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RECENT MEASUREMENTS OF HEAVY METAL REMOVALS USING STORMWATER FILTERS

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This 3-year study was funded by the Water Environment Research Foundation (WERF) (Johnson, et al. 2003) to investigate innovative heavy metal removal from stormwater. During this research, two major areas of heavy metal control were selected for in-depth investigation: media filters and swales. This presentation will focus on the filter tests. The test stormwater used for most of the laboratory tests was collected from a campus parking lot at the University of Alabama during many storm events. The characteristics of the stormwater indicated that the most prevalent metals detected in the runoff were iron, zinc, copper and small amounts of particulate bound lead. Ranges of the metals were within national ranges reported by other studies. The results emphasize the importance of characterizing the stormwater before selecting a treatment media since the type and quantity of metals, pH, and other runoff characteristics can vary a great deal between sites and can affect the pollutant removal characteristics.

In the laboratory-scale filter studies, twelve media were chosen for initial evaluation. Equilibrium and kinetic studies were completed to assess their performance in capturing metals from stormwater. Twelve media were initially evaluated by means of batch equilibrium and kinetic tests to compare the rate and extent of metals capture. The three best performing media: peat-sand mix, compost, and zeolite, were then selected for an in-depth column studies using parallel upflow columns in packed media beds. Metal removal efficiency was examined for different rates of flow and influent conditions. Upflow columns proved more effective than downflow columns in the control of detention time, reduction in clogging of the media by solids, and associated head loss in the column.

Studies on the effect of anaerobiosis on metal retention by filter systems indicated that heavy metals were not mobilized from filter systems under anaerobic conditions. However, some nutrients were lost after the media became anaerobic. It was found that metal retention by the filters was not different from what was observed in oxygenated environments. Tests also indicate that the heavy metals of concern remain strongly bound to the particulates during long exposures at the extreme pH conditions likely to occur in receiving water sediments. TCLP (leachability) tests were also performed on spent media to assess disposal options, with results indicating that traditional, non-hazardous-waste disposal should be acceptable for most stormwater applications. Finally, the effect of microbial growth on metals capture and head loss in the column was investigated.

Several of these filter media were also tested in a pilot-scale device using water from a detention pond that drains a medium-density residential area in Birmingham, Alabama. Testing on the pilot-scale filters was performed in a manner expected to simulate the intermittent use typical in full-scale runoff applications (eight filtering events on eight non-consecutive days over a four week period). The test water was pre-settled stormwater pumped from the detention pond, similar to how the filters may be used for post-treatment. Pre-settled runoff was selected because prior experimentation with laboratory-scale filters found that significant pre-treatment was needed to prevent the filters from becoming clogged well before the media's chemical capacity was met and to ensure a reasonable run time before required maintenance. In this series of tests, the runoff water metals concentrations in the influent were extremely low. On those occasions where the metals concentration was higher, the filters proved effective at removing influent concentrations down to a level of approximately 10 – 15 μg/L. They were ineffective if the influent metals concentration was less than 10 – 20 μg/L.

Much information has been collected during this WERF-sponsored research project that can be directly used for the design of stormwater controls for the reduction of heavy metals. Tests on the treatability of stormwater heavy metals examined the associations of metals with different stormwater fractions. These associations are also useful in predicting the performance of sedimentation controls in removing heavy metals. This information can be used to predict the heavy metal control associated with a wet detention pond that is designed to remove particles of a specific minimum size. Additionally, material on the selection of filtration media, the clogging rates under different conditions, and the metal retention capacity of the different media are all available in the final research report. A simple approach in the selection of filtration media and the sizing of stormwater filters is also outlined. Investigations of several important issues associated with the upflow filters are also presented. These filters hold promise in being much more cost-effective than the more-commonly-used traditional slow filters. Additional tests were also conducted that investigated the clogging behavior of the different media under upflow conditions.
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Document Type: Research Article

Publication date: 2004-01-01

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