I am not an expert in biogas installations, but I will require a working knowledge to be able to provide a holistic (waste)water treatment and management service. I will likely partner with some of the companies mentioned below. I have not done thorough research yet, and as such do not explicitly endorse any companies listed here.
For small ('package plant') domestic sewage treatment, ensure the provider is a member of SewPackSA (http://www.sewpacksa.co.za/#!members/c1oll). Selectra, listed there, converts municipal waste into biogas.
Southern African Biogas Industry Association (SABIA)
Rural, household, small scale
Agama (the agama website is now aimed towards green building consulting, however you would get more information the "biogaspro" website...unfortunately down at the moment).
GCX Africa (they have a new unit which they have installed in Stellenbosch)