Here's my own personal listing of some of the best super sento (public spa), in Yokohama. I have commentary on a few of the listing.
Just a little background: I have been frequenting super sento in my area for years, so I am very familiar with my area. When I research a spa the first thing I look for is the quality of water ( i.e. is it 100% natural and is it recirculated. Is there chlorine in the water). Non-chlorinated water is the best. All of my spa selections were based on that one peeve. No chlorine. Of course, these are my own personal observations. Some of the other things I check for is the overall cleanliness of the place and how much for admissions.
For the record, I have no proof whether these establishments use chlorine or not, but, for those who are familiar with that stinging sensation you get in your nose when you enter a spa or a pool, and you know exactly what it smells like, and how it makes your skin dry, then you should be able to attest to how much of a turn off that chemical.
Chlorine is not really the enemy here though, it's warm water. Elderly need to sit longer for therapeutic purposes, and often times this requires spas to keep their water warm, not hot. Elderly have a difficult time managing excessive body heat and could die from heatstroke. Of course, chlorine has to be added for health reason.
When the water is kept at over 42 Centigrade this kills most bacteria and weakens virus strands in water, which in turn eliminated the need for chlorine. If you are relatively healthy then sitting in water at this temperature is OK, I think.
The last thing I look for are the amenities....(e.g. do they have soap, shampoo, conditioner). Is the place checked every 30 minutes to an hour. Is the water constantly being monitored with thermometers. All of these selections meet that criteria. Sure, there are more spas in the greater Yokohama area - I have been to all of them - but the ones I have chosen in this list are my favorites.