Septic Tank Leach Beds v. Trench Drainfields
Septic tanks convert wastewater into a liquid which is suitable for
further treatment and disposal in the soil surrounding a leachfield.
Most parts of the United States depend upon septic tanks with trench drainfields,
but some states (notably Maine and New Hampshire) favor leach beds on their
septic systems. Which type will need pumping, repair and replacement sooner?
Which will fail, and which will continue to operate?
-- If you clear out an area, lay down some gravel, then your perforated pipes,
then more gravel, that is a leach bed.
-- If you dig some wide trenches, put some gravel in them, then your perforated
pipes, then more gravel, that is a trench drainfield.
Which is better to receive wastewater from a septic tank? According to experts,
a leach bed is inferior to a trench system of the same bottom excavated
dimension.
If the total excavated bottom area is the same, say, 500 square feet (50 sq
meters), you might say that both systems are equal. After all, doesn't the
treated wastewater from the septic tank all flow out the bottom of the
leachfield / drainfield?
To put some numbers to it, let's compare two systems:
Leach bed (50 x 10 ft. rectangle, 2 ft. deep)
Bottom Area: 10 ft. x 50 ft. = 500 sq. ft.
Sides Area: 120 ft. x 2 ft. = 240 sq. ft.
Total gravel-soil interface = 740 sq. ft.
Trenches (250 ft. long, 2 ft. wide, 2 ft. deep)
Bottom Area: 250 ft. x 2 ft. = 500 sq. ft.
Sides Area: 250 ft. * 2 ft. * 2 sides = 1000 sq. ft.
Total gravel-soil interface = 1500 sq. ft.
This example makes clear that the sidewalls of trenches contribute a significant
amount of gravel-soil interface area, thereby giving trench systems a longer
life and better treatment than leach beds. In order to compensate, one rule of
thumb is to make the bottom area of leach beds 50% larger than the bottom area
of a comparable trench drainfield.