A grease trap is typically placed before the septic tank in a wastewater system. The purpose of the grease trap is to capture and separate fats, oils, and grease (FOG) from the wastewater before it enters the septic tank. This prevents excessive grease buildup in the septic tank, which can lead to clogs, reduced efficiency, and potential environmental issues. By placing the grease trap before the septic tank, it ensures that the majority of FOG is intercepted and removed, allowing cleaner wastewater to enter the septic tank and undergo further treatment.