- (7) CDS® hydrodynamic separators
California is widely known for its waterways. Millions of tourists and locals flock to these waters every year to swim, play, fish and relax in the sun, resulting in an economic boon for California. Unfortunately, trash generated by human activity on land is washed by rain into gutters and storm drains and makes its way into streams, creeks, rivers and, eventually, the ocean. Cigarette butts, paper, plastic bags, plastic food containers, cans and bottles have all been found in California waters and on beaches following stormwater runoff.
In 2015, the California State Water Board addressed the issue by adopting amendments to their Ocean and Inland Water Plans, commonly known as the “Trash Amendments,” which mandate that trash discharged from every city, county and CalTrans stormwater system is significantly reduced to protect local waterways.
The Bay Area has had trash capture regulations (C10) since 2009. Under C10, San Jose and other Bay Area cities must reduce certain percentages of trash by milestone dates in 2015, 2017, 2018, and 2022. The Trash Amendments require cities to install devices or other measures over a 10-year period beginning in 2019, with 2018 intended to be a year for planning and design.
To tackle the issue of trash, the city of San Jose has implemented a series of initiatives, including cracking down on illegal dumping, shoreline cleanups, ordinances for single use plastic bags and foam food containers, installing new public litter cans, and additional street sweeping.
In the spring of 2017, the City installed nine CDS® hydrodynamic separators from Contech to meet their latest deadline for trash capture. The CDS is a hydrodynamic separator that uses swirl concentration and continuous deflective separation to screen, separate and trap trash, debris, sediment and hydrocarbons from stormwater runoff. CDS captures and retains 100% of floatables and neutrally buoyant debris, effectively removes sediment, and uses a non-blocking screening technology that facilitates easier maintenance.
The city had experience with CDS, having previously installed systems in 15 key locations. After installing the first two CDS units specifically for trash capture in 2010, the City examined the long-term lifecycle costs for using numerous individual catch basin screens compared to a small number of large devices like the CDS. While the CDS units were found to be more expensive initially, their significantly lower maintenance costs and long history of reliability made them the choice for the city’s ongoing trash control efforts.
The city had less than a year to identify installation sites, perform hydraulic analysis, size and design the CDS units, prepare specifications for bid, sign contracts, retrofit their drainage system with the CDS units, and restore the city streets. Contech worked closely with City Public Works engineers and consulting firm BKF throughout the process, and provided hydraulic analysis for multiple scenarios before final designs were agreed upon.
The CDS units included four 12-foot-diameter units with diversion vaults, two twin 12-foot diameter units with diversion vaults, and one 18-foot-diameter unit with a segmented diversion vault. Existing drainage pipes required special designs to prevent upstream flooding. Box culverts were used at two sites instead of standard rectangular diversion vaults to fit within a limited footprint. Total combined flow treated by the nine CDS units is over 550 cfs. Over 5,000 acres of high-density urban San Jose is now protected by CDS units, and trash within these drainage areas will no longer make it into San Francisco Bay.
Most installation activities occurred within a six-week period in April and May of 2017, with two contractors, each working on three sites performing the installations. All installations were retrofits within a crowded urban area. Large trucks, overhead clearance for the cranes, and neighborhood access all had to be managed. One site required partial installation until the electric utility could relocate buried high voltage lines. Following the relocation, the site was re-excavated, and the remaining components were installed.
A Contech Project Coordinator worked to manage all delivery schedules, while a Contech Field Representative was on site providing the contractors support during the installations. The installations required over 60 trucks scheduled across a dozen installation dates, and each installation date changed multiple times. Most loads were oversize with a flag car, which meant they weren’t allowed on the highway in the morning and required six different staging areas throughout the congested city so the trucks could arrive overnight.
Before the trucks could even be loaded, the 38 screen sections had to be fabricated, the 54 fiberglass pieces had to be built, and over 90 concrete pieces had to be poured. Complicating all of this were design changes made during construction, with three of the sites having changes close to the day of installation. Two contractors worked through conflicting schedules to handle the installations. Despite all these challenges, the team met the June 30 deadline, surprising many city officials who thought the goal was not achievable.
- (7) CDS® hydrodynamic separators