Keeping Harvested Rainwater Safe

We are often asked about the necessity of sanitizing harvested water before it is used. After all, when we are talking about clean rainwater coming from a roof, how much risk is there that there will be harmful pathogens in the water? Some remind us that cultures throughout the world regularly harvest – and drink – untreated rooftop rainwater. So if we are using it for non-potable applications, why bother to go through the cost (and maintenance) of adding a sanitizer like chlorine or ultra-violet? (Most understand and can agree that greywater (gray water) has all kinds of potential pathogens, so of course we should sanitize that source.) But what about rainwater

Here’s how we respond to that question. We think the risk that someone will get sick flushing a toilet or watering a lawn with with “raw” rainwater is pretty low. And if the harvester is a homeowner, and the storage cistern is a rain barrel under the downspout, then we agree that going through a sanitation step would be silly. But the systems we design are for commercial and institutional buildings; public buildings. Now the potential for a public health risk goes up dramatically, as does the standard for proper treatment. What could be in rooftop rainwater that could hurt someone? Bacteria, bird flu virus, and other pathogens. An article in this week’s Chicago Tribune underscores that risk – Two Dead From Legionnaire’s Bacteria – Chicago . Legionnaires is caused by a bacteria that grows in warm water, and can be transmitted to humans breathing a sprayed mist from the source (think sprinkler or spray from a flushing toilet).

Legionnaires and other similar health risks from rainwater are very unusual and the risk is low. But who wants to take that risk, especially when we have simple, proven methods for making that risk essentially zero? For a typical commercial system, the cost of adding a chlorination or U.V. step usually adds only 5-10% to to the cost of the system.

Which sanitation method is best? Chlorine and ultra violet are the two most common sanitation methods for harvested water and greywater harvesting. The advantage of chlorine is that once it has disinfected the water, it has a measurable “residual” value that we can use to confirm that the water is properly treated. We can manage that residual to about 5 parts per million, which is about the level in your municipal water supply. And that residual level of chlorine continues to protect the water downstream in pipes, tanks and toilets. Of course, chlorine is a consumable, and these systems need more added every few weeks. Ultra Violet is a great sanitizer in that it is chemical free (green!) and the bulbs last 10,000 hours – so the system can run for 1-3 years without maintenance. But we can’t confirm that UV has done its job correctly unless we send the water out to a lab for testing. And there is no “residual” value for UV, so once the sterile water leaves the UV system, it can become contaminated again downstream.


In addition to all that, we take pride in the systems we design. We want to sell and support rainwater and greywater harvesting systems that do what they are supposed to do: Make a meaningful impact on the water savings for a building. And we want those systems to deliver clean, safe water with minimal maintenance. So we bristle when we are asked to supply a system without a sanitation step – and if we can’t convince the owner otherwise, we often let someone else do the design and build. Better to leave someone else to the risk and concerns of a system that is not properly designed.

For more information on rainwater harvesting sanitation, visit our site: Wahaso.com