Manufactured greywater systems
There are several manufactured greywater systems on the market in the US and Canada, and many more in Australia. They all have the same basic concept: collect the greywater in a temporary storage tank, filter and disinfect it, then pump it out for toilet flushing or landscape irrigation.
In most single family homes with yards, using greywater directly for irrigation provides the greatest water savings for the least cost, embodied energy, energy use, and maintenance. In multi-family or commercial buildings with little irrigation need, reusing greywater to flush toilets can save significant amounts of water. Manufactured systems require regular maintenance, and a 6-month study in the UK found that several systems installed in private residences failed because their users didn't maintain them.
Residential Manufactured Greywater Systems
All manufactured systems use plastic, pumps requiring electricity, maintenance and eventual replacement. If you have plants to water, use untreated greywater outside--you'll save money, time, and energy.
Chlorine disinfectant systems
The AQUS and Brac systems use chlorine tablets to kill microbes and bad odors in greywater. Some users report problems installing and troubleshooting these systems, a chlorine gas odor in the bathroom, and problems with bacterial growth in the tank and pipes if chlorine use is discontinued. You can read about some problems a house had with their system here.
Hydrogen peroxide disinfectant systems
The Water Legacy has a system similar to the Brac that uses hydrogen peroxide and UV as disinfectant instead of chlorine. The units retail for $4944 and have been sold since July 2008. They are made to only be used with shower greywater, and have an inlet filter that requires regular cleaning.
A study of four systems
Kohler recently completed a study on residential greywater systems and how they impact toilets. You can download a presentation of the study here. They installed four different types of systems that use shower water to flush toilets and tested them for one year.