It's common to reuse bathwater in Japan: https://whatjapanthinks.com/2006/11/28/over-half-the-japanese-re-use-their-bath-water/ https://whatjapanthinks.com/200

It's common to reuse bathwater in Japan: https://whatjapanthinks.com/2006/11/28/over-half-the-japanese-re-use-their-bath-water/ https://whatjapanthinks.com/2006/11/28/over-half-the-japanes... I've seen foam bath covers which are used to keep the water warm between uses. AIUI, Japanese people shower first to get clean, then sit in the bath to relax. They don't use soap or shampoo in the bath, but may use bath salts. Therefore, the water in the bath stays reasonably clean. The bath is essentially a tiny, well-kept swimming pool. I don't know how many times they will reuse the water, or if there is any process to clean it. You could imagine a sort of solera system, where you drain away some fraction of the water every time, then refill the bath with fresh water.