A typical singlepass sand filter system consists of a septic tank, dosing tank, sand filter which is

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A typical single-pass sand filter system consists
of a septic tank, dosing tank, sand filter (which
is a lined watertight box filled with specific
sand materials), and the land application system.
The sand filter container can be constructed of
concrete or an excavation lined with a watertight
liner. Sand filters function as a single-pass
system dosing septic tank effluent intermittently
to the sand bed before entering the land
application system. Sand filters generally have
24 to 36 inches of sand for treatment of the
wastewater. The bed is underlain with a bed of
graded gravel and a collection system. Sand
filters can either be open to the surface, called
free access, or buried below ground, as shown
here.