Can't find the original post, so starting a fresh ...
Since I last cleaned my settling tank I've been titrated acid washing every batch. Had some time spare whilst dewatering today, so I thought I'd give it another clean out.
Surprise ... no soap, just a shallow sludgy type of layer (similar to the intermediate layer I posted on this page recently ...
http://www.biopowered.co.uk/wiki/Water_washing_titrated_acid_method) which, if the tank was cone bottomed, would have drained very easily through a valve.
So acid washing could save you a great deal of digging.