Scientific research has shown that the normal concentration of coliform bacteria (e.g. E. coli) in surface water varies between 105 per 100 ml (also expressed as log 5) and 109 per 100 ml for sewer water. Considering that a dose of only 103 is enough to cause an intestinal infection in people with a weak immune system, no more than 1 per 100 ml may remain in treated water, or a reduction of at least 6 log (99.9999%).
Cysts (such as Cryptosporidium and Giardia) are the largest micro-organisms that occur in water (3 – 6 µm). They are found in surface water in concentrations of 10 per litre. They are very infectious and must not be present in water after treatment, which requires 3 log (99.9%) removal.
The concentration of enteroviruses (e.g. polio and rotaviruses) in surface water is estimated at 102 -104 per litre. It is generally accepted that drinking water may not contain viruses (< 1 per 100 litres). The reduction must be at least 4 log (99.99%).
These purification standards are established in the various water supply decrees in Europe.
Since 1976 we have known that ingested bacteria are not the only serious threat to our health. Bacteria (Legionella) that are inhaled with water vapour can cause a serious form of lung infection.
Read more about Legionella pneumophila…
So far, we have been able to effectively treat illnesses that are caused by bacteria with antibiotics. Slowly, bacteria have developed resistance against this with the so-called NDM 1 gene, which neutralises the effect of all known antibiotics. This gene, which makes bacteria resistant, is found in bacteria that cause dysentery (E. coli), for example.