I would suggest installing a waterless urinal, for starters.
- Since men use that most of the time, it reduces potable water use for toilet flushing by over 50% immediately.
- Sloan, Waterfree, and Falcon all make reasonably priced and well-performing systems.
As for composting toilets, I have actually installed them in conventional homes.
- I recommend one that plugs in with a vent fan and mixer.
- Sun-Mar and Biolet are self-contained composting toilets that are readily available commercially and that I have had good experience with.
Greywater systems are a little more complicated in my experience. Sloan's Agus, BioGreen, and Brac System are all packaged greywater systems that collect water used from shower, sinks, etc., and reuse it for toilet flushing or irrigation.
- If you are using a waterless urinal and composting toilet, the greywater system is really not needed.
- You can use a simple rainwater barrel system on the exterior of the house to collect water for irrigation.
For more information:
Read "We want to design a greywater system for bathrooms we're renovating. Can you help?" a Q&A answered by Jesse Terzi.