UV and Carbon Filtration
by Doug Pushard
Many rainwater harvesting systems used for drinking water rely on a combination of sediment filters, carbon filters and ultraviolet (UV) light to remove all unhealthy impurities and ensure the water is potable. This common water purification system has proven effective for decades; however, it is not equally effective in all systems.
A pair of sediment filters — usually a 25-micron filter followed by a 5-micron filter — removes the particulates from the water so that the UV light can do its job disabling the DNA of bacteria in the rainwater. Without these filters, the particles could block the UV light and the bacteria would be able to pass through it unharmed, thereby getting into the drinking water system. Consequently, it is important to change these filters on a regular basis.
UV light has the distinction of adding no harsh chemicals or flavor to the water, so it is a preferred method to kill bacteria versus chlorination or ozonization.
Activated carbon filters have been used to treat the odor and taste problems in water for a long time. Today, they are used in millions of residential and commercial-grade systems. Activated carbon has been proven to be an excellent method of producing better tasting water as well as removing harmful water contaminants.
Activated carbon filters are manufactured from a variety of materials such as coal and coconut shells. A heating process creates active sites on the carbon material that pollutants adhere to as the water flows through the filter. These pollutants become locked in the carbon. In some cases, carbon filters can be as effective as a 1-5 micron filter (this will be indicated on the label).
In many installations, the sediment filters are followed by the carbon filter, which is followed by the UV light. Quite a few manufacturers actually preconfigure these components for use in private wells and rainwater catchment systems
It is this placement of the carbon filter, between the sediment filters and the UV light, that can be an issue in some isolated cases, specifically when the water system is not used often. When the system is not used for long periods of time, bacteria can adhere to the walls of the carbon filter and replicate, turning the carbon filter into a bacterial breeding ground.
This proliferation of bacteria can become overwhelming and spread throughout the drinking water system. This is especially the case in systems in which the UV light is turned off to save energy. In these systems, the water may become cloudy due to this explosion of bacteria in the standing water in and around the carbon filter. When the system is turned back on, this cloudiness in the water can actually protect some of the bacteria as it passes through the UV light. Consequently, bacteria may spread into the drinking water system.
This is the primary reason why the UV light should be left on at all times. Furthermore, turning the UV light on and off may actually shorten its life.
Still, in vacation homes and other homes that are not used often, shutting off the UV light is really the only option. There are several simple solutions if this is the case.
One option is to drain the filter and UV cartridges. With this option, the canisters are removed, stored in a clean environment and sterilized prior to reinstalling when the system is turned back on. Another option is to install new cartridges. Both options require sterilization of the cartridge prior to installation, usually with a bleach solution. (A subsequent article will cover the sterilization procedure.)
Another option is to position the carbon filter on the opposite side of the UV light and install a backflow preventer between the two. The backflow preventer ensures that only water treated by the UV light can flow into the carbon filter. Consequently, bacteria are not likely to build up in the carbon filter. The carbon filter will still be effective in removing taste and odor problems, but it is no longer a potential bacteria farm.
It is important to note that UV lights typically take one to two minutes to come to full strength, so in systems where the light is shut off, it is important to build a time delay into the system so the pump does not begin pushing water past the UV bulb before it is at full strength. A simple relay timer can be used to delay the start of the pump.
A third option is to add a little bleach to the system. For this to be effective, the carbon filter must be removed, the UV light turned off, and of course the proper dosage of bleach used.
There are possibly tens of thousands of installations in which the UV light is positioned after the carbon filter, and when manufacturers preconfigure these components, they position the components in this order. These systems have proven to work consistently for long periods of time. However, in situations where the water is not used regularly (e.g., in vacation homes, homes used only in the summer or homes in which the occupants are often away on travel) this configuration may create unhealthy conditions. For these installations, placing the carbon filter after the UV light can ensure that a problem does not develop at some point in the future.
Many thanks to Charlee Myers of Mountain and Mesa Construction, Tres Piedras, NM for assistance and input into this article.