What is total viable count in water testing and why is it important?

By: James Greenwood on Aug 13, 2020

Water that is supplied to buildings from a mains water supply can contain bacteria. It is the responsibility of water companies to remove all harmful bacteria from the water before it is piped to homes and businesses so that it is safe to consume. However, some bacteria, generally at very low levels, can enter and remain in our domestic systems, and if the bacteria are allowed to breed, it can cause a problem.

A general bacteria test looks at the overall number of microorganisms present in the water that are able to grow. This is known as a total viable count (TVC).

What is TVC in water testing?

Total viable count (TVC) is a test that estimates the total numbers of microorganisms, such as bacteria, yeast or mould species, that are present in a water sample. TVC may also be expressed as aerobic colony count.

The results of a TVC test offer an indication into the general level of contamination within a system and the overall quality of the water.

What’s the difference between TVC and TBC?

While TVC estimates the number of viable or live cells that are capable of growing into distinct colonies, the total bacteria count (TBC) determines the count of all cells both dead and alive.

How is TVC calculated in microbiology?

There are different test parameters for different types of sample.

First, the samples may be incubated at different temperatures depending on the environment being sampled.

For example, samples from a drinking water system should be incubated at 22˚C or 37˚C for 24 hours in accordance with BS EN ISO 6222.

  • At 22˚C, the TVC indicates the number of live bacteria per ml of water at ambient temperature, meaning the count will predominantly be composed of harmless bacteria.
  • At 37˚C, the TVC indicates the number of live bacteria per ml of water at body temperature, and the bacteria that grow and more likely to cause some harm to humans.

Samples taken from cooling tower systems, on the other hand, should be incubated at 30˚C in accordance with ACoP L8, HSG 274 Part 1.

These test conditions are set to isolate the range of organisms that can colonise and cause infections.

What is an acceptable TVC?

There is no strict regulation on the acceptable value for TVC in drinking water. However, the guide limits for bacterial levels are that there should be “no significant increase” from the incoming mains supply.

But what’s most important is being able to identify any irregularities. This is why it may be recommended that TVC testing is carried out regularly, so that businesses can understand baseline contamination levels and identify any significant deviation from the baseline when it occurs.

Why do we take TVC samples?

Businesses have a duty of care to ensure their water systems are adequately protected and safe for users. And TVC testing provides an overall indication of the quality of the water. A high TVC, for instance, may indicate poor quality of drinking water. It may have a stale or stagnant taste or odour.

Testing TVC regularly allows for businesses to understand baseline contamination levels and identify any significant deviation from the baseline. When a deviation occurs, it may indicate the need for further investigation and could point to problems in a system.

TVC testing can also be used to assess the efficiency of the water treatment regime. High results in mains fed water coolers, for example, can indicate that they are not being cleaned properly or that the cleaning regime needs to be carried out more often.

What does a significant deviation look like?

A count of 50 cfu/ml obtained one month followed a count of 60 cfu/ml the next month would not be considered significant. However, a count of 50 one month followed by a count of 5000 the following month may be considered a significant change to the system.

It isn’t uncommon to find seasonal variations. For example, the TVC may be higher during the summer and it may be appropriate to apply a control measure to reduce the temperature of the system throughout the summer months.

Interestingly, when the coronavirus pandemic forced the nation into lockdown, in many London buildings that were shut down, the TVC was found to be significantly higher than in the same period the previous year. Flushing was not enough to maintain normal TVC levels and Public Health England (PHE) recommended that supplementary microbiological samples should be taken to prove regimes are effective and other actions implemented as required.

What control measures are there to reduce TVC?

Ultimately, there is no substitute for good engineering, risk assessment and a robust water management plan. However, in the real world we know that sometimes there are buildings and systems with challenges for various reasons.

There are many different ways of controlling bacteria if the engineering and the temperature control needs enhancing. One of these is WCS Group’s Ultralox40® solution, which has been proved to reduce stubborn TVC within weeks. It is a highly effective low concentration form of chlorine and particularly efficacious fast-acting biocide.

Ultralox40® can be shock-dosed or used as a permanent retrofit secondary disinfection solution. It is suitable for hotels, care homes, schools, offices, commercial buildings, food and beverage manufacturers, leisure centres, and is used widely across NHS sites.

WCS Group provides accredited laboratory water testing and analysis for TVC and other parameters such as legionella, pseudomonas and coliforms. For more information about our water testing services, please get in touch.

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Topics: Water Testing

James Greenwood

Written by James Greenwood

James Greenwood as been working in the Water Treatment and Water Hygiene Industry for over 20 years. He is currently the Sales and Marketing Director for WCS Group the largest water hygiene and treatment Company in the UK. James has been instrumental in bringing significant innovations to the UK market over the years always focusing on enhancing client’s compliance and delivering true return on investment projects offering monetary and environmental savings.