2.0 SYSTEM DESIGN
© Presby Environmental, Inc., Design & Installation Manual, September 2020 Edition
8
Design Example #2:
Single family residence, 3 bedrooms (360 gpd), SLR = 0.25 gpd/ft², 12% site slope, limiting layer at 14 in. The SLR
of 0.25 gpd/ft and the fact that the system will be above grade both require that the system be designed by a
licensed professional engineer.
Step #1: Determine System Sand Bed Area (SSBA) Required
SSBA
= (360 gpd ÷ 0.25 gpd/ft²) = 1440 ft² minimum.
Step #2: Calculate the Minimum Amount of AES Pipe Needed
AES pipe required: 70 ft/br * 3 br = 210 ft minimum.
Step #3: Determine Row Length and Quantity Needed
Using a row length of 70 ft requires 3 rows (210 ft ÷ 70 ft = 3 rows).
Step #4: Determine Pipe Layout Width (PLW)
PLW = [(3
–
1) * (1.5)] + 1 = 4.0 ft.
Step #5: Determine Minimum System Sand Bed Width (SSBW)
Minimum SSBW = 1,440 ft² ÷ (70 ft + 1 ft) = 20.28 ft (round up to 20.5 ft).
Step #6: Verify Final Bed Width Requirements
a)
Beds sloping 5% or less: not applicable.
b)
Beds sloping > 5%: Minimum SSBW 20.5 ft is greater than the (4.0 + 3.5 ft), use 20.5 as the SSBW.
Step #7: Calculate System Sand Extension (SSE):
a)
Beds sloping 5% or less: not applicable.
b)
Beds sloping > 5%: SSE placed entirely on the down slope side of the bed. SSE = 20.5
–
(4.0 + 1) = 15.5
which meets the 2.5 ft minimum extension for this slope.
Illustration of Example #2:
2.4 Design Specifications
The AES system shall be designed in accordance with this Manual and the Rules and can be installed using either
bed or trench design utilizing any of the design configurations outlined in this Manual.
Daily Design Flow (DDF)
In accordance with the Rules, daily design flow (DDF) is calculated as follows: the minimum flow rate shall be one
hundred twenty gallons (120 gals.) per day per bedroom. The minimum volume of sewage from each single family
dwelling shall be two hundred forty gallons (240 gals.) per day. When the occupancy of a single family dwelling
exceeds two (2) persons per bedroom, the volume of sewage shall be determined by the maximum occupancy at a
rate of sixty gallons (60 gals.) per person per day.
