Controlling scaling in membrane filtration systems using a desupersaturation unit

Scaring in reverse osmosis (RO) and nanofiltration (NF) systems is generall y controlled by limiting the conversion and/or the addition of antiscalants . An alternative approach might be the desupersaturation of membrane concen trate before e.g. the last stage of an RO and/or NF system. A desupersatura tion unit (DU) is aimed at reducing the degree of supersaturation in membra ne concentrate stream by forced precipitation of sparingly soluble inorgani c 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 1 00 h was observed in the case of synthetic concentrate. Early breakthrough of barium sulfate crystallization occurred in NF concentrate. This was attr ibuted to the adsorption of organic matter (TOC = 22 mg/l) and subsequent p oisoning 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 pre treated by ultrafiltration. However, break-through was observed from River Rhine water with organic matter (TOC = 8 mg/l) pretreated by ozonation. Irr eversible poisoning of seed crystals was attributed to adsorption of organi c compounds, e.g. low molar mass acids produced by ozone pretreatment of RO feedwater. Inactivated barium sulfate crystals were restored by backwashin g with acidified (pH = 2) water.