At the Cedar Creek Operable Unit 2 (CCOU2) Superfund Alternative site in Cedarburg Wisconsin Infrastructure Alternatives Inc. and J.F. Brennan partnered on the project. The TSCA and non-TSCA sediment was treated at the on-site water treatment system.
At the Cedar Creek Operable Unit 2 (CCOU2) Superfund Alternative site in Cedarburg Wisconsin sediment cleanup operations have been underway since November 2016. The contractor team of J.F. Brennan Company Inc. (Brennan) and Infrastructure Alternatives Inc. (IAI) was selected to remove and treat about 70000 cubic yards of sediment containing varying levels of PCBs some at concentrations regulated under the Toxic Substances Control Act (TSCA). The bulk of the material was removed with hydraulic dredges in 2017 with some mechanical dredging completed in the fall of 2016 via excavators from shore. By the end of 2017 all dredging and dewatering work had been completed. Dewatered material hauling and water treatment operations are currently ongoing but winding down.
According to the U.S. EPA the Cedar Creek site encompasses approximately 5 miles of the creek where it flows through the town of Cedarburg just north of Milwaukee and three ponds: Ruck Columbia and Wire and Nail.
Mercury Marine a division of Brunswick Corporation operated a plan on St. John Avenue in Cedarburg from 1951 to 1982. Fluids containing PCBs leaked from equipment into floor drains which emptied into storm sewers and the ponds and creeks that flow into the Milwaukee River. Mercury Marine voluntarily took on the cleanup project and set an aggressive schedule for the work. The site is in the middle of a suburban residential area unlike many typical Superfund sites and the city of Cedarburg wanted the city park restored back to its original condition by summer 2018.
Hydraulic Dredging Operations
Starting in April 2017 two 8-inch swinging ladder hydraulic dredges Brennan’s Palm Beach and Fox River began removing sediment. Although the remedial investigation for the project showed that the sediment contained less TSCA-regulated material throughout the project area than non-TSCA material the TSCA sediment was spread throughout the pond. Paul Olander project manager J.F. Brennan Company said that removing all the TSCA material at once was not feasible. Instead Brennan would need to move between TSCA and non-TSCA sediment relying on IAI’s intricate on-site dewatering system would keep the two sediment streams separate during treatment.
Each time a dredge moved from TSCA material into non-TSCA material it had to undergo a decontamination process. For 30 minutes the dredge pumped clean pond water through the cutterhead flushing the downstream pipeline to clear any residual sediment from the line. When the flushing process was complete the dredge could begin working in non-TSCA material.
The two pipelines that delivered dredged material from the dredges to the landside Sediment Processing Area also had to be managed to ensure no cross contamination. A lock-out-tag-out procedure was utilized by Brennan and IAI to ensure that the material flowing through each pipeline was being treated in the appropriate area of the dewatering facility.
Sediment Processing Flow
All the hydraulically dredged material was processed in geotextile tubes at the landside dewatering facility or Sediment Processing Area (SPA). The Brennan-IAI team sought to remove and process the sediment as quickly as possible by adding capacity and increasing efficiency wherever possible within the process. Cardwell said that with a properly sized geotextile tube dewatering system and good operations the tubes allowed the dredges to continue working with minimal downtime. And stacking the tubes allowed a greater volume of material to be processed in the same footprint.
“So for every gallon of dredge slurry flowing into the dewatering system from the dredges one gallon was processed in the tubes. The sediment portion of that gallon of dredge slurry was retained in the tubes and the water portion of that gallon filtered out of the tubes through pores in the fabric was captured and treated in the on-site water treatment system” said Brent Cardwell IAI project manager. “The water treatment plant was designed to process geotextile tube filtrate water at the same rate that the dredges were sending flow to the dewatering pad as well as providing extra capacity for stormwater treatment.”
The SPA was a 600-foot by 350-foot temporary containment facility constructed over a portion of a community park inside a residential area. The location of the SPA created an even higher awareness of safety and security measures on the job than normal according to Cardwell.
Inside the SPA two separate dewatering pads were constructed: one for TSCA regulated material and one for non-TSCA material. Dredged material entered the SPA via two separate pipelines. TSCA and non-TSCA material was kept separate through the sediment conditioning and dewatering.
IAI said it took a great deal of pre-planning engineering controls and close coordination with Brennan to ensure that each waste stream was delivered to the correct dewatering pad. Hydrographic surveying located the dredged material and once removed TSCA and non-TSCA material was tracked and monitored through the dewatering process.
IAI monitored the treatment process and tested the raw dredge slurry at each point in the treatment process.
Geotextile Tube Dewatering and Sediment Polymers
To fill the geotextile tubes at a specific rate sediment conditioning was required with a cationic emulsion polymer to aid in dewatering. Cardwell said Solve-137 polymer was injected into the dredge slurry pipeline prior to entering the tubes. “That allowed enough time to properly mix the polymer with the sediment to achieve optimum floc” he said.
As the dredge slurry-polymer mixtures flowed through the pipe runs to the dewatering pads the turbulence homogenized the dredge slurry-polymer mixture and increased the size of the floc particles as more solids were attracted to them.
Polymers helped IAI operators control the rate of flow into the geotextile tubes. Here they are testing the dewatered sediment in the tubes.
The composition of the dredge material ranged from small rock and sand to silty organics depending on the location in the ponds. Cardwell said the goal was to build a strong floc so that the solid material would consolidate inside the geotextile tube and separate from the water in the dredge slurry with minimal residual polymer in the separated water.
To deliver and achieve the correct polymer dose for the material being dredged at any given moment IAI utilized equipment to collect real-time measurements of slurry flow and density which were signaled to a PLC system that controlled the output of the polymer injection system. IAI’s operators performed regular checks of the floc formation in the dredge slurry and combined with their observation of the performance of the geotextile tubes and regular communication with Brennan’s dredge crew the operators were able to tweak the target dose setting in the PLC as needed to achieve proper conditioning of the dredge slurry. The operators took grab samples from the slurry header to analyze the flocculation of the sediment before the sediment entered the tubes. “They were looking at the size of the particles of floc as well as the clarity of the separated water. Polymer dosing is controlled by the operator as a result of the observations in the grab samples” Cardwell said.
Treated water was discharged to Cedar Creek.
In the dewatering pads large circumference GEOSTRUX® geotextile tubes were utilized to dewater and contain the dredged material until disposal. The dredge slurry-polymer mixture was directed into the geotextile tubes via a network of HDPE main header pipe hoses to the fill ports of the tubes and flow control valves. A total of 42 geotextile tubes were filled over the course of the project to dewater 70555 cubic yards of material. Each tube was filled over the course of two to four days and ranged in size from 60 to 82.5 feet in circumference and up to 285 feet in length. The geotextile tubes were stacked in four layers over the two dewatering pads.
The design configuration of the geotextile tube stacks was developed by IAI with data supplied by Brennan including: the volume of each class of material; the available space in the SPA; the dredge flow for each class of material; the anticipated solids content of dredge slurry; the total project days of dredging; and the daily hours of dredging.
The geotextile tube stacks were configured with 10 tubes on the first layer in the non-TSCA pad and five tubes on the first layer in the TSCA pad and each layer above included one less tube each with four layers stacked in a pyramid shape.
IAI operators use a protocol of observation and data tracking to evaluate the performance of each geotextile tube as it is filled. Operators note the flocculation thickness and composition of the material flowing into the tube the volume and appearance of water shedding off the exterior of the tube and the rate at which the tube height is increasing. When a geotextile tube reaches the maximum fill height designated by the manufacturer flow to that tube is stopped and the tube is allowed to passively dewater for a period of time. The length of time required for passive dewatering depends mostly on the volume of consolidated material already inside the tube.
This protocol has been developed by IAI over years of utilizing geotextile tubes to dewater contaminated sediment. The company began using geotextile tubes for contaminated sediment projects in 2003 and has processed 3.8 million cubic yards of material in tubes since then. IAI President and CEO Dana Trierweiler P.E. said “We have learned that geotextile tubes are an adaptable process and can handle high dredge flow rates and variations in dredge slurry percent solids chemical constituents and physical characteristics.”
Between the TSCA and non-TSCA dewatering pads a sump collected the water produced by sediment dewatering activities as well as stormwater that was captured in the lined facility during precipitation events. “All the water generated at the site (filtrate from the TSCA geotextile tubes the non-TSCA geotextile tubes and all stormwater) was collected in one sump and treated prior to discharge” Cardwell said. The water treatment plant influent was also monitored for PCBs to confirm very little if any was entering the system. “The discharge limitation for PCBs in the treated water was non-detect or effectively zero and that limit was met throughout the performance of the project” Cardwell said.
The water treatment system was designed to treat a maximum of 3000 gallons per minute enough to process all the geotextile tube filtrate and stormwater efficiently and incorporated a settling process two-step filtration and carbon adsorption to remove both solids and organics. Chemical coagulation and settling plates aided clarification in lamella clarifiers settling out fine sediment particles followed by multi-media filters and bag filters to remove any remaining solids. The final treatment step used granular activated carbon to capture any remaining organic contaminants in the water. Settled solids from the lamella clarifier were returned to the TSCA dewatering pad for treatment in the geotextile tubes.
Discharge from the treatment plant was regulated by a Wisconsin Pollutant Discharge Elimination System (WPDES) permit. IAI provided a skilled operations staff and a Wisconsin licensed Operator of Record to ensure optimal treatment plant operations and permit compliance. The treatment system maintained an excellent record of compliance throughout the project.
Reflecting on the sediment processing work at Cedar Creek IAI’s Trierweiler said “Our crews attacked the challenges of this site from the start developing a strong design for the dewatering and water treatment systems that could handle high dredge production and accurately segregate TSCA and non-TSCA material. They then precisely executed operations and maintenance of those systems. I am always proud of our team in particular when they show how well they can execute work of this complexity.”