I have pits that are 45"x60"x60" deep, I get them cleaned and the drains sucked out with a special adapter on the 90 deg. ell turned down.
He hooks the hose on the drain, shuts off the valve and lets the tank vacuum all the way up and then opens it and shuts it several times real quick and it cleans the drain from any mud or sediment it has in the line,,,it pulls so hard it will draw the water out of the floor drains in the equip. room.
I get this done for $75.00 per pit. OH! he hauls it off and dumps it too.
Happy big time with this guy!