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Scientific Evidence for Compost
Filter Socks as a
Stormwater Filtration Device (SWFD)
(.pdf
Verison)
Stabilized organic matter, such as compost, has been used for billions of years as one of nature’s principle measures to filter rain and stormwater in the world’s soil and terrestrial ecosystems. Compost and composted mulch are mostly organic matter and approximately 60 to 80% of the stable organic matter content in compost is in the form of humus. Humus, organic matter, and compost have each been well documented in the scientific research literature as effective pollutant filtration devices evaluated by sediment reduction, and absorption and adsorption of contaminants in soil and storm water (Brady and Weil, 1996).
The Difference
between SWFDs and SRDs
Webster defines a filter as a porous material through which a fluid is passed
to separate out matter in suspension. Compost filter socks are a new technology
of an age-old, proven method to filter sediment and other pollutants from
storm water runoff. Compost filter socks use one of nature’s proven
filtration devices (compost) in a contained system (sock) so compost filter
socks can be used in a wider variety of contemporary storm water filtration
and sediment control applications. Traditional sediment retention devices
(SRDs) only remove sediment from storm water (and therefore only the pollutants
attached to sediment particles) by temporarily damming stormwater runoff to
allow solids to settle – like a miniature sediment pond. Compost filter
socks are a stormwater filtration device (SWFD) which allows water to flow
through a porous matrix to separate sediment (same as SRDs) and other chemical
pollutants that become soluble and/or migrate in storm water, including hydrocarbons,
excess nutrients from fertilizers and manures, and some heavy metals –
without temporarily damming the flow of stormwater runoff. Compost filter
socks are more than just sediment retention devices they are stormwater filtration
devices.
The Advantages
and Benefits of Stormwater Filtration Devices
SRDs rely on ponding of stormwater to allow eroded soil in the form of suspended
solids to settle, whereas SWFDs actually filter the moving water with significantly
less ponding, creating higher flow through rates and less hydraulic pressure
behind the compost filter sock than traditional SRDs. The increase in hydraulic
pressure from stormwater is what often leads to failure of SRDs (and poor
installation), unlike SWFDs like compost filter socks. High ponding characteristic
to SRDs, caused by low flow through rates and minor sediment clogging of their
pores, is why engineers (as designers) and manufacturers of SRDs have continually
increased their effective height – because the hydraulic head it creates
often spills over the device. Compost filter socks and SWFDs are not designed
to pond moving storm water, but to allow the water to flow through its three
dimensional matrix and ‘filter’ the water as it moves through
the sock. Traditional SRDs rely on blinding sediment concentrated runoff that
flows to the device, as this occurs the hydraulic pressure increases significantly
and will eventually push over, go under, or around the SRD. Compost filter
socks, based on private and university research though particle size analysis
and distribution, will not pond water as fast, thus relieving hydraulic pressure
and the propensity for failure. Naturally, fast moving and high volume storm
runoff is slowed by compost filter socks and sediment will accumulate on the
up-slope side of the compost filter sock - as well as within the three dimensional
filter. In a study conducted by Connecticut Transportation Institute of the
University of Connecticut and funded by the US EPA, the Joint Highway Research
Advisory Council
(JHRAC) of the University of Connecticut and the Connecticut DOT, and the
Connecticut Department of Environmental Protection evaluating the performance
of erosion control and sediment retention/filtration devices found that sediment
removed from stormwater runoff by mulch filtration devices was trapped up
to 2 inches within the filtration device (Demars and Long, 1998).
More Advantages
of Compost Filter Socks
Compost filter media within compost filter socks can be customized through
particle size distribution to allow for high flow rate conditions or low flow
conditions. Compost generally weighs about 1000 to 1200 lbs per cubic yard
of material. One cubic yard of compost generally fills 27 linear feet of 12
in compost filter sock and 12 linear feet of 18 in compost filter sock. At
this rate, a 12 in filter sock weighs approximately 45 lbs per linear foot
and an 18 in sock weighs about 100 lbs per linear foot – not exactly
the lightest tool in the sediment control toolbox. The weight of compost filter
socks alone makes them very resistant to movement from storm water runoff,
particularly when installed and staked correctly. Additionally, compost filter
socks that get run over by vehicles or heavy equipment will still filter storm
water (to a diminished degree) until it is repaired, unlike most two dimensional
SRDs that are rendered useless when run over by a vehicle. Finally, according
some ecologists compost filter socks likely have less of an impact on wildlife
migration patterns than silt fence, since compost filter socks can be easily
scaled or traversed by terrestrial fauna, whereas silt fence is the equivalent
of a wall to all terrestrial bound wildlife.
Support from Research Literature – the scientific facts
A review of the scientific university and government agency research literature
provides further evidence of the efficacy and environmental benefits compost
and compost filter socks provide in sediment control and stormwater management.
Simulated
Rainfall vs Simulated Runoff
As demonstrated by decades of stormwater runoff and soil erosion research
conducted by universities, government agencies, and private industry the best
way test soil erosion control devices of any kind is under natural rainfall
and/or simulated rainfall conditions. Although simulated runoff tests can
be valuable they leave out very important processes that occur naturally under
rain generating runoff conditions. Only natural rainfall and specifically
calibrated rain simulators can provide total and evenly distributed water
over a soil surface. The hydraulic force of rain impact affects the soil surface
structure and soil aggregates (which effects runoff movement over, through,
and into the soil - in addition to the soils capacity to hold water), and
the rain drops explosive impact detaches soil particles (along with the action
of runoff) and sends these particles in all directions it. In addition, rainfall
often creates runoff simultaneously and uniformly across the soil surface
area (Brady and Weil, 1996) – unlike runoff simulation studies; while
the explosive and damaging impact to the soil surface helps to create the
areas where rills will form from/with sheet flow of stormwater. In fact, soil
is primarily detached by rainfall impact not runoff (Risse, 1999). In addition,
the rainfall impact pulverizes soil particles and soil aggregates and thus
creates more particles suspended in runoff water (i.e. greater suspended solids);
these soil particles and aggregates are of a slightly different size and shape
– notably smaller and therefore harder for SRDs and SWFDs to trap. This
is a natural process and is not simulated in studies that only apply runoff
without rainfall. In addition, runoff simulated studies rarely take into account
the behavior of natural runoff. Runoff normally starts in very slow sheet
flow dispersed evenly both
horizontally and vertically on a slope (not just as a source from the shoulder),
as rainfall continues runoff rate and velocity normally increases creating
interrill and rill erosion, and concentrated rills and channels (partially
caused by the impact of the rain on the soil) which gradually increase as
the rain continues. Additionally, rainfall (natural or simulated) naturally
impacts the SRD or SWFD by the pounding action and corrosive effects of wind
and water. The rainfall itself (aside from the runoff) can contribute to the
undermining of a SRD or SWFD. Do not forget that in a rainstorm, the rain
falls everywhere, and where SRDs disturb the soil surface as a requirement
for installation (silt fence itself can be considered a soil disturbing activity),
the rainfall that falls near and around the SRD will create a disproportionate
amount of sediment – and the erosion on the down slope side of the SRD
will go unchecked and in simulated runoff studies unsampled. SWFDs such as
compost filter socks do not disturb the soil surface upon installation.
In studies that seek to scientifically evaluate SRDs and SWFDs a controlled area is necessary (often using borders to prevent run-on, effectively creating a watershed) to determine the calculable and probable or improbable effectiveness (through statistical significance and probability) of a device over a specific area that is receiving a prescribed (or measurable) quantity of rain water. Studies that do not employ these methods are considered demonstration sites and are not scientifically reputable projects. University published runoff and erosion research studies have been conducted in plots as small as one square meter, and slope length is less of a factor than slope steepness (Brady and Weil, 1996). Field studies that only use simulated runoff do not replicate the natural conditions that occur in rainstorms that generate runoff and therefore are not as reliable for testing the effectiveness and efficacy of SRDs and SWFDs used in storm runoff applications.
Peer-reviewed
Scientific Research Supports Compost and Mulch as a Stormwater Filtration
Device (SWFD)
In independent field research conducted in Portland, Oregon by CH2M Hill,
the Portland Metropolitan Services District, and W&H Pacific evaluating
SWFDs and SRDs under natural rainfall (5 storm events) on 34% slopes 9 ft
wide by 32 ft long found that compost SWFDs had 92% less total suspended solids
(TSS), 73% less total P, and less metals in sediment concentrated runoff compared
to 36 in silt fence (Stewart et al., 1993).
In independent field research conducted by the University of Connecticut and the New England Transportation Consortium comparing the performance of erosion control and SRDs on slopes 5 ft wide by 30 ft long under 11 natural rainfall events reported the following: During a ¾ in. storm event hay bales lost approximately 300 g of solids (total load), silt fence 50 g, and the mulch filtration device 10 g. During a 4.35 in storm event silt fence lost approximately 3200 g solids, hay bales 2900 g, and the mulch filtration device 250 g. The percent of sediment passing through the SRD/FDs as a ratio to bare soil were: hay bale 2.02, silt fence 1.63, mulch filtration device 0.20. On average, the silt fence and hay bale released an order of magnitude more sediment than the mulch filtration device (Demars et al, 2000).
In a two year field study conducting by a team of soil scientists, engineers, and ecologists at The University of Georgia comparing the environmental performance of compost and mulch erosion control and SWFDs to industry standard practices and SRDs on construction sites reported the following results. Under a series of storm events averaging 3.1 in/hr for one hour duration (equivalent to a 50 yr 1hr storm event for Athens, GA) mulch filtration devices had lower total sediment loads than silt fence and averaged over all storm events mulch filtration devices had 35% less TS than silt fence. When the researchers compared the system of yard waste compost to industry standard practices (silt fence with hydroseed), the compost system had nearly 5 times less sediment loss, 27% less runoff, 2.5 times more time until before runoff commencement, 35% more time until peak runoff rates were achieved, 2.5 times less total nitrogen loss, 8 times less nitrate nitrogen loss, 6 times less total phosphorus loss, and 6 times less bioavailable phosphorus loss (Faucette, 2004).
Independent laboratory testing with compost based Filtrexx Filter MediaTM used in compost filter socks at the Soil Control Lab in Watsonville, CA has shown that this tool consistently removes total solids (TS) from simulated stormwater runoff on 3:1 slopes. Out of 27 trials conducted with compost filter socks, TS concentrations were reduced between 95 and 99%. In addition, flow through rates of 20-25 gallons/linear foot/minute of the compost filter socks were regularly achieved without diminished sediment and/or pollutant removal rates.
As a comparison, flow through rates for silt fence subject to high sediment concentrated runoff is 0.3 gal/linear ft/min and for clean water under lab conditions is 10 gal/linear ft/min according to the US EPA National Pollution Discharge Elimination System (2004). The US EPA goes on to say that silt fence should only be used where low level stormwater sheet flow is anticipated (2004). The US EPA also states the effectiveness ranges for silt fence constructed of filter fabric that are properly installed and well maintained are: silt-clay-loam removal = 0 to 20 % [typically the suspended solids fraction in water] and silt loam 50 to 80%” (2004).
As stated earlier, compost is classified as a stormwater filtration device (SWFD) instead of a traditional sediment retention device (SRD) because it functions to remove other pollutants in addition to sediment from stormwater runoff. According to the US EPA (1998) compost has been shown to degrade the following contaminants under controlled conditions and/or in field research programs: petroleum hydrocarbons (gasoline, diesel fuel, jet fuel, oil, grease), polynuclear aromatic hydrocarbons (wood preservatives, refinery wastes, coal gasification wastes), pesticides (herbicides and insecticides), and explosives (TNT, RDX, nitrocellulose). Petroleum contaminated areas amended with compost exhibited degradation rates of 375 mg kg-1/day compared to only 40 mg kg-1/day without compost (Stegmann et al, 1991 and Hupe et al, 1996). At the rate exhibited by the compost amended areas, typical petroleum hydrocarbon contaminated soils (normal range is between 5,000 to 20,000 mg kg-1) would be completely degraded in 14 to 60 days (USEPA, 1998). In fact, hydrocarbon degrading microorganisms are often isolated from compost and used to inoculate bioremediation projects (Civilini et al, 1996 and Castaldi et al, 1995).
Independent laboratory testing with compost based Filtrexx Filter MediaTM at the Soil Control Lab in Watsonville, CA has shown that this material can remove petroleum hydrocarbons (as motor oil) from simulated runoff at consistently high percentage rates. Out of 32 independent tests with motor oil concentrations in water between 100 and 1700 mg L-1, compost based filter media removed an average of 87% of the motor oil concentration in stormwater, while 20 of the 32 composts removed greater than 95% of the motor oil concentration in the simulated stormwater runoff.
In addition to removing and degrading hydrocarbons from soil and storm water, humus can chemically adsorb and bind nutrients from fertilizer applications as well as certain heavy metals, thereby filtering them from storm water and preventing their migration to and pollution of valuable surface waters (Brady and Weil, 1996).
References:
Brady and Weil, 1996. The Nature and Properties of Soils, 11th Edition. Prentice
Hall, Inc, Simon and Shuster Co., New Jersey.
Castaldi, F.J., K.J. Bombaugh, and B. McFarland, 1995. Thermophilic slurry-phase
treatment of petroleum hydrocarbon waste sludges. In: Micorbial Processes
for Bioremediation, by R.E. Hinchee, F.J. Brockman, C.M. Vogel, 231-250. Columbus,
OH: Battelle Press.
Civilini, M.C., M. de Bertoldi, and N. Sebastianutto, 1996. Composting and selected microorganisms for bioremediation of contaminated materials. In: The Science of Composting, by M. de Bertoldi, and P. Tiziano, 913-923. London: Blackie Academic and Professional.
Demars, K.R.,
R.P. Long. 1998. Field evaluation of source separated compost and
Coneg model procurement specifications for Connecticut DOT projects. University
of Connecticut and Connecticut Department of Transportation. December, 1998.
JHR 98-264.
Demars, K.R., R.P. Long, and J.R. Ives. 2000. New England Transportation Consortium use of wood waste materials for erosion control. April, 2000.
Faucette, L.B. 2004. Evaluation of Environmental Benefits and Impacts of Compost and Industry Standard Erosion and Sediment Control Measures used in Construction Activities. Doctoral Dissertation, Institute of Ecology, The University of Georgia.
Hupe, K., J.C. Luth, J. Heerenklage, and R. Stegmann, 1996. Enhancement of the biological degradation of contaminated soils by compost addition. In: The Science of Composting, by M. de Bertoldi, P. Bert, and P. Tiziano, 913-923. London: Blackie Academic and Professional.
Risse, L.M. 1999. Managing runoff and erosion on croplands and pastures. Georgia Farm A Syst, Cooperative Extension Service, The University of Georgia, Athens, GA.
Stegmann, R., S. Lotter, and J. Heerenklage, 1991. Biological treatment of oil-contaminated soils in bioreactors. In: On-Site Bioreclamation, edited by R.E. Hinchee and R.F. Olfenbuttel, 188-208. Boston: Butterworth-Heinemann.
Stewart, B., P. Pommier, J. Lenhart, L. Faha, D. Collins, L. Ettlin, 1993. Demonstration Project Using Yard Debris Compost for Erosion Control – Final Report. W&H Pacific and Portland Metropolitan Services District.
USEPA, 1998.
An Analysis of Composting As an Environmental Remediation Technology. US EPA
Solid Waste and Emergency Response (5305W). EPA530-R-98-008, April 1998: 2-38.
USEPA, 2004. NPDES - Construction Site Storm Water Runoff Control. http://cfpub.epa.gov/npdes/stormwater/menuofbmps/site_30.cfm
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