cesspit; cesspool
A pit or tank for collecting sewage or wastewater. Often used in contexts of sanitation or environmental engineering.
古い家の庭には汚水だめがあった。
There was a cesspit in the yard of the old house.
汚水だめの清掃は専門業者に頼んだほうがいい。
It's better to hire a professional to clean the cesspool.
下水 refers to sewage or drainage systems in general, whereas 汚水だめ is a specific collection point.
Standard spelling using だめ for the second part.
Alternate spelling with the kanji 溜め; less common but still used.
Compound of 汚水 (osui, 'sewage, dirty water') and 溜め (tame, 'accumulation, pool'), from the verb 溜める (tameru, 'to accumulate').