recirculating hot spring bath; closed-loop bath
A bath where water is filtered, heated, and reused, as opposed to a 掛け流し (free-flowing) bath. Common in modern onsen facilities.
See also: 掛け流し
This hot spring is a recirculating bath, so it's always clean.
Do you know the difference between a recirculating bath and a free-flowing one?
掛け流し is a free-flowing bath where fresh hot spring water continuously enters and overflows, while 循環風呂 recirculates and reheats the water.
Compound of 循環 (circulation) and 風呂 (bath).