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  Storm Water Treatment at Critical Areas, Evaluation of Filtration Media (368 pp, 11.7 MB) (EPA/600/R-00/010) July 1999

This document describes research conducted between 1994 and 1996 on filtration media for storm water treatment. (Other volumes in this report series describe the results of field investigations to determine sources of urban storm water runoff pollutants, field investigations of storm drain inlet devices, and development of a prototype treatment device that could be installed at the storm drain inlet in critical source areas.)

Filtration, especially slow filtration, is of interest for storm water runoff treatment managers because filters will work on intermittent flows without significant loss of capability. This research was initially planned to be the optimization of a sand filter installed in the filter chamber of the multi-chambered treatment train (MCTT). However, the poor removals provided by newly constructed sand filters led to the investigation of other media with the potential to retain pollutants.

Storm water filters currently in operation typically use sand, compost, and peat. This research tested the capabilities of the media currently in use, plus others with known filtering capability (activated carbon, zeolite, a cotton milling waste, and a wood waste), under both controlled laboratory and field conditions. Influent and effluent samples from each filter column were analyzed for toxicity, turbidity, conductivity, pH, major anions and cations, and particle size distribution for each test.

The research demonstrated that physical clogging of the filters occurred well before the sorptive capacity of most media is reached when storm water runoff is filtered without adequate pre-treatment. If adequate pre-settling is done, the solids remaining in the runoff are generally very small (colloidal). These filters are capable of removing many of the colloidal-sized particles; however, the percent removals (measured as suspended solids removal) are smaller when there are fewer larger particles in the influent.

Tests using laboratory-scale columns showed that an activated carbon-sand filter is the best at removing storm water pollutants. The range of cumulative suspended-solids loadings is from 200 grams per square meter (g/m2) (peat/sand) to 2,000 g/m2 (carbon/sand) before the hydraulic capacity is reduced to 1 meter per day (m/day). Because these tests were performed using small columns (4.76 centimeter [cm] diameter and 45.72 cm depth) and were not able to completely dry between most of the tests, it is expected that the suspended-solids loadings in full-scale filters will be about five times greater than these values before the filter clogs.

In terms of chemical capacity, results of the testing showed that the activated carbon, peat moss, zeolite, and compost were the most efficient media for removing the toxicants from the runoff and retaining them during subsequent flushings with clean distilled water. Sand, the most common filtering media currently in use, effectively removed toxicants from the runoff; however, the effluent from subsequent distilled water flushings through newly constructed sand filters indicated that the toxicants were displaced from their "trapped" pores by the water. The flushing effluent was significantly more toxic than the clean flushing influent.

Based on historical full-scale installations, aged sand, after being exposed to field conditions for some time, apparently ripens due to deposition of organic and mineral material and can be much more effective than when first installed. Although an effective filter, the compost added an undesirable color to its effluent. The peat moss, also an effective filter, increased the turbidity of and added color to the runoff. The activated carbon was found to be the most effective for removing the toxicants while not increasing the turbidity or color. In all cases, the media had to be mixed with sand to maintain adequate flow rates.

Research is continuing on the ability of filters to treat storm water runoff, and it is anticipated that a future volume in this report series will detail the results of the ongoing work. This new phase of the filter project has two purposes:

  • To quantify the effects that pH, ionic strength, and influent concentration on the removal ability and capacity of the filter media
  • To perform pilot-scale studies using several selected media to determine the applicability of the bench-scale results to full-scale operations

These two steps are required in order to develop design guidelines for storm water filters that will be useful for the engineering community and storm water management planners.


Richard Field

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