Reuse bathwater with a recirculation, reheating, filtration, and disinfectant system

With the drought predictions and water rationing proposals here in California, I've been considering the unnecessary wastefulness of emptying a bathtub down the drain after every use.

If building codes were modified to allow a system of recirculation pump, water reheater, purification system, and filter, then people could briefly shower, then get in the bathtub to soak, then soap down in the shower before a quick final rinse.

It is common practice in Japan to heat the water in a tub, maintain the temperature, and use it for the whole family's bath. No soap or dirt is allowed in the tub. A shower to soap up, shampoo, and rinse prior to entering the tub is mandatory in homes and in the many public and private spas.

With the addition of a filter to remove skin cells, hair, and other small debris, and suitable purification by UV and/or chemicals to kill any microorganisms, the water could be reused for days or even weeks. This is already done with outdoor spas; people do not change their spa water very often.

I'm recommending building codes be modified to allow a similar system in a residence, but requiring electric heating only, preferably solar, to avoid air pollution and CO2 production.