Grab or Catch Sampling:

A single catch sample collected from the sampling spot at any instant to determine the character of the sample at that particular instant is known as a grab sample. The frequency of grab sampling depends upon the magnitude of fluctuation in the quality of the source. Grab samples are required in the following cases :

(a) When frequent variation of certain constituents like pH and chlorine demand occurs, grab samples are collected and tested instantaneously for correcting the operational procedures in the treatment plant.

(b) When the unusual occurrence of certain impurities are noticed.

(c) when the water-quality remains more or less the same over a period of time.

(d) When the sample required immediate analysis after collection, such as tests for dissolved oxygen, carbon dioxide, residual chlorine, temperature, pH and sulphides.