How plant roots transform waste into clean water
IMAGINE a treatment facility that takes all a community's wastes - sewage, garbage, agricultural residues, industrial byproducts - and transforms them into pure drinking water, turning a profit to boot. Sound like the dream of a budget-squeezed city manager? ``No, it's a very realistic possibility in the next 10 to 15 years,'' says Dr. Thomas Hayes, biomass project director for the Gas Research Institute in Chicago. Green plants are the key. Sewage is an ideal plant food. They flourish in it while at the same time their roots act as solar-powered filters that are highly efficient in removing pollutants. The trick is control, to figure out a way to grow plants on sewage so that waste water could be put in at one end and pure drinking water come out the other - at a reasonable cost. William Jewell, an agricultural engineer at Cornell University, says he's done it.
Dr. Jewell's plant-based sewage treatment process relies on a modified hydroponic system called the ``nutrient film technique'' (NFT). In this process, developed in Great Britain in the early 1970s, plants are grown in water-tight troughs with waste water flowing through the roots. ``Within two hours, the NFT can turn grossly polluted sewage into secondary quality water, the standard for most waste-water treatment facilities in the United States,'' says Jewell. It's extremely efficient at removing some of the most troublesome pollutants, such as heavy metals, according to Jewell, although where they go in the plants is still unknown.
The NFT works most efficiently when solid materials and grease already have been removed from the waste water. Separating these out, thereby producing sludge, is a standard first step in all sewage treatment systems. And sludge disposal is a costly problem. Jewell is tackling this as well.
He has designed a device called an ``anaerobic attached film expanded bed reactor.'' In this process, methane-producing bacteria feed on the solids in the waste water, removing them without producing sludge. Using data from a producing model, Jewell estimates that a combined NFT and expanded bed system could take 1 million gallons of sewage a day (that generated by 10,000 people), and produce better than secondary quality water and $350,000 worth of natural gas yearly.
Once optimized, such a system potentially could pay for itself by harvesting the sewage-purifying plants and feeding those into the reactor along with the sewage solids. This combination significantly increases natural gas production while solving the problem of what to do with the plants growing in the NFT. Likewise, the other community-derived wastes can be used as ``food'' for methane production.
But turning a profit depends on the type of plants grown in the NFT. Jewell's system can grow virtually any terrestrial plant, but some remove certain nutrients more efficiently than others. Cattails, plume and elephant grasses work best with grossly polluted sewage, but later treatment stages are excellent for producing cash crops such as roses, chrysanthemums, carnations, and other ornamentals, even trees. These could either be sold to local markets or used as nursery stock for public parks and other community recreation areas. If Jewell's combined plant-based system is as fast, efficient, and economical as he says, why isn't it used commercially? ``Like any new technology, there is a certain amount of inertia against it,'' says Ronald Isaacson, manager of renewable resources for the Gas Research Institute, a sponsor of Jewell's research.
Another reason is that the public tends to think of living systems as inherently fragile and unreliable. But this is not so, according to experts in the field, including Tom DeBusk, manager of the water hyacinth project at the Walt Disney World Resort Complex in Lake Buena Vista, Fla. This experimental floating-plant facility, in operation since the late '70s, uses aquatic plants growing on open ponds to accomplish the same tasks as Jewell's NFT.
DeBusk contends there's little difference between the track records of the NFT, floating-plant and conventional systems. ``It's fairly common for sewage treatment facilities to operate below standard several months of the year, it's just not noticeable.'' An acre pond covered with dying water hyacinths would be, so one goal of the Disney project is to find ways to guard against this.
Four floating-plant waste treatment systems are in commercial operation in Florida, the largest in Orlando. This 30-acre water hyacinth system operates in conjunction with the city's conventional facility as a final step for removing nitrogen.
Pioneering work in the use of large size plants to process waste for the National Aeronautics and Space Administration further confirms the viability of all the plant-based systems. Fifteen years ago, B.C. Wolverton, senior research scientist at the National Space Technology Laboratory in Bay St. Louis, Miss., designed a series of artificial marshes. Some grow floating plants, including water hyacinths, duckweed, and pennywort, and others have rooted plants like bulrush. ``NASA has used these marshes successfully to treat all of the lab's chemical and hazardous wastes, and domestic sewage, for the last 10 years,'' Wolverton says.
These experts maintain that economic factors will hasten commercial acceptance of plant-based waste treatment systems. The key factors Wolverton cites are the comparatively high cost of building conventional systems and the salaries of skilled engineers needed to run them. Mr. Isaacson concurs, adding that the power cost for conventional systems is the the single biggest expense for many municipalities. DeBusk's research shows that floating-plant systems are 40 percent cheaper in overall operating costs and he estimates that this holds for the NFT system as well.