SAFE USE OF CISTERN WATER
An obstacle to the use of rainwater in domestic systems for mitigating present and future water shortages has come to our attention in recent years.
We built a cistern 12' x 14' x7 ' high incorporated into the basement of our new home in 1986, with the intention of watering our garden and having soft water for domestic use. We have municipal uti1ity water for drinking. Our house, plumbing, and cistern plans were approved in 1985 and the completed structures were passed by the city building inspector in 1986. The current city administration has raised a question about the cistern. According to the Wisconsin state plumbing code it is not designated as a "potable water supply". Our plans were originally approved because all three of our sinks and wash basins have a separate cold water faucet supplied by city water; there are no cross connections and there are appropriate one-way anti-siphon devices to prevent inadvertent connections.
Because of this concern we have been motivated to conduct some experiments to determine how difficult it would be to make a modern, enclosed cistern potable. The water is collected from a baked-on enamel roof into a small outdoor collecting and settling tank to get rid of solid material. The water passes through nylon mesh strainers (25x60 holes per square inch) on entering the collecting tank and again at the exit into the cistern. The organic material is primarily leaf residue and is largely removed by the strainers. Every few years we empty and clean the cistern of the very fine material which settles to the bottom. From the beginning we controlled odors by adding ˝ ppm (parts per million) chlorine as bleach to new rain water as it was collected. Our experiments are showing that this routine reduces coliforms to very low levels and has eliminated E. coli. In order to consistently achieve zero colony counts of both coliforms and E. coli it would probably be necessary to chlorinate the entire cistern contents each time a new increment of rain water is added. We have achieved zero counts with ˝ ppm chlorine to the entire cistern recently, even though it has been 4 years since a cistern cleaning.
The repeated chlorination of the entire volume of cistern water with each new collection, however small, can be assumed to produce more than minimal amounts of organic chlorine compounds such as chloramines and halomethanes. Perhaps the hazard to health of these organic chlorine substances exceeds the benefits of heavy chlorination in our context, since there is limited or negligible access to our roof by human pathogens. This concept may be confirmed by inquiry as to documented water-related health problems in parts of the world where cistern rainwater use is prevalent either because of fresh water shortage or local wells contaminated with arsenic, fluoride, petroleum, agricultural chemicals, etc.
An example is Bermuda, a highly developed small country where fresh water from wells or surface water from wells or surface water is simply not sufficient. We have corresponded with the Bermuda Biological Experiment Station and find that in that country individual domestic cisterns: are universal, are seldom chlorinated or otherwise treated, and are not associated with health problems. In fact, having an individual water supply for each household decreases the possibility of water-borne epidemics.
Galvanized iron or even copper plumbing will corrode in a few years at the pH of rainwater. About one pound of food grade sodium bicarbonate per 1000 gallons will correct the pH to the acceptable range of pH7 to 9, sold in 50# bags in agricultural feed stores. For new construction use plastic pipe and a stainless steel pump.
We present this data rather formally because world wide shortages of safe water may require more resourcefulness and more liberal codes to maintain human health. Of course water conservation is the first method to consider in solving the problem. For example, there would seem to be no need for using potable water for flushing toilets, as we were told the Wisconsin code requires. When we visited the Bermuda Biological Station we learned that they have a separate sea-water plumbing system specifically for toilet flushing, and experience no problems with this.
Byron , Australia, subsidizes private cisterns as the most cost effective way to limit excess storm water flow. Their cisterns are constructed double the size needed for household use with a drain half way to drain storm water away gradually but fast enough to be ready for the next storm.
In summary: a few rather minor changes in the plumbing code and perhaps a minimum of inexpensive research could ease impending water shortages by permitting the use of rainwater for at least some domestic use.
John and Mary Frantz 812 22nd Avenue
December 28 1998
Addendum: Feb. 15 2002
Abigail Alyer, University of Illinois at Urbana-Champaign, President of the American Society of Microbiologists, spoke at the American Association for the Advancement of Science meeting in Boston, MA this month: the title of her talk: Are There Medical Implications of Geomicrobiology? She stated:
“Legionella is a pertinent example. It is primarily a disease of soil amoebae and treats our human phagocytes similarly.”
I asked a question about outdoor reservoirs or cisterns harboring Legionella, particularly small above ground cisterns that would quickly attain summertime ambient temperatures. She replied that although it seemed like a potential problem, it had not been reported.
Addendum: September 8, 2004
The mandated limits to chloride content of public water supplies and effluents of sewage disposal systems is getting harder to meet because of winter road salt and water softener salt entering surface water (and ultimately ground water). Rainwater is totally soft without any treatment.
Forget about using rainwater as potable water if your roof is asbestos shingle or contains any lead (even flashing). Obviously even boiling does not correct lead or asbestos. Asphalt shingles are a nuisance because they contribute grit. Rinsing the roof with about 10 gallons of rain per 100 square feet can be arranged automatically into a barrel or two that must fill before the level reaches the inlet of the cistern. Restoring the rinsing capacity before the next rain becomes automatic if you slightly open the discharge valve of the barrel(s) so they empty in a day or so.