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BMP FILTERS: UPFLOW VS. DOWNFLOW
Citation:
Clark**, S., R. Pitt, AND A. Balakrishnan. BMP FILTERS: UPFLOW VS. DOWNFLOW. Presented at ASCE Urban Water Resources Research Council/Engineering Foundation Conf, Snowmass, CO, 8/19-24/2001.
Description:
Filtration methods have been found to be effective in reducing pollutant levels in stormwater. The main drawback of these methods is that the filters get clogged frequently and require periodical maintenance. In stormwater treatment, because of the cost of pumping, the filters are typically operated in a downflow mode. The objective of this research ws to compare upflow filtration with downflow filtration to find out if there is an improvement in the life of these filters maintaining an acceptable flowrate. The different filtration media used for these experiments were sand, old compost, new compost, agrofiber, cotton, activated carbon, reef carbon, and zeolite, mixed 50% media and 50% sand. A clay-water mixture with a concentration of 1.5 g/L was used as the test water, where the clay used to make the test water was that which had passed through a 250 ug screen. Effluent flowrates were determined by measuring the amount of time required to collect a specific volume (typically one liter) of effluent. The turbidity of each effluent sample was also determined. Breakthrough was defined as occurring when effluent turbidity was greater than 100 NTU. Among the nine mixed media used for these experiments, six gave better results with upflow filtration when compared to downflow filtration. Reef carbon gave the best result among the media used for upflow filtration. Activated carbon was the best among the media used for downflow filtration. Comparing the overall results of upflow and downflow filtrations, reef carbon was identified as the best medium (ignoring the cost of the media) as it gave the best values for accumulated suspended solids loading while maintaining high flow rates. Patterns of flow behavior were noted throughout the experiment for the upflow filtration set-up. At least three different classes of filtration behavior were observed for the media when operated in an upflow mode: particles were primarily removed at the surface through a straining mechanism; particulate removal occurred throughout the complete depth of each medium; the peat-sand medium separated into two layers during upflow filtration.