Small amounts of oil leave a fluorescent sheen on polluted water.
Oil sheen is hard to remove, even when the water is aerated with
ozone or filtered through sand. Now, a University of Utah engineer
has developed an inexpensive new method to remove oil sheen by
repeatedly pressurizing and depressurizing ozone gas, creating
microscopic bubbles that attack the oil so it can be removed by sand
filters.
"We are not trying to treat the entire hydrocarbon [oil] content
in the water - to turn it into carbon dioxide and water - but we are
converting it into a form that can be retained by sand filtration,
which is a conventional and economical process," says Andy Hong, a
professor of civil and environmental engineering.
In laboratory experiments reported online this week in the
journal Chemosphere, Hong demonstrated that "pressure-assisted
ozonation and sand filtration" effectively removes oil droplets
dispersed in water, indicating it could be used to prevent oil sheen
from wastewater discharged into coastal waters.
Hong says the method - for which patents are pending - also could
be used to clean a variety of pollutants in water and even soil,
including:
- So-called "produced water" from oil and gas drilling sites
on land. Such oily water normally is re-injected underground.
"If we have technology to clean it, it could be put into
beneficial uses, such as irrigation, especially in arid regions
where oil and gas tend to be produced," says Hong.
- Water from mining of tar sands and oil shale.
- Groundwater contaminated by MTBE, a gasoline additive that
reduces harmful vehicle emissions but pollutes water due to
leaking underground gasoline storage tanks.
- "Emerging contaminants," such as wastewater polluted with
medications and personal care products.
- Soil contaminated with polychlorinated biphenyls (PCBs, from
electrical transformers) or polycyclic aromatic hydrocarbons (PAHs,
from fuel burning). Water and contaminated soil would be mixed
into slurry, and then treated with the new method.
- Heavy metals in soil. Instead of ozone, air and
metal-grabbing chelating agents would be pressurized with a
slurry of the contaminated material.
- Refinery wastewater and oil spills at refineries or on
waterways. The spill could be vacuumed, and then treated with
the new method on-site or on a barge.
Hong conducted the study with two University of Utah doctoral
students - Zhixiong Cha, who has earned his Ph.D., and Chia-Jung
Cheng - and with Cheng-Fang Lin, an environmental engineering
professor at National Taiwan University.
Zapping Oily Water with Microbubbles from Ozone under Pressure
Hong says his method uses two existing technologies - ozone aeration
and sand filtration - and adds a big change to the former. Instead
of just bubbling ozone through polluted water, Hong uses repeated
cycles of pressurization of ozone and dirty water so the ozone
saturates the water, followed by depressurization so the ozone
expands into numerous microbubbles in the polluted water, similar to
the way a carbonated beverage foams and overflows if opened quickly.
The tiny bubbles provide much more surface area - compared with
larger bubbles from normal ozone aeration - for the oxygen in ozone
to react chemically with oil. Hong says pollutants tend to
accumulate on the bubbles because they are not very water-soluble.
The ozone in the bubble attacks certain pollutants because it is a
strong oxidant. The reactions convert most of the dispersed oil
droplets - which float on water to cause sheen - into acids and
chemicals known as aldehydes and ketones. Most of those substances,
in turn, help the remaining oil droplets clump together so they can
be removed by conventional sand filtration, he adds.
In his study, Hong showed the new method not only removes oil
sheen, but also leaves the treated water so that any remaining
acids, aldehydes and ketones are more vulnerable to being
biodegraded by pollution-eating microbes.
"These are much more biodegradable than the parent compounds," he
says.
Hong says the water is clean enough to be discharged after the
ozonation and sand filtration, but that some pollution sources may
want to use conventional methods to biodegrade remaining dissolved
organic material.
Details of the Experiments
Hong conducted his experiments using a tabletop chemical reactor
that contained about a quart of oily water made by mixing deionized
water with crude oil from the Rangely oil field in northwestern
Colorado.
Ozone was produced by passing dry air through a high-voltage
field, converting oxygen gas, which has two oxygen atoms, into
ozone, which has three.
The ozone was pressurized to 10 times atmospheric pressure, about
150 pounds per square inch, which compares with inflation pressures
of about 100 PSI for Hong's bicycle and 35 to 40 PSI for many
automobile tires.
He found oily water was cleaned most effectively by pressurizing
and depressurizing it with ozone gas 10 times, then filtering it
through sand, then putting the water through 20 more pressurized
ozone cycles, and then filtering it again through sand. That was at
flow rates of 10 to 20 liters per minute [about 2.6 to 5.3 U.S.
gallons per minute] in his laboratory apparatus.
Hong tested how well the ozonation worked by measuring chemical
and biological oxygen demands of treated water samples. Both
indirectly measure organic contents in the water. Hong also used
mass spectrometry to identify what contaminants remained in the
water.
He found that his most effective procedure removed 99 percent of
the turbidity from the "produced water" - leaving it almost as clear
as drinking water - and removed 83 percent of the oil, converting
the rest to dissolved organic acids removable by biodegradation.
A Tryout in China
With success in the laboratory, Hong now plans for larger-scale
pilot tests.
"It is economical and it can be scaled up," he says.
One such test will be done in Wuxi, China, where a prototype
desk-sized device capable of treating 200 liters [53 U.S. gallons]
at a time will be tested at three to five polluted industrial sites
that the government vacated for redevelopment, Hong says.
Meanwhile, the University of Utah Research Foundation has entered
into options to license the technology to Miracotech, Inc., of
Albany, Calif., and 7Rev, L.P., a Salt Lake City venture capital
group, so the companies can bring the technology to market.
Hong says other methods of treating oil well "produced water"
have met with varying degrees of success. They include centrifuges,
membranes, regular ozonation and air bubbles to float off
contaminants. But all have drawbacks, such as inability to handle
dissolved oil or high levels of oil, or susceptibility to quickly
getting fouled by the oil.
Neither ozonation nor sand filtration alone has been able to
effectively treat oily "produced water." Hong says long-chain oil
molecules don't react with ozone easily without his pressure
treatment. And sand filters alone cannot remove oil.
For more information on the University of Utah College of
Engineering, see http://www.coe.utah.edu
SOURCE: University of Utah