Abstract
Scaling in reverse osmosis (RO) and nanofiltration (NF) systems is generally controlled by limiting the conversion and/or the addition of antiscalants. An alternative approach might be the desupersaturation of membrane concentrate before e.g. the last stage of an RO and/or NF system. A desupersaturation unit (DU) is aimed at reducing the degree of supersaturation in membrane concentrate stream by forced precipitation of sparingly soluble inorganic compounds on seed crystals. Barium sulfate precipitation was examined in a laboratory scale (DU) using supersaturated synthetic (no organic matter) and natural RO and NF concentrates. Effective barium removal for at least 100 h was observed in the case of synthetic concentrate. Early break-through of barium sulfate crystallization occurred in NF concentrate. This was attributed to the adsorption of organic matter (TOC = 22 mg/l) and subsequent poisoning of barium sulfate seed crystals. Two RO concentrates from surface water treatment pilot plants were compared. Barium sulfate removal was not affected (100 h) by organic matter (TOC = 12 mg/l) in River Rhine water pretreated by ultrafiltration. However, break-through was observed from River Rhine water with organic matter (TOC = 8 mg/l) pretreated by ozonation. Irreversible poisoning of seed crystals was attributed to adsorption of organic compounds, e.g. low molar mass acids produced by ozone pretreatment of RO feedwater. Inactivated barium sulfate crystals were restored by backwashing with acidified (pH = 2) water.