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Swamp Thing PUBLIC ACCESS

Artificial Wetlands Take Waste Treatment Back to its Roots.

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Contributing Editor.

Mechanical Engineering 122(01), 54-56 (Jan 01, 2000) (3 pages) doi:10.1115/1.2000-JAN-3

This article describes features of phytoremediation. Phytoremediation involves planting beds of wetland vegetation and using them to treat or dewater several types of noxious effluents, including sewage sludge, leachates from landfills, and wastes from chemical plants and oil-drilling operations. According to a video produced by New England Waste Systems, more than 60 municipal wastewater treatment facilities in the United States are using reed beds to treat at least part of their sludge. The reed bed can handle only a certain amount of sludge at any one time. Occasionally, Lloyd’s bed has been overloaded, causing some of the green growth to die off, but with patience and light loading the next few times, the reeds have been able to bounce back. In effect, solar energy is replacing fossil fuels and electricity. The costs, environmental and otherwise, of generating electricity are avoided, and all those chemical additives are kept in their bottles. Proponents say the remaining solid material, when ready to be dug up and disposed of, is far cleaner than waste that has merely been put through a filter press.

Waste treatment plants traditionally dewater sludge by pouring it onto sand beds to let the water drain out, or by putting it through filter presses to squeeze out the water. Another method, however, has been gaining ground in recent years. The principle can’t rightly be called new because it relies on the properties of plants developed eons ago, but engineers are now putting those natural forces to work in manmade wetlands.

Initially developed in Europe in the 1970s, the method, known as phytoremediation, involves planting beds of wetland vegetation and using them to treat or dewater various types of noxious effluents, including sewage sludge, leachates from landfills, and wastes from chemical plants and oil-drilling operations. According to Ronald Lavigne, professor of environmental sciences at the University of Massachusetts in Amherst and president of New England Waste Systems Inc. in Northampton, Mass., more than a dozen types of waste can be treated by means of wetlands.

Different applications, of course, need different sorts of vegetation and different architecture, and will require different regimes for the disposal of any treated residue. In the United States as in Europe, the common reed, Phragmites communis, is a popular species for dewatering wastewater sludge. The sludge contains the dead bodies of the microorganisms used in the primary sewage treatment process, and although it appears densely laden with foreign matter, it is commonly as much as 98 percent water.

According to a video produced by New England Waste Systems, more than 60 municipal wastewater treatment facilities in the United States are using reed beds to treat at least part of their sludge. The company claims that this method is far more economical than sand beds or filter presses, yet it reduces the amount of solid residue to a fraction of what a press leaves. The company says that sand beds and filter presses can extract only about 80 gallons of water from 100 gallons of sludge, whereas reed beds can extract as much as 97 gallons of water from the same amount of sludge. Furthermore, the reeds convey oxygen to their roots, making it possible for aerobic bacteria to live below the surface and metabolize volatile solids in the waste.

The Havasupai Indian Reservation in Arizona uses a free water surface system to treat domestic waste.

Grahic Jump LocationThe Havasupai Indian Reservation in Arizona uses a free water surface system to treat domestic waste.

It is possible to convert old sand beds into reed beds. Side walls 3 to 5 feet high are constructed to allow for ice and snow accumulation in wintertime, and the reeds are planted as rhizomes in the spring or fall. For the first 18 months or so, until the reeds are fully mature, the operators slowly increase the amount of sludge applied.

Two years ago, John Jankiewicz, administrator of the water and sewer department in the Hudson Valley community of Lloyd, N.Y., attended a seminar where he heard about these methods. With advice from Lavigne, he installed a pilot bed of Phragmites reeds that covers 1,500 square feet, and has proven sufficient to take care of 20 percent of the town’s sewage needs. “We loaded it lightly the first year,” he said. “After the first year, we were at full loading. In seven or eight years we’ll empty it, and the volume of solids will be considerably less than if we had filter-pressed the material.” After seven years, what would have been 10 tons of filter-pressed sludge requiring disposal will amount to just three tons.

The reeds essentially act as a dewatering device, Jankiewicz explained. Sludge is applied two or three times every month or two, and most of the water is taken up by the reeds, at least in warm weather. A good 75 percent of the water is evapotranspirated through the reeds, Jankiewicz estimates. The rest of the water filters down to drainage pipes below the bed and is sent back to the treatment plant. The solids are left to form an ever-thickening layer. Depending on who is talking, the treated sludge that remains is described as having the consistency of mud or moist soil.

Unlike sand beds, reed beds do not have to be scraped clear before another layer of sludge is added. As the sludge accumulates, more and more water is sucked up by the reeds. Lavigne explained that this is because the reeds produce adventitious roots—roots that grow out of the side of the plant when the level of soil rises. A tree, unable to produce such roots, would die under such conditions.

These roots and rhizomes form a dense mat that provides channels for water to flow down to the underdrains. This material also acts to strain out many of the small waste particles that may be suspended in the sludge. The roots provide attachment sites for bacteria that consume organic compounds in the waste, and also supply the oxygen necessary for oxidizing reactions. Root growth continues even in winter, though the reeds must be cut back. The tops are harvested when the reed bed fieezes over in winter, leaving about 6 inches of growth showing. New sludge is applied year-round—Lavigne’s company recommends 31/2 inches every two weeks— regardless of weather, even if the bed is buried in snow.

Linchpin of the new technology: The common reed, Phragmites communis, is becoming widely used for removing water from sludge.

The reed bed can handle only a certain amount of sludge at any one time. Occasionally, Lloyd’s bed has been overloaded, causing some of the green growth to die off, but with patience and light loading the next few times, the reeds have been able to bounce back.

Lavigne noted that if circumstances arise making it necessary to overload the bed or send undigested sludge into the bed, some of the vegetation may die and the efficiency of treatment may suffer, but the installation should survive if the bed is well established.

New England Waste Systems estimates that one square foot of bed treats approximately 25 to 75 gallons per year of aerobically digested sludge, or about 14 to 41 gal' Ions of anaerobically digested sludge. Bacterial action in the marsh can reduce volatile solids in the sludge by 20 to 70 percent, the company said, reducing the total volume of solids by as much as 50 percent. The initial sludge volume will eventually be reduced by 95 to 98 percent.

Jankiewicz added that construction and operation of wetlands typically comes to one-fourth or less of the cost of conventional systems, which are energy and resource intensive. Not only is there the cost of electricity, but also of lubricants, disinfectants, and other chemicals needed by more conventional operations. For instance, it is customary to add chemical polymers to wastewater before it is sent through a filter press, to facilitate treatment. And labor requirements for the reed bed system are far lower than with other types of treatment. Mother Nature does most of the work.

Advocates assert that the wetlands are also far more environmentally benign. In effect, solar energy is replacing fossil fuels and electricity. The costs, environmental and otherwise, of generating electricity are avoided, and all those chemical additives are kept in their bottles. Proponents say the remaining solid material, when ready to be dug up and disposed of, is far cleaner than waste that has merely been put through a filter press.

Most remarkably, the wetlands are said to be completely free of objectionable odors, because the sludge is kept aerobic. Jankiewicz explained that aerobic bacteria, which require oxygen to live, generally produce end products such as carbon dioxide, water, and more oxygen, and these substances do not have objectionable odors. Anaerobic bacteria, which produce foul-smelling substances, remain well below ground level in the reed bed, and the substances they produce are neutralized by aerobic bacteria before they are able to escape. Jankiewicz observed that Lloyd’s reed bed is only 100 feet from a neighbor’s window, yet it has produced no complaints. Of course, there does have to be land available, but if there is space, a nice reed bed is likely to be regarded as aesthetically superior to other types of sewage treatment facilities.

A network of pipes often underlies artificial wetlands. These are part of the submerged bed system at Apple Blossom Farm in Highland, N.Y.

Grahic Jump LocationA network of pipes often underlies artificial wetlands. These are part of the submerged bed system at Apple Blossom Farm in Highland, N.Y.

Jankiewicz observed that constructed wetlands had also been helpful in dealing with an area on the edge of town that was beginning to see increasing industrial development. It was not economical to provide sewer service so far from town, five miles from the treatment plant, but three separate wetland treatment projects—two for factories that process apple products and a third for a lighting company—have proven to be a useful solution.

One of the projects belongs to Apple Blossom Farm, a fruit juice factory that processes apples and some other fruits and is located in the nearby hamlet of Highland, N.Y. The dry apple squeezings are sold off for cattle feed, but the runoff from hosing down the equipment eventually became too much to pour over the neighboring apple trees.

Fred DeMaio, the president of the concern, looked into machinery to process this runoff and was confronted with capital costs of $200,000 to $500,000. In addition, there would be the cost of hiring an operator to run the thing, testing, energy, maintenance, and so forth. Hearing about Jankiewicz s new reed bed, DeMaio approached Lavigne and the two of them drew up a design for a wetland on a napkin over dinner. This wetland is sown with a plant known as reed canarygrass (Phalaris arundinacea) that can be used when the objective is treatment of wastes rather than just dewatering. The wetland treats water that is hardly very dirty to start with, having been filtered and put through a settling tank before it enters the marsh, but the canarygrass evapotranspirates most of the water and what is left is actually drinkable. The 5,000-square-foot wetland cost $45,000 to set up, a bit under $10 per square foot. “We did a lot of the work ourselves,” DeMaio said. Energy, maintenance, and labor costs are essentially nil.

Another apple products company in the area has since completed its own wetland.

Zumtobel Staff Lighting Inc. also has a factory in Highland, where it produces commercial lighting fixtures. Because of the company’s expansion, its private sewer system was no longer adequate, explained Allison Craig, Zumtobel Staff’s marketing communications manager, and if the firm was to remain where it was, some solution had to be found. Jankiewicz suggested a constructed wetland using reed canarygrass, and it is now under construction.

The use of Phragmites has been criticized by some environmentalists because it is regarded as an invasive species. Lavigne argues that since the plant reproduces by rhizomes, not by airborne seeds, it can never spread from its bed, and no case of it escaping from a reed bed has been reported.

But Joseph Pane, principal fish and wildlife biologist at the New York office of the New York State Department of Environmental Conservation, said of Phragmites: “We do not recommend it at all. It’s a very invasive plant. I’ve seen the shoots reaching across paved roads, and shoots coming right up through the pavement.” In some areas of New York state, operators in search of an alternative have turned to reed canarygrass or cattails, which are favored by the state agency.

Lavigne emphasized that “the key is to know enough about waste to pick an indigenous plant” for a constructed wetland. He has been using a variety of plant species in the course of setting up wetlands in a wide range of localities and dealing with a variety of waste forms. At Shushufindi in the Amazonian jungles of Ecuador, where Lavigne has constructed wetlands to treat sewage for a community of 10,000 that supports oil-drilling operations, he selected a plant used locally as forage for cattle, known as pasta! alemân, or German grass. He is currently advising the drilling companies on the construction of wetlands to clean up the petroleum wastes at Shushufindi, and is scouting the local flora to see what might be suitable. Meanwhile, he is helping to design a wetland to handle the wastewater from shrimp farms on the coast of Ecuador.

He noted that the Havasupai tribe of Arizona, who live near the Grand Canyon, have to dispose of the waste from as many as 300 tourists a night. The tribe’s wetland has been providing satisfactory service for several years, and an expansion of the facility is contemplated.

Lavigne also uses the wetland method for the remediation of landfill leachate, which he calls “a hundred times as powerful a pollutant as raw sewage.” In this case, he favors reed canarygrass, planted in a peat moss—based growth medium. The peat moss, with its large surface-area-to-volume ratio, provides nooks and crannies for the bacteria to live, while the reed canarygrass develops an extensive root system that channels oxygen to the hungry bacteria so they can digest organic materials. Also, both plants readily adsorb the heavy metal ions that help make leachate such a highly problematic substance. According to Lavigne’s firm, this combination of digestion and adsorption provides a 99 percent reduction in total organic compounds, biochemical oxygen demand, and heavy metal concentrations in the water, with residence times as short as 24 hours.

Given the possible combinations of soil, plants, and hydraulics design, Lavigne is highly optimistic about the future of this technology. Jankiewicz is more modest in his assessment, but clearly is a satisfied customer: “It’s very feasible for waste treatment,” he said, adding that, for those dealing with waste treatment issues, “it should be considered as a viable alternative.”

Copyright © 2000 by ASME
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