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 atilization of some mineral salts better characterization of organic matter can me made by such teste as total organic carbon BOD,COD out of suspended within a defined period it may include floating material depending on the technique.