Sewage is a complex mixture of suspended and dissolved materials; both categories
constitute organic pollution. The strength of sewage and the quality of sewage effluent are described in terms of their suspended solids (SS) and biochemical oxygen demand (BOD); these two measures were originally either proposed or devised by the Royal Commission (l898-l9l5). The SS are determined by weighing after the filtration of a known volume of sample through a standard glassfibre filter paper, the results being expressed in mg l-1. Dissolved pollutants are determined by the BOD they exert when incubated for 5 days at
20°C. Samples require appropriate dilution with oxygen saturated water and suitable
replication. The oxygen consumed is determined and the results again expressed in mg l-1.