Dam Cleanup of PCB Contaminated Sediment

CUSTOMER PROBLEM:

In 1969, a government agency’s staff working at a dam disposed of three old electrical capacitors by dumping them into a river. Each capacitor contained between 10 and 12 gallons of oil heavily laden with PCBs. The result was PCB-contaminated sediment covering over an area of several acres along the river bank adjacent to the dam. The agency’s consultants conducted a study to determine the best way to address this problem and determined a diver assisted suction dredging operation was the best removal method.

In the Fall of 2007 they spent $1.9 million to clean up the sediment. The dredge operation was to include divers, working 3 ½ -hour shifts, armed with a 4-inch suction dredge pipe removing PCB-tainted sediment. The muddy water is piped into a treatment system; spanning three barges tied side-to-side that removes PCBs. The initial dewatering specification called for just 13 tanks for dewatering the dredge stream, and then decanted fluid was to be pumped into a sand filter then to activated carbon vessels to remove the PCB prior to direct river discharge. In addition, the operation was to take no longer then 45 days due to salmon spawning season.

RFR SOLUTION:

We responded to the bid request from the engineering company, with a series of relevant technical concerns that suggested the initial dewatering plan would not accomplish the required effluent water quality within the time scheduled. The initial treatment system would not allow continue dredge operation due to the slow settling time of the solids and PCB’s in the tanks.

The Rain for Rent team performed field jar test (tow & shows), brown-bag presentations, and evaluated engineering/geotechnical test data. With the assistance of their RFR filtration system representative, the team was able to rewrite the technical specifications to offer a better technical solution. They had to sell the improved system design to the engineering consultants, US Army Corps of Engineers staff, several State and local Water Quality Regulators, US Fish & Wildlife, and associated Native American associations.

The Rain for Rent technical solution included the use of 13 dewatering boxes for primary sediment settling. A DV-100 was then used to pump into three bi-level tanks. Due to the small particle size and the high solids concentration a Chitosan flocculent injection system was used to accelerate settling in three bi-level tanks.

The flow was then pumped into three weir tanks for additional settling. Then a DV150SA controlled by a radar gauge, mounted in a weir tank, and ANDRU Box pumped into the Portable Water Quality Monitoring Box to measure turbidity and re-circulate if the NTU out of the weir tanks was above 400 NTU set point, thus insuring proper settling.

The next filtration step was a 48-4 sand filter, BF-1000 using 5 micron bags, PF-1000 using 1 micron cartridges, and then a 5,000 pound activated carbon vessel. The 500 gpm flow was measured again for turbidity prior to final discharge. Typical effluent was 0.1 NTU and the river background was a 3 NTU.