There are usually two key issues that are often raised regarding the safety risks associated with using harvested rainwater as a source of drinking water: 1) Lead contamination; 2) Sludge/biofilm contamination.

Lead Contamination

The type of roofing material, particularly the use of lead flashing on roof structures, may result in water that is harvested from the roof surface becoming contaminated with toxic heavy metals that may be harmful to your health.

A scientific study conducted by Magyar et al, titled 'Influence of roofing material and lead flashing on rainwater tank contamination by metals', which was published in the Australian Journal of Water Resources (2014), found that lead contamination in rainwater harvesting storage tanks is widespread. The main contributor to this contamination is from lead flashings on rooftops, which prevent water from penetrating into the cracks and crevices of a roof junction. Consequently, it is recommended that if a roof surface is used for rainwater collection for drinking water, any lead flashing that is present should be painted with a non-toxic sealant to prevent lead from leaching into the harvested rainwater supply, or better still, it should be removed and replaced with a lead-free alternative.

Contamination by Sludge and Biofilm

While water diverters and sediment strainers can play a role in removing unwanted debris, particulate matter will still accumulate in the water storage tank over time. This debris will eventually form a layer of sludge at the base of the tank, which can potentially provide ideal conditions for harmful bacteria to breed (Magyar et al). However, others (Coombes & Spinks) argue that this sludge can be beneficial to the quality of harvested rainwater, helping to eliminate both lead and harmful pathogens. In an article aptly titled "Rainwater Health Debate", Smit & Coombes discuss the various arguments. According to the authors, who studied water quality of harvested rainwater across Australia over a 15 year period:

"It turns out that a decade of independent research confirms the rainwater treatment train that includes the natural processes of flocculation, settlement, biofilms (including the sludge) and competitive exclusion of bacteria (where more resilient environmental bacteria eliminate more fragile potential pathogens)."

According to the authors, biofilm that forms in rainwater storage tanks can be very effective at removing contaminants such as lead from the water stored in the tank. Their research shows that the layer of sludge is bound together by a tacky substance consisting of polysaccharides, which effectively absorbs lead and other pollutants from the water column. They recommend that rather than cleaning this layer of sludge out regularly, this biofilm layer should be left undisturbed so that it can continue to serve as a natural filtering agent.

A well maintained rainwater harvesting system that is fitted with water diverters and mesh strainers is unlikely to have a thick growth of green or black slime with a foul odor as one would commonly find in a pond. Coombes points out that the communities of microbes present in biofilms are typically harmless bacteria commonly found in soils and the environment, which feed on nutrients, chemicals and other bacteria. Since nutrient levels in rainwater storage tanks are low, these microbes are essentially hungry and will readily consume whatever comes their way, including harmful bacteria and chemicals, improving water quality within the tank as they do so.

Ensuring Rainwater is Safe to Drink

However, besides lead and bacteria, other airborne pollutants such as industrial chemicals or pesticides can settle onto your roof surface and get washed into your water storage tank. To be on the safe side it is best to err on the side of caution. We recommend filtering any stored water using a good quality drinking water filter, such as a Berkey Filter, that is capable of removing bacteria, heavy metals such as lead, as well as industrial chemicals, pesticides and a wide range of other pollutants that could potentially contaminate your harvested water.