HomeMy WebLinkAboutDosing Design Calculations Revisd 07.21.2026DOSING DESIGN CALCULATIONS, for
Mountain Springs Ranch Lot 29, Garfield County, CO
7/13/2026
Step 1: Flows
3 Bedrooms Table 6-1
0 Bedroom (future)
6 assumed people Septic Tank Size:
450 gallons per day 1000 gallons per Table9-1
0.5 average day in gallons per minute
4 doses per day 0.94 peak day in gallons per minute
113 gallons per dose 1.88 peak hour in gallons per minute
Step 2: BOD5
6 persons
450 gpd used by residence
BOD5 for domestic uses:
0.2 lb/person/day from CDPHE 1.2 lb BOD5/day
Step 3: Dosing Basin
Third Compartment of Septic Tank
60 inch length
40.5 inch width
16.88 square feet 126.2 gallons per foot of depth
depth of dose volume = 0.89 feet of depth in manhole
Invert at dosing tank 8132.20
Sump = bottom of third compartment 8128.12
Pump "Off" 1 8129.12
Pump On "Lead" Float 1.89 8130.01
Pump On "Lag" Float 2.39 8130.51
High Alarm (min) 2.89 8131.01
High Alarm (max) in -12" 1.00 8131.20
minutes of run time per dose = 3.2 35 gpm
(5 minute rule of thumb)
Step 4: Elevations, Orifices, and Head loss
Septic Tank Rim Elevation = 8135.5
High Point Elevation = 8122.5
Invert at dosing tank= 8133.2
Sump = 8128.12
Elev Diff = -5.6 feet
Length from pump to leach field of 2" = 90 feet
Length of 1-1/2" in leach field = 300 feet
2" fittings in equivalent pipe length:
90 90 check 90 gate tee 45 2 - tee
5.2 5.2 17.2 5.2 1.4 10.3 11.2 20.6 76.3 feet
Equivalent 2" Length from pump = 160.7 feet
from Fig F, using 1.81 ft of friction head per 100' of 2" plastic pipe = 2.9 head loss @ 30 gpm
from Fig F, using 0.5 ft of friction head per 100' of 1-1/2" plastic pipe = 1.5 head loss @ 7.5 gpm
Dynamic Head (TDH) -1.2 system condition use TDH of 10 feet
1/8 " dia. orifice @ 5' of Head yields 0.45 gpm
4 Laterals 18 orifices per lateral 32.4 gpm
Step 5: Velocity verification
35 gpm in 2 inch pipe
0.022 = area
0.078 = flow in cfs
3.575 = velocity in feet per second, must be greater than 2.0 feet per second
but less than 10 feet per second
8.75 gpm in 1.5 inch pipe
0.012 = area
0.019 = flow in cfs
1.589 = velocity in feet per second
Step 6: Pipe Volume & Drain Back
no drain back