Septic system, also known as an onsite sewage system or septic tank system, serves as a sewage disposal system. If you are looking for synonyms or alternative phrases for the term "septic system," you may consider using terms like wastewater treatment system, sewage treatment plant, sewage disposal system, or effluent treatment system. Additionally, you can use phrases like decentralized sewage system, private sewage system, or individual sewage disposal system. The term "subsurface sewage system" or "on-site system" can also be used as a synonym for a septic system. No matter the terminology you use, the primary purpose of all these systems is to treat and dispose of domestic wastewater.