As mentioned, the minimum number of square feet of "effective leaching area" required per bedroom increases as the perc rate decreases. (The slower the soil gets rid of water the broader the area of soak-down you need.)

   Each type of leaching structure is given credit for a certain number of square feet of Effective Leaching Area (ELA). For example, a Mantis 536-8 Low Pro unit is allowed 6.5 square ft. per linear ft.; a 4' wide x 12 high leaching trench is allowed 3 SF/LF.
  The total number of square ft. required for your house, based on the perc rate, can be calculated from the chart below:

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