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Oceanside's Department of Harbor and Beaches pays Bio Clean Environmental Services Inc. $500/month to maintain its “modular wetland.” Photo: Zach Kent

Oceanside Harbor in Oceanside, Calif., is home to a marina lined with fine restaurants, a renowned yacht club, tourists, and locals ranging from surfers to families. As a key destination for those looking for sand, surf, and scenery, it's also a popular stop for boaters making their way along the Pacific coast.

The harbor's 20,000-square-foot boat ramp area includes a two-lane washing station and two waste dumps for RVs and boats. In the summer, the constant washing of boats and jet skis dumps up to 4,000 gallons of nuisance flows into the harbor daily.

To combat the pollutants generated by all that wash water, the city's Department of Harbor and Beaches installed an oil-water separator to capture and treat runoff before it is discharged into the harbor. But while the system removes sediment and hydrocarbons, it doesn't weed out bacteria and dissolved metals.

“The wash-down area is very different from traditional stormwater treatment situations,” says Mo Lahsaie, Clean Water Program coordinator for the Oceanside Water Utilities Department. Even with the dumping stations, “a small amount of wastewater still finds its way into the storm drain. This wastewater can contain high amounts of bacteria that finds its way to the harbor.”

Few water treatment systems are designed to deal with continuous nuisance flows in addition to high concentrations of bacteria, so the department looked at biological filtration. Bioretention systems are effective in traditional treatment situations, but they need to drain to maintain their aerobic treatment zones, making them ineffective for treating continuous nuisance flows.

One biological treatment system, how ever, seemed to be effective at both stormwater and wastewater treatment: subsurface flow gravel wetlands, in which the amount of water is reduced as it travels through a rock and gravel bed. Contaminated water travels horizontally through a contained wetland cell that's filled with gravel, sorptive filtration media, and native plants. The combination of physical and biological processes captures, removes, and transforms pollutants. Used for hundreds of years to passively treat wastewater, the concept has been used since the early 1990s to manage stormwater runoff.

In April the city installed a hybrid treatment system that incorporates subsurface flow gravel wetlands with screening, hydrodynamic separation, and media filtration. Similar to a wastewater treatment plant's multistep “treatment train,” in which each stage removes different pollutants, runoff flows through the three pretreatment stages before entering the wetland.

At the harbor, wash water enters a screening device to capture trash and debris, and then enters a separation chamber that allows sediment to settle. From there, it passes through a perimeter filter that contains a specially engineered mix of inert oxides that removes particulate pollutants. The pretreated water then enters the wetland chamber, which removes the remaining pollutants.

Finally, the filtered water is drained to the middle of the harbor through an existing discharge pipe.

Department managers estimate the harbor is exposed to more oil and grease in one week than a typical street is in an entire year. The system's screening baskets fill up in a few weeks, and it takes just a month for the sediment chamber to accumulate a few hundred pounds of pollutant-laden sediment. Typically saturated with about 80% hydrocarbons, the perimeter filter's media are replaced monthly.

The project costs approximately $28,000: $21,000 for the unit and $7,000 for installation by an outside contractor. In a typical new stormwater setup, the installed cost is about $25,000/acre.

“It's definitely doing what it's supposed to,” Lahsaie says. “It's prevented pollutants from reaching the wetland chamber. This situation fully demonstrates the importance of having a four-stage treatment system. It's a perfect example of nature and technology working together in perfect harmony.”

— Zach Kent is a design and compliance consultant and stormwater specialist with Bio Clean Environmental Services, Inc., in Oceanside, Calif.