That's the folks you see standing beside their broken down car on the freeway
I used a bio-degradable de-greaser intended for cleaning bars. I used it in app 1:10 with hot water, worked like a dream.
I filled app. 30 l. of hot water/degreaser into the tank, and used a pump with a hose to circulate and flush down from the man hole. After some minutes I pumped most of it to a barrel.
I then emptied the tank completely with a wet vacuum cleaner and had it dried out over some days.
are from 1987, and there was a remarkable amount of sludge in those. Never had any problems though, but I am sure that would have had eventually.