In 1937, a patent was issued to Leroy C. Lind for the Servisoft water softener (U.S. No. 2,087,157). The invention used a regeneratable granulated mineral, such as zeolite, which was already in use by domestic water softening systems using a permanent bed of the material. Leroy Lind devised a method that would be easier and faster to maintain. It would be less costly, and enabled the spent mineral to be regenerated collectively from a number of installations at a central location. The softener was packaged in water pervious, flexible walled cartridges or sacks sized to stack several vertically within a tall cylindrical tank connected to the water supply line. The contents of the sacks would settle to seal against the interior surface of the tank. The tank had a removable top to facilitate switching with renewed sacks.«