Is Water Tank Sludge Unsafe or Harmful to Rainwater Quality?

Recommended monitoring and maintenance procedures by the Australian Health Department include inspecting every 2-3 years for the presence of accumulated sediments. If the bottom of the tank is covered with sediment the tank should be cleaned. This sediment is normally comprised of organic matter which turns into a slimy biofilm layer at the bottom of your tank, hence it is commonly called “tank sludge.” It is assumed that tank sludge is detrimental to water quality, however there is professional opinion that these naturally forming biofilms in rainwater tanks are safe and effective at removing lead and other contaminants from the water column. In this article I explore the research on both sides of the debate.

The Debate: Is Tank Sludge Good or Bad?

Rather than share my own opinion on the matter, I’m interested here in professionally written papers discussing whether tank sludge is bad (i.e., a source of contamination), or good (i.e., helpful to increasing tank water quality). Dr. PJ Coombes defends that sludge in good his discussion paper of Magyar et al (2014), Influence of roofing material and lead flashing on rainwater tank contamination by metals, which argued that there was widespread lead contamination in rainwater harvested in tanks. To summarise, there appears to be much disagreement on two main points:

1. Whether lead in rainwater tanks accumulates from roofs without lead flashing, and
2. Whether contamination is had from the sludge that accumulates which becomes a breeding ground for harmful bacteria.

All agree that lead flashings found on rooftops, which stop water penetrating junctions, should be avoided. There is very good evidence that lead flashing is damaging to rainwater quality and so should be avoided if collect rainwater for consumption. Where lead flashings are present the priority should be to have them replaced before harvesting rainwater. While I’m interested here in the debate on sludge, studies arguing sludge is bad generally focus on high levels of lead present in tanks. These studies indicate that 23% of urban rainwater tanks have lead contamination. If true, one might wonder why there aren’t more widespread reports of lead contamination amongst the 2.3 million Australians drinking rainwater? This lead Dr. P. J. Coombes of Urban Water Cycle Solutions to conduct his own research of rainwater tanks found on old inner-city properties within industrial areas. Interestingly, he found no significant lead contamination inside the actual water column of rainwater tanks. Regarding sludge present in tanks, Coombes concluded that this biofilm layer caused by natural processes increases the quality of rainwater in the water column by absorbing heavy elements like lead and other contaminants. To quote his own words:

A decade of independent research has discovered and confirmed 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). So is the quality of rainwater poor because lead and other contaminants might be found in the biofilm layer known as sludge? The answer is no because the biofilm is highly effective at removing lead and other contaminants from the water column.

In contacting Coombes to discuss his paper, he emphasised that the microbial communities found in sludge are predominantly harmless soil or environmental bacteria that consume other bacteria, nutrients and chemicals. Since rainwater tanks are a low nutrient environment, the biofilm is hungry which means it is very good for preserving water quality. So then, it seems sludge may not be as harmful as many thought. Still, sludge isn’t particularly appealing as a smelly thick green or black slime. Some report rainwater plumbed into their toilets having a slight colour, others report an unpleasant odour. Using water diverters and strainers, and a reasonably maintained tank shouldn’t develop such. Buying and installing a self-cleaning tank will definitely keep sludge to a minimum and ensure fresh-tasting rainwater if consumed.