80 mg/l. This represents a reduction in concentration of over 85% when compared with the solubility of calcium sulphate in water at 20¦C and atmospheric pressure.
This technology has been extensively trialled in South Africa. Costs of around ú0.20 per m3 treated are reported.
I am presently evaluating reports of proven industrial scale biological sulphate treatment.
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