Dear a student,
I am not sure if I understand your problem exactly. 
You are operating an anaerobic digester connecting to pit latrine. 
I wonder how you connect the digester to pit latrine.
The pit latrine is designed for the liquid to soak away and sludge to remain. 
Anaerobic digester is designed to treat the remaining sludge, which could be hauled and pumped into the digester.
If your system is different, could you describe your system? If your sludge is too old to further be degradeable, no gases come out.
Hope this would help your study.

E. Choi
Professor

Dear Geof

Do you have a drawing or photos of the failed soakpits?  I learned a few lessons about soakpit design the hard way many years ago and could possibly make suggestions for the design.

Regards

Dave

Greetings,
I am a student researching the performance of soak pits connected to a new type of biodigester tanks in Uganda, which are facing increasing reports of failure. I’m looking for case studies or literature on failed soak pits to guide my site selection for sample collection.
Any help would be appreciated.
Thank you!