One Solution to Roadway Pollution

If asked for a method to control air pollution, most people would describe a filter or chemical to trap or change the fouling agent. Describing a method for controlling water pollution is not as simple because it is caused by contaminants from many sources. Experts will say the best management practice is to reduce pollutants and then to contain and treat contaminated water or channel it away from aquifers and waterbodies. They may also ask, "What kind of polluted water?" Controlling pollution in roadway stormwater runoff, for example, is challenging because rain is erratic and the composition of roadway pollutants varied. Runoff comes from a broad area and accumulates more pollutants as it moves. Besides, it contains heavy metals, one of the major contributors to water pollution throughout the world.     

 In some parts of the US, roadway runoff is controlled at grade by plants that absorb pollutants as water passes through or by retention basins that promote dilution and evaporation.In other parts of the US, it is controlled by filtration through pavement or channeling to a wastewater treatment facility, but none of these effectively removes heavy metals from water pouring directly from an elevated roadway into a waterbody. For Louisiana, this circumstance is significant because the state has more elevated roadways than any other and many are constructed over fish nurseries - wetlands and estuaries. Dr. John Sansalone of the Louisiana State University Civil and Environmental Engineering Department is studying a method for filtering runoff from these roadways before it enters an estuary.      

"Roadway runoff is the most significant source of heavy metal in the environment today," he said. "Vehicles contain components full of lead, cadmium, zinc, nickel, copper, and chromium - all potentially toxic. "In fact, the annual level of pollution from roadway storm water runoff actually exceeds that from untreated domestic and industrial wastewater combined.Roadway runoff contains significant pollutants, such as these heavy metal molecules may be partially filtered by soil beside inland roadways but it flows directly into estuaries from elevated roadways. The direct discharge of this pollution into the wetlands can be toxic to many aquatic species, especially to invertebrates like oysters or juvenile shrimp.     

 Sansalone's solution is to filter the storm water runoff before it enters the estuary. "First we created a bench-scale filter system in the lab," Sansalone said. "Now we are field testing and monitoring its performance on a roadway over an estuary."     

 He chose a biofilter - a sorptive floating bead clarifier - developed for aquaculture systems by Dr. Ron Malone, also of LSU's Department of Civil and Environmental Engineering.The hourglass shape defines the three working zones in the bead clarifier - a filtration chamber, a washing throat, and an expansion chamber. Water enters the filter at the bottom of the lower chamber and is moved upward by natural hydraulic head pressure through the washing throat and a bed of one-millimeter plastic beads in the filtration chamber. The packed bed of beads captures varying sizes of solid particles while the surface of each bead attracts dissolved or suspended pollutants in the water. The "cleaned" water moves on through the system and pours out while the solids and pollutants are either consumed or accumulated in the bottom of the clarifier.      

Naturally forming bacteria on the surface of the beads consume much of the waste materials in an aquaculture system but they are not useful for removing heavy metals. Through lab experiment, Sansalone identified an oxide coating and baked it onto the beads' surfaces. The coating helps the beads attract and hold huge quantities of heavy metal ions. "Oxide increases each bead's surface area by about 100 times," Sansalone explained. "This improves adsorption and filtration using a net negative surface charge that attracts heavy metal cations (positively charged ions)."    

  How does this work on an elevated roadway? The filter is attached to a "gutter" system, often built into elevated roadways to channel runoff safely and efficiently. "The hydraulically washed bead system permits large volumes of wastewater to be treated using a relatively compact filter," Sansalone said. As storm water accumulates and flows in the gutter system, it develops a hydraulic head that moves the water up through the filter system, trapping solid particles and attracting heavy metal cations as the water flows through. The massive capacity to adsorb cations allows these tiny beads to accumulate a large volume of various heavy metals and solid particles. When the rain stops and hydraulic pressure disappears, a vacuum is created that draws the beads and particles to the bottom and then opens an air-inlet check valve. The narrow constriction in the hourglass shape plus bubbles created by the air mixing in the remaining water "scrub" the beads, and all waste is drawn down into a particle sludge at the bottom of the clarifier.Once all the beads have passed down into the lower expansion chamber, the valves readjust to the filtration mode and, when the rain comes, water again carries the beads into the upper chamber and filtration begins. "When a filter like this is permanently installed on an elevated roadway, a technician will periodically have to empty out this sludge," Sansalone observed.     

 This concept is currently being tested on a 600-meter elevated portion of I-10 over City Park Lake in Baton Rouge, Louisiana.Two three-foot tall filters and associated equipment are located on a retaining wall under an overpass and surrounded by a chain link fence.One filter has coated beads, one uncoated.At the site, Sansalone and research associates Brent Duet and Jonathan Hird are collecting and analyzing runoff samples before and immediately after the clarifier to determine the filter's and the bead coating's effectiveness. The Louisiana Water Resources Research Institute, the Louisiana Transportation Research Institute Center, Louisiana Sea Grant and the US EPA are all supporting this research. Eventually, if the results are positive, Sansalone will refine the design so that the filter will be incorporated directly into a roadway during construction. "We want to create infrastructure that is part of the solution rather than part of the problem," he said.