Natural Treatment for Sewage Treatment Facility
by Dave Maddux
In October 2001 the Borough was given a Notice of Violation by the Alaska Department of Environmental Conservation (ADEC) for surpassing water quality discharge standards. Subsequently the Matanuska-Susitana Borough let an RFP for an upgrade to the system. The upgrade to the town’s water and sewer system was funded by the USDA and administered by the Matanuska-Susitna Borough.
The entire project included improving and modifying the existing lagoon flow control system; converting the percolation pit into a third lagoon; installing a new lift station and modifying two existing lift stations; building a constructed wetland treatment system; and camera inspection and cleaning of the existing force main. In addition, a previously non-existant SCADA monitoring system was installed for remote monitoring and control of all pumps and controls in the lift stations. The prime contractor was Construction Unlimited and the landscaping contractor was Evergreen Landscaping, both based in Anchorage, Alaska.
After a competitive bid process, a group consisting of Bob Gilfilian, P.E., of Gilfilian Engineering, Dr. Dave Maddux of Applied Wetlands Technology and Mark Sherman, P.E., of ASCG, Inc. were awarded a contract to design and oversee construction of a surface-flow constructed wetland. This system was a replacement for the percolation pit in the final treatment of effluent before its discharge to the receiving waters of the Talkeetna Slough. The following discussion deals only with the constructed wetland portion of the project.
Benefits Of Using A Constructed Wetland
Selection of a constructed wetland treatment system provided several benefits. When construction was completed, operation and maintenance costs were estimated at $2,000 per year; effluent discharge would meet DEC standards; and the treatment system created an attractive green space that complimented its natural environment.
The construction cost of the constructed wetland portion was approximately $440,000 which equates to $13 per square foot of constructed wetland surface area. This facility is capable of treating 14.6 million gallons of effluent per 145-day treatment season which easily meets the current needs of the town and allows for future expansion.
Discharge Water Quality Requirements
The ADEC sets the discharge permits for treated sewage wastewater discharge to receiving waters in Alaska. There are three main requirements; 1) reduction of fecal coliforms to a 30-day average of 20 cfu/100 ml, 2) 5-day BOD concentration maximum of 65 mg/L and 3) a total suspended solids maximum of 70 mg/L. Dissolved oxygen has a minimum of 7 mg/L and a maximum of 17 mg/L.
Climatic Conditions Are A Challenge In Alaska
Constructed Wetland Design Overview
Once the effluent is delivered to the treatment facility, the flow process is a simple gravity-fed discharge from one lagoon to the next. The pumped effluent flows into lagoon #1 and travels the length of the lagoon before being discharged through gravity flow to lagoon #2. The effluent then travels the entire length of lagoon #2 before being discharged through gravity flow into lagoon #3. Finally, the effluent is discharged seasonally to the constructed wetland through a buried, four-inch HDPE line by opening mechanical distribution header valves.
The constructed wetland is a continuous system comprised of six cells with an operating depth of 12 inches. Each cell is separated from the following cell by a deep water trench that is 4 feet deep and acts as a flow redistribution zone. This zone allows for remixing of the effluent, maximizing an even flow across the entire width of the cell and minimizing the potential for channels (short circuiting) developing through the wetland.
This free-water surface flow system has a surface area of 35,000 square feet and a volume of 2,618,000 gallons. Current flow-through is 395 m3/day (105,000 gpd) with a theoretical hydraulic detention time of 1.86 days. Discharged effluent flows through a V-notched weir to an 8” HDPE buried line that discharges to the Talkeetna River slough, an anadromous fish stream.
It was expected that cells 1 and 2 would be subjected to the highest loads of ammonia. Cattails and bulrush were placed in these cells because both species have shown a remarkable ability to thrive while withstanding high pollutant loads that include ammonia. These species also produce a significant volume of biomass which provides an available carbon source to microbes in the substrate.
Placement of the common sedge in cell 3 was based on the known ability of the sedge to rapidly colonize the open areas and provide a large surface area for periphyton attachment. Both species of sedge used in this constructed wetland do well in polluted water of medium to low concentrations. Cell 3 is far enough along the treatment train that any high concentrations of ammonia should be reduced to levels that will not adversely affect the plants.
Cell 4 was planted with calla lily, which is primarily a floating plant. Calla is slightly rooted at its base in the substrate and sends out thick, floating stems sometimes ten feet in length, with large leaves. The remarkable attribute of this plant is the huge amount of subsurface aerial root production along each stem. This species has more biomass in the roots floating in the water column than in the leaves and stems above the water line. Not only does the plant supply a thick mat of vegetation below the water surface to filter out suspended solids, it also produces a large volume of leaves that decompose rapidly upon senescence and add to the detrital, vegetative mat. Furthermore, since it spreads out and covers the entire water surface, it minimizes the oxygen transfer between the atmosphere and the water surface, providing a near perfect environment for denitrification to occur.
Cells 5 and 6 were planted with blue-green sedge and common sedge respectively, both of which can be found growing along the banks of the receiving water. These two species will grow thick, luxuriant stands of plants, which will effectively filter out seeds and vegetative propagules from the cattail, bulrush and calla lily. Although these three species are indigenous to the state, they are not indigenous to the local area and spreading of the species by accident is not desired. These two species of sedge provide final polishing of the effluent before its discharge to the receiving waters.
Site Preparation For The Constructed Wetland
A layer of top soil 12-inches thick was placed on the liners on the bottom and side slopes of the cells. The soil was manufactured to a minimum specification organic content of 10% and a maximum of 20% by dry weight of finely chopped, well mixed organic materials. To keep clumps and rocks out of the mix the non-organic portion of the planting soil was required to be less than 2 mm in size.
Once topsoil was in place the planting crew began laying out the various planting grids and then using hand-held pluggers to remove soil plugs for planting purposes. Spacing for the various plants was either 12-inch, 18-inch or 24-inch centers depending on the species being used.
Plants And Planting Process
The planting process was the same for all plants except the calla lily. For the cattail, bulrush and sedge, a hole was punched in the soil, the soil plug was removed, then approximately 2 cups of diluted Humi-Zyme®, a liquid fertilizer, was poured into the hole. The plant plug was inserted into the hole, tamped down with some soil and a final watering of diluted liquid fertilizer was applied. Since the calla lily was harvested by hand from a wild stock, it was not rooted in a tube or pot. Each plant consisted of rhizome-like stalks approximately 18 inches to 24 inches long with substantial aerial roots along each rhizome. These plants were simply placed on the substrate with a shovel-load of soil placed on the end of the rhizome structure and then tamped down by foot.
After each cell was planted it was thoroughly watered but not flooded. Once all the plants were in, the cells were flooded to a depth of 1 inch for 10 days, then the flooding depth was raised to 6 inches for 20 days. The depth was then raised to the operating depth of 12 inches for the following 30 days. No effluent flowed into the constructed wetland during this 60-day acclimation period.
Water Quality Results For The First Operating Season
The effluent exiting the lagoons to the constructed wetland inlet was low in TSS and BOD and the discharge concentrations to the receiving waters averaged well below the ADEC discharge standards. Fecal coliform concentrations exiting the lagoon to the constructed wetland inlet were not as high as was expected except for two sampling events in the fall when concentrations were over 28,000 cfu/100 ml. The FC discharge concentrations to the receiving waters for three sampling events in 2004 were well above the desired ADEC water quality standards.
Another problem which was not anticipated was migration of one of the species due to wind. The calla lily has large leaves which protrude like a flag on the surface of the water. Three days after flooding the cells to 12 inches, a stiff wind caught the large leaves like sails and pulled the plants from their mooring, blowing most of them into the upwind end of the cell. This left a large amount of open water available to waterfowl, which can have the effect of increasing the fecal coliform and BOD concentrations in the water.
A drawback from the client’s point of view is that constructed wetlands do not show their full potential for pollutant removal until about three years of growth have passed. Unlike engineered systems which are expected to work as soon as they are turned on, biological systems take time to mature. Full plant colonization that allows the various plant and microbe communities to establish themselves and fill all the required niches for efficient pollutant removal takes several seasons.
Part of the performance of the landscaping effort was a guarantee that 90% of each species would survive transplant for a period of 60 days. For the most part this occurred, but two areas of plantings for one species in particular was adversely affected and had to be replanted by the contractor. As of the termination of the 2004 season those replanted areas still had not reestablished themselves. Either that particular species had a problem with the seed stock used by the greenhouse or those areas where the planting did not survive had soil that was toxic in some way. Planting procedures followed by the contractor were identical to the successful plantings, so no fault could be found with the contractor.
Acquiring all the necessary permits was somewhat complex because the Talkeetna River Slough is designated an anadromous fish stream. As such, the Alaska Department of Fish and Game became involved in the permitting process, along with the ADEC and the Army Corps of Engineers. Coordination and negotiation with all the agencies involved caused some problems which may have been avoided with more comprehensive permit planning.
Under a competitive bid process we were unable to select the contractor of our choice and there was some concern regarding unknown performance. Additionally, this was the first commercial constructed wetland built in Alaska and therefore no Alaskan contractor had specific experience with these systems. However, we worked with an excellent general contractor, construction crew and landscape contractor, all who made a potentially difficult project into a straightforward one. The construction schedule for this project worked well and there were very few changes I would make for a similar project.
For more information contact Dave Maddux, Ph.D., Wetland Ecologist, Applied Wetlands Technology, P.O. Box 81091, Fairbanks, Alaska 99709, e-mail: email@example.com.
©2004 - 1998 Land and Water, Inc.