If your property has a septic system any and everything that is drained, rinsed or flushed flows into your septic tank. These tanks are usually 750-1050 gallons and can be made of concrete, fiberglass or plastic.
Water flows from your house into the tank thru the inlet pipe. From there solids sink to the bottom creating sludge while things like grease and toilet paper form a scum layer on top. In between the sludge and scum layer is the water, known as effluent. In a properly working system the water will flow out of the back of the tank thru the outlet pipe to the drain field where it then seeps down thru the filtration system to eventually be absorbed back into the groundwater.