Field Evaluation of a Stormwater
Sand Filter
by
Ben R. Urbonas, John T. Doerfer and L. Scott Tucker
The July, 1996 issue of the APWA Reporter had an article summarizing the findings of a two year effort to test a sand filter as a structural BMP (Urbonas, et. al., 1996). At this time an expanded discussion on this topic is presented and readers' comments are solicited by the authors.
The use of media filter basins, mostly sand filters, for stormwater quality enhancement was first reported by Wanielista et al. (1981) and Veenhuis et al. (1988). Since then the use of filters has expanded, with most uses reported in the State of Delaware, the Washington DC area, Alexandria, VA and the Austin, Texas area (Anderson et. al., undated; Bell et. al., 1996; Chang et. al., 1990; City of Austin, 1988; Harper and Herr, 1992; Shaver and Baldwin, 1991; Trung et. al., 1993). Recently filter media such as peat, compost and goetextiles have also been tried (Farham and Noonan, 1998; Galli, 1990; Stewart, 1992). Some designs, such as in the City of Austin, are defined by local criteria, including an ingenious sand filter inlet suggested by Shaver and Baldwin. In many designs, a detention basin is often provided upstream of the filter to capture the runoff and even out the flow through the filter.
It is evident that sand and other media filters are gaining popularity in the United
States as stormwater BMPs. While the literature contains reports about their ability to
remove pollutants, very little has been reported in the literature on their long-term
hydraulic performance.
Since 1994 the Urban Drainage and Flood Control District, in cooperation with the City of Lakewood, has been conducting field and laboratory investigations about water quality enhancement and hydraulic performance of stormwater sand filters. The District supported the laboratory testing by one of its student interns of a number of different sand gradations (Neufeld, 1996). This resulted in the selection of concrete sand (ASTM C-33 Mix) as providing the best balance between constituent removals and hydraulic conductivity. This sand mix was then field tested in Lakewood, CO. ASTM C-33 sand is a commonly available gradation of sand and appears to be used today at many of the existing stormwater filters reported in the literature.
The water quality performance characteristics of the District's test sand filter were
found to be comparable to those reported in the literature, especially for total suspended
solids (TSS) (EPA, 1983; Veenhuis, 1989; City of Austin, 1990). However, this was true
only for the fraction of the runoff that actually flowed through the filter. This is not
true for all of the runoff that by-passed the filter. Early in the field test program the
sand filter began to quickly lose hydraulic conductivity. This led the authors to conclude
that, unless there is substantial detention volume upstream of sand filters that will
capture and slowly release stormwater, or the sand filter is much larger or it is sized to
be in balance with the available stormwater capture volume upstream of it, much of the
runoff will either by-pass the filter or cause large and prolonged ponding areas on the
land surfaces adjacent to the filter.
The Figure shows the test filter's unit hydraulic flow-through rate (feet per hour per square foot of sand filter) degrades as the total TSS removed by each square foot of the filter's surface accumulates over the test season. Immediately after the filter was installed, its flow-through rate was in excess of 3-feet per hour. This diminished to less than 0.075 feet per hour as 0.4 to 0.6 pounds of TSS per square foot of filter area accumulated on its surface (0.4 lb./sq.ft. of TSS accumulation equals approximately 1/16 inch layer). The final flow-through rate of 0.05 feet per hour (0.006 gpm/sq.ft.), which in the test facility was reached after only a few storms, is approximately equal to one-tenth to one-fifth of the design rate recommended by several local stormwater BMP criteria manuals that permit or require sand filters as BMPs. The rapid reduction in flow-through rate is a "red flag" about the use of this practice today. Clearly, hydraulic design questions need to be better answered before this type of BMP is recommended by District for use.
During the 1995 summer season of operation, over one half of all runoff events exceeded the combined capacity of the filter and the upstream surcharge volume of the test facility to "capture" the storm's entire runoff. This resulted in a TSS removal rate during the summer monitoring season of less than 15% after all the flow by-pass volume was accounted for. This compares poorly with the more than 85% TSS removal rates reported in the literature, and confirmed by District's field tests, for that portion of the runoff that actually flowed through the filter.
Flow bypasses were anticipated in the design of this test installation because it was designed to be "stressed" in order to more quickly reveal how much maintenance a sand filter BMP would need. The rate at which it sealed and lost hydraulic conductivity was a surprise however. Clearly, if these findings can be extrapolated to other installations, three concerns emerge: (1) there probably is a need for an aggressive maintenance program to keep such filters operational, (2) the filters probably need to be sized larger than most current design recommendations suggest, and/or (3) the filters need formal stormwater capture volume basin upstream (Urbonas and Ruzzo, 1986; City of Austin, 1988) that balance the filters' flow-through rates with the population of storms for which the filters are being designed. Any of these have significant economic and operational consequences. We suggest all of these be considered whenever sand filters or other stormwater quality BMPs are being selected.
A recent report by Bell et. al. (1996), in his very extensive report on the
performance of sand filters in the Alexandria, VA area, also reported on some bypasses of
flow around filters, but he did not address the fraction of the total annual runoff that
bypassed them. Bell's group primarily field tested the Delaware filter that was originally
suggested by Shaver and Balwin. Their findings suggested a longer clogging period than was
found by the authors although their data were insufficient to judge if the clogging rates
were similar. It is not surprising that the Delaware filters did not clog as rapidly as
the Denver test suggest since they were much larger in proportion to the tributary
impervious area, had larger storage volumes above the filters, and the inflow at their
test sites had much lower average event mean concentrations (EMC) of TSS than were found
at the test site in the Denver area (60mg/l vs. 400 mg/l). This suggests that adequately
sized filters, ones that are sized with maintenance frequency, the average annual runoff
and its average EMC of TSS in mind, can have acceptable performance for more that one
season. However, the three basic points about filter maintenance and design made earlier
cannot be forgotten whenever media filters are considered for use.
Clearly, filters can be popular BMPs where land area is at a premium, but they need regular maintenance to keep working. It is important to recognize that a media filter, once clogged, will drain at very slow rates (i.e., falling head of approximately 0.5 inches per hour) and stormwater will either pond upstream of the filter of bypass it. Either condition is unacceptable. In the first case the ponding water may be a nuisance or create dangerous situations. In the latter, only a fraction of the stormwater that arrives at the filter actually receives the treatment efficiencies reported for media filters.
To compensate for this potential problem it is necessary to properly size the filters for the expected maintenance cycle that matches both the average annual runoff volume and the average annual EMC of TSS in its runoff. If the filter cannot be made large enough to pass through the design event without backing up water when it is partially clogged, provide sufficient stormwater capture detention volume upstream that is in balance with the filter's clogged flow-through rate. Making the filter larger will reduce its needed maintenance frequency. Also, remember that media filters, without upstream detention, have no effect on stormwater runoff flow rates. As a result, they have do not attenuate the increases in runoff rates that result when land urbanizes.
When a media filter is located within an underground vault, such as a water quality inlet, it is out-of-sight-and-out-of-mind and is likely to not receive the needed maintenance attention of a visible surface facility. As a result, regular inspection programs are a must. Otherwise there is nothing to insure that the filter will continue to operate properly.
Sand filter inlets suggested by Shaver and Baldwin and by City of Austin, while
effective, are expensive to construct. Above ground filter basins can be more expensive to
build than simple extended detention basins. It has been argued that media filters are
most likely to be used where land costs are very high. However, comparisons made by the
authors of filter basins designed with clogging and minimal maintenance in mind against
the detention basins and retention ponds revealed that filter basins often require similar
land areas to construct as do extended detention basins. If this is the case, and recent
findings suggest it is, the life cycle costs of functional media filters may actually be
more than those of extended detention basins.
The authors acknowledge the help of Richard Ommert, engineering student intern with the
District and Curtis Neufeld, formerly an engineering student intern with the District and
now a project engineer with Greenhorne and O'Mara. Their hard work and dedication in
collecting and analyzing the field data is much appreciated. The cooperation and
assistance of Jerry Goldman and Jay Hutchinson of the City of Lakewood are also
acknowledged. Without their help, the installation, maintenance and testing of this
facility would not have been possible.
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