Environmental Engineering Reference
In-Depth Information
public water supply. On average, approximately 700,000 cubic meters of
water a day is drawn from the Potomac River, the only water source capable
of supplying that volume of water, and treated at Dalecarlia and McMillan
Water Treatment Plants (U.S. Army Corps of Engineers 2005). The treatment
consists of removing the solids, largely river silt, from the Potomac River
water and disinfecting the finished water for distribution throughout the
Washington Metropolitan area.
The treatment process removes an average of 125 tons of solids daily from
the river water. For decades these residual solids were settled out as part of
the water treatment train and then discharged back to the Potomac River,
from whence they came. This process continued even after enactment of
the Clean Water Act and the U.S. EPA establishment of the NPDES permit-
ting system to limit discharge of pollutants to waters of the United States.
However during the 1990s, there was pressure on the U.S. EPA to curtail the
unconstrained discharge of solids from the two Washington Aqueduct water
treatment plants to the Potomac River. EPA was reluctant to ignore this pres-
sure because they felt compelled to regulate another government agency (i.e.,
U.S. Corps of Engineers) with the same vigor with which they regulated pri-
vate industries and municipalities. Thus in 2003, they issued a new NPDES
permit which severely limited discharge of water treatment residual solids
to the point that Washington Aqueduct would be forced to intensely manage
the solids and construct significant additional infrastructure.
The Washington Aqueduct had concerns over this new requirement for sev-
eral reasons. The water treatment plants, although initially built in relatively
undeveloped land, by 2000 were surrounded by single-family residential
development, some of it high density and some of it occupied by influential
Washington insiders. The construction and operation of the additional infra-
structure could impact the aesthetics and land use in the neighborhoods and
generate conflict and opposition. Also, under most scenarios after the resid-
ual solids were dewatered they would be trucked off-site, at a rate of up to
20 trucks a day. The truck traffic could not only generate unwanted noise but
also significant traffic and possible safety concerns in the adjacent residen-
tial neighborhood streets. There was reluctance to impact the neighborhood
and generate opposition if some benefit was not achieved by instituting an
intense residual management operation.
The Washington Aqueduct and its parent organization, Baltimore Division
of the Corps of Engineers, were not initially convinced that an intense residual
solids management and disposal operation would achieve an environmental
benefit that was commensurate with the adverse impact to the neighborhood
generated by the action. In order to evaluate the question of adverse environ-
mental effects from the discharge, they conducted extensive environmental
tests on the residual solids, including chemical analysis and toxicity testing.
The results of the testing demonstrated that the discharge would not result in
severe impacts to the aquatic environment, which was not unexpected because
the process just returned the solids back to the river. These findings combined
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