HomeMy WebLinkAboutSanitary Sewer Demand Calculations
INTRODUCTION
The site plan proposes a single building with one residential unit, one warehouse unit and one
warehouse-support/retail unit located on Lot 5, Block 2 of Glen Lake Commerce Subdivision [Plat
J-642] in Bozeman, Montana. The property is zoned as M-1 (manufacturing and industrial district).
The property is fronted by a city street, with Iron Horse Road being adjacent to the North-East
property boundary. Water and sanitary sewer mains are located within Iron Horse Road e to which
a single water and sanitary sewer service will connect to the proposed building as shown on civil
sheet C3.0. No fire lines are proposed.
Sanitary Sewer Design Load
The flow rates used herein are according to the City of Bozeman Design Standards and
Specifications Policy (DSSP) dated March 2004 with approved Addendum No. 7 approved March
13, 2020.
Based on Table V-1, the sewer discharge rate for a manufacturing and industrial use property (M-
1) is 960 gallons/acre/day. As such, this 0.173-acre lot would result in a sanitary sewer discharge
rate of 166.08 gallons per day. The peaking factor for the design area is determined by figuring
the equivalent and inserting the population into the Harmon Formula. An estimated total of 24
employees and customers is assumed for the combined population of warehouse and warehouse-
support/retail units.
Harmon Formula: Peaking Factor = (18 + P0.5)/(4 + P0.5)
where: P = Population in thousands
Peaking Factor = (18 + 0.0240.5)/(4 + 0.0240.5)
Peaking Factor = 4.3695
Warehouse and Retail Peak Flow Rate = (166.08)(4.3695) = 726 gal/day
The peak flow rate is calculated by multiplying the City's design generation rate by the peaking
factor.
Based on City of Bozeman Design Standards and Specifications a population of 2.11 population
with an average daily flow rate of 89 gallons per capita per day is estimated for the single dwelling
unit.
Harmon Formula: Peaking Factor = (18 + P0.5)/(4 + P0.5)
where: P = Population in thousands
Peaking Factor = (18 + 0.002110.5)/(4 + 0.002110.5)
Peaking Factor = 4.4603
Dwelling Unit Peak Flow Rate = (89)(2.11)(4.4603) = 837.6 gal/day
The peak flow rate of the dwelling unit is calculated by multiplying the population by the peak
flow rate by the peaking factor.
Assumed infiltration rate = 150 gallons/acre/day = 150 (0.173 acres) = 26 gal/day
The assumed infiltration rate is calculated by multiplying the Bozeman Design Standards and
Specifications assumed 150 gallons/acres/day infiltration rate by the lot size.
The Overall Peak Flow Rate = 726 + 838 + 26 = 1,590 gal/day
= 1.104 gal/day
= 0.00246 cfs
The overall peak flow rate is calculated adding the warehouse and retail units peak flow rate, the
dwelling units peak flow rate and the infiltration rate.
The capacity of a 4-inch service is checked using Manning’s Equation:
Qfull = (1.486/0.013)AR2/3S1/2
For an 4-inch service:
Manning's n = 0.013 for PVC Pipe
Minimum Slope = 0.02 ft/ft
A = area = (3.14/4)d 2 = (3.14/4)(4/12)2 = 0.0873 ft2
P = perimeter = 2(3.14)r = 2(3.14)(2/12) = 1.0472 ft
R = hydraulic radius = A/P = 0.0872/1.046 = 0.0834 ft
R2/3 = 0.1908 ft
S = 0.02 ft/ft
S1/2 = 0.1414 ft/ft
Qfull = (1.486/0.013)(0.0873)(0.1908)(0.1414) = 0.2691 cfs
Q75% = 0.75*Qfull = 0.75*0.2692 cfs = 0.2019 cfs
Connection:
Q/Qfull = 0.00246 /0.2692 = 0.012 or 1.2%
Based on these calculations, a 4-inch sewer line has adequate capacity to carry the design flows
for the project. The City of Bozeman will confirm that their wastewater system has the capacity to
handle this peak flow downgradient of the project.