Sanitizing Gutters and Down Spouts:

part 2 in a 3 part series on Minimizing Rainwater Contaminants
by Dr. Dennis J. Lye, EPA Research Microbiologist

ARCSA's last newsletter issue included best practices to achieve clean rainwater collection on rooftops that are properly designed with appropriate surface materials and are periodically rinsed and well maintained. The next issue will feature tips on the sanitary storage of harvested rainwater. This issue focuses on its safe transport from roof to cistern or other storage system, in other words, assuring gutters and down spouts are relatively clean when the blessed downpours come. Sanitation's goal is to substantially limit harmful contamination; in environments exposed to the elements and wildlife, sterilization or rigorous purification is obviously impractical.

Gutters and down spouts are the repository for everything that runs off the collection area. Properly designed guttering and spouting should convey rainwater to a storage system without adding to the level of contaminants freed by rain falling on the catchment area. Thus, an emphasis on excellence in the design, materials and maintenance for roofs applies equally to gutter and down spout systems. Adherence to these three principles will significantly decrease the levels of both chemical contaminants and microbiological communities that might reach storage areas. Our over arching objective is to optimize water quality system-wide.

In a typical rain storm, the greatest risk of chemical and organic contamination from the guttering system comes at the storm's onset, known as the first-flush phenomenon. Diverting this first-flush can drastically increase the quality of water delivered to the storage area. Studies have shown that neighboring collection areas may require the diversion of different volumes of initial rainwater depending upon the intensity and/or direction of rainfall, local geography, flora and fauna, orientation of the collection area, properties and parameters of the catchment area, slope of the catchment area, etc.. Each collection system is unique; what works for one may be inadequate for a similar system, even if located nearby.1  Nevertheless, all rainwater harvesting (RWH) systems should have some type of first-flush diversion system.

Even after diverting the first-flush rinse water, (there are devices on the market to do this automatically) unless all gutters are sufficiently sloped, they can contribute biological contaminants during rain and snow melt events. The Golden Rule of design, therefore, is to install all gutters, spouts and connections so that there is no place that even a tiny amount of water can puddle once the rains subside. The drainage route from roof to tank must be downwards without any deviation.

Otherwise, standing water is a risk with the companion danger for growth of microorganisms that will not only cause taste and odor problems, but may breed bacteria and viruses that can sicken anyone consuming an otherwise wonderful source of drinking water.2  As an added precaution, filtering systems can be purchased to block organic particulates released from the roof surface such as feathers, fur fibers and fecal material, from entering the gutters.

Freezing climates and coastal RWH installations exposed to the salinity of ocean spray may require gutter and spout materials that differ from hot, inland locales, for example. Since all construction materials are vulnerable to leaching when moistened, usually acidic rainwater can present special concerns. To minimize leaching, coated metal or plastic are the preferred construction materials in most areas and bare metal surfaces in collection areas, gutters and spouts are always discouraged.
Lastly, routine maintenance is essential to assure the delivery of the highest quality water. Of the many different types, sizes and shapes of gutter and down spout accessories available on the market, each may present advantages and disadvantages for a customized RWH installation. Always bear in mind during the design and construction phases that the overall system should offer ease of access for routine maintenance. Thus, gutters and down spouts are likely to be the most problematic component for water quality assurance in a RWH system and the most difficult to repair and remedy once the system is operational.

Footnotes 1 Kim et al, 2005, 'Pollutants in rainwater runoff in Korea: their impacts on rainwater utilization' in Environmental Technology, 26:411-420
2 Di. Lye, 2002, 'Health risks associated with consumption of untreated water from household roof catchment systems' in Journal of the American Water Resources Association, 38(5): 1301-1306

Article reprinted from the American Rainwater Catchment Systems Association Quarterly Newsletter Volume 1, Issue 2, June 2010