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Chapter 8*: SANITARY INSPECTION AND MICROBIOLOGICAL WATER QUALITY - page 31 / 52

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Percentage compliance

To assess percentage compliance (EEC, 1976) the regulatory percentage of the total number of data “n” obtained from a sampling station has to be calculated. The individual results of the set of data that comply with the established standards have to be counted in order to see if they are higher or lower than the compliance level. This is the approach used in the European Union. For thermotolerant coliforms the guideline standard is 100 cfu per 100 ml in 80 per cent of the samples and the mandatory standard is 2,000 cfu per 100 ml in 95 per cent of the samples (EEC, 1976). This approach is very easy to calculate but does not take into account the absolute values of all the microbiological counts and does not produce any average numerical value of the concentration of micro- organisms in the bathing area. For the worked example of Table 8.8, 50 per cent and 90 per cent of 12 samples is 6 and 10.8 (=11) respectively, which signifies the number of samples that must have counts under or equal to the standards associated to those percentages. In the case of bathing area A, only one result complies with the 50 per cent standard instead of the six needed, and only eight results comply with the 90 per cent standard when there should be 11. This bathing area is therefore failing the compliance assessment. Bathing area B complies with both the 50 and 90 per cent standards (Table 8.8).

Table 8.9 Basic statistics of worked examples from two sets of data obtained for thermotolerant coliforms from two bathing areas

1

16

1

1.20

92

10

1.97

2

170

2

2.23

1,600

12

3.20

3

3,390

11

3.53

36

8

1.56

4

450

4

2.65

<1

1

0.00

5

450

5

2.65

140

11

2.15

6

590

6

2.77

4

4

0.70

7

740

8

2.87

<1

2

0.00

8

190

3

2.28

36

9

1.57

9

1,180

9

3.07

4

5

0.70

10

6,700

12

3.83

8

6

0.95

11

2,800

10

3.45

<1

3

0.00

12

600

7

2.78

32

7

1.52

Total 17,276

78

33.31

1,952

78

14.32

Average 1,440

-

2.7758

162.7

-

1.1933

SD4

1,965

-

0.6934

455

-

0.9883

SD Standard deviation

1

cfu per 100 ml

2

Ranks are given in ascending order

Sample

Bathing area A

Bathing area B

Count1 Rank2 Log count Count Rank Log (count + 1)3

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