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Most existing wastewater treatment plants have been designed to remove only organic matter using bacteria to eat the sewage. (Organic matter is living or dead material, including microorganisms, made up of carbon-based compounds.) Oxygen must be supplied to the treatment systems for the process to produce non-polluting carbon dioxide and water, which are released into the environment. The bacteria in the waste sludge are incinerated, or treated so that the sludge may be used as compost or fbr crop irrigation.
Organic matter aside, phosphorus is being removed in some treatment plants, including those in four areas in Virginia - the Potomac River basin, the Occoquan basin, the Chickahominy basin, and the city of Roanoke.
These methods are costly - a lot of energy is needed to pump in oxygen, chemicals have to be bought, and their use results in increased quantities of sludge. Moreover, nitrogen pollutants are not removed and are discharged.
Recent research in the United States, however, has shown that bacteria can be used to get rid of nitrogen and phosphorus, as well as organic matter, in a process that costs less than the conventional one.
The technology was developed in South Africa, and improved by Virginia Tech civil engineering Professor Clifford Randall and colleagues Kenneth Branna at The Citadel and Larry Benefield at Auburn University.
The one-year experiment was completed last August, and the project is now in the final stages of design. “The real advantage of a plant with the new technology,” Dr. Randall says, “is that it’s cheaper to operate because it needs less oxygen.” In addition, the volume of sludge is reduced because no chemicals are used.
The new technique passes and recycles sewage through three bacteria-laden tanks, the first two containing no oxygen. These remove the bulk of the organic matter while producing very little sludge. The third, an aerobic tank, completes the job.
Meanwhile, nitrogen pollutants are converted into ammonia, then nitrates, which are recycled into one of the tanks. Bacteria break down the nitrates into harmless nitrogen gas which is vented, and oxygen which is used by the bacteria. Phosphorus is also taken up by the bacteria which are then removed with the sludge.
The proposed new plant at Norfolk, which would process 50 million gallons a day and cost about $60 million, would be the first in Virginia to be designed and built using the new technology. (There are probably not more than four such plants in the whole country.)
Existing sewage plants, however, can be upgraded to provide for biological removal of nitrogen and phosphorus. For example, a wastewater treatment facility on the York River in Virginia was upgraded in three months for $50,000.
So far five sewage plants in Washington, D.C., Maryland, and North Carolina have expressed interest in using the new technology.
But Dr. Randall doubts that it will be readily adopted on a wider
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