noun
water-purification tank
General term for a tank that purifies water; can refer to various treatment systems.
この工場では浄化槽を使って排水をきれいにしている。
This factory uses a water-purification tank to clean its wastewater.
noun
septic tank
Specifically a tank for treating domestic sewage, often in areas without a central sewer system.
田舎の家では浄化槽を定期的に点検する必要がある。
In rural houses, septic tanks need to be inspected regularly.