The amount of CFU- Colonie Forming Units – describes the microbiological activities in a liquid. Very simplified it is the bacterial or fungal reproduction in water during a time.
All organisms are able to multiply under certain conditions. So in theory one cell is enough to start multiplying and to populate a water system. As bacteria or fungus in water might be dangerous for the health for the consumer of the water, it is closely controlled. This is not only true for drinking water, but as well for cooling water.
In cooling water or special cooling towers the CFU is measured because of legionella in the water. People got sick in the wind drift of the cooling tower.
To analyze water how many CFU are contained, a sample of the water is taken and put in a so called Petri-Dish. Depending on what type of bacteria or how much bacteria are to be expected to be in the sample, the water sample will be diluted before putting on this plate. This plate is then put to defined conditions for a certain time. Temperature and time are set to get comparable results. After this growth period it is possible to calculate the amount of growing units in the water. This is usually done by counting the bacteria in the petri-dish. The result is given in CFU/mL.
In drinking water the CFU/mL should not be above 100 CFU/mL. In technical water used for cooling, the amount of bacteria should be less then 1000 CFU/mL.
This values are set and very often controlled by the authorities. There is no international binding standard, they might differ from country to country.