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Chapter 8*: SANITARY INSPECTION AND MICROBIOLOGICAL WATER QUALITY - page 14 / 52

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is rather time consuming (taking between five and seven days), several laboratories prefer it to other methods of water analysis because it is applicable to all sample types.

The MPN technique is generally conducted in three sequential phases (presumptive, confirmatory, and complete), each phase requiring 1 to 2 days of incubation. In the initial or presumptive phase, three volumes of samples (usually 10, 1, and 0.1 ml) (Table 8.2 and Figures 8.1 and 8.2) are inoculated into 3, 5, or 10 tubes containing the appropriate medium to allow the target bacteria to grow (Figure 8.2; Tables 8.3 and 8.4). In this test, it is assumed that any single viable target organism in the sample will result in growth or a positive reaction in the medium.

Sewage

10-2

10-3

10-4

10-5

10-6

10-2

10-3

10-4

10-5

Secondary effluent

10-1

10-2

10-3

10-4

10-5

1

10-1

10-2

10-3

Contaminated bathing

10

1

10-1

10-2

10-3

10

1

10-1

Table 8.2 Recommended serial dilutions for water samples in relation to the degree of microbiological contamination and type of indicator

Serial dilutions for faecal streptococci

10-6

100

10

1

Type of water

Serial dilutions for thermotolerant

coliforms1

Figure 8.1 Preparation of a series of dilutions

water

Clean water

E. Coli Source: Anon, 1983 1

10

1

10-1

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