Conclusions - trel02-380038

Conclusions

The principal objectives of the project were to evaluate the efficiency of the

0.6-m (24-in.) MDS in producing a sand fraction with fines and PCB

concentrations sufficiently reduced to permit beneficial use, and to evaluate the

correspondence of contaminant levels predicted by bench-scale testing versus

field-scale operation. The target product (sand) specifications were less than 10

percent fines by mass and PCB concentrations less than 1 mg/kg. The underflow

fraction produced averaged over 92 percent sand, as measured by a Coulter

counter, and 0.144 mg/kg PCB 1242 and 0.0119 mg/kg PCB 1260. Based on

statistical analysis of the results, the contaminant concentrations predicted for the

sand fraction by the bench-scale testing were essentially equivalent to that

achieved in the field operation. This is particularly significant since the process

feed concentration of PCB 1260 was statistically greater than the bulk sediment

concentration for the bench-scale testing. This indicates a somewhat higher

efficiency of PCB removal for the MDS compared with that of wet sieving of the

material. This may be attributable to the presence of coarse organic particles,

which would report with the sand on a wet sieve, but would report with the

overflow of a hydrocyclone. This is supported by the higher TOC concentration

measured in the process overflow compared with that of the silt/clay fraction of

the bulk sediment used in bench-scale testing. Distribution of metals was

somewhat more variable than for PCBs, but metals were reduced by an order of

magnitude in the sand fraction, with the exception of selenium, silver, and

molybdenum. Selenium, silver, and molybdenum were reduced by factors of 2.6

to 7.8. In the absence of specific criteria establishing acceptable levels of metal

constituents, partitioning theory could be used to evaluate the magnitude of

potential release of metals in the beneficial use environment. Predicted releases

could then be compared with applicable water quality criteria and necessary

dilutions estimated.

Chapter 3 Conclusions