We (in North America) live in ever present fear of one of the longest lived and most adapted life forms… the bacteria. We sterilize everything, from head to toe, and as such we are missing out on the benefits they provide, symbiotic benefits with synergistic outcomes.
Sure bacteria can be harmful… that is the harmful ones can be harmful. But following from the science bacteria are mostly beneficial. A snapshot of the human body shows number of bacteria that live on the inside and out vs the human cells outnumber human cells 10:1. Look to the research in autism and they are using diet to change gut bacteria to alter toxins from the bad bacteria to allow better functioning. In the face of antibiotics where many of the bad bacteria have developed immunity, we are looking to Russian science and the use of phages (introduction of bacteria) to combat infections from the bad ones through out competing. Permaculture science and soils also demonstrates the importance of promoting bacteria in high diversity for soil and plant health. So what does this have to do with rainwater?
Using bacteria to clean and filter rainwater is a biological approach that removes the reliance upon manufactured disposable consumer items. Of coarse the natural world has figured this one out long ago, but us humans are a little slower on the uptake.
Aerobic Bacteria – These bacteria us oxygen to convert organic materials to more organic materials and carbon dioxide. As they do this they reduce the Biochemical oxygen demand and create an environment that allows higher level life forms to exist including ciliates, flegellates, protozoa and, rotifers. (ref. 1)
Anaerobic Bacteria – Four families of bacteria that function in low to no oxygen play a huge role in cleaning water. The stages are hydrolysis, acidogenisis, acetogenesis and methanogenesis. Bacteria in the hydrolysis group are responsible for breaking down complex organic polymers into simple monomers like sugars, amino acids and fatty acids. Acidinogenic bacteria further break these down through fermenting and create ammonia, hydrogen sulfide and carbon dioxide. Acetogenic bacteria then break these items down further into acetic acid and hydrogen. The methanogens then convert the final products to carbon dioxide, methane and water. (ref. 2)
What ways can we use bacteria?
- Biosand filters (slowsand filters)
- Constructed wetlands
- In situ with cisterns
Biosand Filters
These filters rely on passing water slowly through a deep bed of submerged sand, where upon the waters entrance to its exit both aerobic and anaerobic bacteria combined with removal of oxygen, virtually purify the waters. These systems are used in developing countries to clean non-potable cloudy water. Refer to Figure 1: Water enters at the top layer, where a scum layer known as a schmutzdecke naturally develops (an aerobic bacterial layer), beginning the initial process of clarifying; this aerobic layer also oxidizes and removes minerals that can contaminate water . From here water migrates into a deep bed of saturated sand moving from fine aggregate to coarse – during this process both mechanical and biochemical purification occur. (ref. 3)
Biosand filters can be used in the home or for a remote lodge, and need to rely on a system that has four stages: primary storage, biosand filter, secondary storage, pump and filtration for polishing.
Constructed Wetlands
Think of natural wetlands, often considered the kidneys of the natural world. Surface-water flows are impacted by UV light and oxygen, waters in aquatic ponds rely on bacteria in the same way that a Biosand Filter works, using biochemical, biological and mechanical means to 
purify the water. These natural systems are so effective that watersheds like the CRD (Capital Regional District) rely on the biology of the landscap and aquatic ecosystem to purify the drinking water reservoirs serving over 300,000 people on Southern Vancouver Island. The natural processes require minimal final polishing to ensure potable quality.
Water Cisterns – the life that is hidden
Beginning with the premise that water collected in a cisterns has been pre-filtered to remove coarse and fine debris to within a range of 100 microns (done through re-usable screens), the waters that are in a tank can be cleaned considerably via the bacterial life in the tank. Within a water cistern there are two layers of bacteria… the upper layer that contains aerobic life (floating bio-film) and the scum on the bottom that is the anaerobic life. Aerobic bacteria begin by converting organic matter with a byproduct of carbon dioxide, while anaerobic convert and create methane and nitrates as noted above in the discussion on Biosand filters.
A preferential design to use cisterns to treat water is to daisy-chain two or more together, having an air pump and diffusor add oxygen to the first tank and supporting the activities of aerobic bacteria on initial inflow of organics; followed by the second tank which the water is allowed to be anaerobic. From this second tank water is pumped and filtered. The benefit of the two tank system, other than the storage, is the ability to take advantage of retention time.
Viruses
A discussion on bacteria should include a comment on viruses. Viruses are smaller particles, to 0.1 micron, and can die when not in their prime environment. Retention time is important, but sunlight is one of the best ways to scramble the genetics of a virus so considerations should be given to fine 0.1 micron filter, or exposing to Uv light.
Survival of the fittest. When there is a whole host of life forms, no one particular bacteria form will out compete the rest, in essence all being kept in balance. But when life forms become less varied there becomes more opportunity for one life form to take control and cause havoc.
References:
1 – https://www.ebsbiowizard.com/2011/08/aerobic-vs-anaerobic-treatment-in-wastewater-systems-part-1-2/
2- https://en.wikipedia.org/wiki/Anaerobic_digestion
3- http://www.biosandfilter.org/biosandfilter/index.php/item/320
