Between 40 and 65 % of the solids in an average
wastewater are suspended. Settleable solids, expressed
as millilitres per litre, are those that can be
removed by sedimentation. Usually about 60 % of
the suspended solids in a municipal wastewater are
settleable. Solids may be classified in another way
as well: those that are volatilised at a high temperature
(600 °C) and those that are not. The former are known as volatile solids, the latter as fixed solids.
Usually, volatile solids are organic.