River's Years-Old PCB Contamination Removed
September 29, 2009
In 1969, employees working at a dam in Oregon disposed of three old electrical capacitors by dumping them into a nearby river. Each capacitor contained 10 to 12 gal of oil heavily laden with polychlorinated biphenyls. Over the years, the capacitors corroded, leaking PCBs and contaminating sediment over several acres in and along the river.
An agency and consultants conducted a remediation study, concluding that the pollutants would be most efficiently removed by diver-assisted suction-dredging at a cost of $1.9 million. Its proposal for the job included:
- Divers with suction dredge-pipe to remove PCB-tainted sediment.
- Sediment piped into treatment and filtration system staged on three barges tied side-to-side.
- 13 tanks to dewater the dredge stream.
- Decanted fluid pumped through sand filter and activated carbon vessel to remove PCBs prior to direct the discharge back into the river.
- Completion within 45 days to avoid interfering with upcoming salmon spawning season.
Rain for Rent offered a new set of technical specifications, which had to be approved by the project's consultants, the U.S. Army Corps of Engineers, several state and local water quality regulators, U.S. Fish and Wildlife Service, and local Native American associations.
With project approval obtained, Rain for Rent set up two DV-100 4" Power Prime pumps: one to pump silt at 400 gal/min into 13 dewatering boxes for primary sediment settling, and one to pump muddy water into three bilevel tanks. Due to the small particle size and the high solids concentration, a Chitosan flocculent injection system was used to accelerate settling.
The flow was pumped into three weir tanks for additional settling. A 6" DV-150 sound-attenuated Power Prime pump, controlled by a radar gauge, was mounted to a weir tank, pumping the flow into a portable water quality monitoring system. This system was equipped with an AnDru box, which monitored turbidity and recirculated the flow if the discharge was above the established 400 NTU. The next step in the filtration process included one 48-4 sand filter, one BF-1000 with five micron bags, one PF-1000 with one micron cartridge, and a 5,000 lb activated-carbon vessel. The 500 gal/min flow was again monitored for turbidity prior to final discharge.
Rain for Rent completed the entire dredging and filtration project within the 45-day time limit, and the salmon-spawning season was uninterrupted.