The Rainwater Observer

Is there a health risk associated with using rainwater in an irrigation system?

Rainwater harvesting is a safe alternative for irrigation water and indoor water use (including drinking water) as long as the proper components are in place to eliminate harmful bacteria and contaminants in the water.  In all applications where rainwater may potentially be consumed (shower water, sink water and drinking water), it is essential that a sterilization treatment such as ozone, ultra-violent light or chlorination be used before it reaches the end use. If there is any question regarding the safety of your water source, have a water test performed immediately.

Legionella bacteria

To ensure maximum water quality, a well-designed rainwater system should include:
1. Gutter filtration (we use Gutter Glove),
2. Pre-tank fine sediment filter (we use a Vortex Filter),
3. Calming Inlet,
4. Floating suction filter.

The main health concern for rainwater is Legionella bacteria which can be harmful if breathed in. In the case of a rainwater system designed for irrigation use, it is quite possible that the rainwater will be aspirated by anyone who comes within range of the spray mist from a conventional irrigation system. Fortunately, underground tanks do not allow stored water to reach a high enough temperature where Legionella thrive (above 75 deg. F).

Since the gutter filters and pre-tank vortex filters remove all of the organic matter, they generally prevent the build-up of bacteria. It is also dark- so the water is kept cold and dark and in many respects clean. Properly designed systems use calming inlets and floating intake filters to reduce stirring of the tanks (stirring encourages bacteria growth). This also helps to retain higher than normal oxygen levels, promoting healthy aerobic bacteria.

Water in an underground tank is much colder than the lower limit for growth as researched by the Reliance Worldwide Organisation. The water is generally clean before it enters storage and is kept under conditions that are generally safe. The same can not be said for all above ground rainwater systems. In this case, it is wise to employ a post-tank sterilization method.

To improve the quality of water after it leaves the tank, there are sterilization options such as UV light, ozone treatment and chlorination which can bring the water quality up to a potable grade. These treatments are effective, but they are also costly and require ongoing maintenance.

In summary, all current research on the subject points to this conclusion: For irrigation purposes, underground tanks where the rainwater is pre-filtered using rainwater screens, a vortex filter, a calming inlet and floating intake filter, do not need additional filtration to be safe for use on landscapes.

To alleviate any smell associated with the water, a post-tank fine sediment and carbon filter should be installed. These are much less costly then the other sterilization options and do not create costly ongoing maintenance of the system.

More information on the subject from the Texas A&M AgriLife Extension Agency.