Compact filtration system treats Surf City stormwater

Huntington Beach, Calif., internationally known as Surf City, showcases miles of beautiful beachfront, a world-renowned pier, lavish restaurants, and an array of fine arts and cultural activities. With more than 200,000 residents and 11 million visitors a year, city officials realized that expansion was inevitable.

Heil Avenue widening and stormwater treatment, Huntington Beach, Calif.

Product application
Modular Wetlands stormwater filtration system fits constrained space in a retrofit application.

Part of that expansion was the Heil Avenue Widening, CC-1230 Project. The widened street would provide one additional lane in each direction from Beach Boulevard to Silver Lane. However, runoff from the existing and additional lanes would need to be treated, and the city wanted to exceed federal and state water quality requirements. It was at this time that the city’s civil engineers began researching natural stormwater filtration systems. In February 2008, the city of Huntington Beach contacted Modular Wetlands Stormwater Engineer Zach Kent to discuss possible retrofit options.

The challenge
The Heil Avenue Widening Project, like many other retrofit projects, faced one common challenge: space constraints. The existing catch basin for the surrounding streets is located on the northeast corner of Heil Avenue and Silver Lane. This catch basin collects runoff from Heil Avenue and surrounding streets. The city needed a natural stormwater filtration system that would allow it to continue using the existing catch basin. The only possible space to integrate a system would be on Silver Lane behind the existing catch basin and sidewalk. The city was concerned with treating metals, oil and grease, oxygen-demanding substances, sediment, and trash and debris. A single system would need to address all of these issues while satisfying the space constraint.

The 22-foot-long by 5-foot-wide, vault-type Modular Wetland System-Linear fits Huntington Beach’s space constraints and compliments the existing catch basin.

With such limited space, and the need for a natural stormwater filtration system, the city’s stormwater department turned to Modular Wetlands for assistance. Because the Modular Wetland System-Linear (MWS-Linear) is only 22 feet long and 5 feet wide, its vault-type system would compliment the existing catch basin.

“We worked closely with the city on all aspects of this project,” said Kent. “We assisted them with the design and drawings, as well as provided guidance to the contractors at the time of the install.”

Polluted runoff enters the existing catch basin. It is then diverted via a trough that directs the runoff into a pipe that enters the MWS-Linear. Low flows continue through the Modular Wetland System then back into the catch basin where the clean water is discharged. The first flush and low flows are diverted to the system while high flows bypass over the trough and proceed into the catch basin’s outflow pipe.

The MWS-Linear provides multiple stages of treatment. Sediment, organic matter, total suspended solids (TSS), metals, nutrients, and bacteria are treated. Before polluted runoff enters the last stage of treatment — a subsurface flow wetland — it is processed through two pretreatment stages: hydrodynamic separation and media filtration. The hydrodynamic separation allows large sediments and TSS to settle out. The runoff is then directed through a perimeter filter, which contains BioMediaGREEN filter media. This media physically, chemically, and biologically removes a wide array of pollutants including dissolved metals and hydrocarbons. Only after being filtered by the pretreatment stages does the polluted runoff enter the subsurface flow wetland.

Installation of the MWS-Linear took place in mid January 2009. “Considering the space constraints, we are quite pleased how it turned out,” said Kent. “We had very limited space and needed our system to connect with the existing catch basin. We were confident that the MWS-Linear would be a perfect fit for this project.” The Modular Wetlands system can treat as much as 120 gallons per minute.

The finished treatment system supports native vegetation that is well suited for stormwater runoff conditions.

From the January installation to present, the native plants used in this system have been thriving. “We always use plants that are aesthetically pleasing and well suited for stormwater runoff conditions,” added Kent. The system was designed with input from various city maintenance crews. The design is meant to allow for simple and fast cleaning by hand or with the use of a vacuum truck. This treatment train concept minimizes maintenance costs because each treatment stage protects the next stage from clogging.

This article was contributed by Modular Wetlands, Oceanside, Calif. (

Posted in Uncategorized | January 29th, 2014 by

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