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The sample container being used must be sterile, either through reusable non-reactive borosilicate glass or plastic bottles being properly cleaned, rinsed, and autoclaved, or using pre-sterilized plastic bags designed specifically for water sampling. To ensure the accuracy of results representing conditions at the time of sampling, a reducing agent, such as sodium thiosulfate, must be added to the sample containers used for water containing chlorine or other halogens. This reducing agent will neutralize any halogen residuals, preventing continuation of bactericidal activity. Select a tap that is supplying water from a service pipe connected directly to the main.

Select a tap that appears to be clean, and without leaks to the outside of the faucet. A 100 mg/l chlorine solution may be sprayed on the faucet if it is specifically required for sampling, otherwise find another suitable tap. Mixing faucets with screens or splashguards also need to be handled with additional precautions of removing the attachments and running the hot water first, then normal flushing on the cold water line.

Open the tap fully and let run for 2 to 3 minutes, or a time sufficient to allow clearing of the service line. Reduce the flow to allow filling of the bottle without splashing. It is important that the container be kept closed until immediately before sampling, and the sample is representative of the water system being tested. Fill the container without allowing fingers, tools, or other foreign objects to come in contact with the water. Leave sufficient headspace in container to allow for mixing by shaking before examination.

When sampling a pool of water, a stream, or other type of water source, caution must be taken to avoid contamination of the sample. Always hold the sample container downstream from the opening of the sample bottle. If necessary, hold the sample container with a clean clamp or tool to avoid contaminating the water with your hands.

The final step in sampling is to assure the samples arrive for analyses in the proper condition, either through delivery within one hour of sampling, or refrigerate below 10 degrees C.

In summary, the sample must be representative of the system to be tested, and factors that will affect the accuracy of that representation must be dealt with. Not only do you want to know that positive results are accurate, but that negative results truly represent safe conditions.