SOP: Collection and Transport of Water Samples for Bacteriological Examination
GIFSQ — Global Institute for Food Safety & Quality
Principle
The reliability of a bacteriological examination and the interpretation of its results depend largely on how the sample is taken: the sample must truly represent the supply and must be collected without extraneous contamination. The collected water is held cold and examined promptly so that the bacteria present do not multiply or die off before the tests begin.
Apparatus required
- Clean sterilized bottles of neutral glass (200–250 ml capacity), provided with ground-glass stoppers with an overlapping rim
- A strip of paper to relax the stopper (placed between stopper and neck to prevent breakage during sterilization or jamming)
- Paper or parchment covers to protect the stopper and neck of the bottle
- Blow-lamp, or cotton soaked in methylated spirit with means of ignition, for sterilizing taps
- Length of string for lowering the bottle into rivers, lakes and wells
- Insulated ice box for transporting samples
Reagents
a. Sodium thiosulphate solution (3%) — used when the water to be sampled contains or is likely to contain chlorine; add 0.1 ml of the solution, or a small crystal of the salt, to the bottle before sterilization.
Sample preparation
No preparation of the sample is needed beyond collecting it as directed; do not open the sampling bottle until the moment it is required for filling.
Procedure
Collect the sample from a tap in regular use:
Open the tap fully and allow the water to run for at least 2 min, flushing the nozzle and discharging the stagnant water in the service pipe
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Fill the sample bottle from the stream of water from the tap, avoiding splashing
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Collect the sample from a tap not in regular use:
Sterilize the tap by heating it with a blow-lamp, or with an ignited piece of cotton soaked in methylated spirit, until the tap is too hot to touch
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Cool the tap by allowing the water to run before collecting the sample
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Fill the sample bottle from the stream, avoiding splashing
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Collect the sample from a well fitted with a pump:
Pump the water to waste for about 2 min
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Collect the sample from the pump delivery or from a tap on the discharge
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Collect the sample from a river, lake, reservoir, tank or well:
Attach a string to the neck of the bottle, wrap the bottle completely in paper and sterilize it
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Remove the paper cover without allowing the sides of the bottle to touch anything, and tie a long clean string to the end of the sterilized string
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Lower the bottle into the water, allow it to fill, raise it and replace the stopper
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Alternatively: hold the bottle at the bottom and plunge it neck downwards below the surface, then turn the bottle until the neck points slightly upwards and direct the mouth towards the current; where there is no current (as in a reservoir), push the bottle horizontally forward away from the hand to create one
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Once the bottle is full, lift it quickly clear of the water and replace the stopper
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Transport and storage:
Begin the bacteriological examination as soon as possible after collection
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Where this is not feasible, keep the sample in ice until it is taken up for analysis, and complete the analysis of all such iced samples within 48 hr of collection
Results
A sample is fit for bacteriological examination only when it was collected as a true representative of the supply without extraneous contamination, the bottle was opened only at the moment of filling, and the examination began promptly or the sample was held in ice and analysed within 48 hr of collection. Samples not preserved and analysed in this manner are not suitable for bacteriological examination of hygienic quality.
Precautions
- Take samples from rivers and streams neither too near the bank nor too far away from the point of draw-off.
- Avoid collecting samples from leaky taps; water running down the outside of the tap may enter the bottle and contaminate the sample. If a leaky tap cannot be avoided, clean the outside of the tap and flame it sufficiently to ensure sterility.
- Samples not preserved in ice and analysed within 48 hr are not suitable for bacteriological examination of hygienic quality.
- When the water contains or is likely to contain chlorine, add the sodium thiosulphate to the bottle before sterilization.
- Keep the stopper relaxed with the paper strip during sterilization to prevent breakage or jamming, and keep stopper and neck covered with paper or parchment until filling.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
Manual of Methods for the Examination of Water, Sewage and Industrial Wastes, Special Report No. 47, Indian Council of Medical Research, New Delhi (1963) — cited in the book.
Standard Methods for the Examination of Water and Wastewater, 13th edn., American Public Health Association, Washington, D.C. (1971) — cited in the book.