Roof Harvested Rainwater in the Adelaide Region, South Australia

Author: Chirhakarhula Chubaka

Chubaka, Chirhakarhula, 2019 Roof Harvested Rainwater in the Adelaide Region, South Australia, Flinders University, College of Science and Engineering

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In the context of this thesis, rainwater is defined as any rain that falls on building rooftops and is collected and stored for future use. Rainwater differs from stormwater, which is water that has runoff from land surfaces into waterways and includes non-harvested roof rainwater. Rainwater is part of a complex hydrological system which acts like a filtration system removing impurities. However, during collection and storage, rainwater can become contaminated from the catchment areas (contaminants on roofs and gutters) and the associated plumbing systems (pipes and taps, etc., leading to and from the rainwater tanks). This study investigated the level of contamination in rainwater that is being preferentially used as primary source of drinking water by many households in the Adelaide region. Sampling corridors were selected across the metropolitan area based primarily on land use. Samples collected from rainwater tanks in the sampling corridors were tested for microorganisms and trace metals to assess against the Australian Drinking Water Guidelines. The capacity of filters to remove microorganisms and metals from rainwater was also assessed. Finally, householders’ motives to drink potentially contaminated rainwater when they have access to clean municipal water supplied by water utilities was investigated.

Irrespective of rainwater’s clean appearance, indicator microorganisms and metals were detected in rainwater samples collected in the Adelaide region above permissible levels in drinking water. Lead was found to be the predominant metal found in rainwater samples at concentrations above the drinking water guidelines. In 47 out of a total 53 tanks investigated, lead was found above the NHMRC threshold of 0.01 ppm for drinking water. Similarly, 28/53 tanks investigated were found to contain Escherichia coli. E. coli should not be detected in a 100 mL sample of any water intended for drinking. The level of E. coli and lead in rainwater samples collected in the Adelaide region was consistent with the results of other studies carried out on rainwater in urban and rural Australia.

A relationship was found between rainwater harvesting environment (hanging tree canopies and TV roof mounted antennas), and E. coli presence in stored rainwater. The study found similar trends in microorganism numbers in rainwater in summer and winter months, and a decline in microorganisms after a prolonged dry period. The tank materials, water pH, the presence of first flush diverters, and the bottom tank sludge drainage were found to have no relationship with rainwater microbial content. However, a relationship was found between building roof structure material and lead concentration in stored rainwater. In tanks that had filters attached, filtered and unfiltered samples showed that filters were not successfully removing E. coli, nor trace metals such as lead and zinc, although a slight reduction of metals was observed in filtered water. In contrast, an experimental water filtration unit installed in the laboratory removed E. coli from contaminated rainwater samples to 0 MPN/100 mL, the standard required of drinking water, but the filter became blocked at less than half the filter cartridge’s advertised lifespan. It was evident from this study that filters’ capacity to remove metals from rainwater was low. The difference between the laboratory study and field samples could be due to improper maintenance or installation of filters or recontamination of the faucet after filtration.

This study also investigated the drivers that cause an important fraction of the public to preferentially drink potentially contaminated rainwater when they have clean municipal water supplied. It was found that taste was a primary determinant, and parameters like the addition of fluoride and/or chlorine to municipal water were central in people’s preference for rainwater over municipal water, even when they were aware it may contain contaminants.

There have been previous epidemiological studies that have indicated that drinking rainwater containing E. coli is not likely to cause health effects. However, there is a need for future epidemiological studies to assess the health effects of drinking rainwater that contains elevated levels of lead. Other future studies arising from this work, include the assessment of different filters’ capacity to remove metals in rainwater to acceptable drinking water standards, and the assessment of blood lead of householders who primarily drink rainwater with elevated lead concentrations to determine whether the levels in rainwater consumed are of biological significance.

Keywords: Rainwater, Escherichia coli, total coliforms, pathogens, trace metals, lead, cadmium, Australia, outbreak

Subject: Water Management thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Dr Kirstin Ross