My plastic holding tank is a hair over 2 CF so it ought to hold 15 gallons. Yet on pump outs it appears to consistently yield 10 gal even when filled with a hose. Does it make any sense that sludge could take up 1/3-rd of the capacity? How about the theory that outlet pipe projects 4" above the bottom of the tank (12" high)? Would there be any design reason to do that?
Assuming sludge, any suggestions on chemicals or additives to help dissolve it ?
Our tank nominally holds 40 gallons... but the real limit is about 24 gallons... partly due to tank measurements and wall thicknesses (40 was optimistic in the first place) and partly due to the location of the inlet hose from the toilet (about 3" lower than the top of the tank).
You might check both wall thickness (to see if it matters) and the inlet hose location.
OTOH, I'd guess could be complicated by sludge build-up, too.
Selby Bay, South River, Chesapeake Bay, USA.