The Dow Filmtec BW30-4040 brackish water reverse osmosis membrane elements provide consistent and outstanding system performance in light industrial applications. Elements with a hard shell exterior are recommended for systems with multiple-element housings containing three or more membranes, as they are designed to withstand higher pressure drops. From the Manufacturer The Dow Filmtec BW30-4040 brackish water reverse osmosis membrane elements provide consistent and outstanding system performance in light industrial applications. Elements with a hard shell exterior are recommended for systems with multiple-element housings containing three or more membranes, as they are designed to withstand higher pressure drops.