CRC Street Sweep Eval 1999.pdf( 489.9 KB ) - Clearwater

COOPERATIVE RESEARCH CENTRE FOR CATCHMENT HYDROLOGY
Executive Summary
Street cleansing is a common (and expensive)
practice undertaken by most urban municipalities
with annual expenditure by a municipality often
exceeding one million dollars. Street sweeping,
essentially the operation of large trucks for cleaning
street surfaces, is primarily performed for aesthetic
purposes. It is, often perceived to lead to
improvements in the environmental conditions of
urban waterways by preventing pollutants deposited
on street surfaces from reaching the stormwater
system. There is, however, little available evidence to
quantify the extent to which street sweeping can
improve stormwater quality. This report investigates
the effectiveness of street sweeping for stormwater
quality improvement.
The effectiveness of street sweeping for stormwater
pollution control is examined for two types of
pollutants, gross pollutants (> 5 mm) and sediment
(including associated pollutants). The research
literature on street cleaning indicates a general dearth
of studies that address the issues of gross pollutant
management. Most studies predominantly examine
the effectiveness of street sweeping for sediment and
associated contaminant removal. This study looks at
the effectiveness of street sweeping for gross
pollutants using the results of Australian field studies,
while sediment and other suspended solid removal is
investigated with interpretation of results from
overseas studies.
Experimental studies overseas found street sweeping
to be highly effective in the removal of large solids
greater than 2 millimetres under test conditions.
However, field conditions are expected to
significantly reduce the efficiency of solid removal
because of limitations with sweeper access to source
areas (mainly due to street design and car parking),
sweeping mechanisms used and operator skills. Field
studies undertaken by the Cooperative Research
Centre for Catchment Hydrology (CRCCH) in
Australia found significant stormwater gross pollutant
loads generated from source areas in spite of a daily
street sweeping regime.
An earlier CRCCH study, involving analysis of gross
pollutant loads from a 50 hectare urban catchment of
mixed residential, commercial and industrial landuse,
found a clear relationship between the gross
pollutant load in the stormwater system and the
magnitude of the storm event. The shapes of the
curves relating gross pollutant load to event rainfall
and runoff were found to be monotonically increasing
and representable by a logarithmic function. The
shape of these curves suggests that the limiting
mechanism affecting the amount of gross pollutants
entering the stormwater system is rainfall dependent
(ie. the available energy to re-mobilise and transport
deposited gross pollutants on street surfaces) rather
than being source limiting (ie. the amount of available
gross pollutants deposited on street surfaces).
Overseas studies indicate that street sweeping is
relatively ineffective at reducing the street surface
load of fine particles (below 125 µm). The particle
size distribution of suspended solids conveyed in
stormwater in Australian conditions typically range
from 1 µm to 400 µm with approximately 70% of the
particles smaller than 125 µm. Therefore, street
sweeping as it is currently practiced cannot be
expected to be effective in the reduction of suspended
solids and associated trace metals and nutrient
concentrations in stormwater.
The study concludes that the performance of street
sweeping for stormwater pollutant control is limited
and must be accompanied by structural pollutant
treatment measures to effectively reduce the
discharge of gross and sediment associated pollutants
in stormwater. The incremental benefits in increasing
the frequency of street sweeping beyond what is
required to meet street aesthetic criterion is expected
to be small in relation to water quality improvements.
As a result, there seems little benefit in conducting an
in-depth field-based study into the effectiveness of
street sweeping for stormwater pollution control.
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