Fact sheet on colony counts at +22 °C and +36 °C in drinking water

General
The colony count at +22 °C and +36 °C is often also referred to as the "total viable count (TVC)". It provides information on the number of microorganisms in tap water that multiply optimally under aerobic conditions at the incubation temperatures of +22 °C and +36 °C.

• More specific information on important microorganisms in drinking water: see other fact sheets

Origin / Occurrence

• The colony count examination at +22 °C and +36 °C covers a wide variety of different microorganisms. The cause of occurrence and the origin vary depending on the microorganism (e.g. soil, plants, animals, humans)

General significance

• Drinking water is not sterile and a certain colony count is unavoidable
• An increased colony count does not necessarily represent a direct health hazard, but it is a signal of an adverse change that needs to be addressed

Significance of colony count at +22 °C

• This parameter tests drinking water for the natural microbial load
• An exceedance of this parameter indicates a negative change in operating conditions in the distribution system, such as deposits, increased nutrient availability, or disturbance of the biofilm

Significance of colony count at +36 °C

• This parameter is an indicator in drinking water of contamination introduced from outside into the distribution system. These organisms can also include potentially pathogenic microorganisms
• An exceedance of this parameter indicates external contamination such as the ingress of insects, soil, plants, or entry of faecal contamination

Growth conditions

• Temperature:
  • Optimum: between +20 °C and +40 °C
  • Minimum: usually +5 to +10 °C (depending on the organism, exceptions down to 0 °C)
  • In general no growth above +50 °C (exceptions are thermophilic bacteria)
• pH value: growth generally between 5.0 and 9.0 (depending on the organism, also possible at significantly lower pH values)
• a_w value: most bacteria multiply at medium to high water activity of at least 0.88 (in rare cases also at significantly lower a_w values)
• Oxygen requirement: many bacteria grow with and without oxygen (facultative anaerobes). There are, however, bacteria that can only multiply under aerobic or anaerobic conditions.

Inactivation by heating

• Most microorganisms are killed at +72 °C for at least 2 minutes (Note: monitor core temperature)

1. If an aerator is present: remove and disinfect. Replace depending on condition
2. Remove contamination and/or biofilms
3. If possible, flame (heat-sterilize) the last section of the water outlet after cleaning
4. Verify success by follow-up testing

For drinking or process water (cause in the pipe network)
Pipe network (building installation) contaminated, stagnant water in the system, or contains a biofilm

1. Perform stepwise investigations if necessary to better identify the location of the contamination in the pipe network
2. Hire a specialist company or building services to identify causes
3. Carry out disinfection
4. Verify success by follow-up testing

1. Thorough visual inspection of the treatment system
2. Check maintenance intervals, replace filters if necessary
3. Hire a specialist company or building services for inspection/maintenance
4. Verify success by follow-up testing

For drinking water from private water supply systems, i.e. own wells and springs

Contamination due to intervention/repair in the spring collection chamber/well

Contamination after heavy rain (contamination by surface water)

Contamination due to construction defects in the spring collection chamber/well (leakage, age-related wear)

1. Thorough visual inspection of the spring collection chamber/well and surroundings
2. Regular inspection of the private water supply system, especially after possible weather-related impacts (heavy rain, prolonged drought)
3. Carry out the inspections required by the public health authority at the prescribed intervals
4. Hire a specialist company at appropriate intervals for cleaning and disinfection of the spring collection chamber/well
5. Verify success by follow-up testing

For flaked ice, crushed ice and ice cubes
Hygiene deficiencies:

in the machine including utensils
or

in the connecting hose (feed water)
or

in the pipe network (building installation)

1. Thorough visual inspection of machines and connecting hoses. This usually requires dismantling parts of the machines
2. Remove contamination or biofilms
3. Thorough cleaning and disinfection of machines and individual parts (possibly by a specialist company)
4. Thorough cleaning and disinfection of utensils (e.g. scoop)
5. Regularly replace the connecting hose between the machine and the mains
6. Immediately replace contaminated or damaged connecting hoses (e.g. brittle, porous)
7. Test the drinking water from the mains (before inlet to the machine)
8. Verify success by follow-up testing

Mains-connected water dispensers
Hygiene deficiencies:

in the dispenser
or

in the connecting hose (feed water)
or

in the pipe network (building installation)

1. Thorough visual inspection of the water dispenser including the connecting hose
2. Remove contamination or biofilms
3. Thorough cleaning and disinfection of the water dispenser (usually by a specialist company)
4. Possibly replace the connecting hose between the machine and the mains at regular intervals
5. Immediately replace contaminated or damaged connecting hoses (e.g. brittle, porous)
6. Test the drinking water from the mains (before inlet to the dispenser)
7. Verify success by follow-up testing