Page 78 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

ENVIRONMENTAL ISSUES 61

Sewer Sludge Ash. More than 15,000 municipal wastewater treatment plants in the
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country produce over 8 million tons (7 10 kg) of dry solids of sewage sludge.
Following dewatering, sludge cake contains between 18 and 24 percent solids consist-
ing mostly of nitrogen and phosphorus, but may be contaminated from various waste-
water streams. Much of this sludge cake is incinerated, producing about 1 million tons
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(0.9 10 kg) of ash a year. Sludge ash has the potential for use as an asphalt filler and
use in brick manufacturing. Studies indicate that with heat treatment, the ash can pro-
duce lightweight pellets that can increase concrete compressive strength by 15 percent
when replacing aggregate. Sewage sludge ash has been used as a mineral filler in
asphalt paving in Iowa, Minnesota, and other states. Sewage sludge can be composted
for agricultural uses such as soil amendments, compost, or fertilizer. Recycled munic-
ipal sewage sludge can be a health and safety concern for highway workers using it in
landscaping.

Scrap Tires. In 1994, NCHRP published findings of a 5-year review and synthesis of all
of the states’ highway practices involving the use of waste tires. This document, entitled
Uses of Recycled Rubber Tires in Highways, is the result of a compilation of over 500
sources of information on the topic. The discussion in this section is a synopsis of the infor-
mation provided in that document. A copy of the document can be obtained through the
Transportation Research Board of the National Research Council 2101 Constitution
Avenue NW, Washington, DC 20418.
It is estimated that 2 to 3 billion waste tires have accumulated in the United States,
about 70 percent of which are dumped illegally throughout the countryside or disposed
of in unauthorized, uncontrolled stockpiles. Also, scrap fires account for about 2 percent
of the solid waste that is disposed in regulated landfills. Each year an additional 242 million
more scrap tires add to the nation’s solid waste dilemma. Scrap tires are regulated under
RCRA Subtitle D as a nonhazardous waste. However, if they are burned, the resulting
residue, which may consist of oils, carbon black, and metal-concentrated ash, may be
hazardous. In addition, leachate from tire-based products may also be a hazardous
or toxic concern. Potential uses of scrap tires in highways and related facilities are
numerous.
Table 1.17 identifies the uses of tires in transportation facilities in several states. The
environmental implications of the use of scrapped tires in pavement are issues of emis-
sions from the manufacture and placement of rubber asphalt. Leachate is also a major
concern, particularly of metals (arsenic, barium, cadmium, chromium, lead, selenium,
and zinc) and PAHs (polyaromatic hydrocarbons). A Minnesota study conducted in wet-
land areas concluded that the use of waste tires in asphalt-rubber pavements may affect
groundwater quality. The study’s results were comparable to two other studies with
regard to metal leachates, but PAH leachate concentrations were not confirmed by the
other studies. Mitigation measures suggested in the Minnesota study would be to place tire
materials only in unsaturated zones of the subgrade or fill areas and not below the water
table or within surface water boundaries. A Wisconsin study that scrap, shredded, and
crumbed tires were not hazardous, nor did they release significant amounts of priority
pollutants. Several studies have indicated that the emissions in asphalt-rubber operations
are not significantly higher than with conventional asphalt concrete. The one exception to
this may be the release of methyl isobutyl ketone, which appears to be consistently
slightly higher than with the conventional mixture. The results of these studies should be
used with caution, in that the tires from which asphalt rubber is made are not of the same
chemical composition, and are continuing to change. The rubber-asphalt formulation
process also varies significantly, changing the emissions and leachable properties of the
asphalt rubber. Comparison difficulties are compounded in that the composition and