New Option For Stormwater Treatment Maximizes Usable Land Area For Redevelopment

In June 2002, construction started on a 15,000 square foot office building complex in Vancouver, Wash., known as First Investments Plaza. Owned by First Independent Bank, the site was already home to one of its regional branches. The majority of the new office space in the complex would be leased out, but the Bank also planned to use some of the space to consolidate its banking and administrative services in one location. In addition to a new building, access to the site would be improved and additional parking would be added to accommodate the increased traffic.

During the design phase, engineers from Mackay & Sposito, Vancouver, Wash., recognized that a bioinfiltration swale on the west side of the existing site was taking up approximately 2750 square feet of buildable land. Trying to maximize the available land space and keep total costs down, they determined that the most economical solution would be to replace the swale with an alternative underground system that would both free-up above-ground area and treat runoff from the site's soon-to-be-increased impervious surfaces.

Ultimately, the engineers selected an underground stormwater treatment system that could be easily installed, offered a high-level of pollutant removal, met local jurisdictional requirements, and allowed them to replace the swale with seventeen much needed parking spaces.

Options for stormwater treatment
The existing biofiltration swale was already the smallest of the natural treatment systems on the approved Best Management Practice (BMP) list maintained by the City of Vancouver, Wash., the governing jurisdictional authority for the property. Other approved natural systems, such as wet ponds or filter strips, would require one-and-a-half to four times as much land area in order to provide the same level of treatment. Because the engineers wanted to maximize parking on the site, these above-ground systems did not meet design requirements. More feasible options on the BMP list included two underground systems, a sand filter and a cartridge filter that utilized a compost-based media to remove pollutants from site runoff.

"The swale was taking up so much room," commented Tadeusz "TZ" Biegien, PE, Engineering Manager, Associate, at Mackay & Sposito. "We had to go with an underground system. With the need for extra parking, and the other site design requirements, there was no other way."

While the sand filter did offer a higher level of treatment than the swale, installation and maintenance would be complicated. The basic operation of a sand filter requires water to percolate through a horizontal bed of sand that filters out pollutants, including sediments and oil and grease. However, the process of sand filtration is very slow, and therefore the installed system must be sized to hold a water volume over time to accommodate this extended filtration rate. This is typically accomplished through either a two-part or a single, larger-size system. In the two-part system, water first enters a wet vault that collects the water volume and allows some settling to occur before it directs a regulated flow to a second unit that holds the sand bed. Alternately, in a larger-size system, a single unit is used that can store the total water volume on top of the sand bed. While installed underground, both of these sand filter configurations take up substantial space.

Furthermore, once installed, the system would require regular maintenance to remove polluted sand before it became clogged to a point where it no longer allowed filtration. Typically, sand filter maintenance requires manual removal of the top layer of the sand bed without substantially disturbing or mixing it with the underlying layer. Once removed, the polluted material is lifted to ground-level for disposal, an extremely time-consuming process.

Maintenance was of particular concern for this project because the site was zoned as private property. The City of Vancouver has specific restrictions for installation of stormwater treatment systems on private property, including a requirement that the installed system come with a long-term maintenance program. In addition to the complicated and time-consuming nature of sand filter maintenance, there is currently no designated party that offers maintenance services for sand filters.

The alternative stormwater treatment option being considered, available in the form of a proprietary technology approved by the City of Vancouver and 360 other jurisdictions across the United States as an effective BMP, was the cartridge filter that utilized a compost-based media. Named The Stormwater Management StormFilter®, this passive filtration system, consists of an underground concrete structure that houses rechargeable, media-filled filter cartridges with integrated surface-cleaning mechanism. The system works by passing polluted stormwater runoff through the filter cartridges that are customized to remove site-specific pollutants such as sediments, metals, nutrients, trash and debris, and oil and grease. Having already used this system at other sites, the engineers at Mackay & Sposito knew that integration of the system into the site plans would be straightforward; and the fact that the system could be customized to target specific pollutants reconfirmed that it would provide the necessary level of treatment for parking lot runoff.

"In the past we had tried designing sand filters into our designs, but the systems ended up being so large and expensive," said Biegien. "We had previously used the StormFilter and were starting to use it in our projects on a regular basis. Because the systems are so small, they are really better than a large sand filter." System maintenance, recommended once a year, would also be simple, requiring removal of collected sediment and debris from the structure's floor and replacement of the filter cartridges, which easily unscrew from the under-drain manifold cast into the vault floor, when necessary. The system manufacturer, Stormwater Management Inc., Portland, Ore., also offered a full line of maintenance services, including a long-term maintenance program.

"Because of the City and County maintenance requirements, the StormFilter was really our only choice," said Biegien. "Because of the maintenance program, it was easy for us to get approval to install this system."

To treat a design water quality flow rate of 0.5 cfs from the site, an 8' x 16' precast filter system with 16 compost-media cartridges was required. The structure could be designed with H-20 rated manhole lids, allowing it to be installed in a traffic area directly under a parking space. The manhole lids provide easy access for inspections and maintenance. As well, each cartridge's integrated surface-cleaning mechanism, activated when the water level in the vault reaches a certain level on the cartridge hood, would help to maintain the permeability of the filter surface and enhance the overall performance and longevity of the system.

Reviewing their options, the engineers selected the cartridge filter system for the site, as it would increase the buildable land area, was the most straightforward system to install, and met the City's maintenance requirements.

Installation
Once specified on the site plans and approved for use by the City of Vancouver, George Schmid & Sons, Inc., Washougal, Wash., the subcontractor on the project, handled the construction and installation of the filter system. They purchased the filter unit directly from Stormwater Management who provided them with system shop drawings for review and approval. Upon receipt of approved shop drawings in late September 2002, Utility Vault, Wilsonville, Ore., was authorized to proceed with fabrication of the precast vault.

This process wasn't new to Jim Schmid, Project Manager for George Schmid & Sons, who has installed many other vault and catch basin systems designed by Stormwater Management. "We've used StormFilters all over the county. We use them in subdivisions and in development for retail or commercial, or office buildings, where you're trying to save space," remarked Jim. "This is actually the second or third one we've used where there was an existing system that we took out and replaced with the filter vault."

With delivery of the completed unit four to six weeks out, George Schmid & Sons prepared to replace the swale. Existing surface runoff still required treatment during construction; so, the swale could not be filled immediately. However, the swale had been directing treated water to a downstream dry well that was located where the new filter system would be installed. To address this complication, a secondary dry well was installed nearby to collect the runoff coming from the swale, allowing the existing dry well to be removed and excavation for the filter system to begin.

George Schmid & Sons performed the excavation using a 325 Caterpillar Excavator. An additional challenge of the site was the depth of the existing conveyance pipes that would need to tie into the new treatment system. The existing swale was already four to five feet deep, and the vault itself was approximately seven feet deep. Therefore, in order to set the system at the required elevation, the total area requiring excavation was roughly 12 feet deep.

The completed vault was delivered in November 2002 directly to the job site, where George Schmid & Sons used a crane to set the unit in place. Existing inlet and outlet conveyance pipes were connected to the vault, and all joints between vault sections and pipe penetrations were grouted. Finally, the Caterpillar was used to backfill around the vault. The entire installation took just one day.

To prevent clogging of the filter system with construction debris, the filter cartridges were not installed in the system until the remaining site construction was completed, including construction of the building pad and addition of the parking lot. "We had temporary systems in place to keep construction debris out of the catch basins and the vault itself to prevent plugging," said Schmid.

Because the swale was still being used during construction, paving had to be completed in two sections. First, the area around the filter vault was paved; then, the swale area was filled with clean granular material, graded, and paved. In June 2003, site construction was completed, and the compost media filter cartridges were installed, putting the system into operation.

Jim Schmid estimates that the total cost for the purchase and installation of the system was $35,030, including $27,150 for the vault and the remaining cost for relocation of the dry well, excavation, placement and tie-in to the drainage system, and removal of sod and filling of the swale. Though, Schmid sees the trade-off of more land area for development as a value to property owners that counters the cost. "The main benefit of the filter system is that it takes up no room because you can bury it, drive over it…with the swale, you lose valuable property. Being able to reclaim and reuse this property by installing an underground system, really offsets part of the system cost," commented Schmid.

Current Operation
The new building at First Investments Plaza is now open and operating, and tenants and passers-by are enjoying the benefits of all the work that went into the project. With the addition of the new facility and parking lot, new landscaping was added and traffic flow to the bank has been improved.

"The new building is really a ‘show piece'," said Richard Wehring, Purchasing and Facilities Manager for First Investments Bank. "People are really impressed with the look of the new building. The construction also allowed us to put in a new entry on the West side of the property. This, combined with the additional paved areas, has really helped reduce traffic and has opened up the roadway."

Now operating for almost a year, the system's first annual maintenance is coming up in June, but Wehring feels the maintenance won't present him with any challenges. "We have another Stormwater Management system installed out at our Camas, Wash., branch, that's been there for the last four or five years. We've had no problems with that; so, I don't expect any problems with this system."

The project was also beneficial for engineers at Mackay & Sposito. Near completion of the project at First Investments Plaza, the engineers began work on the property adjacent to the west side of the Bank. This site also had a swale, though of a different design where the swale was recessed two to three feet deep between retaining walls. The original site redesign included construction of a concrete sidewalk parallel to the exiting swale. For safety reasons, this would require the addition of some fencing between the sidewalk and swale. To address safety concerns and site aesthetics, Mackay & Sposito recommended that the property owners follow the example of First Investments Plaza and install an underground StormFilter. Thanks to their experience at the Bank, the engineers were able to show the property owners the direct value of replacing the swale with an underground filter system, gaining approval from the City for this other project was straightforward, and, by having George Schmid & Sons do the construction, installation went smoothly.

Source: Stormwater360