Monday 2 January 2023

Instructions for taking mains water samples for bacterial counts

 Image: By José Manuel Suárez, CC BY 2.0,

Back-to-basics microbiology for the New Year. Here are some simple instructions for sampling mains water for bacterial counts, using an approiate neutralizer type and concentration.

To samples mains (tap or portable) water, use a 1 x sterile 500 ml plastic bottle containing an appropriate neutraliser to neutralize any residual disinfectant in the water.


The most commonly used neutralizer, which is appropriate for chlorinated or brominated water systems and those using ozone or hydrogen peroxide, is sodium thiosulphate (thiosulfate - U.S. spelling) (an inorganic compound; sodium thiosulfate reacts with bromine, removing the free bromine from the solution and it also reduces hypochlorite). The amount of neutralizer will vary. For example, with cooling towers, 180 mg/L (i.e. sufficient to neutralise 50 mg chlorine per litre) must be used. 


For mains water to be classed as ‘potable water’ it must be disinfected. Chlorine is the preferred chemical and it effectively kills bacteria and most viruses and maintains a residual to protect the water supply through the supply network. Alternatively, bromine is used. Bromine is a more effective disinfectant in water than chlorine when the pH is greater than 8 (which is the case in many cooling waters).



The sampling strategy should determine the sampling technique. If the quality of water as delivered from the tap (i.e. including any bacteria that are colonising the tap) is of interest, then the tap should not be sanitised and the sample should comprise the first portion of water delivered  preferably immediately after a period of no, or minimal, use.


If only bacteria present in the system prior to the tap are sought, the tap should be sanitised and run for 2 – 3 minutes before sampling. When attempting to ascertain the origin of contamination, samples before and after sanitisation and flushing may be appropriate.


The following sampling procedure should be followed (to assess bacterial counts in the water system):


  • If possible, ensure that the tap is in good condition, with no leaks.
  • Remove any internal and external fittings such as hosing.
  • Clean the end of the tap thoroughly with a clean disposable cloth (and detergent if necessary).
  • Disinfect with sodium hypochlorite solution (sufficient to give 1% available chlorine) made up on the day of use, or chlorine dioxide foam, such as by using a wash bottle to spray hypochlorite solution onto the outside and inside of the tap spout. Leave for 2-3 minutes before rinsing.
  • Turn on the tap gently to avoid unnecessary aerosol production and run water to waste for two to three minutes.
  • Label a sterile bottle (1 litre or 500 ml bottle containing neutraliser) with the location andsample details, sender’s reference, sampling officer and date and time of sampling.
  • Aseptically open the bottle, fill almost to the brim with water, replace and tighten the lid and shake the bottle to distribute the neutraliser.

Water samples should be stored between 1 and 8oC. They should be submitted to the laboratory to ensure that they are examined promptly, ideally the same day, but always within 24 hours of collection.

1 comment:

  1. Sir very well presented the with many basics to sensitise even very less educated need more posts to improve the matter Dr TV Rao


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