by Water Research Bhutan, 15 July 2024 waterresearchbhutan@gmail.com
Numerous government and non-government agencies have promoted rainwater harvesting as a ‘sustainable’ means to address water shortage woes. However, there is a lack of awareness of the risk of using stored rainwater for potable and non-potable uses. A few years ago, popular pilgrimage sites like Drakarpo in Paro had many older people using stored water for day-to-day domestic use. Exposure of the vulnerable people to low quality potable water use can seriously impact their health. The situation at Drakapo has improved since then, but there are many places in remote Bhutan where similar situations still exist today.
Photograph © Bhutan Foundation website
A global review of the microbiological quality and potential health risks associated with roof-harvested rainwater tanks by Hamilton et al. (2019) published in a highly-reputed peer-review journal has shown that roof-harvested rainwater can contain contaminants such as microorganisms, chemicals, nutrients, and heavy metals that pose health risks. Pathogens such as Escherichia Coli (E. Coli) have consistently been found in roof-harvested rainwater, indicating that it does not meet the criteria for suitable drinking water quality. E. coli is a group of bacteria that can cause infections in the gut, urinary tract, and other body parts.
In addition, pathogenic E. coli, total and faecal coliforms, other pathogenic species such as Campylobacter, Salmonella, Legionella, Pseudomonas, Cryptosporidium, Enterococci, Giardia, Aeromonas, and Mycobacterium avium Complex (MAC) have been detected in rainwater storage tanks (Chubaka, et al. 2018).
To mitigate drought effects on the sustainability of available water resources, many Australian states have introduced regulatory requirements and incentives for installing rainwater harvesting systems. However, when the effectiveness of such an initiative was reviewed by Chubaka et al. in 2018, traces of poisonous heavy metals such as arsenic, cadmium, chromium, lead, and iron were present in Australian rainwater; these metallic elements were generally found below the health limit guideline. But it would all depend on the quality of the roofing materials and paints on roofs. In addition, harvesting of rainwater near industrial and mining sites further increases the risk of heavy metal concentration in the rainwater.
If we go by the international WHO standards, the rainwater stored in rusted tar containers converted to ‘storage tanks’ in remote places would even be unfit for non-potable uses or even not suitable for long-term irrigation standards.
Regular cleaning of rooftops and gutter systems is reported to reduce fecal contamination significantly. Installing devices to prevent birds from perching on rooftops can also help reduce contamination. Regular cleaning of rainwater tanks, diverting the first flush, and maintaining filters can improve the water. Applying an appropriate amount of disinfectants such as chlorine (which comes in various forms of commercial names) would help reduce the risk of pathogens but increase the risk of the formation of other harmful secondary contaminants such as disinfection by-products (DBPs). Thus, there is no magic bullet for improving water quality, such as rainwater, when stored for long duration.
Of course, the risk of contamination by various pathogens varies by geographical region; we do not have any data on the water quality of rainwater harvested in Bhutan. Based on the current scientific literature, we propose relevant agencies to work towards the development of general guidelines on rainwater harvesting and its use including development of technical guidance on rainwater tank installation in the Bhutanese context. The proposed initiatives help create public awareness and improve water quality in places like rural Sarpang and Danaga dzongkhags, where rainwater harvesting is promoted as an effective climate-smart technology for addressing water scarcity without much consideration to overall water quality.
Further, the water quality is critical when CSOs are promoting the rainwater harvesting as a means for irrigating organic farming. Risk of numerous chemical contaminants such as PFAS leaching from use of low quality HDPE pipes for water distribution, and accumulation of heavy metals may contaminate the organic produce.
The article is published based on personal experiences and observations by a group of water researchers from Bhutan www.waterresearchbhutan.org
The article was also published in the National Newspaper of Bhutan (Kuensel). The link https://kuenselonline.com/how-safe-is-roof-harvested-rainwater/
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