Messenger - Vol. 1, No. 3, Page 10
Spring 1992
A safer sludge

     Seeking to maintain the environment as we know it, Steven Dentel
boldly goes where few researchers dare to tread.
     An environmental engineer working in the oft hidden world of
wastewater management, Dentel, an associate professor of civil
engineering, is exploring the uncertain fate of chemical additives in
     Under a $300,000 grant from the Water Environment Research
Foundation, Dentel and his four graduate students are investigating
the use and overuse of polymers intended to remove water from sludge.
     The goal of all sewage treatment plants is to separate and purify
the water contained in municipal sewage. Polymers, long repeating
chains of carbon compounds, are routinely added to "de-water" the
precipitated, semi-solid sludge, which even in its final stage is
still 70-80 percent water.  Because they have a positive charge, the
polymers bind tightly to negatively charged particles in the sludge,
pulling the particles together and "squeezing out" the water.
     Although this treatment has been in use for some 20 or 30 years,
Dentel says no one has determined how the optimum polymer dose should
be found, and studies also are needed to establish the ultimate fate
of polymers that make up 1 percent of the total solids in the
de-watered sludge.
     "Polymer overdosing is fairly typical in a variety of de-watering
operations in the Delaware Valley," he says, and overdosing allows
some unattached polymers to slip into the water that will be recycled
into local rivers and bays. "We know some polymers are toxic to fish
at very low levels (less than one milligram per liter)," he says, "and
some monomers remaining after the polymerization reaction are highly
carcinogenic." And no one knows what happens to the polymers when the
sludge is dumped into a landfill or spread out as a soil supplement on
the landscape, he says.
     Dentel's research will result in a manual to help sewage
treatment plant personnel evaluate and select the appropriate polymer
products and the second phase will develop a technology to monitor
proper doses of polymers for optimum effect. "Polymers can represent
up to 25 percent of the cost of sewage treatment," Dentel says, "so by
carefully monitoring the dose, cost savings are expected, as well as
the decreased release of polymers into the environment."
     How polymers interact with other solids in the sludge and how
they can be separated or extracted from these solids is the most
challenging aspect of this research project, Dentel says. He expects
some analogies with another on-going project in which he and Herbert
E. Allen, professor of civil engineering, are trying to assess the
impact of surfactants on sludge. "We've always assumed that
surfactants, which are frequently used in laundry detergents to remove
oil, were harmless," he says, "but researchers have reported that
surfactants lift pollutants off the soil and transport them into the
     Testing on artifically produced sludge will take place in the
laboratory. The researchers will study the "real stuff" on site at
sewage treatment plants in Wilmington; Elkton, Md.; Warminster, Pa.;
and Hatfield, Pa.
                                        -Cornelia Weil