A technical publication of the Water Group of HDR
Volume 14, Number 2
Summer 2003
Innovative Wastewater Management Creates Wetlands
and Irrigation Supply for the Oregon Garden
By Bruce Willey, PE
T
he City of Silverton, Oregon, needed to
comply with tightening environmental
regulations while encouraging economic
development and maintaining affordable rates.
Inventive thinking and fortuitous circumstances
resulted in an affordable water reclamation plan
that combines wetlands creation with irrigation of
a world-class botanical garden.
Silverton is a residential community of 6,500
people nestled in the western foothills of the
Cascade Mountains, overlooking the fertile
Willamette Valley of Oregon. Located 30 miles
southeast of Portland, it is best known as home to
a scenic state park, where trails take hikers behind
spectacular waterfalls.
In 1993, the City faced several challenges. The
Oregon Department of Environmental Quality
required the City to minimize – and possibly cease
– discharging treated wastewater to Silver Creek,
a water-quality limited stream. The City’s
treatment plant had insufficient capacity and was
under a state order to improve performance.
Finally, like many other small towns, the
community had limited resources to pay for the
needed improvements.
· The Oregon Nurseryman’s Association agreed
to locate the Oregon Garden in the city, but
needed irrigation water for the project. The
Oregon Garden was conceived as a botanical
display, garden and nature area that would
showcase plants of the Northwest. The garden
would provide an educational tool, a tourist
destination and a source of inspiration for
gardeners.
To meet these needs, HDR developed a plan to
convey part of the effluent during the summer to
the Oregon Garden. After conveyance to the site,
the effluent passes through a series of wetlands
and is used to irrigate approximately 240 acres,
sustaining the plants during the dry summer
months without drawing on drinking water
resources. This approach, coupled with advanced
treatment capabilities, sharply reduces summertime
loadings to Silver Creek during critical, low-flow,
summertime periods. During the winter, when
stream flows are high, chlorine-free secondary
effluent is discharged to Silver Creek.
The wetlands application was the first in Oregon.
Using treated wastewater to create wetlands was
previously prohibited, but the state now recognizes
the multitude of benefits, due in part to this
project. Irrigating the Oregon Garden with
reclaimed water creates a high-profile opportunity
for public education about water conservation and
reuse.
Design of Wetlands Sites
HDR managed a multidisciplinary team for design
of the treatment plant, effluent conveyance and
wetland/irrigation facilities. To meet seasonably
variable limits for ammonia and nitrate-nitrogen,
HDR designed a unique continuous-flow, highrate, activated-sludge reactor. The reactor is
configured in a complete-mix, racetrack format,
with air cycled on and off to create the aerobic and
Water Reuse Provides Solution
To address these issues, HDR initiated a facility
planning process that was founded on the
community’s values: preserving natural resources
while providing economical service for ratepayers.
A wide range of effluent management strategies
was considered, including conventional reuse
applications and the export of treated effluent to
another river. In the end, the recommended plan
was influenced by two key developments:
· The City developed an industrial park that
required wetlands mitigation.
Part of the effluent from the City of Silverton, Oregon’s wastewater treatment plant is conveyed to the Oregon Garden, providing sustenance to
plants during the dry summer months.
anoxic zones needed for nitrification and denitrification. Other treatment
improvements include new headworks facilities, flow equalization, ultraviolet
(UV) disinfection facilities, effluent pumping facilities and sludge thickening. A
one-mile-long force main conveys the reclaimed water to the Oregon Garden.
The wetlands system occupies approximately 16 acres of the garden. Three
interconnected complexes contain a chain of 25 cascade cells linked together
by discharge swales. Water elevation in each cell is maintained by a vegetated
berm until it flows over a spillway from one cell to the next. The vegetation in
the spillways acts as a natural filter to improve water quality.
The wetlands system is divided into three unique complexes, each with
distinctive design objectives: Complex A, where the reclaimed water is first
introduced, is a high-rate monoculture wetland with an emphasis on nutrient
removal capability; Complex B is a low-rate, uplands wetland; and Complex C
is a large, low-rate terraced wetland. The three wetland types were developed
to increase the diversity of plant life and provide greater research capabilities
regarding the functionality of wetlands for pollutant removal and habitat
creation.
Irrigation water for the garden is withdrawn after wetland Complex A. This
provides the water at a high elevation on the site and allows reduced hydraulic
and nutrient loadings to wetland complexes B and C.
HDR coordinated multiple federal, state and local agencies to obtain timely
approval and secure funding. The project benefited from low-interest loans and
grants from such diverse sources as the U.S. Environmental Protection Agency,
the Rural Development Agency, the Oregon Lottery, the Oregon Department of
Transportation, and the Oregon Department of Land Conservation and
Development. Costs to the community were further reduced through generous inkind contributions of piping material, wetlands plantings and labor to plant the
wetlands.
Success in Oregon
Construction of the treatment plant, reclaimed water pipeline and wetlands
systems was completed in 1999. The treatment plant performance has been
exceptional, routinely producing a high-quality effluent with summertime
The Oregon Garden – shown here in 2000 – is irrigated with reclaimed effluent, providing a win-win
situation for the world-class botanical garden and the City of Silverton, Oregon.
ammonia-nitrogen concentrations of less than 0.5 mg/L and nitrate-nitrogen
concentrations of six to 10 mg/L.
The Oregon Garden formally opened to the public in June 2000 and has become
a highly popular attraction. This year, more than 200,000 people are projected
to visit the garden and wetlands.
Bruce Willey can be reached in HDR’s San Diego, California, office at
(858) 712-8306, fax (858) 712-8333 or email [email protected].