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Water quality field testing conventionally depends on membrane filtration, but due to the cost and complexity of the equipment involved, this article presents alternative methods.
Key field tests[edit | edit source]
A water quality field test should be simple, rapid, inexpensive, and accurate. These tests estimate the amount of fecal Coliform present in the water. Fecal Coliform is not a pathogen. It is used as an indicator of pathogen presence because Coliform bacteria are very common in the feces of warm blooded animals and their presence is relatively easy to test for. Coliform are acceptable in water sources so long as their concentration is relatively low.
Table 1 shows fecal Coliform ranges and the actions which should be taken depending on the Coliform count
|Table 1: values for water treatment|
Membrane Filtration[edit | edit source]
- Requires 45 Celsius incubator
- Takes 24 hours
- Sterilization of equipment is required
- Various chemicals and other materials are required
Presence/Absence (P/A)[edit | edit source]
This test does not count the number of colonies, but simply detects the presence or lack of Coliform Bacteria in a relatively small (~10ml) sample.
- A powder is added to the water sample
- The mixture is incubated at 35 Celsius for 24 hours
- If the liquid is colorless, there are no Coliform
- Yellow indicates Coliform
The Most Probable Number can be calculated by using multiple P/A samples.
- Due to the lower incubation temperature, samples can be incubated on the body
- Easier to use than Membrane Filtration
Petrifilm[edit | edit source]
Petrifilm is a commercial product of the 3M Corporation which comes prepared for testing.
- A 1ml sample is applied to a petrifilm plate and spread evenly across it.
- Incubated at 35 Celsius for 24 hours
- The Coliform colonies are counted and the MPN is calculated
- Only capable of accurately determining MPN above 100cfu/100ml
- Portable, inexpensive and easy to use
Recommended approach[edit | edit source]
- 3 10ml samples should be taken from every potential water source and a P/A test conducted.
- If these samples are all negative, the water source is usable.
- If there are positive results from the P/A test, a Petrifilm test should be conducted
- Since the petrifilm range is limited to above 100cfu/100ml, it is possible to get negative results from this step. If this is the case, the water is not deemed safe, but must be tested with 10 10ml P/A samples to assess the MPN.
Conclusion[edit | edit source]
The major advantages of Petrifilm and P/A over standard Membrane filtration are that they are easier to use and can be incubated at lower temperatures. It is recommended that the samples be kept in a pouch such as a money belt for the 24 hours. Experience shows that the human body temperature of 37 Celsius is adequate for performing these procedures.
Jargon[edit | edit source]
MPN - Most Probable number of colony forming units per 100 ml of water (cfu/100ml)
References[edit | edit source]
Médecins Sans Frontières (1994) Public Health Engineering in Emergency Situation. Médecins Sans Frontières: Paris.