Introduction to testing water quality

We can begin to recognise trends and patterns in Chao Phraya water quality by taking physical, chemical, and biological measurements of the river.

In an attempt to devise a system to evaluate water quality for easy comparison between different sites, the National Sanitation Foundation (NSF) created and designed a standard index, called the Water Quality Index (WQI).

The WQI is one of the most widely used of all existing water quality indices. It was developed in 1970 and can be used to measure water quality changes in a particular river over time, compare water quality from different sections of the same river, and even compare water quality of different rivers. The results can be used to determine if a particular stretch of the river is in a good healthy state or not.

To determine the WQI, nine tests are performed. These include measuring dissolved oxygen, faecal coliform bacteria, pH, biochemical oxygen demand, temperature change, phosphates, nitrates, turbidity, and total dissolved solids.

A water sample for WQI analysis is collected (source:

After completing the nine tests, the results are recorded and converted to a unit-less number called the Q-value. Each parameter has its own weighting curve chart for making this conversion. This is the only way that the data from different parameters can be compared.

The data is then processed further by multiplying the Q-values by its weighting factor. The weighting factors allow us to account for the varying levels of significance of the different tests. Finally, these values are added together to obtain the WQI which scores the cleanliness of the water on a scale from 0-100.

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