Septic tank
While a properly maintained and located septic tank does not pose any more environmental problems than centralized municipal sewage treatment, certain problems can arise with septic tanks in unsuitable locations. Since septic systems require large drainfields, they are not suitable for densely built areas.
Mud tank
A mud tank is an open-top container, typically made of square steel tube and steel plate, to store drilling fluid on a drilling rig. They are also called "mud pits", as they were once simple pits in the earth.