The following points highlight the top two techniques used for determining the potability of water. The techniques are: 1. Coliform-Test-Technique 2. Membrane-Filter-Technique.
1. Coliform-Test-Technique (Multiple-Tube Fermentation Test):
Coliform-test-technique (or MTFT) is a standard method followed all over the world to determine whether the water is potable or faecally polluted. The technique involves three successive steps, namely, presumptive test, confirmed test, and completed test.
In the presumptive test, lactose broth tubes are inoculated with three different water volumes to give an estimate of the most probable number (MPN) of coli forms in water.
Tubes which are positive for gas production are inoculated into brilliant green lactose bile broth in the confirmed test, and positive tubes are used to calculate the most probable number (MPN) value. Presence of coliform bacteria in water samples are established in completed tests. A general scheme of coliform-test technique is presented in Fig. 31.2.
2. Membrane-Filter-Technique:
This technique used for bacteriological examination of water to determine its potability was developed in Germany during World War II and, at present, is being considered advantageous over coliform-test-technique because of its significant advantages.
In this technique, a thin membrane filter-disc is used. The filter-disc consists of cellulose derivatives and can retain on its surface all bacteria from the water sample. The water is filtered through filter-disc and the disc is then transferred with a sterile forceps on to a thin absorbent pad that has previously been saturated with the appropriate medium (generally Endo-broth [GM-9] medium) and accommodated within a Petri dish.
The Petri dish containing absorbent pad and filter-disc is incubated at 37°C for 18-24 hours. The medium diffuses through the pores of the filter-disc and provides nutrient to the bacteria. After the incubation is over, one can see colonies developing upon the filter-disc. The characteristic colonies of different bacteria could now be studied to determine water potability.
Advantages:
(i) A large volume of water can be analysed in a short period of time without much expenses.
(ii) The membrane filter-disc can be transferred from one medium to another to differentiate organisms.
(iii) Quantitative estimations of certain bacterial types, e.g., coliforms can be done using appropriate selective media even when the bacterial types in question are present in small numbers.
(iv) This technique requires much less equipment and, therefore, can be operated direct in the field.
Disadvantages:
(i) High turbidity waters limit volume sampled.
(ii) High population of background bacteria result in overgrowth.
(iii) Metals and phenolic compounds can absorb to filters and inhibit growth.