How Rainwater Cisterns Filter Water

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 hypothesis is extremely useful in permaculture design and one of the most hopeful pieces of information within the regeneration space. It has 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: In this article and thesis, Anthony Spinks reviews all of the incidental mechanisms that act as treatment processes within rainwater harvesting systems: biofilms in rainwater tanks filter and clean contaminants out of rainwater.

 

I want to focus this blog on case study 3 as I find it incredible and very useful within permaculture design.

In Dr. Spinks’ thesis, he goes through the various incidental treatment mechanisms that exist within a rainwater harvesting system, which 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 tyndallization in the hot water tank

 

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

Spinks found through his research project that rainwater collected off of 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 – 11,000 fold, depending on where the biofilm sample was taken within the tank.

 

Here are some of the concentration levels that were found in his research. The number listed below is the magnification level detected within the biofilm (Magnification is the number differential in concentration between the rain water and biofilm: 10,000 indicates 10,000 times higher concentration in the biofilm than in the water).

 

Lead -> 

Cadmium -> 100-400 times greater in biofilm

Nickel -> 500 – 32,400

Silver -> 700 – 32,400 

Mercury -> 20 – 100

 

It is essential to understand that this is not saying that rainwater is naturally high in these heavy metals. The concentration of heavy metals in rainwater you collect will be determined based on 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.

 

Spinks also looked at the influence on sludge layers in rain tanks and found equally incredulous findings. The same metals were shown to concentrate at even higher rates; here are the highlights.

 

Lead -> 66,860 – 343,000 times greater in biofilm

Cadmium -> 1,000 – 15,000

Nickel -> 13,000 – 45,000

Arsenic -> 9,600 – 61,200 

Copper -> 16,000 – 203,000

 

One of the reasons this research was so important was that it provided insights into how to manage rain tanks to best enhance the effect of the sludge layer and biofilms that contradicted conventional wisdom.

 

These insights included:

1) Do not take rain tanks out of service to remove sludge layers and clean biofilms.

2) Topping rain tanks up 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 minimize sludge layer disturbance. 

 

Leonardo Davinci said, “if you don’t copy nature, you’re wasting your time.” Looking at how biofilms and biologically active sludge layers 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 book and course on rainwater harvesting.