Using Rainwater in Your Home

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Photo by Pixabay/artyangel
Using rain indoors can cut your water bill in half.

The Water-Wise Home (Storey, 2015) by Laura Allen examines every crack and crevice of your home, looking for ways to radically reduce water use in your home, and shave a few bucks off your water bill in the process. Allen’s guide shows you how to use every drop of water in your home efficiently, making sure nothing goes to waste. The following excerpt is her guide to using rainwater indoors.

Using rainwater indoor is the best way to maximize water savings, especially in Mediterranean climates where there’s no outdoor irrigation need during the wet season. Indoor rainwater systems are more complex than irrigation systems, and it’s important to seek professional advice. However, if you have professional experience or are a very handy DIYer, there is good technical information available to help you with the installation, although a consultation with an experienced installer is recommended.

Treatment or Purification Systems

All over the world rainwater is used for washing, cleaning, and drinking, without filters or disinfection. However, in the U.S. and Canada most potable systems do include filtration and disinfection. An increasingly popular option in homes connected to a municipal water supply is to use rainwater for non-potable indoor uses: toilet flushing and washing machines. These systems are easier to get permitted, cost less, and don’t require as much treatment as whole-house potable water systems.

For nonpotable use of rainwater (e.g., irrigation or toilet flushing), filters remove particles in rainwater to prevent clogging in a pump, drip emitter, or plumbing fixture. Nonpotable indoor systems also require filtration to prevent odor or discoloration in toilets or washing machines. Drinking water systems require filtration as well as disinfection so there are no disease-­causing organisms in the water.

Types of Filters for Indoor Water

NSF International (NSF) certifies filters and provides a list of drinking water treatment units with NSF certification. Drinking water systems typically use three types of filters:

Particle filters to remove sediment and grit from the water. Particles in rainwater are filtered out by cartridge filters. Rainwater is pumped through a filter that captures particles of different sizes. Installed in a row, each filter screens out smaller and smaller particles.

Carbon filters to improve taste and eliminate odors. An activated carbon filter captures microscopic particles in the filter’s small pores, removing odors and color as well as some types of organic compounds, contaminants (e.g., pesticides and solvents), and volatile organic compounds (VOCs).

Ultraviolet (UV) disinfection for potential disease-­causing organisms. UV disinfection is a widely used technology that destroys bacteria and viruses without any chemicals. Class A UV filters are installed after particle and carbon filters. Water must be sediment-free for UV light to be effective, because pathogens can be shielded by particles in unfiltered water. UV disinfection requires maintenance (cleaning the quartz sleeve) and annual lamp replacement. UV lamps are rated according to the gallons per minute they disinfect. There are other disinfection options, such as chlorine, though it’s toxic to produce and forms carcinogenic trihalomethanes if it combines with organic matter. Ozone and sand filters are other options, as are under-the-sink drinking water filters.

City Rain: Clean or Not?

Rain is distilled water, free of pollutants. As raindrops fall through the atmosphere, they dissolve the carbon dioxide in the air, forming a weak acid with a pH of 5.7 or so. When rain falls through polluted air, it picks up particles of soot, dust, and smoke. If the air is polluted where you live, using rainwater catchment for irrigation is of no more concern than the rain falling directly from the sky and landing on your garden; the same pollutants will end up in the garden either way. However, with a roofwater catchment system, you should use a first-flush diverter to minimize the introduction of additional pollutants from the roof to the garden. Any drinking water system requires additional filters to remove particulates.

Sample Treatment Methods for Indoor Use of Rainwater

Specific filtration and disinfection requirements are determined by local regulations and installer preferences, but here are some common treatment methods for different end uses:


• Filtration: Minimum 50-micron sediment filter ­(prevents grit from interfering with toilet valves)

• Optional: Carbon filter to address any color or odor issues

• Permitting agencies may require 5-micron filters and disinfection

Washing Machine

• Minimum sediment and 5-micron carbon filter

• Permitting agencies may require disinfection

Potable Use (showers, sinks)

• Sediment filter

• 5-micron carbon filter

• 0.5- to 1-micron filter

• Class A UV disinfection

Slow-Sand and Ceramic Filters

Internationally, the low-tech filtering option of slow-sand and ceramic water filters is widely used for drinking water treatment. The group Potters for Peace teaches people how to construct these low-cost and highly effective ceramic filters. Simply pour water through the filter and collect purified water below with a jug. Regarding permits, this filter falls outside plumbing codes — it’s like using a portable water ­filter to purify creek water while camping.

Potable Systems: Test the Water

Before drinking from your rainwater system, it’s a good idea to test it. Tests may include bacteria (total coliform and fecal coliform), pH, and turbidity. Instead of relying on expensive testing for common pathogens like giardia and cryptosporidium, use filters that will remove them. Choose a certified lab and follow their collection methods so you obtain accurate results.

More from: The Water-Wise Home

Identifying Your Home’s Greywater Sources

Excerpted from The Water-Wise Home © by Laura Allen. Used with permission from Storey Publishing.

Related Video: 4 Ways to Harvest Rainwater

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