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This is the holy grail for on-site sanitation and nobody has really got the answer. Some are a lot further from a solution than others.
The key contact at UN HABITAT would be Graham Alabaster.
The other person to contact would be Steven Sugden at the London School of Hygiene and Tropical Medicine who has much more up to date knowledge and experience than I:
https://www.lshtm.ac.uk/people/sugden.steven
I would also recommend anybody wanting information about sanitation and hygiene to make their first port of call Duncan Mara’s website:
https://www.personal.leeds.ac.uk/~cen6ddm/
He has a load of stuff on pit emptying: https://www.personal.leeds.ac.uk/~cen6ddm/PitEmptying.html
One thing to consider when taking this approach is the soil stability. There is a danger that the trench sides will collapse if you suck the shit out. So even though it is an appalling job, a first pilot using hand methods (with appropriate H&S precautions) should be considered. It may well be that emptying will not be feasible and if there is no room for new trenches, a completely different approach will be needed.
Regards
Jeff Broome