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Influent samples from each of the three test portions were collected and mixed to
result in a 500 mL influent sample. A 500 mL sample also was collected from the
effluent container. The samples were analyzed for total suspended solids (TSS). Crebbin
(1988) reported filtration efficiencies which ranged from 87% to 91%. A comparison of
these efficiencies with those reported by manufacturers indicated that the AOS parameter
was not a valid parameter to indicate efficiency and that the standard VTM-51 filtration
efficiencies reported by manufacturers were influenced by the gradation of the soil in the
test slurry mixture (Crebbin, 1988).
Kouwen (1990) also completed a laboratory evaluation of the effectiveness of silt
fences using a model filtration apparatus which was similar in principle to the flume
utilized for the VTM-51 and Crebbin’s test. However, the method of slurry introduction
was substantially more technical and filtration efficiencies for straw bales and rock berms
could also be evaluated. A series of stilling tanks allowed the capture of test water and
solids. A jet pump was placed near the bottom of the primary settling compartment and
an electric pump was positioned near the water surface at the final settling bin. The two
pumps were operated in tandem to provide constant slurry flows of sustained duration.
The ancillary equipment allowed Kouwen (1990) to monitor efficiencies and operational
characteristics over test periods of five hours. The test sediment was a poorly graded
number 56 Barnes silica sand with an average particle size of 0.2 mm (medium sand).
Influent and effluent samples were collected and analyzed for TSS. Reported
filtration efficiencies were 99% to 100% over extended periods of evaluation.
Efficiencies for five centimeter rock berms degraded rapidly over time and ranged in
value from -125% to 100%. Straw bales typically performed well, although in a single
experiment no solids were removed. No discussion of negative efficiencies were included
in his report (Kouwen, 1990).
Kouwen (1990) concluded that geotextile silt fences were effective filtration
media; however, properly installed straw bales rivaled geotextile performance. He also
reported that less permeable fabrics facilitate greater system efficiencies, but the
increased possibility of clogging and over-topping which must be considered in fabric
selection.
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