A sewer pipe is a large pipe that carries wastewater from a building to a treatment plant or septic tank. A drain pipe is a smaller pipe that carries wastewater from sinks, showers, and toilets to a sewer pipe. The main difference between a sewer pipe and a drain pipe is the size of the pipe. Sewer pipes are typically much larger than drain pipes. Sewer pipes are also made of different materials than drain pipes. Sewer pipes are typically made of concrete or clay, while drain pipes are typically made of PVC or ABS plastic.