In general, it’s more cost-effective and sustainable to replace your toilets with the most-efficient toilets on the market. Toilets that reuse graywater from a sink directly above it are on the market as well. Rainwater to flush toilets [PDF] is a more cost-effective and long-term option to consider.
The California Plumbing Code allows graywater to be used for flushing toilets but the graywater must be treated to meet certain water quality requirements. These systems require a permit, inspection, backflow protections, and cross-connection control. Check with your local city planning or building department for more information.
Since even relatively clean graywater (like from your clothes washer) has particles, lint, hair, etc. it’s essential to not only filter but regularly maintain them. Graywater for toilet flushing is more appropriate in commercial or other high-density properties where dedicated staff can maintain it over time.
In general, toilet-flushing greywater systems usually require frequent maintenance, manual cleaning of filters, and chemical disinfectant to prevent odors in the bathroom. They also tend to be relatively complicated, and it’s critical that they be designed and installed properly.
If you are considering such a system try to find people to talk to who’ve had the systems installed in their homes for at least a year, and be sure to find out the maintenance requirements of the system. Consider maintenance contact with the installer (if you can afford it) for any system that requires more than annual maintenance.
Read more about Manufactured Graywater Systems (whole-house and toilet-flushing graywater systems) and specifications at greywateraction.org.