Process for reducing the sulfate concentration in a wastewater stream

A method for reducing a sulfate concentration in wastewater stream comprising:

• directing a wastewater stream containing sulfate ions through a membrane and producing a membrane permeate stream and a membrane reject stream, the membrane reject stream containing sulfate ions;
• directing the membrane reject stream containing sulfate ions to a first precipitation reactor;
• adding a calcium source and a calcium salt seed material to the precipitation reactor, wherein the calcium salt seed material comprises calcium sulfate hemihydrates;
• mixing the membrane reject stream, the calcium source, and the calcium salt seed material in the precipitation reactor;
• precipitating calcium sulfate in the precipitation reactor and adsorbing sulfate ions in the membrane reject stream onto the precipitated calcium sulfate and onto the calcium salt seed material to form a treated reject stream containing the precipitated calcium sulfate with adsorbed sulfate ions;
• directing the treated reject stream to a first solid-liquid separator and separating the precipitated calcium sulfate from the treated reject stream and forming a first treated effluent;
• directing the precipitated calcium sulfate to a first settling tank, settling the precipitated calcium sulfate in the settling tank, and forming a supernatant in the settling tank;
• wherein settling the precipitated calcium sulfate in the settling tank further comprises settling heavier calcium sulfate precipitants having a larger particle diameter to the bottom of the settling tank and settling the lighter calcium sulfate precipitants having a smaller particle diameter to an intermediate portion of the settling tank;
• recirculating the precipitated calcium sulfate from the settling tank to the precipitation reactor;
• wherein recirculating the precipitated calcium sulfate comprises separately recirculating a portion of the lighter calcium sulfate precipitants from a portion of the heavier calcium sulfate precipitants to the precipitation reactor;
• directing the first treated effluent stream from the first solid-liquid separator to a second precipitation reactor;
• adding an aluminum salt to the second precipitation reactor;
• mixing the first treated effluent stream and the aluminum salt in the second precipitation reactor;
• precipitating calcium sulfoaluminate in the second precipitation reactor to form a second treated effluent stream containing precipitated calcium sulfoaluminate;
• directing the second treated effluent stream containing precipitated calcium sulfoaluminate to a second solid-liquid separator and separating the precipitated calcium sulfoaluminate from the second treated effluent stream and forming a third treated effluent stream;
• directing the precipitated calcium sulfoaluminate to a second settling tank, settling the precipitated calcium sulfoaluminate in the second settling tank;
• and
• recirculating the precipitated calcium sulfoaluminate from the second settling tank to the second precipitation reactor.