1.
sewage, wastewater
Dirty water that flows from houses and buildings into drains.
2.
sewer, drainage system
The system of pipes and channels that carries away sewage.
下水 covers both the wastewater itself (sense 1) and the system that carries it away (sense 2). A compound of 下 (lower) + 水 (water).
COMMON COLLOCATIONS:
- 下水道: sewer system
- 下水処理: sewage treatment
- 下水処理場: sewage treatment plant
- 下水工事: sewer construction
- 下水が詰まる: sewer gets clogged
CONTRAST:
NOTE:
下水道 (with 道) specifically means the sewer system/infrastructure, while 下水 alone can mean both the water and the system.