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Technical Guidance on Microbiological Contaminants

Clostridium Perfringens – Guidance for Local Authorities

Description and Background

Clostridia are a group of anaerobic bacteria found in the environment and the gut of many warm blooded mammals, including a significant proportion of humans. Clostridium
perfringens is the characteristic species, and the one tested for under the Water Intended for Human Consumption (Private Supplies) (Scotland) Regulations 2017. Its presence in drinking water indicates that there is faecal contamination of the supply.

Clostridia are spore forming and do not multiply in the water but spores recovered from water samples may be incubated under special conditions in the laboratory. All Clostridia spores are very resistant to harsh conditions, including disinfection chemicals and UV light, which means they can persist through the disinfection process and last for long periods in water supplies. This characteristic means that they are commonly used as
indicators of historic faecal contamination, both before and after the disinfection process.

The size of Clostridia spores means that they can be a useful surrogate for Cryptosporidium and their presence on a system with a physical barrier process such as  filtration may indicate that it has been less than effective and, consequently, the supply may be at risk from Cryptosporidium.

Health Significance

Clostridium perfringens can cause gastro-intestinal problems when consumed in large quantities. Low levels of Clostridium spores, in themselves, are unlikely to present a significant risk to healthy individuals directly from consuming contaminated drinking
water. The main risk from this organism is where spores from contaminated drinking  water are able to multiply in incorrectly cooked or stored foodstuffs. In these circumstances serious gastrointestinal disease can result.

The value of monitoring for Clostridium perfringens in drinking water comes from its role as an indicator of historic or intermittent faecal contamination and the effectiveness of any filtration process. Detections of this organism should serve as a trigger for a thorough investigation of the supply and any treatment process.

Where large numbers (more than one or two) Clostridium perfringens are detected in a sample, or if in any doubt, medical advice should be sought. 

Risk Assessment and Monitoring

Sampling for Clostridium perfringens is only a statutory requirement for regulated supplies. The sample frequency is enhanced to check monitoring where the water source is, or could be, influenced by surface water. However, it may be useful to sample for it for exempt supplies if there are concerns about surface water infiltration and the consistent microbiological safety of the supply and a thorough investigation is required.

What if it fails?

Failures for Clostridium perfringens should trigger an investigation of the supply and any treatment. As any disinfection process is unlikely to be effective against this organism, its presence on its own (i.e. without coliforms or E.coli )should not be taken to indicate
that disinfection has been compromised.

It should be remembered that where Clostridium perfringens is detected, there is also the potential for Cryptosporidium to be present, with potentially more serious implications
for health.

Check the following:

• Potential contamination of the source, or infiltration of dirty water or sewage into raw water pipework;
• The effectiveness of any filtration or barrier process such as a membrane – if Clostridium is being detected, the process may be compromised; even if it is currently operational, is this consistent and could it have been compromised in the past?
• Is there any potential for contamination of the supply post-treatment, for example via ingress into pipework or tanks?

Options for resolving at source

Usual options for minimising the microbiological loading at source, for example by preventing animal access to the source.


If there is no filtration / barrier process, the repeated detection of Clostridium in a supply could indicate a significant (and unmonitored) risk from Cryptosporidium. In order to fully
protect the health of those consuming the supply, a filtration system capable of removing particles down to a size of less than 1µm (and, ideally, 0.5µm ) should be installed. Such a barrier treatment process is also the only means of ensuring Clostridium spores are
removed from the supply and that samples taken for this parameter are compliant with the regulations. A correctly installed and maintained UV treatment system offers protection from Cryptosporidium.

It is unlikely that simple sand filtration alone will consistently meet the required standard of filtration. Professional advice should be sought as to the most appropriate sort of filter to use on a specific supply. Options for effective treatment include :

  • Cartridge filters. It will be necessary to install a number of these in series with descending levels of filtration down to less than 1µm. A suitable size for first stage in the filtration process will depend on the quality of the incoming water,
    but 20µm would be a typical value.
  • Membrane treatment. Membranes of ultrafiltration and finer (i.e. UF, NF and RO) should be capable of removing Clostridium and Cryptosporidium from a supply. These will incur a significant installation and ongoing maintenance cost.
    Water losses from the reject stream may be an issue where water resources are tight.
  • Mechanical filtration. These are filters consisting of fibres or other fixed materials that are automatically and repeatedly cleaned to maintain effectiveness. Various designs are available, with some being more appropriate than others in different situations. Many automatically wash, meaning they can be a good option for larger supplies with a high amount of particulate in the water. There is evidence that UV treatment may have some efficacy against Clostridium spores, however their high resistance means that the UV fluences required are likely to be far higher than for the other bacterial pathogens. Values of 95 mJ/cm2 have been quoted in order to achieve 2 log removal (i.e. a 99% reduction), against UV fluences of 9
    mJ/cm2 for E.coli 1 . Consequently, UV should only be considered for treatment of Clostridium with extreme caution, and expert advice should be sought to ensure any
    units are correctly sized and operated. 

1 Smeets , Rietveld, Hijnen , Medema, and Stenström 2006
Microrisk: Efficacy of Water Treatment Processes, European Union
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