Active Stormwater Treatment Solutions for a Waterfront Break-Bulk Terminal
Northland Services Inc. operates a 70-acre marine cargo handling facility at Port of Seattle’s Terminal 115 adjacent to the Lower Duwamish Waterway. The facility is the largest dry cargo barge facility on the West Coast, focused on domestic import and export and storage of shipping containers and break bulk materials. Stormwater runoff from the site is regulated under the State of Washington Industrial Stormwater General Permit (ISGP) and like many the other industrial facilities in Washington, triggered stormwater treatment. Runoff from paved and unpaved portions of the facility were measured to contain elevated levels of metals and very fine particulate very difficult to prevent from entering storm drains.
To determine an appropriate stormwater management approach, four different treatment alternatives were evaluated, including biofiltration, enhanced media filtration, chitosan-enhanced sand filtration (CESF), and electrocoagulation. Ultimately, Northland selected and implemented a CESF treatment system for its competitive cost and minimal required footprint required for treatment, and positive bench scale test results. Both temporary (interim) and permanent full-scale treatment systems were implemented with the goal of treating stormwater runoff to the degree necessary for meeting ISGP benchmarks.
The interim treatment system was designed to treat 400 gallons per minute (GPM) from a 10-acre unpaved area with all components intended to be reused in the full-scale system. In addition, the interim stormwater collection and conveyance piping were designed and constructed so no permanent modifications to existing infrastructure were necessary to bring the interim system online. The full-scale treatment approach includes two treatment systems (Basin 3 and Basin 4) to capture the entire 44-acre drainage area. Basin 3 treatment system is designed for 1,200 GPM and the Basin 4 system capable of treating an additional 725 GPM.
It should be noted that significant conveyance revisions were necessary to convey stormwater runoff to the two systems for treatment including flow splitter structures, pumps, associated valves, influent and effluent piping, etc. It is critical for site owners/operators to consider conveyance and other site improvements that may be necessary as part of the selection process for any treatment system as, in many cases, the costs of conveyance system improvements can be far greater that the cost of the treatment systems themselves.
Treatment system construction was substantially complete by the end of 2014 and quarterly sampling results for Basin 3 and Basin 4 CESF treatment systems have proven to be well below discharge benchmarks for all ISGP required parameters.