MPCA Design Guidance for Large Subsurface Wastewater ...

Attachment 4B — Loading rate table
This chart determines a soil dispersal system loading rate based on infiltration /absorption capabilities of the
soil. The final loading rate may be less if another design constraint (e.g., ground-water mounding) is excessive
at the loading rates provided in this table.
The soil condition at the proposed depth of the soil dispersal system is used for determining the loading rate. If
more restrictive layers are present below the system, those values may also be used for sizing (but are not
required). However the more restrictive layer may result in unacceptable ground-water mounding at the lower
loading rate.
Soil
classification
Soil texture
(USDA)
Soil structure
(USDA)
BOD 30 to 220
mg/L and TSS
30 to 150 mg/L
(gpd/ft 2 )
BOD < 30 mg/L
and TSS < 30
mg/L (gpd/ft 2 )
1 Coarse sand Single grain 1.2 1.6
2 Medium sand, loamy sand Single grain 1.2 1.6
3 Fine sand, loamy fine sand Single grain 0.6 1.0
4 Sandy loam Weak to strong 0.8 1.0
5 Sandy loam Massive or platy 0.6 0.7
6 Loam Moderate to strong 0.6 0.8
7 Loam Weak or platy 0.5 0.6
8 Loam Massive 0.4 0.5
9 Silt loam Moderate to strong 0.5 0.8
10 Silt loam Weak or platy 0.4 0.6
11 Silt loam Massive --- 0.24
12
13
14
Sandy clay loam, clay loam,
silty clay loam
Sandy clay loam, clay loam,
silty clay loam
Sandy clay loam, clay loam,
silty clay loam
Moderate to strong 0.45 0.6
Weak or platy 0.3 0.3
Massive --- ---
15 Sandy clay, clay, silty clay Strong 0.24 0.24
16 Sandy clay, clay, silty clay
Weak to moderate,
massive, or platy
--- ---
* The soil structure must have a moist consistency of loose, very friable, friable, or firm as determined by the Field
Book for Describing and Sampling Soils (NRCS, USDA).
Design Guidance for Large Subsurface
Minnesota Pollution Control Agency
Wastewater Treatment Systems April 2010
43