Canadian Environmental ProtectionCanadian Environmental ProtectionCanadian Environmental ProtectionCanadian Environmental ProtectionCanadian Environmental Protection March 2005March 2005March 2005March 2005March 200533333

Engineered wetlands are playing aleading role in the new, “green”treatment infrastructure of the 21st

century. As experience with wetlands hasgrown, new types of wetland systems havebeen developed. Within the last 30 years,there has been a progression from usingnatural wetlands to the use of man-madeconstructed wetland systems. Today, evenmore highly engineered wetland reactorsare being developed. The greater treatmentcapacity of these “engineered wetlands”expands the application of wetlands tosolve environmental problems around theworld.

The cost benefits of engineeredwetlands can be summed up in a simplephrase: “Plants and bacteria work forfree; people and machines don’t.”

What makes wetlands unique from othertreatment processes is that they employvegetation as part of the treatment process.Because they are a land-based technology,wetlands cannot be used everywhere.However, on sites with adequate space, thebenefits of wetlands can create substantialcost savings, especially for systems thathave to operate over long periods of time.Wetland systems, through their complexassemblages of plants and bacteria, canprovide treatment of domestic wastewaterand recalcitrant, difficult-to-degradeindustrial compounds.

Types of wetland treatment systemsThere are three major types of constructedwetlands; free water surface (surface flow, oropen water), horizontal subsurface flow(vegetated submerged bed, root-zone or rock-reed filters), and vertical flow systems. Freewater surface wetlands are man-madeequivalents to natural marshes. Thesesystems provide the most wildlife habitat.Horizontal subsurface flow wetlands were

Engineered wetlands: Effective coldEngineered wetlands: Effective coldEngineered wetlands: Effective coldEngineered wetlands: Effective coldEngineered wetlands: Effective coldclimate wastewater treatmentclimate wastewater treatmentclimate wastewater treatmentclimate wastewater treatmentclimate wastewater treatmentby Scott D. Wallace, P.E., Executive Vice President, North American Wetland Engineering LLC

Constructedwetlands arecovered withan insulatingmulch layer topreventfreezing —allowing thewetlands tooperateeffectivelythroughout thewinter months.

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developed in Germany in the 1960’s. In thesesystems, water flows horizontally through agravel bed planted with reeds. Because nowater is exposed during the treatmentprocess, these wetlands are ideal forresidential sewage treatment. The newesttypes of wetlands are vertical flow processes;they provide the greatest amount of treatmentwithin a given area and are ideal for situationswhere space is limited.

Engineered wetlands for residentialdevelopmentIn many areas, the loss of open space dueto suburban sprawl is a growing concern.One solution is the use of “smart growth”platting, where homes are clustered withinthe development and the remaining area ispreserved as permanent open space. Bypreserving large areas as open space,wildlife habitat can be protected, trails andother recreational amenities can bedeveloped, and environmental impactsfrom residential developments can beminimized.

There are challenges in successfullycreating open-space developments. Onemajor challenge is sewer service. In mostcases, traditional sewer service (the BigPipe) requires high-density housing tobecome cost effective. Because open-spaceplats preserve large amounts ofundeveloped land (often 50 percent orgreater), conventional sewer designs oftenresult in very high per-lot costs.

The smaller lot sizes used with openspace developments can also present prob-lems for individual on-site septic systems.With on-site septic systems, the setbacksrequired from the property lines, structures,wells and other features, combined withthe need for a dedicated back-up area re-sult in space requirements that exceed thedesired lot size. The solution is to matchthe infrastructure to the development den-sity; a concept that in 1997 was termed de-centralized wastewater management bythe United States Environmental Protec-tion Agency. In the case of open-spacedevelopments, that means using commu-

March 2005March 2005March 2005March 2005March 2005 Canadian Environmental ProtectionCanadian Environmental ProtectionCanadian Environmental ProtectionCanadian Environmental ProtectionCanadian Environmental Protection44444

Alexandria, Ontario

The lagoon system at theAlexandria WWTP in N. Glen-

garry Township is being upgradedto improve the quality of the facil-ity effluent. The design approachused in upgrade is the result of col-laboration between the Canadianengineering firm, JacquesWhitford, and the American firm,North American Wetland Engi-neering LLC (NAWE).

The new 5,500 m3/day facilitycouples optimization of anaeration lagoon with polishing byengineered wetlands. The key tothe design is conservation of heat; theaeration lagoon will be insulated witha floating cover and the engineeredwetland will be insulated with a layerof mulch. In each case, the insulationlayer is designed to keep the heat inthe water so that cold wintertemperatures do not overly hinder thebiological processes. It is the use ofthe heat conservation measures alongwith advanced lagoon design andwetland engineering that has permittedthe Township to make cost-effectiveimprovements to the facility and meetstringent discharge limits.

St. Stephen, New Brunswick

The Town of St. Stephen, NewBrunswick recently constructed a

new wastewater treatment facility to re-place two undersized, aging sewagelagoons. The Town has proposed to con-vert the two old lagoons into free watersurface wetlands for wildlife habitat andfor effluent polishing. This would createa wetlands park along the St. Croix Riverin the centre of town. The project pro-posed by the Jacques Whitford/NAWEteam would effectively transform the li-ability of the obsolete sewage lagoonsinto an asset for the community.

Canadian cold climate wetland designs in process

The Town of St. Stephen has proposed to converttwo old lagoons into free water surface wetlands,creating a wetlands park along the St. Croix River.(Photo courtesy of Jacques Whitford.)

nity sewer systems. These systems can in-clude engineered wetlands.

Most often in residential applications,subsurface flow wetlands are the designof choice. Because the water is not exposedduring the treatment process, subsurfaceflow wetlands do not produce mosquitoesor odours. They can be insulated with amulch layer in areas where winter freezingis a concern. This makes them ideal for resi-dential applications, where the wetland canbe combined with a septic tank and soilabsorption system.

The primary factor that limits theperformance of wetland systems is theavailability of oxygen for treatment. Thishas lead to the development of aeratedwetland systems. These systems eitheruse a fluctuating water level ordistributed aeration, such as in thepatented Forced Bed Aeration process,to introduce oxygen into the treatmentzone. These aerated wetland reactors arefar more efficient in removing thepollutants commonly found in residentialwastewater. Because they can beinsulated, they are suitable for cold-climate applications.

Cold climate wetland design andperformanceIntroduction of engineered wetlandtechnology into Canada and the northernareas of the United States has been limitedby the ability of conventional wetlandsystems to operate without freezing duringthe winter. A new design approach is touse horizontal subsurface-flow and verticalflow constructed wetlands that are coveredwith an insulating mulch layer to preventfreezing.

Properly designed insulation of thewetland bed is effective in preventingfreezing and resulting hydraulic failure.Relying on snow and ice cover does notprovide reliable insulation during coldperiods with limited snow pack. Placinga mulch layer over the system preventsfreezing and allows the wetland to oper-ate effectively throughout the wintermonths.

The type of mulch insulation used canstrongly affect the performance of the sys-tem. Only well decomposed organic mate-rials can be used without degradingtreatment efficiency. To be effective, insula-tion must be uniform in coverage, whichrequires that it be designed as an integralpart of the wetland system.

Treatment in most wetland systems islimited by low oxygen transfer rates.Another advancement in engineeredwetland technology has been thedevelopment of aerated wetland systems.(This technology, termed Forced Bed

Aeration is patented in Canada and theU.S.) Aerated systems are not limited intheir ability to transfer oxygen to thewastewater and as a result, are veryeffective in ammonia removal. CombiningForced Bed Aeration with insulationproduces engineered wetlands that canwork effectively year-round, even undervery cold winter conditions.

ConclusionNorth America has not seen residentialgrowth equal to the last decade since theend of the Second World War. At thesame time, aging water and sewerinfrastructure must be upgraded andreplaced, leading to a number ofredevelopment challenges. The creationof open space for recreation and wildlifehabitat is often a top priority in creatinglivable, people-friendly communities.Successfully meeting these challengesrequires combining the talents of avariety of professionals includingengineers, landscape architects,planners, environmental scientists andcontractors.

The potential of wetland technology isstill better accepted in Europe than in the

United States and Canada. Some processes,such insulated wetlands and Forced BedAeration are gaining popularity forresidential cluster system designs and forindustrial waste treatment. Moredemonstrations of these systems areneeded at the single-family level to promotegreater acceptance and gain a betterunderstanding of how these systems canoperate on a smaller scale.

The new challenges in landdevelopment (and redevelopment) areforcing everyone involved to bereceptive to innovative approaches toinfrastructure service. By using new,“green” technology, as embodied byengineered wetlands, environmentalprofessionals can continue to solve theirclient’s problems in effective,environmentally sound, and creativeways.

North American WetlandEngineering LLC

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For more information, contact ScottWallace. Email: swallace@nawe-pa.com.