By Michael A. Stern

With the news media reporting an oncoming worldwide shortage of potable water, many new companies are entering the water vending machine business. If you are thinking about doing so, or are already there, here are a couple of new features you should consider incorporating into your vending machine. Such additions can greatly improve the performance for your machine and the quality of the water being dispensed.

How they work
The water vending machines I am discussing here are the many stand-alone units that are placed in front of most supermarkets, selling bulk water by the gallon and dispensing it into the customers’ own container. These machines internally process municipal water through a series of filters including a reverse osmosis membrane. Then the processed or filtered water is held in a storage tank until ready to vend. Most water vending machines then use an ultraviolet lamp as a final disinfecting stage prior to vending.

These machines normally use two electronic controllers. One controller is used to operate the RO system and keep the storage tank full and the second is a vend controller which interfaces a coin acceptor and flow meter and takes care of all the functions necessary to control the accurate vending of water into a container.

Many machines in service today have certain inherent problems that have not been addressed by their manufacturers. I would like to discuss two of these problems, and suggest an innovative method for improving upon them.

A common problem that occurs with many machines is the vending of hot water into a customer’s container. A customer expecting to get one gallon of cool refreshing water can be very surprised to see hot water coming out of the machine, filling his container. This occurs because a certain amount of water always remains inside the UV lamp housing. Since the UV lamp generates heat and is powered all the time, the standing water left in the UV lamp housing can become very warm, even hot. Thus, the next person to put his money into the machine gets this slug of hot water. This problem is most pronounced if a long period of time elapses between vends.

This problem can be corrected by providing a periodic flush cycle to remove the hot water from the UV housing before it can be vended into a customer’s container. This periodis lush cycle should be long enough to remove the hot water and frequent enough to keep the water from becoming too hot. I suggest a frequency of approximately every 30 minutes between vends, with a three-second flush. Three seconds seems to be just enough time to remove the warm water from the UV lamp housing and refresh the internal water lines. A three-way flush valve can be added to the system to route the flushed water back into the storage tank to conserve water. If a flush valve is not used, the flushed water can be directed out the vend spout and down the drain. The flush cycle interval time should be reset each time a new vend occurs, as the vending cycle itself will remove the water currently residing in the UV housing. (See Figure 1).

A second problem present in most water vending machines is the accumulation of organic growth inside the water storage tank. This occurs because there is nothing present in the water, or the tank, to prevent such growth. Although these organisms may not be harmful to humans, it is very undesirable to have an uncontrolled growth present inside the tank. To help reduce this problem the same flush cycle, previously discussed to remove hot water, can be used to periodically disinfect the water held inside the product storage tank. To implement this feature, the three-way flush valve should be used to route the water back into the storage tank. The idea here is to periodically run a certain amount of water from the storage tank past the UV lamp and back into the storage tank. In this case, the flush duration should be set to a longer period of time, such as 10-20 seconds, which would allow one-half to one gallon of water to periodically circulate from the storage tank, past the UV lamp and back into the storage tank every 30 minutes. Depending on the size of the storage tank, most of the stored water in the tank can pass by the UV lamp every 24-48 hours.

This may not seem necessary for machines that have a high volume of water sales, however, it can be beneficial if very little vending occurs during the late night and early morning hours.

Adding a periodic flush cycle to a water vending machine has a twofold benefit. First it helps freshen the water lines and removes hot water that may have accumulated in the UV lamp housing, second it helps disinfect the filtered water held in the store tank.

About the author
Michael A. Stern is the president of Electronic Systems Design, Inc., located in San Fernando, California. For many years his company has been designing and manufacturing controllers for RO systems and water vending machines exclusively for private companies.

For more information, please contact Electronic Systems Design, Inc., 1010 N. Maclay Ave., San Fernando, California, 91340. Phone: (818) 365-0864, Fax: (818) 365-1308, or email:


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