What is a linear loading rate for a septic system?

Answer 1

Basically linear loading rate refers to the ability of the soil to accept wastewater and not become anerobic. It was determined that drainfield beds of widths over 5 or 6 feet had dead zones in the center where oxygen was not available to the aerobic soil bacteria below the drainfield to treat the wastewater. The result was that the center of these wide beds easily ponded and failed. Therefore a calculation had to be made to figure out what the proper size of a drainfield should be to maintain an oxygen supply in the soil for different soil textures and structures. The linear loading rate is measured in gallons of effluent per feet of drainfield (not square feet, but feet).

An example would be a moderately structured silt loam soil having a linear loading rate of 3 feet. If serving a 3 bedroom residence that produces 150 gallons per day per bedroom then there is 450 gallons per day to treat in the soil. 450 gallons per day divided by the linear loading rate (3') says that you need a minimum 150 feet of drainfield length. Thìs can be in one long drainfield or multiple cells of equal length.

The other consideration is soil loading rate to determine the width of the drainfield. The above soil example might have a soil loading rate of 0.6 gallons per day per square foot. Therefore the final size must be a minimum of 750 square feet. Using the length that was arrived using the linear loading rate (150'), 750 divided by 150' equals 5 feet wide.

Therefore the sizing of the drainfield would be 5' wide by 150' long (but probably 3 cells each 3' by 50' with spacing in between of perhaps 3' or more) and be expected to provide sufficient oxygen to the bottom of the entire drainfield.