Treatment of Stormwater Runoff by Geotextile Filters via Suspended Solids Capture

Abstract

Suspended solids in stormwater runoff create a range of water quality problems; their removal lessens the deleterious impact of stormwater runoff on aquatic ecosystems. In this study, three geotextiles were tested in laboratory column tests with influent suspensions having hydraulic loading rates, total suspended solids (TSS) concentrations, and particle size distributions (PSDs) similar to those reported for urban highway stormwater runoff. After a short ripening period, the geotextile filters removed TSS from 100-200 mg/L to below a target concentration of 30 mg/L. A lower geotextile permittivity resulted in an increased percentage of TSS captured by the filter; however, the total mass of solids captured was unaffected because lower permittivity resulted in lower total solids loaded to the filter overall. In general, the effectiveness of the geotextile filter at retaining suspended solids increased as filter opening sizes decreased and as influent particle sizes increased. The hydraulic conductivity of a geotextile filter, which was related to TSS captured via a power function, was higher for geotextiles with higher permittivity and larger opening sizes and for larger influent particle sizes. Overall, the filter with the second highest permittivity (0.8 s-1), NW2, was the most successful geotextile tested. TSS removal in the geotextile laboratory tests was comparable to reported values from sand filters in literature under similar loading conditions. Sand filters in laboratory tests had greater TSS removal than geotextile filters. However, the sand filters clogged at a lower total solids loading than the geotextiles. The applicability of existing filtration criteria for geotextiles for stormwater treatment is addressed, and four new retention criteria ratios specifically for stormwater filtration which use two filter opening sizes and two particle diameters, such as (095/D60)/(O30/D10) > 0.05, are introduced. Results from field testing are analyzed; the geotextile filter reduced TSS concentrations in runoff by an average of 84% with input TSS event mean concentrations (EMCs) ranging from 22 - 185 mg/L and output EMCs ranging from 1.7 - 22 mg/L. A mathematical model is developed which estimates hydraulic conductivity as a function of solids captured. The model and filtration criteria are used to assess the field testing results.