VPE 511: Environment and Environmental Hygiene (2+1)

FACTORS TO BE CONSIDERED WHILE SAMPLING

• Sample site: The point at which sample is to be drawn should be decided appropriately as it largely reflects on the inference of the analysis. Usually samples have to be collected from two or more points for comparison of results.
• Quantity of sample: In general, sample of about 250 ml is sufficient, but more quantity may be required based on the type of analysis.
• Sample size and Sample interval:  For samples from a distribution system supplying to cities, towns and other areas, the following guidelines may be followed.

• Maximum interval between successive samples and minimum number of samples to be taken :

  Population Served	Maximum interval between successive samples	Minimum number of samples to be taken from whole distribution system, each month
  <20,000	1 month	1 sample / 5000 population
  20,000-50,000	2 weeks	1 sample / 5000 population
  50,000-1,00,000	4 days	1 sample / 5000 population
  >1,00,000	1 day	1 sample / 1000 population

• Container: The sampling container may have significant effect on the results of water analysis. Eg. Plastic absorbs silver, oil or grease, glass is unsuitable for borates and silicates as these can be contributed by glass itself. Other factors like ease of handling, cleaning, sterilization, disposable nature should also be considered while selecting a container.   
• Use of Preservatives: For microbiological assay if water is likely to contain chlorine, chloramine, or other compounds thiosulphate solution (0.1 ml 3% solution or a small crystal) should be added. If heavy metals are present (e.g. copper) chelating agents like Ethylenediaminetetra-acetic acid (EDTA) can be added.
• Representative Sample: The sample should be, as far as possible representative of the source. For, this the container may be rinsed with the water from the source before taking the sample. In deep lakes or stream samples may be collected at different levels using suitable equipment.