TVC (Total Viable Count)

Accreditation: UKAS accredited for potable & process waters.

Sample volume: 100mls of sample is prefer.

Turnaround time: 1 – 3 days depending on the test

Expression of results: cfu/ml or cfu/0.1ml.l

 

This test is a general indication on the overall quality of the water and does not indicate if a sample will be harmful to drink or not. A raised TVC in a drinking water can impart a stale/ stagnant taste and/or odour to the water. Typically the TVC test is undertaken at 22oC and 37oC for drinking water and most other types of water, and at 30oC for cooling tower samples.

The UK water supply is governed by an EU directive on water supply and water quality. This directive has been introduced in the UK as the Water Supply (Water Quality) Regulations 2000, and is regulated by the Drinking Water Inspectorate.

When the regulations were first issued the TVCs were required as an indication of the overall bacteriological water quality. At this time the guide limits for bacterial levels were set at 10cfu/ml at 37oC and 100cfu/ml at 22oC. These were only ever guide limits; there has never been a statutory limit. Some mains water supplies will always have counts that exceed these limits, particularly surface derived waters. In 2005 revised guidance notes were issued for the 2000 Regulation where the above guide levels were removed and it was stated that counts should not differ from what is normally expected.

To quote section 6.25 paragraph (v) of Regulation 19 in the guidance:

Colony counts (Standard: no abnormal change)

Colony counts are enumerations of the general population of heterotrophic bacteria present in water supply. In environmental waters these represent bacteria whose natural habitat is the water environment or those that may have been washed from soil or vegetation. It is well recognised that only a small fraction of the viable heterotrophic bacteria population is estimated by enumeration on nutrient rich media with incubation at 22oC and 37oC. However, monitoring of water supplies for colony count bacteria can be useful for monitoring trends in water quality and detecting potential sudden deterioration in water quality. The colony count at 22oC generally represents those bacteria naturally present in water and are not of sanitary significance. They may however be of greater relevance to the food and drinks industries where high numbers may impact on the quality of products. An increase in the colony count at 37oC can be a sensitive indicator of ingress and further investigations should be undertaken to establish the source. Colony counts may be useful in assessing the efficiency of water treatment and the cleanliness and integrity of the system. In all cases the value on monitoring is to establish data which characterises a water supply in terms of seasonal and longer term changes. Drinking water supplies derived from surface waters tend to support higher numbers of heterotrophic bacteria than those derived from ground water sources. The onset of significant changes in colony count results against the normal range established for that water supply is much more significant than the absolute values of individual results”