This project researched techniques to remove dissolved dispersant from
Ohmsett water after the tank has been used for a series of chemical
dispersant effectiveness experiments. The primary objective of the proposed
study was to develop and test a simple, inexpensive system for expediently
removing dissolved dispersant from Ohmsett tank water. The second objective
of the study was to select and refine an analytical technique for
determining the concentration of dissolved dispersant in the tank salt
Powdered activated carbon (PAC) was determined to be the best answer for the
removal of dissolved dispersant from the tank water after chemical
dispersant tests at Ohmsett. The selected analytical technique was to
measure the interfacial tension of the water against a highly refined
mineral oil (USP or Technical grade) with a DuNouy ring tensiometer
following procedures laid out in ASTM D971.
A prototype treatment system was built and tested following a series of
cold-water dispersant experiments at Ohmsett in March 2003. The system
consisted of three modified ISO shipping containers placed end to end in the
tank, spanning its width. The containers held well-mixed slurry of activated
carbon through which the tank water flowed from South to North under the
influence of the filtration pumps.
The top and both sides of the three containers were cut away. The sides were
replaced with an expanded metal mesh that supported a very fine filter cloth
clamped around the edges of the opening. PVC air piping to bubble the slurry
and maintain the PAC in suspension was laid along the bottom of the
container. An additional air pipe was placed directly at the bottom of the
downstream, or exit, side to provide agitation to that filter cloth to
ensure is did not become clogged by a cake of PAC. A square was cut in the
bottom of the container and covered with mesh and cloth to permit the
container to drain completely when removed from the tank. The intervening
spaces between the ends of the containers were sealed with rubber wipers.
Each container initially had 200 pounds of PAC added to it, and then the air
bubbler system was activated. The head created by the filter pump moving
water from the North end of the tank to the South end forced water through
the filter cloth on the upstream (South) side of the containers and out the
downstream (North) side at an average velocity of 0.5 ft/min. In the 15
minutes that the water spent inside the 8-foot wide containers it was
thoroughly mixed with the PAC slurry. A second 200 pounds of PAC was added
to each container on the second day of operation, for a total of 1200 lbs.
The system was operated unattended for 72 hours.
Grab samples of the tank water taken before and after installation of the
prototype system showed that it reduced the concentration of Corexit 9527
dispersant in the water (about 20 gallons had been sprayed during the
testing program) from a range of 5.5 to 9 ppm before to below the detection
limit (1 ppm) after. Combining the three to five days required to operate
the leaf filter to reduce dispersed oil concentrations to below 10 ppm) and
a two- to three-day period of treatment with PAC, it should be possible to
return the tank to a quality that meets the standard for equipment testing
in one week following a dispersant test series.