Black water is waste from toilets or kitchens and can present risks to human health, the community and the environment as a whole if not treated correctly.
Small scale treatment systems are generally uneconomic and impractical to manage/maintain.
Water from treatment centres if controlled to a consistent standard may be used for similar applications as grey water.
The recent case study DBP00132, may also be of interest. This deals with cleaning kitchen waste water at source before returning it to the mains system.