ADPH – PUBLIC HEALTH AREA VIII TRAINING FOR PROFESSIONAL ENGINEERS And Associated Onsite System Professionals

1. BOD vs Hydraulics

2. Sewage -- waterborne or non-waterborne waste of
similar composition and strength as may be found in
the typical residence (dwelling). The EPA Onsite
Wastewater Treatment Manual defines typical
residential wastewater component median
concentrations as BOD5 - 250 mg/l, Total Suspended
Solids - 350 mg/l, Ammonia - 10 mg/l and Total
Phosphorus - 9 mg/l.
Sewage (High-strength) -- waterborne or nonwaterborne waste from establishments that are of
similar composition but of higher strength than would
be found in a typical dwelling. This may be permitted
at the discretion of the Board. Historically this has
been kitchen waste from establishments.

3. .35 – A system that handles any amount of high
strength sewage – engineer design required.
.36 – Three places to gather data
Table 1
Accepted literature
Most common – EPA Manual
Actual data from similar type facility
.38 – Calculate EDF using Hydraulic AND BOD
Larger field size is the required area

4. Determine if sewage is high strength.
Generally, any BOD of greater than 250 mg/l
would be considered high strength.
Formula – used to calculate BOD or mg/l
Gal/day X 8.34 lb/gal X mg/l = lbs/day
1,000,000

5. (a) Determine the average design load in lbs of BOD/day from
Table 1 (or other appropriate engineering literature, as
identified by the engineer and approved by the Board).
(b) To calculate the BOD load to the field assume that the septic
tank will remove 40% of the BOD from sewage and 30% of BOD
in high-strength sewage.
(c) Divide the BOD load to the field calculated in paragraph (b)
by the appropriate figure from the column headed “Field Sizing
for Establishments Primary Effluent” in Table 3 or Table 3a. This
is the size of the field based on BOD loading.
(d) Next divide the design flow in gpd from the establishment
by the appropriate number from the column headed “Field
Sizing for Establishments with/without Secondary Effluent.”
This is the required size of the field based on hydraulic loading
to the soil.
(e) The larger field size computed above is the required bottom
area.

6. A restaurant in downtown Wherever has 20 seats.
The primary EDF area has a percolation rate of 60
mpi. The engineer is using values from Table 1 in
the rules. Size the system..

7. BOD/TSS Design = 20 seats X .2 lbs BOD/day = 4 lbs
BOD/day
Septic tank removes 30% HS = 4 lbs-(.30 x 4) = 2.8 lbs
BOD
Hydraulic Design flow = 20 seats X 50 gal/day/seat
= 1,000 gal per day flow
Septic tank with 2 day retention (2 x 1,000) = 2,000 gal
tank

8. Field size based on BOD:
2.8/.0008 = 3,500 sq ft
Field size based on hydraulic loading:
1,000/.500 = 2,000 sq ft
USE LARGER = 3,500 sq. ft.

9. An engineer is wanting to design an onsite system
for a catfish restaurant. He has obtained data from
a similar type facility where the flow is 800 gpd
and the BOD is 750 mg/l. The estimated
permeability is a Group 2 soil. Size the system.

10. Convert 750 mg/l to lbs/BOD.
800 gallons/1,000,000 = .0008. .0008 X 8.4 = .00672.
.00672 X 750 mg/l = 5.04 lbs BOD
5.04 lbs BOD X .70 (septic tank removes 30%) = 3.528
lbs BOD
3.528 lbs BOD/.00096 lbs BOD/sq ft/day = 3675 sq
ft. of EDF required
800 gpd/0.60 gal/sq ft/day = 1333 sq ft based on
hydraulics.
MUST USE BIGGER - 3675 sq ft

11. An application proposes a 10 seat lunch counter at
a convenience store. The permeability is a Group 4
soil. Design a system.

12. First, you have to know the percolation rate for
Group 4 soils – use 80.
10 seats X 50 = 500 gallons per day/0.366 = 1366 sq
ft
10 seats X 0.2 = 2 lbs BOD X .70 = 1.4 lbs BOD (after
septic tank)/.00059 lbs BOD/sq ft/day = 2373 sq ft
USE LARGER FIELD = 2373 sq ft

13. A meat & 3 is going to be built off a major highway
and will have 20 seats. The permeability is a Group
3 soil. Design a system.

14. 20 seats X 140 gpd/seat = 2800 gpd
20 seats X 0.7 lbs/day = 14 lbs
2800 gpd X 8.34 X mg/l = 14 lbs
1,000,000
.0028 X 8.34 X mg/l = 14 lbs
.023352 X mg/l = 14 lbs
= 599 mg/l
599 mg/l is high strength
Since flow is greater than 1200 gpd and high strength,
must use treatment (420-3-1-(8)(b)).
Therefore, design on hydraulics and use column with
calculation for treatment
2800 gpd = 3921 sq ft
0.714

15. Should advanced treatment be used, the only
reduction allow is for treatment
Table 3/3A – Field Sizing for Establishments or
Large Flow System with/without Secondary
Effluent gal/sq ft/day USE THE FIRST COLUMN.
MUST ALWAYS USE THE LARGER EDF
SIZE.