Oils, fats, waxes and fatty acids are the major constituents included in this category in domestic wastewater. The presence of a significant amount of oil and grease in wastewater hinders the transportation of wastes through pipelines. It causes scum in aeration basins of activated sludge plants, which interferes with the biological oxidation of wastes and produces a low quality settling sludge.

These substances are determined by extracting them with an organic solvent 1,1,1-trichloroethane. The two immiscible solvents (organic solvent and water) make separate layers. The solvent containing the oil and grease fraction of the wastewater is separated from the aqueous layer. It is dried and evaporated to determine the extractable residue.