Each time a tank-type toilet is flushed, three and one-half to eight gallons of water are used. Dams for reducing the amount of water used in one flush are known and have been used. Dams are flexible panels pressed into place in the tank. They block off a portion of the lower part of the tank thus reducing the amount of water released into the bowl. The dams allow the tank to refill completely, maximizing water velocity. Dams can save more water and do not interfere with the toilet's ability to flush away waste and are, therefore, better to use rather than switching to smaller tanks or displacing some water volume within the tank. In any case, there are patented variations on the way dams have been used to conserve water during a flushing cycle.
The U.S. Pat. No. 4,143,430 to Joshi et al. is a water conserving device which shows a flush tank with a single partition which traps and releases reserve water using a valve unit which opens and closes a single port in the partition. When continued rotation of a trip lever moves a flush arm, reserve water is permitted to flow through the port in the partition and exit the tank. However, as the reserve water level drops, the valve unit closes off the port. The water trapped by one partition is not used during each flush, but only when further flushing is required.
U.S. Pat. No. 4,152,793 to Mills discloses an inner reservoir within a conventional flush tank formed by a partition having a port. The port is prevented from allowing fluid to flow through it during a portion of the flush cycle by an air pump outside the flush tank which displaces a sphere to either open or close off said port. Again, the port opens and closes depending on whether more water is required to flush the waste.
U.S. Pat. No. 3,820,170 to Kern, et al. shows a partition system for flush tanks where the partitions would have to be fabricated to create a minimal, optimal water loss.
The U.S. Pat. No. 3,719,958 to Wilhelm shows a stand pipe having a series of small openings at the top through which water from the tank can replenish the water in the stand pipe to achieve a fractional flush.