noun
sewer pipe
Refers to a pipe that carries sewage or wastewater, typically underground.
下水管が詰まって、水が流れなくなった。
The sewer pipe got clogged and the water stopped draining.
道路の下に新しい下水管を敷設する工事が始まった。
Construction to lay new sewer pipes under the road has begun.
排水管 is a drainpipe or drainage pipe in general, not necessarily for sewage; 下水管 is specifically for wastewater/sewage.
Compound of 下水 (sewage, wastewater) and 管 (pipe, tube).