A Proposed Method For Performance Testing of Street Sweepers
At last!
Pacific Water Resources has developed a procedure that emphasizes using
a large enough statistical sample.
by Roger Sutherland, P.E.
One of the looming problems in sweeping is the lack of a testing procedure
specifically designed to show how effective particular sweepers are at picking
up different types of debris. What usually happens in a demonstration for
a road sweeper, for example, is that a very large amount of material is
laid down. Then, while an assortment of municipal officials watch, each
competitive sweeper takes a few passes. The winner is often the one which,
subjectively, seems to leave the least on the ground. Of particular significance
is that there is currently no way to evaluate which sweeper has done the
best job at picking up fine sediment, the material known to produce the
highest level of heavy metals and other pollutants in stormwater. It is
widely recognized that the only way sweeping will be able to enter the 21st
century as a Best Management Practice (BMP) for stormwater control is if
a uniform testing procedure is developed.
Roger Sutherland, P.E., president of Pacific Water Resources,
is an Oregon-based stormwater consultant who took part in the National Urban
Runoff Program's (NURP) sweeper evaluations in the early 1980s. He is also
one of the foremost authorities on sweeping as it applies to stormwater
pollutant control. Here are his thoughts on the topic of sweeping methods
and tracking:
The big mistake with most of the sweeping demos that municipalities and
others put on is that they're simply not real. Most of the time, what I
see being put down onto the pavement is an amount of material that no sweeper
is going to pick up in one pass.
For example, they designed and conducted a sweep-off in Palm Springs.
It was a great idea, but was based upon sweepers going over the same territory
five times. Who in the real world is going to get five shots at sweeping
the same area?! Then, the final test to see how well each sweeper did was
based upon hand vacuuming one square foot of pavement. That, alone, ensured
no empirical comparison could possibly be made from a stormwater perspective.
The minimum that should have been hand vacuumed after each sweeper is about
400 to 500 square feet of pavement.
When I worked with Gary Minton of Resource Planning Associates in Seattle,
Washington, who did the sweeper effectiveness testing field work, our hand
vacuumed test plots were 2,000 square feet each. Everyone was real sick
and tired of hand vacuuming by the time we got done. But, when you monitor
that size of area, you then have a shot at getting significant results.
Still, you can't really fault the Palm Springs people, because they were
at least trying to come to some realistic conclusions. The real problem
is there is not yet a sanctioned testing procedure that anyone testing a
road sweeper can go by.
In terms of stormwater pollutants, if a street looks
dirty, it probably has a sediment load of thousands of pounds per curb mile.
Through the multiple studies we have done, I have become convinced that
'stormwater sweeping' can be an effective BMP component for stormwater pollutant
control. It will take a different way of looking at sweeping, however; it
can no longer be done just for cosmetic reasons. When a street actually
appears to be dirty to someone with an untrained eye, it is probably holding
thousands of pounds per curb mile, instead of a more classic loading of
just a couple hundred pounds per that distance. If we want to make an impact
on stormwater runoff pollutants, then we can't let the debris loading get
that high between sweepings. And, if the sweeping program you are implementing
is not doing an effective job of keeping the volume of dirt down, then what's
out there on any given day is a very significant loading.
A realistic testing procedure must also be developed, one that can be
adapted to make sense in virtually every part of the country. It must be
one that takes into consideration other variables which can dramatically
affect the performance of the sweeper. By this, I'm talking about pavement
condition: gutter design, pitches, etc. If a sweeper has a gutter broom
that can vary its pitch, for example, used correctly it makes a substantial
difference. Variable gutter brooms are an example of a new technology on
sweepers, something that didn't exist when the NURP studies were performed.
Back then, if you wanted to change the pitch of a gutter broom, you had
to get out of the truck with a wrench and spend half an hour - and the same
to set it back to the previous position. You can imagine how often operators
would actually take the time to do that.
Actually, enough previous studies have been done that the information
exists to allow us to come up with formulas for these kinds of issues. The
only thing stopping it, really, is money and a sanctioning body. I agree
with the American Sweeper view that the American Public Works Association
(APWA) may be the organization which needs to get the ball rolling, along
with some oversight from perhaps the EPA and state environmental agencies
such as Oregon state's Department of Environmental Quality. Collating the
studies and developing a standard testing procedure is certainly possible
with the information that now exists. Until the process is developed, however,
there is really no way for public works directors to have a standard. There
is no way for them to test for sweeping effectiveness. Nor is there a way
for such officials, especially in smaller communities, to be able to evaluate
the water quality benefits between purchasing one brand of sweeper and another.
The two components needed are methodology - the way to test - and what
debris to use for the testing. For the latter, if there is no 'representative'
street dirt available, procedures need to be developed which will describe
how to produce a simulant (or substitute material) for street dirt. This
is nothing more than a guideline for making something that mimics the physical
characteristics of street dirt. Actually, that part's not simple. Sand is
easy; however, you also need to include coarse fractions, silt fractions,
extremely fines and more. Essentially, base material needs to be obtained,
then sieved into fractions, and then a certain weight of these different
fractions is measured and they all get mixed up. Then, the resulting mixture
could be laid down as street dirt. The location in the country, though,
would determine the types of soils and base material that would need to
be included to put the simulant together easily and correctly.
We have come up with a fairly well developed
method for how to conduct the actual testing procedure. This is backed up
by our SIMPTM computer modeling. The key is having ample area, so you can
collect material from a sample area before sweeping, as well as find out
what gets left on the ground by each sweeper after it sweeps. Without going
into details, you collect your samples with a vacuum that's attached to
a push broom. That way when the broom is pushed and dust kicks up, the dust
is captured, too. As a control, dirt is collected with that device in an
adjacent plot prior to any sweeping. Then, each sweeper collects what it
can, and the push broom/vacuum follows after it in the same area. With that
methodology you end up with a 'before sample' (from the adjacent plot),
and an 'immediately after sample.' For that last part, you must hand vacuum
at least 400-500 square feet. And, the tests shouldn't be run if there are
winds of more than a mile or two per hour.
When you hand vacuum an area after the sweeper has gone by, depending
upon how effective your sweeper is, the area has to be 3 or 4 times bigger
than the 'before' test. Otherwise there won't be enough sample material
picked up to be representative, or to sieve and weigh with accuracy. That
is a synthesis of the very specific methodology we've developed on how to
conduct the process.
Unless the big players -- the EPA, Ecology, APWA
and others -- come into the process, we won't have the impact needed
to get sweeper testing going.
I think it's quite important that correct, sanctioned procedures be developed
for doing this sort of testing. Such a procedure will also provide something
on which manufacturers can base their own testing. Unless some of the big
players come into the process, however, like the EPA, Ecology, APWA and
probably some others, we won't have the impact we need to get sweeper testing
going around the US.
Stormwater runoff pollution is a big problem, right now, in many parts
of the country. And it's going to get nothing but more important. That,
you can count on. If sweeping can show that it is an effective alternative
to other methods of non-point pollution control, which it appears to be,
it could usher in a whole new era in how sweeping is conducted throughout
the country.
To contact Roger Sutherland at Pacific Water Resources, call 503-671-9709.
The company may also be reached via email at: roger.sutherland@pacificwr.com
This article is reprinted from American Sweeper magazine, Volume 6 Number 1.
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