Sampling Procedure of Drinking Water
Location of sampling points
Samples must be taken from locations that are representative of the water source, treatment plant, storage facilities, distribution network, points at which water is delivered to the consumer, and points of use. In selecting sampling points, each locality should be considered individually; however, the following general criteria are usually applicable:
- Sampling points should be selected such that the samples taken are representative of the different sources from which water is obtained by the public or enters the system.
- These points should include those that yield samples representative of the conditions at the most unfavorable sources or places in the supply system, particularly points of possible contamination such as unprotected sources, loops, reservoirs, low-pressure zones, ends of the system, etc.
- Sampling points should be uniformly distributed throughout a piped distribution system, taking population distribution into account; the number of sampling points should be proportional to the number of links or branches.
- The points chosen should generally yield samples that are representative of the system as a whole and of its main components.
- Sampling points should be located in such a way that water can be sampled from reserve tanks and reservoirs, etc.
- In systems with more than one water source, the locations of the sampling points should take account of the number of inhabitants served by each source.
- There should be at least one sampling point directly after the clean-water outlet from each treatment plant.
Sampling sites in a piped distribution network may be classified as:
— fixed and agreed with the supply agency;
— fixed, but not agreed with the supply agency; or
— Random or variable.
Each type of sampling site has certain advantages and disadvantages. Fixed sites agreed with the supplier are essential when legal action is to be used as a means of ensuring improvement; otherwise, the supply agency may object to a sample result on the grounds that water quality may have deteriorated in the household, beyond the area of responsibility of the supplier.
Sampling frequency
The recommended minimum frequencies for these critical measurements minimum sample numbers for piped drinking water in the distribution system are shown in Table 1. and in unpiped water supplies are summarized in Table 2
Table 1 Minimum sample numbers for piped drinking-water in the distribution system
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Population served No. of monthly samples
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<5000 1
5000–100000 1 per 5000 population
>100000 1 per 10 000 population, plus 10 additional samples
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Table 2 Minimum frequency of sampling and analysis of unpiped water supplies
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Source and mode of supply |
Minimum frequency of sampling and analysis |
Remarks |
|
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Bacteriological
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Physical/chemical |
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Open wells for community supply
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Sanitary protection measures;bacteriological testing only if situation demands |
Once initially for community wells
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Pollution usually expected to occur
|
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Covered dug wells and shallow tube wells with hand-pumps |
Sanitary protection measures;bacteriological testing only situation demands
|
Once initially, thereafter as situation demands
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Situations requiring testing: change in environmental conditions, outbreak of waterborne disease, or increase in incidence of waterborne diseases |
|
Deep tube wells with hand-pumps
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Once initially, thereafter as situation demands
|
Once initially, thereafter as situation demands
|
Situations requiring testing: change in environmental conditions, outbreak of waterborne disease, or increase in incidence of waterborne diseases |
|
Protected springs
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Once initially, thereafter as situation demands
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Periodically for residual chlorine if water is chlorinated
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Situations requiring testing: change in environmental conditions, outbreak ofwaterborne disease, or increase in incidence of waterborne diseases |
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Community rainwater collection systems
|
Sanitary protection measures; bacteriological testing only if situation demands
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Not needed
|
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Sampling methods for bacteriological testing
All samples should be accompanied by an appropriate collection form; a model sample collection form is illustrated in Fig. 1. When water samples are collected for analysis, care should be taken to ensure that there is no external contamination of the samples. Unless valid samples are collected, the results of the subsequent analysis may be misleading.
Fig. 1 Model sample collection form
Sampling containers and its preparation
Several types of bottle may be used for sampling, but glass bottles are best. These should have securely fitting stoppers or caps with nontoxic liners, and both bottles and stoppers should be sterilized. Each cap should have a metal sleeve clear of the screw thread to ensure that the risk of contaminating the water sample is minimized. Cotton wool plugs and paper caps should be avoided as they tend to fall off during and after sampling and increase the risk of contamination. The bottles should hold at least 200 ml of water. Whenever chlorine is used for disinfection, chlorine residual may be present in the water after sampling and will continue to act on any bacteria in the sample; the results of the microbiological analysis may therefore not be indicative of the true bacteriological content of the water. To overcome this difficulty, it is common procedure to add sodium thiosulfate to the sample, which immediately inactivates any residual chlorine but does not affect the microorganisms that may be present. The sodium thiosulfate should be added to the sample bottles before they are sterilized. For 200-ml samples, four or five drops of aqueous sodium thiosulfate solution (100 g/litre) should be added to each clean sample bottle. The stopper is loosely inserted into the bottle, and a brown paper or aluminium foil cover is tied to the neck of the bottle to prevent dust from entering. The bottle is then sterilized in a hot-air oven for 1 hour at 160 or 170C for 40 minutes or in an autoclave at 121 0C for 20 minutes. If no other facilities are available, a portable sterilizer or pressure cooker can be used, but sterilization will then take 30–45 minutes. To prevent the stopper from getting stuck during sterilization, a strip of brown paper (75 3 10 mm) should be inserted between the stopper and the neck of the bottle. For reasons of cost, bottles should be reused. After the samples have been analyzed in the regional or central laboratory, bottles should be resterilized and, if possible, returned to the sender.
Water can be divided into three basic types for the purpose of sampling:
1. Water from a tap in a distribution system or from a fixed pump outlet, etc.
2. Water from a watercourse (river, lake, etc.) or a tank
3. Water from a dug well, etc., where sampling is more difficult than from an open watercourse.
1.Sampling from a tap or pump outlet
A. Clean the tap
B. Open the tap
C. Sterilize the tap
D. Open the tap before Sampling
E. Open the sterilized bottle
F. Fill the bottle
G. Stopper or cap the bottle
2. Sampling from a watercourse or reservoir
A. Open the sterilized bottle as described in section.1.
B. Fill the bottle
3 Sampling from dug wells and similar sources
A. Prepare the bottle
B. Attach the bottle to the string
C. Lower the bottle
D. Fill the bottle
E. Raise the bottle
1. Sampling from a tap or pump outlet
A. Clean the tap
Remove from the tap any attachments that may cause splashing. Using a clean cloth,
wipe the outlet to remove any dirt.
B. Open the tap
Turn on the tap at maximum flow and let the water run for 1–2 minutes.
Note: Some investigators do not continue to stages C and D but take the sample at this stage; in this case, the tap should not be adjusted or turned off, but left to run at maximum flow. The results obtained in this way will provide information on the quality of the water as consumed. If the procedure is continued to stages C and D, however, the results represent the quality of the water excluding contamination by the tap.
C. Sterilize the tap
Sterilize the tap for a minute with the flame from a gas burner, cigarette lighter, or an ignited alcohol-soaked cotton-wool swab.
D. Open the tap before Sampling
Carefully turn on the tap and allow the water to flow for 1–2 minutes at a medium flow rate. Do not adjust the flow after it has been set.
E. Open the sterilized bottle
Take out a bottle and carefully unscrew the cap or pull out the stopper.
F. Fill the bottle
While holding the cap and protective cover face downwards (to prevent entry of dust, which may contaminate the sample), immediately hold the bottle under the water jet, and fill.
A small air space should be left to make shaking before analysis easier.
G. Stopper or cap the bottle
Place the stopper in the bottle or screw on the cap and fix the brown paper protective cover in place with the string. A small air space should be left to make shaking before analysis easier.
2. Sampling from a watercourse or reservoir
A. Open the sterilized bottle as described in section.1.
B. Fill the bottle
Holding the bottle by the lower part, submerge it to a depth of about 20cm, with the mouth facing slightly upwards. If there is a current, the bottle mouth should face towards the current. The bottle should then be capped or stoppard as described previously.
3 Sampling from dug wells and similar sources
A. Prepare the bottle
With a piece of string, attach a clean weight to the sampling bottle.
B. Attach the bottle to the String
Take a 20-m length of clean string rolled around a stick and tie it to the bottle string. Open the bottle as described in section
.C. Lower the bottle
Lower the bottle, weighed down by the weight, into the well, unwinding the string slowly. Do not allow the bottle to touch the sides of the well.
D. Fill the bottle
Immerse the bottle completely in the water and lower it well below the surface without hitting the bottom or disturbing any sediment.
E. Raise the bottle
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