BOD vs Hydraulics

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 non-waterborne 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.

.35 – A system that handles any amount of high strength sewage – engineer design required..36 – Three places to gather data

Table 1Accepted literature

Most common – EPA ManualActual data from similar type facility

.38 – Calculate EDF using Hydraulic AND BODLarger field size is the required area

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/day1,000,000

(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.

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..

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

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.

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 gpdand the BOD is 750 mg/l. The estimated permeability is a Group 2 soil. Size the system.

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 BOD5.04 lbs BOD X .70 (septic tank removes 30%) = 3.528 lbs BOD3.528 lbs BOD/.00096 lbs BOD/sq ft/day = 3675 sq ft. of EDF required800 gpd/0.60 gal/sq ft/day = 1333 sq ft based on hydraulics.MUST USE BIGGER - 3675 sq ft

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

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 ft10 seats X 0.2 = 2 lbs BOD X .70 = 1.4 lbs BOD (after septic tank)/.00059 lbs BOD/sq ft/day = 2373 sq ftUSE LARGER FIELD = 2373 sq ft

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.

20 seats X 140 gpd/seat = 2800 gpd20 seats X 0.7 lbs/day = 14 lbs

2800 gpd X 8.34 X mg/l = 14 lbs1,000,000

.0028 X 8.34 X mg/l = 14 lbs

.023352 X mg/l = 14 lbs= 599 mg/l599 mg/l is high strengthSince 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 ft0.714

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.