2.0 SYSTEM DESIGN
© Presby Environmental, Inc., Design & Installation Manual, July 2020 Edition
7
Task 2: Determine Minimum System Sand Bed Area (SSBA) Required
Continuing with Table B, find where the system’s perc rate (mpi) and number of bedrooms intersect to determine
the minimum SSBA needed for the DDF or calculate manually by dividing the gpd design flow by the assigned soil
loading rate for a given percolation rate.
Task 3: Determine Multiple Bed System Layout
If using multiple beds, calculate the SSBA for each bed by dividing the SSBA by the total number of EF units
required for all beds, then multiply by the number of EF units in each bed. Note: If individual beds are located in
different soils (perc rates) the minimum SSBA may be calculated separately to accommodate different SLRs,
resulting in different sized dispersal areas within the system. Follow the remaining tasks for each bed individually.
Note:
Treatment fins can interlace with the fins from adjacent EF unit(s) as long as there is a minimum of 6 in of
system sand between them. This will result in odd dimensions for the unit’s length and width. Although allowed,
the standard layout is recommended for ease of design and installation.
Task 4: Calculate the Full Depth System Sand Bed Area
Calculating the full depth SSBA is done in three steps as follows:
a)
To find the length of the full depth SSBA, use the following formula:
full depth SSBA (along the long axis)
of EF = (# of EF units -1) x C/L ft + 11 ft
.
)
b)
To find the width of the full depth SSBA use the following formula
full depth SSBA (along the short axis)
of EF units = (# of EF Units – 1) x C/L + 4.5 ft.
c)
Calculate full depth SSBA: multiply result from a and b above.
Example
:
a)
The field is two EF units long
on the long axis, the SSBA
length needed to extend
beyond the units along this
axis is: (2 units -1) x 10.5 ft +
11 ft = 21.5 ft.
b)
The field is two units long
on the short axis, the full
depth SSBA width needed to
extend beyond the units
along this axis is: (2 units -1) x 4 ft
+ 4.5 ft) = 8.5 ft.
c)
Full depth SSBA = 21.5 x 8.5 =
182.75 ft
2
.
Task 5: Determine if System Sand Extensions (SSEs) are needed
a)
Level beds:
Choose sand bed dimensions to accommodate site constraints. Long and narrow beds are
preferred. Sand bed dimensions must be at least those calculated in Tasks 4 and have the minimum SSBA
required from Task 2. The sand bed that extends beyond the dimensions calculated in Tasks 4 is called the
SSE and only needs to be 6 in thick. The dimensions from Tasks 4 must be within 20 ft of the final sand
bed dimensions.
i.
If the full depth SSBA from Task 4c is equal or larger than the minimum SSBA from Task 2 then there
will not be any SSEs needed.
ii.
If the SSBA from Task 4c is smaller than the minimum SSBA from Task 2 then a SSE will be needed.
Tocalculate the SSE, divide the minimum SSBA from Task 2 by the full depth SSBA length (Task 4a)
subtract the full depth SSBA width (Task 4b) and divide the result by 2. This results in the width of the
SSEs, which are a minimum of 6 in thick and are applied to each side of full depth SSBA.
Please note, the term “axis” only refers to the orientation of the EF units
themselves. The EF units may be oriented with its long axis horizontally
or vertically depending on site constraints.