Making the Best of Wastewater

Two Mississippi State University researchers are exploring the possibility of extracting oil from microorganisms fattened up in wastewater treatment facilities to produce biodiesel.
By Susanne Retka Schill | September 16, 2008
A feedstock with the potential to produce 10 billion gallons of biodiesel per year is literally under our noses in our nation's wastewater treatment facilities. Results of first-generation studies conducted by two Mississippi State University researchers established that the existing microorganisms in wastewater treatment facilities yield about 7 percent oil from the dried secondary sludge. Chemical engineer Rafael Hernandez and his colleague Todd French, a microbiologist, did the math and found that if 70 percent of the nation's municipal water treatment facilities captured that oil, it could be converted into about 800 million gallons of biodiesel a year. Tapping into industrial wastewater treatment facilities in addition to municipal wastewater, and utilizing microorganisms selected for their oil-producing capabilities could boost biodiesel production to the 10 billion gallon mark, which is more than three times the nation's current biodiesel production capacity.

Primary sludge from wastewater treatment facilities is well known as a source of low-quality fats and grease that can be removed from the water prior to treatment. However, the primary sludge comprises a relatively small proportion of the waste stream. Of interest to Hernandez and French is the secondary sludge-the dirty water that needs to be treated before it's released into rivers or surface waters. Modern wastewater treatment facilities employ aerobic treatment systems where oxygen is bubbled through the wastewater in bio oxidation chambers. In the aerobic system, native microorganisms grow on the nutrients in the dirty water and essentially fatten up as they clean up the water. In their first-generation studies, the researchers established the secondary sludge's oil content when dried down to 95 percent solids and tried several methods for oil extraction and biodiesel reaction with conversion rates up to 6 percent. The highest conversion rate was achieved using in situ transesterification where the oil extraction and biodiesel conversion processes are combined in the same step, using a mixture of alcohols and solvents. "If you convert that to gallons, we were getting 1 gallon of biodiesel for every 100 pounds of dry sludge," Hernandez says. The researchers estimated the cost of production at $3.11 per gallon of biodiesel, which included $2.06 per gallon for the centrifuge, drying and extraction processes.

In the second-generation tests, the researchers identified a consortium of high oil-producing (oleaginous) yeast, bacteria and fungi that when supplemented with sugar in the wastewater boosted oil yields. The experiments involve using the sugars from treated lignocellulosic biomass. "We think those sugars could be used more efficiently in producing oils than ethanol," Hernadez says. "We would introduce these sugars to wastewater treatment facilities to get that secondary sludge to accumulate more oil." Their microorganisms use both glucose and xylose, French adds. "To produce ethanol from lignocellulosic sugars, [researchers] have to genetically modify the organisms," he says. "We don't have to modify our bugs and they use all the sugars." The researchers speculate that a cellulosic ethanol facility might use the more easily fermented sugars for ethanol, and divert the xylose as food for the oleaginous bugs in their process.

Another huge difference between the two systems is that the oleaginous microorganisms are self-replicating, unlike the enzymes used in ethanol processes. French adds their microorganisms are readily freeze-dried so if a facility had a wash out-where a high rainfall event flushes the system-the lipid accumulation chamber could be re-inoculated with freeze-dried bricks.

Envisioned Process: The New Biorefineries


Different From Algae
While the system being investigated by the Mississippi research duo may resemble work being done with algae, there are significant differences. "Algae are dependent on carbon dioxide, sunlight and water, and are more efficient and quicker at making sugars," French explains. Those sugars are then metabolized into oils, he says. However, the yeast, bacteria and fungi that comprise their oleaginous consortium, which are not dependent on sunlight, excel at removing the organics found in wastewater, a task that algae aren't as efficient at. "Algae are good at taking the polished water that comes out of a wastewatertreatment facility," he adds. That could lead to synergies that may make it possible to produce even more oil by adding algae and carbon dioxide to the water coming out of wastewater treatment facilities after the oleaginous microorganisms have cleaned the water and converted the organics to oil.

Another major difference from proposed algae systems is that French and Hernandez plan to use existing infrastructure. "We would not need the large ponds or land and water requirements that algae will need in order to produce large quantities of oil," Hernandez says. Instead, they aim to use the bio oxidation chambers already in place at wastewater treatment facilities, and the temperature and residence parameters already used. "We should be able to operate within every parameter I've seen for wastewater treatment facilities," French says. Water temperatures range from 15 to 35 degrees Celsius (59 to 95 degrees Fahrenheit) and the water resides in the bio oxidation chambers from 6 to 10 days. French says the oil content in the organisms they are studying can go from 4 percent to 50 percent in about 48 hours. Generally, the organisms start at a 10 percent oil content and can reliably reach 40 percent oil levels. "We get 50 percent oil regularly, and even as high as 60 percent," French adds.

A key challenge both algae developers and the Mississippi researchers share is the economical extraction of oils from the watery medium. "That's the problem that is going to plague us, and that's going to plague algae," French says. For the first-generation tests, the Mississippi researchers dried the secondary sludge to 95 percent solids before doing their extraction and reaction experiments-a step that contributed about one-third of the cost estimates. They are now experimenting with a number of approaches to collect the oils at the 30 percent to 50 percent solid levels for treated secondary sludges that are commonly found in water treatment facilities. They even have one promising extraction technology that would work at the 4 percent solid levels of the secondary sludge coming out of the first bio oxidation chamber. "We run electricity through the water, which encourages pores in the cell walls and the oil leaks out," French says. "When the electricity is gone, the pores close up and the organism goes back to doing its normal operation."

Although still in the preliminary stage, the concept is already of interest to a business entity, he adds.

Next Steps
French and Hernandez are in the middle of research funded through a $200,000 U.S. EPA grant they received this summer. They are continuing their work with the second-generation microorganisms, and are conducting tests to establish whether the new consortium of oleaginous microorganisms will clean up wastewater as effectively as current systems. In addition, they hope to establish that the final sludge remaining after the water treatment process will qualify as Class A sludge. A Class A sludge is free of pathogens, and has a higher value as a fertilizer. Also included in this round of research is an analysis to ascertain if it would be more economical to transport extracted oil from several wastewater treatment facilities to a centralized biodiesel plant, or to build biodiesel processors into existing systems.

The researchers are excited about the potential for transforming wastewater treatment facilities into energy producers, and quite hopeful that the resulting biodiesel will be a quality fuel. "Our tests show the fatty acid profile is between an animal fat and vegetable oil," Hernandez says. "One very interesting result in the lab is that depending on the operating conditions and sugar mixtures, there's a possibility of controlling the ratio of saturated versus unsaturated fatty acids." French adds that the colder the water, the more unsaturated fatty acids are produced in the oil. "It's been proven in bench studies," he says. "That's why we need the pilot system to see if it follows through when we're moving water around."

The current round of studies needs to be completed and more funding is required for the pilot-scale studies. The researchers estimate it will be two years before the pilot studies are complete. "After that we should be able to get interest from industry to commercialize the technology," Hernandez says. "And get it into a wastewater treatment plant," French adds.

Susanne Retka Schill is a Biodiesel Magazine staff writer. Reach her at or (701) 738-4922.
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