Geoscience Reference
In-Depth Information
from this study is that a regular street-sweeping programme, which frequently removed RDS,
should be an important component of a city's pollution management.
Relevant reading
Brinkmann, R. & Tobin, G.A. (2003)
Urban Sediment Removal: The Science, Policy, and Management of Street
Sweeping
. Kluwer, Dordrecht, 168 pp.
The routine physical removal of RDS is car-
ried out in many urban environments, both for
aesthetic reasons and to limit the impact of RDS
on watercourses and sewer systems. Removal
is accomplished by mechanical street sweeping.
Early street-sweeping procedures were carried
out predominantly for aesthetic reasons, and
removal efficiencies were low (Sartor et al. 1974).
Subsequent studies have shown that regular
removal of RDS by street sweeping may lead to a
significant reduction in both sediment contamina-
tion levels and contamination of surface runoff
(e.g. Sartor & Gaboury 1984; also see Case
Study 6.4). It can, therefore, be a highly effective
method of urban pollution management. It has
been shown that street sweeping is most effec-
tive at removing pollutants in climates where
long periods of dry weather lead to pollutant
accumulation (Sartor & Gaboury 1984). Street
sweepers can be based on vacuum systems or
on a rotary brush system, and comparisons have
been made on the effectiveness of each type for
RDS removal. Generally, although the rotary type
removes a greater proportion of RDS from street
surfaces, vacuum-based models are better at
removing the finer grain fractions (Brinkmann
& Tobin 2003). This is an important considera-
tion as it has been shown that the fine fractions
contain the highest contaminant loading (see
section 6.3.1). There is, therefore, a trade-off
between an increased volume of sediment removal
or more efficient contaminant removal. In general,
street sweeping is much less efficient at remov-
ing the finer-grained fractions of RDS than the
coarser-grained fractions. This has implications
for pollution management as the finer-grained
fraction generally contains the highest loading of
contaminants. In a study in Hawaii, Sutherland
(2003) showed that street sweepers removed
only 62% of Pb from RDS, primarily as a conse-
quence of low efficiencies of fine-grain sediment
removal (Table 6.3). The resulting waste produced
from street-sweeping can be reused as ground
cover or disposed of on land or to landfill, but
this waste has not been widely assessed for its
suitability for such. The limited studies that have
been undertaken (e.g. Viklander 1998; Clark et
al. 2000; German & Svensson 2002) have con-
cluded that the sweepings material, owing to
high contamination levels, should be treated prior
to its reuse or disposal on land.
6.6.2 Management of sediment in gully pots
The management of gully pots, and their associ-
ated sediment, forms an important part of urban
water quality management. Most authorities
regularly empty and clean gully pots, thereby
removing the sediment from the urban drain-
age system. This is commonly carried out to
minimize flooding and drainage issues, rather
than for pollution management reasons. Memon
& Butler (2002a) modelled the efficiency of gully
pots in urban drainage networks and showed
that gully pots can reduce the suspended sedi-
ment content of water entering sewer systems
(and ultimately receiving water bodies) by
40%, with even larger reductions being possible
with improved gully pot design. The model also
showed, however, that reduction in pollut-
ants (such as ammonium and chemical oxygen
demand) was minimal.
