Carefully look into the regulations in your area. In some cases , the regs are written so that its human waste that is regulated, wash water from a sink or shower drain can be handled by a simple hand done drainage system, it may require proof of biodegradeable products being used and then your grey water can be handled by a very small simple leachfield.
As to the black tank contents, look into an incineration toilet.
Normal black tank contents are reduced to ash which is harmless and sterile.
www.incinolet.com
is one example.
