Cesspool or septic system? Design ideas?

[HADave]

Do we know how large the average size system is in terms of excavation. Lets use a 3 bdrm home with a daily flow of approx 270 gallons. I thought I heard someone say that a cesspool has a cement cover. Here in Maine my system (drain field) takes up 900 sq ft that approx 20' x 40' They don't make cement covers that big , do they? The photos I've seen tend to suggest that cesspools are covered by a 4ft dia cover or so. Then again I might not understand how the cesspool is designed. I thought it was just a hole in the ground and the water drained out from there. Maybe someone could explain just how porous lava is and is all lava the same?

Dave