One of the most important considerations when designing a rainwater harvesting system is what you intend to do with the water. Almost everything keys off of this aspect. The size of the tank, the method of delivery, the quality of the water and the overall cost of the system are all affected by this decision. Consider carefully the changes that may come in the future when making this determination. If there is a chance that the rainwater could someday be used for drinking, then the components of the collection and storage facility will need to be approved for this use. It is not a good idea to install a secondary use system and then press it into potable service. It would be better to add a second discrete system for drinking water, in this case. If you intend to use any of the water indoors, it is probably best to consider the system to be potable even if you never expect to drink your harvested water.
Water efficiency experts that tracked data from three water districts over two years have come to the conclusion that a rain harvesting system, used for irrigation only, that supplants 40% of the water needed for this purpose will pay back its cost in about 4 years. Indoor water conservation techniques including low flow faucets and shower heads, high efficiency dual-flush (HET) or ultra-low flow toilets (ULFT), water conserving appliances and composting toilets were also tracked with varying results. The bottom line is that integrated systems cost more and take longer to pay back. Outside only use has the best return on investment but lacks versatility. You must decide what is important to you.
Irrigation and Other Outside Uses
Irrigation is among the first things that come to most people’s minds when thinking about collecting rain. It makes a lot of sense not to buy drinking water to irrigate with. In fact, there is some evidence that chlorinated water damages the biological health of the soil and thus decreases production. Other outside water uses that work well with rain is vehicle washing since it is soft and does not leave scale deposits that harm delicate finishes and water features like fountains and ponds that don’t benefit from chlorination.
If your water district enforces outside water use restrictions, they do not apply to harvested rainwater. Rainwater use does not incur a sewer charge or strain the aging infrastructure that is reaching maximum capacity in parts of the country and the world. We can all play a part in extending the amount of time our government entities have to deal with this issue by using less tap water for purposes that do not require it.
Secondary Indoor Use
Toilet flushing and clothes washing comprise about 40% of indoor water use and do not require tap water quality. Again, softness is a plus when it comes to these fixtures in the home. Less soap is needed to obtain the same level of cleaning and clothes are not damaged by the deposits left behind when hard water is used. Toilet valves and trap ways are kept cleaner when soft water is used. The sewer bill is based on consumption so your water bill is reduced in two ways when rain replaces tap water for indoor secondary use.
Bathing, cooking, dishwashing and drinking are considered potable use in most cities and water districts and some states restrict the use of harvested rain for these uses, especially when the property is connected to a public water supply. Check your local codes and ordinances before attempting to use rain for indoor potable use. If your property is not connected to a public supply, then there are several university studies that have been published that verify that rainwater is safe for human consumption if collected and stored carefully. About 30,000 years of human habitation of the planet backs those studies up with historical data. Rainwater quality is a function of air quality, so there are areas where heavy industrial facilities, livestock operations or volcanic activity could create a problem but these can be counteracted by proper treatment. If you suspect that something like this is going on in your area, have the water in your cistern tested and corrected, if necessary.
Rainwater is slightly acidic; in most areas averaging about 6.5 pH. This can be buffered by adding a small amount of a USP grade of calcium carbonate. The less desirable sodium bicarbonate (baking soda) can be used but it also adds sodium to the water. If you have an unlined concrete cistern, the calcium present in the concrete will buffer the pH to some degree. Most people don’t bother to add anything to their cistern and it is very important not to add disinfectants like chlorine unless an unusual event happens to contaminate the water with toxic microbes.
Homeowners have fished drowned raccoons out of their cistern without disinfecting the water and didn’t notice any problem. There is a limit to how much of this kind of thing can be tolerated but a minimally maintained cistern develops a healthy biofilm on the inside which contains a colony of beneficial microbes that control the less desirable ones that enter with each rain or wildlife event. If rats enter the cistern, however, they carry a disease causing microbe in their urine that may need to be counteracted.
For safety’s sake, it is recommended that a potable rainwater system use some method of disinfection or purification after the cistern. This can be as simple as a point of use or faucet-mounted device or as elaborate as a whole-house ultra filter or UV water treatment unit. The most popular choice is a UV system with a carbon per-filter. These units are easy to install and maintain but it is important to have good water clarity and to change the UV bulb once a year to ensure that they are effective. Ultra filters do a different job, they purify the water by filtering the microbes out rather than deactivating their DNA and leaving them in the water but unable to reproduce as UV does. The effect is the same but the difference is significant to some people. Adding chlorine has multiple effects in addition to disinfection such as reacting with other elements that could be present in the water and leaving byproducts that could be harmful. Few homeowners use chlorine for anything other than cleaning the cistern and pipes before initial use. This ensures that nothing is resident in the system that could flourish in untreated water. After that, adding chlorine should be considered a dramatic measure reserved for drastic circumstances.
An integrated system that combines drinking, secondary, grey and black water recycling is the ultimate in water efficiency and self reliance. Few of these systems exist, but more are coming online every day. The Leadership in Energy and Environmental Design (LEED) and the Living Building Challenge movements are driving more and more people to reevaluate the way in which they choose to live on the planet and how they interact with society. Integration of all aspects of life, reaching way beyond just survival needs is being considered.