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Is Tank Sludge Harmful to Rainwater Quality?

If you own a rainwater tank, proper tank maintenance procedures recommended by the Australian Health Department include inspecting inside your tank every 2-3 years for sediment build-up. This sediment build-up is normally comprised of organic matter such as leaves which turns into a slimy biofilm layer at the bottom of your tank called “sludge.”Is Tank Sludge Harmful to Rainwater Quality? Many automatically assume this layer is detrimental to tank water quality, yet there is growing descent as new research reveals the biofilms in rainwater tanks appear to be highly effective at removing lead and other contaminants from the water column. This article delves into the research to present two alternative positions on sludge affecting water quality.

Tank Sludge and Rainwater Quality

Like a coin which has two sides, there are often two sides on any given issue. With the sludge that develops at the bottom of a tank, there has been much discussion between those who view sludge as a source of contamination on one side, and those who see sludge as a valuable part of treating and protecting rainwater quality on the other. It is often good to start with agreement, and in the debate both sides agree that lead flashings, which stop water penetrating rooftop junctions, should be avoided when harvesting rainwater. The evidence is strong that lead on roofs is damaging to rainwater quality, and so should be avoided especially when harvesting rainwater for consumption. Thankfully, Australia now has building controls in place with address concerns being found in lead flashings. Nonetheless, many home owners may not be aware that lead was used in flashings of older homes, so removing such if they exist should be done if harvesting rainwater. Where there is passionate disagreement is on two fronts with the beliefs that:
  • high levels of lead in rainwater tanks will still accumulate from roofs without lead flashing.
  • contamination will be had from the sludge that accumulates which becomes a breeding ground for harmful bacteria.

Sludge and Biofilm Contamination

You might be wondering why such disagreement? Different studies conducted, and perhaps motivated subjective opinions of the facts on either side, are often what lead to disagreement. We don’t attempt to take one side or the other, but simply present the issue for the reader to decide. With that said, studies by those who believe sludge is a source of contamination and often focus on high levels of lead being generally found in rainwater tanks, it is suggested are motivated by water monopolies who control centralised water supplies and so stand to lose money if everyone began becoming self-sufficient for their own water. While motivation may/may not be a play, it is important to look at the facts in the studies, and read over the discussion had. Some studies have indicated that 23% of urban rainwater tanks have lead contamination, yet then why isn’t there more widespread reports of lead contamination amongst the 2.3 million Australians drinking rainwater? Dr. P. J. Coombes of Urban Water Cycle Solutions studies of old inner city residence located within industrial areas found no significant lead contamination. In fact, studies on over a thousand tanks in many parts of Australia in the past 15 years rarely revealed lead contamination. As summarised in Urban Water Cycle Solutions’ article, Rainwater Health Debate:
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).
In other words, the natural process which create the biofilm layer (sludge) behaves as a water treatment absorbing any heavy elements like lead and other contaminant from the water column (where water in a tank is accessed from). It is important to understand that the microbial communities found in biofilms 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 actually means it is good for preserving water quality. Some very poorly maintained water tanks, ones without water diverters and strainers often required by councils in urban areas, may develop the presence of a smelly, thick green or black slime. Yet, a reasonably installed and maintained tank will normally not develop such. In addition, the use of tank self-cleaning systems which continually remove any and all sediment build-up, would render any discussion of sludge contamination trivial to a tank owner.