Performance of a Stormwater Filter and Bacteria Inactivation Using Biocidal Media

Author

Alexander Scott Bowerman, California Polytechnic State University, San Luis ObispoFollow

DOI: https://doi.org/10.15368/theses.2010.29
Available at: https://digitalcommons.calpoly.edu/theses/260

Date of Award

3-2010

Degree Name

MS in Civil and Environmental Engineering

Department/Program

Civil and Environmental Engineering

Advisor

Yarrow Nelson

Abstract

There are many possible ways to mitigate stormwater pollution, but this study focused on the DrainPacTM catchment basin insert and the feasibility of integrating N-halamine biocidal brominated beads into the filter system. This study was divided into three sections. The first section involved testing a DrainPacTM filter for treatable flow rates, head loss, and removal of solids, oil, and bacteria. The DrainPacTM filter is designed to be installed in a stormwater catch basin. The filter is composed of a 12 x 41 inch metal frame with textile filter media attached to it in a basket shape. The upper portion of one panel of the filter basket is made from a plastic mesh to allow overflow if the filter is overloaded. The second section of this study involved testing N-halamine brominated biocidal beads in laboratory-scale columns, and the third section involved integrating the beads into the DrainPacTM filter and testing it full scale.

For the DrainPacTM filter tests, the unit was installed into a custom-built test flume which was designed to mimic the conditions that would be encountered in a real stormwater application. The flume was supplied with a gravity-fed stream of water from a retention pond located on the Cal Poly, San Luis Obispo campus. The initial tests were conducted to determine the amount of head loss produced by the filter. First, the clean filter was subjected to flow rates between 20 and 200 GPM. The filter showed very minimal head loss (0.5 to 9.1 cm for 20 to 200 GPM) when not loaded with solids. Next, the filter was subjected to 200 GPM flow with a solids concentration of between 80 and 100 mg/L until it failed (overflowed). This occurred after 625 g of solids had been added to the filter. After the filter had been loaded with solids to the point of overflow at 200 GPM, it was tested to determine what flow rate could be filtered with the solids present. The fully loaded filter was able to pass a flow rate of up to 80 GPM before overflowing.

The DrainPacTM filter removed solids at a range of efficiencies from 83 to 91% at flow rates between 20 and 200 GPM. The higher removal efficiencies were achieved at the lower flow rates. The filter removed oil at efficiencies ranging between 40 and 80%. The oil removal efficiency did not appear to depend on the flow rate. The DrainPacTM filter did not remove bacteria under the test conditions.

Following the DrainPacTM experiments, 0.3 mm and 0.8 mm diameter N-halamine brominated biocidal beads were tested in the lab using a laboratory glass column. At flow rates between 0.28 and 1.4 mL/sec, a 1 cm bed height of the 0.3 mm beads was found to produce head losses between 19 and 51.7 cm. The 0.8 mm beads produced head losses ranging from 11.9 to 47.7 cm when tested over the same range of flow rates. These flow rates represent nominal velocities between 0.36 and 1.8 cm/sec which would be expected in the DrainPacTM filter. The beads were then tested to determine how effectively they inactivate bacteria in a stream of water. Contact time after flowing through the column was found to be the key factor in how efficiently the beads worked. When the effluent samples were instantly quenched with sodium thiosulfate, the bacteria removal results matched those observed for the control (beads without bromine). When the samples were quenched directly after collection by adding the sodium thiosulfate to the sample as soon as the desired sample volume had been collected (95 to 285 seconds depending on flow rate), between 95 and over 99 percent of the bacteria were inactivated. After 10 minutes, all of the bacteria were inactivated.

The final test involved integrating the N-halamine brominated beads into the DrainPacTM filter for a full scale test. Two sleeves containing 1400 grams of beads were laid into a DrainPacTM filter which was custom built to concentrate the flow through the beads. This system was tested using pond water with an average of 298 CFU/100 mL coliform bacteria at a flow rate of 36 GPM. The results of this test were very similar to the results of the lab scale testing. Contact time again proved to be necessary for bacteria inactivation. The filter with integrated N-halamine beads removed between 72 and 100% of bacteria with contact time between 30 seconds and 10 minutes.