In Permaculture, Rainwater Harvesting, Water

One of the first questions we field when we’re talking about rainwater harvesting is Isn’t rainwater unsafe? Don’t you need to purify it with chemicals?

We respond, Well, actually… and explain that a well-designed rainwater collection system filters and cleans the water naturally. Stunned looks ensue…

To all who want to know how this natural purification works, here’s your answer!

One of the fascinating concepts in biology is the microbial infallibility hypothesis. In his 1951 textbook, British microbiologist Ernest Gale put forward a hypothesis which stated:

“if there is energy to be gained from a compound, a microorganism will figure out how to extract it and create a niche for itself.”

This is one of the most hopeful theories within regenerative agriculture. It’s been tested worldwide in many different situations; here are a couple of interesting examples that will lead you far down the rabbit hole if you are interested in seeing it for yourself.

Case Study 1

In this blog you will see how Paul Stamets uses oyster mushrooms to “eat” petroleum and turns it into mycelial mats and mushrooms. 

Case Study 2

This Michigan State University article discusses how Geobacter can “soak up” uranium. 

Case Study 3

Biofilms in rainwater tanks filter and clean contaminants out of rainwater. In this article and thesis, Anthony Spinks reviews all of the incidental mechanisms that act as treatment processes within rainwater harvesting systems.

I want to focus this blog on case study 3 as I find it incredible and very useful within permaculture design. But first, here’s a great video where my partner and co-author, Michelle Avis, interviews Dr. Peter Coombes about his twenty years of reasearch into rainwater harvesting and biofilms. 

Michelle and Peter hosted a Rainwater Harvesting Masterclass, which happened on January 25th, 2023. More information here.

Now… back to Case Study number 3! In Dr. Spinks’ thesis, he goes through the various incidental treatment mechanisms that exist within a rainwater harvesting system. These include:

  • UV radiation on the roof surface
  • pre-rainwater filtration (the rain head)
  • the biofilm and sludge layer in the rain tank
  • the pressure change in the pump and sterilization in the hot water tank

In my opinion, the most interesting treatment process is the impact of undisturbed sludge layers and the biofilms that form on the walls of the rain tank. 

Spinks found through his research project that rainwater collected off clean roofs and run through a rain head has minimal nutrients, making it hard for microbes to survive. When the water enters the rain tank, this essentially makes the microbes that exist in the rainwater hungry, which means they will sorb whatever they can get “energy” out of. His research compared the concentration of lead, cadmium, nickel, silver and mercury in the rainwater relative to the concentrations within the biofilms on the tank surfaces.

What he found was astonishing. The biofilms had concentrated all of the heavy metals they tested for to extraordinary levels, with lead being the highest. Lead concentrations varied from 500 – 10,000-fold times higher than the rainwater column depending on where the biofilm sample was taken within the tank.

Here are some of the other concentration levels that were found in his research. 

Cadmium: 100 – 400 times greater in biofilm

Nickel: 500 – 32,400 times greater in biofilm

Silver:  700 – 32,400 times greater in biofilm

Mercury: 16,000 – 203,000 times greater in biofilm

This is not saying that rainwater is naturally high in these heavy metals! The concentration of heavy metals in rainwater you collect will depend upon local and global air pollution at the time of testing and the materials you use on your roof and piping system. The research says that if you have trace elements of these metals in your water, biofilms represent a passive, low-energy solution to cleaning rainwater to a very high level of quality.

Spinks also looked at the influence the sludge layer at the bottom of rain tanks and found equally incredible findings. The same metals were shown to concentrate at even higher rates; here are the highlights.

Lead: 66,860 – 343,000 times greater in sludge than in rainwater column

Cadmium: 1,000 – 15,000 times greater in sludge than in rainwater column

Nickel: 13,000 – 45,000 times greater in sludge than in rainwater column

Arsenic: 9,600 – 61,200 times greater in sludge than in rainwater column

Copper: 16,000 – 203,000 times greater in sludge than in rainwater column

One of the reasons this research was so important was that it provided startling, counterintuitive insights into how to manage your rain tank and protect and enhance the effect of the sludge layer and biofilms. 

These insights included:

  1. Do not take your rainwater tank out of service to remove sludge layers and clean biofilms.
  2. Topping up your tank with chlorinated municipal water harms biofilms and hampers their ability to treat rain water.
  3. Set the tank outlet line above the sludge layer to avoid disturbing the sludge layer. 

Leonardo DaVinci said, “if you don’t copy nature, you’re wasting your time.” Looking at how biofilms and biologically active sludge layers capture contaminants in rain tanks is a perfect example of this. 

If you want to learn more about rainwater harvesting and how to put these biological forces to productive use on your urban or rural homestead, check out our resources, books and courses on rainwater harvesting here.

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