M.L.S.S. stands for "Minimum Leaching System Spread". It is a calculation of three factors which are multiplied together, resulting in a number which indicates how many linear feet across the slope the leaching system should be spread.
The idea is that the shallower the restrictive layer (which "restricts" water from soaking down), and the gentler the slope, the farther across the slope the system must be spread to avoid overloading the soils. The assumption is that the water traveling underground along the top of a restrictive layer is traveling in the direction of the slope, and the steeper the slope and the faster the perc rate of the soil above, the faster the water is travelling. Conversely, with a gentle slope, if you serially-fed multiple short rows over a high restrictive layer the saturation of the upper rows could OVER-saturate the receiving soils of the lower rows. Remember, the bottom of the leaching trenches or structures is at least eighteen inches above the restrictive layer so that the effluent will be purified by passing through this soil. M.L.S.S. deals with the ability of this intervening layer to receive the effluent.
M.L.S.S. applies to the design of all new systems. "B-100" drawings showing a "code-complying" system for the purposes of building an addition to your house must also show compliance; however, in this case there is a last resort, one-time work-around that allows only fifty per-cent compliance. For repairs of existing systems we try to meet the M.L.S.S. requirement within the constraints of the lot, because, as with most of the code, M.L.S.S. ultimately has to do with function.
An alternative to using the M.L.S.S. calculation is hiring an engineer to conduct an in-depth hydraulic analysis. This is done during the wettest times of the year and requires monitoring of water levels over an extended period of time.
To calculate the M.L.S.S. formula you need the field data and three charts. The formula is:
M.L.S.S. (in feet) = HF x FF x PF
HF - Hydraulic Factor = "Factor based on hydraulic gradient and depth of restrictive layer within and down gradient of the leaching area." (hydraulic gradient is the slope calculated as a percentage.)
FF - Flow Factor = "Factor based on the design flow." (2 bedrooms = 1; add .5 for each additional bedroom.)
PF - Percolation Factor = "Factor based on the percolation rate of the receiving naturally occurring soil." (such as, if the perc rate is 1" in 5.1 to 10.0 min. then PF = 1.2; if 1" in 10.1 to 20.0 min. PF = 1.5)
For the charts and more information consult the State Code, a pdf which will open in a new tab...