The rainbow effect caused by varying thicknesses of oil film on water’s surface might be pretty to look at but is indicative of polluted water. This “oil sheen” proves especially difficult to remove, even when the water is aerated with ozone or filtered through sand. But 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.
The new process created by Andy Hong, a professor of civil and environmental engineering at the University of Utah, uses two existing technologies – ozone aeration and sand filtration – but significantly changes the former method. Instead of attempting to turn the entire hydrocarbon (oil) content in the water into carbon dioxide and water by just bubbling ozone through polluted water, the new process converts it into a form that can be retained by sand filtration.
To achieve this 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.
Compared with larger bubbles from normal ozone aeration, the tiny bubbles provide much more surface area for the oxygen in ozone to react chemically with the oil. Hong says pollutants tend to accumulate on the bubbles because they are not very water-soluble and 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 a sheen – into acids and chemicals known as aldehydes and ketones. Most of those substances, in turn, help the remaining oil droplets to clump together so they can be removed by conventional sand filtration, he adds.
Hong says his method could be used to clean a variety of pollutants in water, including refinery wastewater and oil spills at refineries or on waterways where the spill could be vacuumed, and then treated on-site or on a barge. Hong also says so-called "produced water" from oil and gas drilling sites on land, which normally is re-injected underground, could instead be treated and put to beneficial uses, such as irrigation, especially in arid regions where oil and gas tend to be produced.
The process could also be used to clean contaminated soil says Hong. Soil contaminated with polychlorinated biphenyls (PCBs, from electrical transformers) or polycyclic aromatic hydrocarbons (PAHs, from fuel burning) would be mixed into slurry, and then treated with the new method. Meanwhile, soil contaminated with heavy metals could be treated by replacing ozone with air and metal-grabbing agents, which would be pressurized with a slurry of the contaminated material.
In his study, Hong showed the new method not only removed oil sheen, but also left the treated water so that any remaining acids, aldehydes and ketones were more vulnerable to being biodegraded by pollution-eating microbes.
"These are much more biodegradable than the parent compounds," he says.
Hong 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. Although the water is clean enough to be discharged after the ozonation and sand filtration, he says that some pollution sources may want to use conventional methods to biodegrade remaining dissolved organic material.
Having achieved 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 200L (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.