Particles in highway runoff from three sites with high annual
average daily traffic
300,000 vehicles/day were characterized
by particle size, density, and settling velocity. Particles with the
same diameter exhibited a range of settling velocities and the
overall particle settling efficiency was much lower than calculated
with Newton’s law.
Simulations using data from 16 storm events from three highway
sites were used to estimate particle mass reductions for a
two-compartment settling tank. One compartment was used to
store the initial runoff to prolong the period of settling and the
second compartment was used as a continuous flow clarifier.
Particle reduction rate was optimized by adjusting the fraction of
storage compartment over a range of design storm sizes, ranging
from 1.6 to 26 mm total rainfall. Generally a 3:1 ratio of storage
to continuous flow compartment surface areas optimized removals.
Overall particle mass removal increased from 70 to 80%
as the design storm increased from 1.6 to 13 mm. Larger storage
compartment surface areas increased removals of particles with
diameters 25–41 m by as much as 26% depending on the design
storm size. Particles larger than 100 m were generally
well removed regardless of compartment volumes. The existence
of a particle first flush increased small particle reduction rate
from 5 to 16% for particles ranging from 2 to 41 m. Total
chromium, iron, and lead, which are more associated with
particles fd=0.21, 0.03, and 0.07, respectively, had removals
of 50–70%, depending on specific conditions, whereas cadmium