Final Report - Flathead County, Montana

population projections shown in Table BF-1 and using an annual average per capita
flow of 85 gpcd and an average per capita summer flow of 100 gpcd.
With the new and larger capacity WWTF, the District will not only be able to
produce a high quality effluent that meets or exceeds existing and future DEQ permit
conditions, but it will be able to take on wastewater from previously unsewered areas
with high densities of septic tank and drainfield systems. The most critical of these
areas is the Woods Bay area in Lake County where there is a high density of septic
systems most of which are in soils consisting of very coarse and highly permeable sand
and gravel and which are thought by many to be a significant source of contamination
and nutrients in Flathead Lake. The Greater Woods Bay Planning Area is currently in
the process of looking for funding sources to help pay for the costs of installing
infrastructure to collect and convey wastewater to the Bigfork Water & Sewer District’s
WWTF.
WASTEWATER COLLECTION AND CONVEYANCE:
The wastewater collection and conveyance system serving Bigfork consists of 8”,
10” and 12” gravity sewer mains and a total of 16 “major” lift stations with 4”, 6” and 8”
force mains. There are a number of individual or privately owned grinder pump systems
but they generally serve only one or two residences or commercial structures and are
not considered a part of the public sewer system. There are also several low pressure
grinder pump systems scattered around the District that were designed to serve a small
number of residences in specific areas that could not be served by gravity sewer.
These systems, located in Mayport Harbor (McDowell Drive), Eagle Bend, Eagle Rock
and Bear Hollow, were designed to serve a specific area and were not intended for
expansion to serve a broader area.
A. Gravity Collection System:
As mentioned, the gravity sewer system consists of 8”, 10” and 12” mains with
the vast majority being 8”. The only 10” gravity sewer runs north of the wastewater
treatment facility (WWTF) along Montana Highway 35 and is not a continuous run as it
is interspersed with several sections of 8” gravity sewer. The only 12” gravity sewer
was recently installed west of the WWTF along Harbor Hts. Boulevard to Beach Road.
Morrison Maierle conducted a study of “trunk” sewer lines within the District and
there are several sections of existing 8” sewer main reported to be “flat”, less than
minimum grade. Some mains are significantly less than minimum grade. This results in
a reduced conveyance capacity, slower velocities and more maintenance by the District.
In fact, the District indicated that they have to flush and clean a section of sewer main
along Bigfork Bay that runs through Marina Cay at least once each year.
Sewage Treatment in the Flathead Basin 12