Abstract: The filter is made of monofilament polypropylene, a comparatively fragile material but chemically adapted for the purposes, in the shape of a pillow case with a long edge opening and oppositely, adjacent an end thereof, a conduit opening. All edges are reinforced with a strip of felted polypropylene, a comparatively heavy and strong material and also chemically adapted for use with caustic chlorine. The filter is additionally reinforced with pads of the same felted material where supported in a container. In use, a flat filter leaf with a conduit is inserted into the filter through the opening with the conduit opening coinciding with the conduit, and the long edge opening tightly sewn together. Caustic chlorine is introduced into the filter through the conduit opening and filter leaf, and escapes into the container through the filter free of salt. Both the monofilament and felted polypropylene define minute interstices that are impervious to salt but not to chlorine.