Why and How is Bathing Water Quality Assessed?

Intestinal enterococci and Escherichia coli are measured in colony-forming units (CFU) per 100 mL of water.

The standard methods to be used for analysis have been set out: ISO 7899-1 or ISO 7899-2 for analysing intestinal enterococci and ISO 9308-3 or ISO 9308-1 for analysing Escherichia Coli.

In the good and excellent quality ranges, classification is determined by evaluating, to the 95th percentile, the normal data probability density function log₁₀ of the microbiological data. It is assumed that the results found during the four previous years follow a statistical law known as "normal log". The 95th percentile is the value to which 95% of the data (microbiological analysis findings) is lower.

In the "sufficient" quality range, classification is determined by evaluating, to the 90th percentile, the normal data probability density function log₁₀ of the microbiological data, the 90th percentile being the value to which 95% of the data (microbiological analysis findings) is lower.

See sample calculation illustrated with graphs (here).

the percentile value is calculated as follows :

• i) Take the log₁₀ value of all the bacterial counts in the data sequence to be assessed (if the value found is zero, use the log₁₀ value of the minimum detection threshold of the analytical method used).
• ii) Calculate the arithmetical average of the log₁₀ values (µ).
• iii) Calculate the standard deviation of the log₁₀ values (σ).

The upper 90th percentile point of the data probability density function is derived from the following equation: upper 90th percentile = antilog (µ + 1,282 σ).

Calculate the standard deviation of the log10 values (s). The upper 95th percentile point of the data probability density function is derived from the following equation: upper 95th percentile = antilog (µ + 1,65 σ).