This invention relates to a vacuum toilet system.
Vacuum toilet systems have been in use for many years. Because they require considerably less rinse water than conventional gravity toilet systems and employ small gauge piping and fittings, vacuum toilet systems have proved particularly useful for passenger transport applications such as in aircraft, ships and trains. Vacuum toilet systems are also being used increasingly in domestic housing projects.
A conventional vacuum toilet system comprises a number of waste receiving toilet bowls, each connected via a sewer valve to a sewer, which comprises a sewer main and sewer branches connected to the sewer valves respectively. The interior space of the sewer can be placed under a substantially lower pressure than exists in the interior of the toilet bowls. When a flush button associated with one of the toilet bowls is actuated, a disposal cycle is initiated. During the disposal cycle, the sewer valve is opened and pressure difference between the sewer and the interior of the toilet bowl causes waste in the toilet bowl to be evacuated from the toilet bowl into the sewer and transported to a sewage collecting tank, which is emptied from time to time. To assist in the evacuation of waste from the toilet bowl, and to improve cleanliness and hygiene, rinse water is fed into the bowl during the disposal cycle.
In order to achieve a satisfactory evacuation of waste from the toilet bowls, and to ensure effective waste transport through the sewer to the collecting tank, the level of vacuum in the sewer must exceed a minimum threshold value, which in many systems is about 40 kPa. As used in this description and in the appended claims, ambient pressure of a toilet bowl means the pressure existing in the toilet compartment that contains the toilet bowl, and vacuum level in a sewer means the amount by which the pressure in the sewer is below ambient pressure of a toilet bowl served by the sewer. In a large vacuum toilet system, such as the one described in U.S. Pat. No. 4,184,506, a constant partial vacuum is continuously maintained in the sewer. In a small system, for example having only one or two toilet bowls, such as is described in U.S. Pat. No. 4,297,751, the volume that must be put under vacuum is relatively small, and in this case the necessary vacuum can be satisfactorily generated each time a disposal cycle is initiated and need not be maintained continuously.
In a conventional small vacuum toilet system, the vacuum generator starts operating when a disposal cycle is initiated and continues to operate until a vacuum measuring device detects that a predetermined level of vacuum has been obtained. Thus, the vacuum generator operates for as long as necessary to produce the predetermined level of vacuum. The length of this operating time will vary according to the free vacuum volume of the system, that is, the volume that is placed under vacuum by operation of the vacuum generator.
In a system in which the sewer is in open communication with the sewage collecting tank, the free vacuum volume is composed of the volume of the sewer and the portion of the volume of the tank that is not occupied by waste, and depends on the level of liquid present in the tank. The smaller the system is, the greater is the relative change in the free vacuum volume of the system as the volume of sewage in the tank changes, because the tank volume is large in comparison to the volume of the sewer. Particularly in a small system of this kind, the optimum vacuum, that is, the vacuum required for reliably achieving a satisfactory evacuation of waste from a toilet bowl and ensuring its effective transport through the sewer to the collecting tank, is not constant but varies according to the free vacuum volume. At low tank liquid level (large free vacuum volume), a smaller vacuum is sufficient, and at high tank liquid level (small free vacuum volume) a greater vacuum is required. Hence, the constant vacuum level generated in a conventional vacuum toilet system is not the optimum solution for a small system of the kind referred to.
To solve the problem of always obtaining an optimum vacuum level, a control system responsive to the level of liquid in the collecting tank could be used to adjust the operating time of the vacuum generator. However, a control system to provide a varying operating time would be expensive and, due to its complexity, might not be totally reliable. The problem, therefore, is to design a small vacuum toilet system of the kind referred to, in which the level of vacuum generated on each disposal cycle is not constant but is dependent on the free vacuum volume in a functionally favorable way, without unduly increasing the cost of such a system or reducing its reliability.