Filtering Rainwater

We have seen in the pages concerning rainwater quality that the physicochemical quality of rainwater stored in a concrete / stone / masonry cistern is close to ideal. A cistern that is built with these materials artificially reproduces the characteristics of a natural rock cavity, where water conservation works well.

It is therefore necessary to take great care in the design and construction of a cistern. Read more on this at the PLUVALOR system. 

«Safe Water» for Non-food Purposes

A fundamental principle of sustainable water management is to adapt water quality to its end-use. It is unreasonable to use potable water for all purposes. In reality, for human consumption, only 3 to 5 litres of high quality water is needed per day, per person. For other uses, including personal hygiene, a lesser quality water is suitable. In the PLUVALOR system, this is what we call «safe water», but it is not potable water in the legal sense [1].

The physical and chemical qualities of «safe water» will be faultless, but for the presence of a few hundred benign bacteria and sometimes of a dozen fecal contaminated bacteria (coliform bacteria, streptococci) per 100 ml. Based on the daily practice of thousands of households over the years, accidental absorption of such water is not harmful to one’s health. A normal person’s immune system perfectly controls such bacterial intake. Even admitting a health risk (which is yet to be scientifically demonstrated), the risk is very weak since such water is not to be used for food or drink. Thousands of infants are washed in such water and yet do not appear to be in lesser health than others.

To speak out on the «dangerous custom» of using domestic rainwater becomes less credible in a country like Belgium, for example, where more than 750 000 people have been using filtered rainwater for personal hygiene, for many years. (Of those, 100 000 have been drinking it.)

In daily life, we absorb water and food that contain more bacteria, said pathogenic, than what is contained in «safe water». That will be the case, for example, of the last rinse water on a lettuce head in the grocery. To demand legally compliant water for all domestic uses is scientifically incoherent. However, this does not mean that one should drink all water, no matter which. Between the unconditional hygienic approach to eliminating all bacterial life around us and a total lack of cleanliness, there is a just middle ground: adapting water quality to its end-use. The problems with chlorine disinfection are treated on the Chlorine in water page.

Safe (i.e. sanitary) water is obtained by installing a filter with 10-micron porosity. This filter must be placed downstream from the well pump system.

It sometimes happens that cistern water contains particles that convey a slight turbidity to the water. These are fine particles that come from dust, micelles (bacterial clusters) and plant residues (leaves, moss). To protect the 10-micron filter, a primary sediment filter with a porosity of 25 to 50 microns must be placed before it, right after the pump system. When selecting a sediment filter, choose one with a washable (and reusable) filter such as a nylon mesh, instead of dispensable cartridges.

For food and drink, an additional filtering system is required. On this matter, go to the Making rainwater potable page.

To see how NOT to reclaim and reuse rainwater, click here

To continue reading, go to the Making Rainwater Potable page.

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