2.0 SYSTEM DESIGN
© Presby Environmental, Inc., Design & Installation Manual, July 2020 Edition
11
Task 4: Determine Full Depth System Sand Bed Area (SSBA)
Arrange circular EF unit(s) to best fit site constraints. Find the diameter of the EF unit(s) without sand, from outer
edge of fin on one side to outer edge of fin on the other. Add one foot for the 6 in of system sand around the
outside of the fins. This is the full depth SSBA diameter. Using curving fins provides the smallest diameter of 9 ft
and using straight fins will result in a diameter of 11 ft. Calculate area using A =
π
(d/2)
2
Task 5: Calculate Minimum System Sand Bed Diameter
Using the following formula:
2
𝑥𝑥
√𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆
÷
𝜋𝜋
and the SSBA from Task2 for single bed applications or Task 3 for
multiple bed applications, find the minimum system sand bed diameter.
Task 6: Calculate System Sand Extensions (SSE)
a) If
the diameter of the
minimum SSBA (Task 5) is larger than the diameter of the full SSBA (Task 4), then
there will be a 6 in deep, SSE around the full depth SSBA. Subtract the full depth SSBA diameter (Task 4),
from the minimum SSBA diameter (Task 5), then divide by 2. This is the distance of the SSE beyond the
edge of the full depth SSBA.
b) If
the diameter of the
minimum SSBA (Task 5) is equal or smaller than the diameter of the full SSBA (Task
4), then there will not be an SSE.
Note:
The system sand bed can utilize many different shapes and configurations as long as there is a minimum of 6
in of system sand around the perimeter of the treatment fins and the appropriate minimum SSBA associated with
the design flow & soils (perc rate) and the number of EF units within each bed of the system.
Design Example #3 (Circular Beds): Single family residence, two bedrooms, 15 mpi perc rate, level site, due to a
site obstacle the EF units will need to be divided to two locations.
Task 1: Determine Number of EnviroFin Units Needed
EF units required from Table B = 4.
Task 2: Determine Minimum System Sand Bed Area Required
Total
SSBA from Table B requires 67 ft
2
.
Task 3: Determine Multiple Bed System Layout
Total
SSBA from Table B requires 67 ft
2
. SSBA per unit = 67 ft
2
÷ 4 units = 16.75 ft
2
per unit minimum.
For this example, assume 1 fin will be in a separate location using a
round layout configured with fins radiating straight outward from
FDU and the remaining 3 EF units will be placed in one location using
a horizontal rectangular layout. Bed sizing will be calculated for the
circular bed using the following steps and bed sizing for the
rectangular bed will continue with step 4 from the rectangular bed
design procedure.
Task 4: Determine Full Depth System Sand Bed Area
SSBA diameter at full depth using straight fins is 11 ft.
Calculate area A = π
(11/2)
2
= 95.03 (round up to 96
ft
2
).
Task 5: Calculate Minimum System Sand Bed Diameter
Using the following formula:
2
𝑥𝑥
√𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆
÷
𝜋𝜋
=
2
𝑥𝑥
√
16.75 ÷
𝜋𝜋
= 4.62, ft diameter min. 11 ft diameter fin
layout exceeds the required minimum SSBA of 16.75 ft
2
from Task 3.
Task 6: Calculate System Sand Extensions
No SSE is required because the area from Task 5 is greater than the area needed from Task 3.