Total solids include both the suspended solids and the dissolved solids which are obtained by separating the solid and liquid phase by evaporation.

Suspended solids are a combination of settleable solids and nonsettleable solids, which are usually determined by filtering a wastewater sample through a glass fiber filter contained in a Gooch crucible or through a membrane filter. Settleable solids are those which usually settle in sedimentation tanks during a normal detention period. This fraction is determined by measuring the volume of sludge in the bottom of an Imhoff cone after 1h of settling.

Solids remaining after evaporation or filtration are dried, weighed, and then ignited. The loss of weight by ignition at 600°C is a measure of the volatile solids, which are classed as organic material. The remaining solids are the fixed solids, which are considered as inorganic (mineral) matter. The suspended solids associated with volatile fraction are termed volatile suspended solids (VSS), and the suspended solids associated with the mineral fraction are termed fixed suspended solids (FSS).