The determination of odour has become increasingly
important, as the general public has become
more concerned with the proper operation
of wastewater treatment facilities. The odour of
fresh wastewater is usually not offensive, but a
variety of odorous compounds are released when
wastewater is decomposed biologically under
anaerobic conditions. The principal odorous compound
is hydrogen sulphide (the smell of rotten
eggs). Other compounds, such as indol, skatol,
cadaverin and mercaptan, formed under anaerobic
conditions or present in the effluents of pulp
and paper mills (hydrogen sulphide, mercaptan,
dimethylsulphide etc.), may also cause a rather
offensive odour. Odour is measured by successive
dilutions of the sample with odour-free water until
the odour is no longer detectable.