On the basis of its sampling and testing programme, each year the EPD ranks each beach into one of four categories. Ranking is based on sampling taken over the entire swimming season from March to October, so individual week to week fluctuations in water quality are less important than the overall geometric mean for the year. One of the advantages of this ranking system is that it is easy to spot trends of water quality improvement or deterioration at Hong Kong beaches over the longer term.

To compile its ranking system, the EPD first consolidates all the readings taken from each beach from March to October, and from them extracts the geometric mean of E. coli levels. The most important benchmark figure here is 180 per 100mL, the key Water Quality Objective figure. Beach water samples which generate a geometric mean of more than 180 are not considered to have met the WQO.

In addition, the EPD provides a more fine-grained system. Beaches that meet WQO standards are further ranked into ‘Good’ or ‘Fair’ categories, while those that do not meet the WQO are ranked as either ‘Poor’ or ‘Very Poor’. This fourfold ranking recognizes public demand for information on which beaches have consistently high or consistently low water quality.

Hong Kong's annual beach ranking system

Rank E.coli count per 100mL* Minor illness rate** (Cases per 1,000 swimmers) WQO Compliance Good <=24 UD Compliant Fair 25-180 <=10 Poor 181-160 11-15 Non-compliant Very Poor >610 >15 * Geometric mean E.coli count calculated based on all data collected during the bathing season. ** Skin and gastrointestinal illnesses UD Undetectable

The EPD’s beach grading system uses similar labels to the annual ranking system, but in fact measures much more short-term data on the water quality of individual beaches. It is especially important for swimmers who want to know the state of any beach at a particular time in the year, so they can make an informed decision about swimming there.

To get as accurate a picture as possible of the current water quality of a beach, the EPD calculates the geometric mean E. coli levels of the five most recent samples it has taken. Most gazetted beaches are sampled at least weekly, so a typical beach grading figure represents the state of a beach’s water over the previous month or so. This geometric mean figure allows the beach to be graded into one of four categories: Grade 1 (Good), Grade 2 (Fair), Grade 3 (Poor), and Grade 4 (Very Poor). Any single sample that shows current E. coli levels exceeding 1,600 per 100mL overrides the geometric mean figure and immediately triggers a Grade 4 rating. This means the beach grading system can take into account rapid fluctuation in water quality caused by sudden events like heavy rainfall or localized pollution.

Hong Kong's beach grading system

Grade Beach water quality E.coli count per 100mL* Minor illness rate ** (Cases per 1,000 swimmers) 1 Good <=24 UD 2 Fair 25-180 <=10 3 Poor 181-160 11-15 4 Very Poor >610 or last reading >1,600 >15 * Uniess otherwise indicated, the E.coli count represents the geometric mean of the 5 most recent sampling occasions. ** Skin and gastrointestinal illnesses UD Undetectable