HomeMy WebLinkAboutLift Station Capacity Analysis October 25, 2022
City of Bozeman Engineering Dept.
20 E. Olive Street
P.O. Box 1230
Bozeman, MT 59771-1230
RE: Lots 4 & 5, Block 18 Baxter Meadows, Baxter Meadows Lift Station Capacity
Calculations.
Dear Simon Lindley,
The following information is provided to address the ability of the existing Baxter Meadows
Sanitary Sewer Lift station and force main to provide service to the proposed development of Lots
4 and 5, Block 18 of Baxter Meadows Subdivision, Phase 2A.
Original Design Capacity
Lots 4 and 5 are located within the original service area of the Baxter Meadows lift station within
a 139-acre area identified for Business Park (BP) development within a Planned Unit
Development (PUD) with an underlying zoning designation of B-2. The original design
calculations for the lift station assumed that the lands zoned B-2 would generate wastewater at a
gross rate of 3,495 gallon per day per acre. At the time of design this was the largest projected
flow rate for any type of development within the City of Bozeman. Given that this project was a
part of a planned unit development, 30% of the land area was removed from the gross area to
account for the 30% open space requirement associated with the PUD. No subtractions were
made to the assumed wastewater generation rate for road right-of-ways, storm water
retention/detention, wetlands, stream corridors, trails or public parks.
To account for the gross area calculation used in the original lift station capacity calculations we
will add one-half of the adjacent Equestrian Lane right of way width at the lot frontage and one-
half of the rear alley width to the gross development area associated with Lots 4 and 5. Using
the original design generation rate of 3,495 gallons per acre per day this 18,467 square foot area
was assumed to generate approximately 1,482 gallons per day.
The original design report used a peaking factor of 3.01 calculated from an equivalent service
population of 8,747 people. Peak hour flow including an infiltration rate of 150 gallons/acre/day
from Lots 4 and 5 equates to 4,524 gallons per day using a peaking factor of 3.01.
Proposed Wastewater Generation
Lots 4 and 5 are proposed for mixed use development with each Lot containing 3,058 square feet
of office space on the main floor with two 1-bedroom living units and one 3 bedroom living unit on
the second floor. The City of Bozeman does not publish a design rate for office square footage,
T: 406.581.4097 MCOTTERMAN@CADDISENG.COM I PO BOX II8o5,BOZEMAN, MT 597191 WWW.CADDISENG.COM
so we will use typical wastewater flows from Montana Circular DEQ 4 for office space (13
gpd/employee), assuming 10 employees. The two 1-bedroom apartments are assumed to have
one occupant each and the one 3-bedroom apartment will have 2.11 persons per dwelling unit
per City of Bozeman Design Standards. The projected sanitary sewer rate for the proposed
development of Lot 4 is calculated as follows:
3,058 sq it of office space = 10 employees @ 13 gpd/employee = 130 gallons per day
1 Bedroom Apt = 1 person/br @ 64.5 gallons per capita per day = 64.5 gallons per day
1 Bedroom ,Apt = 1 person/br @ 64.5 gallons per capita per day = 64.5 gallons per day
3 Br Apt = 2.17 people per dwe1lling unit (c)- 89 gallons per capita/day= 140 gallons per day
Wastewater Generation Fate = 399 gallons per day per Lot
Total Average Daily Wastewater Flow = 2 Lots(399 gallons/day/Lot)
= 798 gallons/day
The proposed 798 gpd wastewater flow rate is less than the 1,482 gpd flow rate assumed with
the original lift station design proving there is sufficient capacity reserved for this project. The
peaking factor for a service population of 28.34 persons is calculated to be 4.36. This equates
to a peak flow rate of 3,543 gallons per day (including infiltration) which is still less than the
approximately 4,524 gallons per day peak hour flow assumed with the original lift: station design.
It's worth noting that the original design capacity calculation presented herein is conservative
because it only makes allowance for the one half of the abutting r/w width at the lot frontage and
does not consider the undeveloped area associated with side streets, storm water facilities,
wetlands, stream corridors, trails and parks. If the flow from these undevelopable spaces were
to be allocated evenly to the developable lots the calculations would show significantly more
wastewater capacity available to said lots.
Please don't hesitate to contact me if you have any questions or need any additional information
to further substantiate these calculations.
Sincerely,
Matt Cotterman, P.F_., P.L.S.