Undissolved substances are referred to as solid or particulate substances or matter. According to [DIN38409:1987] they are divided into suspended and settleable solids.

• Suspended solids solids means any solid matter contained in effluent that is retained on a filter of 2.0 micrometer (μm) or smaller size (Wastewater Systems Effluent Regulations SOR/2012-139). According to [DIN38409:1987] are particles that can be quantified after an ignition process used to remove any moisture. These include particulate matter and suspended particles. Note that in [ASTM D1129-13], the residues from the ignition of particulate or dissolved matter, or both, is called “Fixed Matter”

• Settleable solids according to [DIN38409:1987] are suspended particles that, given the right circumstances and time, settle out of a still fluid.