We all know that most people use water in their homes for drinking, washing dishes, laundry, bathing, and flushing their toilets. The resulting dirty water from these "domestic users" usually drains from homes into an extensive system of underground pipes that lead to a wastewater treatment plant. Typically, the treatment plant is designed to clean such domestic wastewater.
On the other hand, so called "non-domestic users," include industries and businesses that may use water to manufacture products and provide services. If, for example, a manufacturer uses water to clean metal parts before they are painted, that non-domestic wastewater often is quite different than wastewater originating from residential homes. It may not be able to be adequately treated at a typical wastewater treatment plant. That's where "pretreatment" comes in. In other words, pretreatment refers to the process of treating this "non-domestic" wastewater before it reaches the wastewater treatment plant.