I recommend The Mysteries of the Vortex as well.

If water is really at such a premium, it would be well worth doing a test. I'd think a slot washer, e.g., a standing, "archival" type would be the best for batches of 12 prints. Trays are just too much of a hassle with larger batches. One with a small volume would be best for water saving.

Then, fill your washer, shut off the water flow but keep the washer full, and insert the prints to be tested (you could use regular prints with adequate borders for testing, or use fixed and HCAd white paper). Agitate by hand every, say, five minutes by lifting and dropping them. After 10 minutes, pull a print and test for adequate washing using a residual hypo test (e.g., Kodak HT-2). Change the water in the washer by draining and refilling. Pull a print and test again after another 10 minutes. Dump and refill again. Keep going in 10 minute intervals till you have a well-washed print then add a safety margin. I imagine you will come out at about two changes of water and at 25-30 minutes total wash time.

Best,

Doremus