coping with drought
by Angela Spivey

ater is something you don't think about — until there's not enough. North Carolina's summer came up dry, and many counties and cities restricted water use. The city of Durham made such decisions using mathematical models that Donald Lauria, professor of environmental sciences and engineering, developed especially for Durham. After city staffers plug in current water levels and demand, the models predict the probability of not being able to meet that demand before the reservoir fills. Terry Roland, Durham's director of environmental services, says his staff used the models weekly during the summer to decide when to implement water restrictions and to weigh options such as buying water from other towns.

About 70 miles away, the town of Carthage needed help evaluating a more drastic solution. The reservoir was just about dry, and the town had been buying water from Southern Pines. To save money, officials in Carthage considered recycling the water that had been used to clean their new membrane filtration system. They would put this "backwash water" back into the reservoir to be treated and used. The only problem was, the backwash might contain nasty stuff such as Giardia and Cryptosporidium — microbes that can make you sick. Fran DiGiano and Mark Sobsey, both professors of environmental sciences and engineering, tested the level of these microbes in the backwash water. All water that goes into the reservoir is eventually treated, of course, but over time the contaminants could build up. DiGiano says, "The issue is, is this a safe practice to allow these microbes to accumulate endlessly by returning this backwash water to the reservoir? The higher these levels, the more likely is penetration of a significant number through the subsequent treatment processes, increasing the risk of exposure in the drinking water."

He and Sobsey found that the microbe levels were fairly high and advised that the backwash get an extra treatment to inactivate them before going into the reservoir.

Then, it rained. In early September Carthage's reservoir was full. But, if drought threatens again, Carthage may revisit the recycling idea.

Angela Spivey is the associate editor of Endeavors magazine.
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