Biodigester and water treatment


Does anyone know of a design for a domestic biodigester, I would like to try and build a system that will produce some gas for cooking and treat sewage, but will also produce fertiliser for the garden. I would like to try and put the overflow water through a reed bed to clean it up and reuse on the garden. What I need is something small scale, most of the designs I have seen are for larger communities where as I need something for a single house. Anyone had any experience of something like this?got any suggestions?