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Chapter 8*: SANITARY INSPECTION AND MICROBIOLOGICAL WATER QUALITY - page 30 / 52

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Table 8.8 Comparability of methods for assessing complia quality criteria in two bathing areas

nce with microbiological

Method of assessing compliance Interim criteria of quality1 Bathing area A

= 100 cfu per 100 ml 50% Bathing area B

Percentage compliance

Non-compliance (only 1 result complies)

Compliance (10 results comply)

Ranking method

590 non-compliance

8 compliance

Geometric mean2 (95% confidence intervals)

597 non-compliance (216, 1,646)

16 compliance (4, 66)

Log normal distribution method3

680 non-compliance 597 non-compliance

13 compliance 15 compliance

50 percentile point 90 percentile point

50th percentile 90th percentile

595 non-compliance

20 compliance

Percentage compliance

Non-compliance (only 8 results comply)

Compliance (11 results comply)

Ranking method

3,390 non-compliance

140 compliance

Geometric mean2 (95% confidence intervals)

597 non-compliance (216, 1,646)

16 compliance (4, 66)

Log normal distribution method3

4,700 non-compliance

530 compliance

50 percentile point 90 percentile point

4,618 non-compliance

288 compliance

50th percentile 90th percentile

5,707 non-compliance

1,162 non-compliance

Method of assessing complia

nce Interim criteria of quality1 = 1,000 cfu per 100 ml 90%

Bathing area A

Bathing area B

The sets of data for faecal coliforms obtained consecutively from the two bathing areas are as follows (in units of cfu per 100 ml): A (16; 170; 3,390; 450; 450; 590; 740; 190; 1,180; 6,700; 2,800; 600) and

B (92; 1,600; 36; 0; 140; 4; 0; 36; 4; 8; 0; 32)

According to the UNEP/WHO (1985) Interim Criteria for Recreational Waters, the concentrations of thermotolerant coliforms in at least 10 water samples should not exceed 100 cfu per 100 ml in 50% of the samples and 1,000 cfu per 100 ml in 90% of the samples 1

The regulation that applies the geometric mean has only one standard in the interim criteria and not two as in the example and the geometric mean is calculated from at least 5 samples equally spaced over a 30 day period (running geometric mean) (US EPA, 1986) 2

3

Data extracted from WHO/UNEP (1994b) and Anon (1983)

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