Since 1967, San Diego Galvanizing has been hot-dipping structural steel, pipe and railings in zinc. Product applications include commercial and residential ranging from shipyards, bridges and highways, to residential utilities.
The galvanizing process requires dipping parts into a galvanizing kettle that holds 110 tons of molten zinc. A fan draws about 80 percent of the zinc chloride and ammonium chloride fumes coming off the galvanizing bath into a 3-ft. diameter duct inlet where most of the zinc is removed by a wet scrubber. Some fugitive emissions containing zinc escape onto the roof where they settle, or are swept out of the air into stormwater runoff.
To comply with air pollution control rules in the mid-1980s, San Diego Galvanizing installed a wet scrubber. With the onset of National Pollution Discharge Elimination System (NPDES) permit requirements in 1999, the company initiated stormwater sampling. Initial stormwater samples showed 10 to 20 milligrams per liter of zinc. The source of the zinc was, however, uncertain. An environmental compliance consultant for the company, Mike Cairns of PBS&J Engineering (San Diego), explained that the company’s stormwater runoff compliance goal for zinc was 0.2 milligrams per liter (mg/l).
“The NPDES General Industrial Permit in San Diego does not have numeric effluent limits at this time,” Cairns said, “and the San Diego Basin Plan only addresses zinc with regard to drinking water standards. However, the California Ocean Plan lists an instantaneous maximum concentration water quality objective of 0.2 mg/l zinc. The San Diego Regional Water Quality Control Board has directed that this guideline shall serve as the stormwater pollutant discharge goal for San Diego Galvanizing.” The facility also has responsibility for monitoring total suspended solids (TSS), oil and grease, nitrites, specific conductance and pH.
Further sample analysis by PBS&J revealed the facility’s roof downspouts as the principle source of zinc in stormwater runoff.
Learning that, management decided to install holding tanks to collect stormwater runoff from the roof, and then recycle the stormwater back into the galvanizing process, as was being done with the scrubber condensate. Two surplus 3000-gallon tanks were fixed to the gutters and downspouts in the facility’s front yard and back alley. “But we found that managing that water was very difficult,” said Lewis Wise, vice-president of San Diego Galvanizing. “Because the tanks weren’t large enough to handle consecutive storm events, while we were consuming water collected from the first event, the second event would quickly exceed tank capacity. In any case, sampling during both normal and overflow conditions was still in the range of 5-10 milligrams of zinc per liter.”
“We hadn’t been cited, but we heard concern from the Regional Water Board, the county, and then the city,” said Wise. “Mike Cairns had already prepared a stormwater pollution prevention plan, and was looking to cost effectively reduce zinc pollution in our runoff. He’d heard that Contech. has products to achieve further abatement of zinc, so we contacted them.”
The engineer determined that the first step for the zinc problem was installing holding tanks to collect stormwater for reuse in the galvanizing process. When managing the water proved difficult, and zinc reductions at the outfall were still inadequate, the company added DownSpout StormFilters from Contech. This was done to treat stormwater via gravity from tank overflow pipes.
A two-stage filter system was placed near the holding tank at two locations. Each DownSpout StormFilter consists of a two-stage unit installed with stormwater holding tanks. They are customarily installed with two cartridges, allowing treatment for runoff from rooftops up to 15,000 square feet.
Installing new DownSpout StormFilters in the front yard and the back alley dramatically reduced zinc content in the stormwater. Initial sampling showed an 87 percent reduction.
The filter systems are easily maintained and readily upgradable by changing to new media cartridges during maintenance. The number of filter units and cartridge configurations are tailored to site requirements. For applications requiring a higher degree of treatment, such as this one, two-stage units were ideal.
Inflow from each holding tank spills into an overflow pipe that empties onto the open grate cover of the first stage of a two-stage filtration unit. Filtrate from the first stage is piped through the sealed top of the second filtration stage for final treatment. Both the first and second stages of the treatment system use MetalRx™ high-capacity organic media designed to achieve a high level of dissolved metals removal. The system is gravity actuated and requires no power or chemicals to operate.
Based on initial test results with the DownSpout StormFIlter passive treatment system in place, the facility may see effluent levels even lower than the goal set by the Regional Board.
The inflow to the filter measured 2.5 milligrams per liter of zinc, and the discharge showed 0.33 milligrams per liter, a reduction of 87 percent. On another occasion at the outfall, the company found a reading of 0.18 milligrams per liter.
“These results are very encouraging, since we are now close to or below compliance levels,” said Wise. “We had researched five other technologies for stormwater filtration before finding this one to be the most effective and the most cost-effective. We are confident that we are using the ‘best available’ pollution control technology.” The DownSpout StormFilters have been in service for more than two years and have yet to require maintenance.