Engineered wetlands can be effective cold climate wastewater treatment
By Scott D.Wallace, P.E.
British Petroleum wetland in Casper, Wyoming
North America has not seen
residential growth equal to
that of the last decade since
the end of the Second World
War. At the same time, aging water and
sewer infrastructure must be upgraded
and replaced, leading to a number of
redevelopment challenges. The creation
of open space for recreation and
wildlife habitat is often a top priority
in creating livable, people-friendly
communities. Successfully meeting
these challenges requires combining
the talents of a variety of professionals
including engineers, landscape architects,
planners, environmental scientists,
and contractors.
The new challenges in land development
(and redevelopment) are forcing
everyone involved to be receptive
to innovative approaches to infrastructure
service. Engineered wetlands are
playing a leading role in the new, green
treatment infrastructure of the 21st
century.
Types of wetland treatment systems
There are three major types of constructed
wetlands: free water surface
(surface flow, or open water), horizontal
subsurface flow (vegetated submerged
bed, root-zone or rock-reed filters),
and vertical flow systems. Free
water surface wetlands are man-made
equivalents to natural marshes. These
systems provide habitat for the most
wildlife. Horizontal subsurface flow
wetlands were developed in Germany
in the 1960s. In these systems, water
flows horizontally through a gravel
bed planted with reeds. Because no
water is exposed during the treatment
process, these wetlands are ideal for
residential sewage treatment. The
newest types of wetlands are vertical
flow processes; they provide the greatest
amount of treatment within a given
area and are ideal for situations where
space is limited.
Cold climate wetland design and
performance
Introduction of engineered wetland
technology into Canada and northern
areas of the United States has been
limited by the ability of conventional
wetland systems to operate without
freezing during the winter. A new
design approach is to use horizontal
subsurface-flow and vertical-flow constructed
wetlands that are covered with
an insulating mulch layer to prevent
freezing.
Properly designed insulation of the
wetland bed is effective in preventing
freezing and resulting hydraulic failure.
Relying on snow and ice cover
does not provide reliable insulation
during cold periods with limited snow
pack. Placing a mulch layer over the
system allows the wetland to operate
effectively throughout the winter
months. The type of mulch insulation
used can strongly affect the performance
of the system. Only well decomposed
organic materials can be used
without degrading treatment efficiency.
To be effective, insulation must be
uniform in coverage, which requires
that it be designed as an integral part of
the wetland system.
Treatment in most wetland systems
is limited by low oxygen transfer rates.
Another advancement in engineered
wetland technology has been the
development of aerated wetland systems.
(This technology, termed Forced
Bed Aeration™ is patented in Canada
and the U.S.) Aerated systems are not
limited in their ability to transfer oxygen
to the wastewater and as a result,
are very effective in ammonia
removal. Combining Forced Bed
Aeration with insulation produces
engineered wetlands that can work
effectively year-round, even under
very cold winter conditions.
Scott Wallace,
P.E., is Executive
Vice President,
North American
Wetland Engineering.
He
is the principal
author on the
WERF design
manual, “Feasibility, Design Criteria, and
O&M Requirements for Small-Scale
Constructed Wetland Wastewater
Treatment Systems” scheduled for publication
in 2005.
Contact: swallace@nawe-pa.com.
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