Rain water

Question

In our new building we have to reuse rainwater. As a preventive measure, is it not better to flush your toilet with tap water instead of rainwater? Does this increase the risk of contamination?

Answer

Dear Madam, sir,
Thank you for this question. Reusing rainwater is more and more done, especially the systems where rainwater is collected from the roof. Several studies have examined the safety of the use of this water source, especially when the water source is also used for food and cooking. As can be expected rainwater contains more bacteria than tap water, but the concentration is rather low. However, when the rainwater is used in food or for cooking, there is an increased risk of gastrointestinal infections. Most microbiological studies are mainly focused on the study of the presence of gastro-intestinal bacteria. Your question relates to the use of rainwater mainly to flush the toilet. In this context, the presence of low concentration of gastro-intestinal bacteria is of course of no importance. In a few studies the presence of important germs in cystic fibrosis, such as Pseudomonas- like bacteria or mycobacteria has been examined. Those germs were not frequently found in rain water sources, and if they were found, it was in a low concentration. We can conclude that there is no contraindication to use rainwater for flushing the toilet or to do the washing. However do not forget that tap water is not sterile.
We hope this answers your question. More information can be found in the scientific article of W. Ahmed, T. Gardner, S.Toze: “Microbiological quality of roof-harvested rainwater and health risks: A review” uit de J. Environ. Qual. 40:1-9 (2011).
Prof. dr. Marijke Proesmans

20.10.2015