- ONSTRUCTED wetland technol- 3 has advanced dramatically in

the last ten years. New wetland designs now have the capability of

A treating high-strength wastes nd functioning even in subfreez-

&I the 1970s ("Was that system kind of a I shallow pond with cattails?") Those surface

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ADVANCED DESIGNS FOR CONSTRUCTED

flow (also known as free water surface) wet- lands are well established, but their use is limited in cold-climate applications when year-round treatment is required.

Another type of wetland, termed a sub- surface flow constructed wetland, is more suitable in cold-weather applications. This type of design was originally developed in Germany during the 1950s and uses a grav- el bed, with water flowing horizontally through the gravel. The gravel bed is then planted with aquatic plants, and their root systems grow down into the treatment zone. Typically, the water level is kept below the

I top of the gravel, minimizing odors and mosquitoes. The basic subsurface flow wet- land has found widespread application in the United States and Europe.

PROTECTION AGAINST FREEZE AND ODORS However, freezing of the gravel bed will oc-

cur in winter if the weather is cold enough. The extent and severity of freezing is strong- ly influenced by the amount of snow or other insulation ~resent. with cold tem~eratures and no snob resulting in the worst freezing. Starting in 1994, our firm, North American wetland Engineering (NAWE), began devel- oping insulation designs that incorporated a permanent insulation layer. These insulated systems aren't dependent on an intermittent snow cover for protection. Using computer- ized data loggers to measure in-situ temper- atures has repeatedly shown successful oper- ation of these insulated wetlands in freezing environments. By incorporating energy bal- ance methods into the design, the depth and extent of insulation can be predicted.

Insulation goes both ways - in hot cli- mates, sun-scald and mortality of young plants can be a problem; use of a thin mulch layer can alleviate these problems. Insula- tion of wetlands has also shown another big benefit: odor control. Organic mulch materi- als are very effective in controlling odors. When selecting a mulch, it is important to avoid. amone other things. ~oorlv demaded

Treatment wetlands for commercial and industrial liquid residuals are using such

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materials th i t adversely affect system per- formance. Preferred materials such as peat or high quality yard trimmings compost of- fer the best of both worlds - freeze- roof OD-

technologies as vertical flow, Reciprocating AS a result and effective of these advances, year-round '

Bed and Forced Bed Aeration in areas use of constructed wetlands has crossed over to the permafrost zone.

considered "too cold" a few years ago. HIGH-STRENGTH WASTE? NO PROBLEM!

Scott Wallace By the early 1990s, wetland designers be-

gan to recognize the limitations of the basic

The forced bed aeration design uses direct injection of air bubbles to improve wetland alant root arowth. A aravel

14dJown (rig&) and the vegetation planted on top (below).

technology variants. Surface flow wetlands functioned well during the growing season, but the exposed water surface could result in pathogen exposure and odors. Subse- quently, most surface flow designs began to focus on polishing of secondary (or tertiary) treated effluent; multifunction wastewa- terlwildlife projects became the norm. Sub- surface flow systems were well suited to pro- cessing of primary (settled) sewage; but atmospheric diffusion of oxygen into the gravel bed was not sufficient to support both biological oxygen demand (BOD) and am- monia removal (nitrification).

The last half of the 1990s saw a variety of new wetland designs geared to high strength waste applications. Three wetland technolo- gy variants have emerged as major players in the marketplace -vertical flow, Recipro- cating Bed, and Forced Bed Aeration.

Vertical Flow Vertical flow wetlands use a gravel bed

planted with wetland vegetation. As the name implies, water flows vertically within the gravel bed. Vertical flow wetlands began to find wide acce~tance in North America af- ter 1996, when-studies conducted at Nia- gara-on-the-Lake, Ontario demonstrated successful nitrification (ammonia oxidation to nitrate) in a downflow mode and success-

This vertical flow wetland ful denitrification (conversion of nitrate to installation at a single-family nitrogen gas) in an upflow mode. Vertical residence is constructed using flow wetlands are currently being intensive- a gravel bed with perforated ly studied in Europe, and basic parameters distribution pipes. such as loading rates and clogging dynamics

are being quickly determined. Vertical flow systems may use surface

flooding (typical in Europe) or perforated dis- tribution pipes (typical in cold climates). The vertical flow wetlands may be operated in an upflow or downflow mode, depending on the application, and flow recirculation is often employed. At NAWE, we have developed a patent-pending flow-recirculated vertical flow wetland system optimized for Total Ni- trogen removal. This process is used when

the effluent must meet stringent nitrogen standards (typically 10 mg/L Total Nitrogen):

Reciprocating Bed Reciprocating Bed (ReCip) wetlands (US

Patent 5,863,433) was originally developed by the Tennessee Valley Authority (TVA) to dial with high-strength aquaculture waste. The basic process uses two gravel-filled wet- land cells. Wastewater is pumped back-and- forth between the wetland cells. The alter- nate filling and draining of the wetland draws atmospheric oxygen into the gravel pore spaces, enhancing oxygen transfer. ReCip wetlands are being used successhlly to treat a variety of liquid residuals including domes- tic wastewater, deicing runoff, food process- ing wastewater and aquaculture wastes.

Forced Bed Aeration Forced Bed Aeration (US Pa ten t

6,200,469) is a design variant of the subsur- face flow wetland process that uses direct in- jection of air bubbles on a diffuse grid to achieve high levels of oxygen transfer. This air injection has been demonstrated to dra- matically improve wetland plant root growth. A spinoff of the decentralized resi- dential wastewater management field, the process has found wide application in treat- ment of high-strength wastes. Of the three advanced wetland designs featured in this article, it is the only one that can operate strictly by gravity flow.

Forced Bed Aeration was originally devel- oped to provide nitrification and denitrifica- tion in a single-stage reactor. Recent studies by the University of Iowa indicate that those wetlands are capable of nitrification of land- fill leachate at very low temperatures.

CASE STUDIES Cheese Plant Retrofit

In 1997, NAWE designed an insulated subsurface flow wetland to treat 1,400 gal- lons per day of employee washroom waste at the Eichten Cheese Plant near Center City, Minnesota. However, soon after installa- tion, routine monitoring revealed that the antiquated piping system within the plant allowed large amounts of cheese processing solids to enter the septic system. As a result, the wetland was experiencing a dramatic overload of high protein waste.

Reasonable construction costs combined with low annual operating expense make wetland technologies attractive on a life cycle basis.

In order to protect their investment in the wetland system, the owners decided to retrofit the wetland with a Forced Bed Aer- ation (FBA) system in 1998. After installa- tion, the FBA system was operated inter- mittently from April through June, 1999 during startup. This system is now operated full time. The result has been a dramatic turnaround in system performance.

Restaurant Food Residuals To replace an existing noncompliant sep-

tic system, NAWE designed a vertical flow wetland for the Island View Golf Club in Waconia, Minnesota. Because of space lim- i ta t ions and soil restr ict ions, NAWE

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sought (and received) a variance from the Minnesota Pollut ion Control Agency (MPCA). After primary treatment (settling tanks), waste-water flows into the vertical flow wetland for treatment, with treated ef- fluent being disposed of in a subsurface drip irrigation system. The system was de- signed to treat up to 7,000 gallons per day of wastewater.

Island View produces a high-strength waste typical of restaurant applications, with level of fats, oils, grease (FOG), biolog- ical oxygen demand (BOD), and total nitro- gen levels much higher than typical domes- tic strength waste. Operating performance of the Island View vertical flow wetland has demonstrated an average BOD removal rate of 98 percent, while the NAWE nitrifica- tionldenitrification recirculation process has demonstrated Total Nitrogen removal rates between 78 and 91 percent.

Petroleum Contact Waste To combat the high cost of waste treat-

ment, Williams Pipeline elected to install a FBA system at their terminal facility in Wa- tertown, South Dakota in 1998. Since oper- ation, the system has consistently shown dramatic reductions in BOD. ammonia. and benzene-toluene-ethyl b inzene-xdene (BTEX).

Because of the continued successful oper- ation of the FBA system, Williams has since modified their permits a t other terminal fa- cilities to allow treatment a t the Watertown FBA wetland facility.

IMPLICATIONS OF THE NEW WETLAND TECHNOLOGIES

Constructed wetlands are recognized as simple, easy-to-operate treatment systems. They have construction costs comparable to other treatment technologies. This reason- able construction cost, combined with low annual operating costs, make them very at- tractive on a life cycle cost basis.

As facility managers gear up for the "long haul" a t many solid waste landfills, the ex- pedient decisions of the past, such as truck- ing of leachate and mechanical physical1 chemical treatment works need to be exam- ined again. Similarly, in order to make the best decisions about allocating budget re- sources, utility managers need to carefully review waste treatment costs.

As these few case studies show, the role of wetland treatment systems is dramatically expanding. Treatment wetlands are now be- ing successfully operated in areas that were considered "too cold" a few years ago. Also, the new wetland treatment systems are be- ing successfully used to treat waste previ- ously considered to be "too strong". As a re- sult, constructed wetlands are currently being used to lower treatment costs in a va- riety of previously unexplored applications in Europe and North America.

Fax: 201/457-3339 www.allugroup.com

Scott D. Wallace, P.E. is vice president ofNorth American Wetland Engineering in Forest Lake, Minnesota. (E-mail: nawe@visi.com)