‘‘Fixed solids’’ is the term applied to the residue of total, suspended, or dissolved solids after
heating to dryness for a specified time at a specified temperature. The weight loss on ignition is
called ‘‘volatile solids.’’ Determinations of fixed and volatile solids do not distinguish precisely
between inorganic and organic matter because the loss on ignition is not confined to organic
matter. It includes losses due to decomposition or volatilization of some mineral salts. Better
characterization of organic matter can be made by such tests as total organic carbon (Section
5310), BOD (Section 5210), and COD (Section 5220).