Scientists at Lehigh hope to “turn garbage into gold” by using waste products to grow algae that can be converted into a fuel.

Their goal is to reduce society’s dependence on fossil fuels while finding new roles for carbon dioxide and other pollutants.

The researchers—from the Energy Research Center (ERC) and the earth and environmental sciences department (EES) —aim to use CO₂ from power plants and wastewater from water-treatment plants to grow Nannochloropsis, a green microalgae that produces oil.

They have built a prototype photobioreactor (PBR) to test the optimal conditions for growing and harvesting Nannochloropsis. If successful, they could enable cost-effective production of biodiesel fuel from waste CO₂, which now must be captured and stored through chemical and physical methods, and removal of excess nutrients from treated wastewater before it is discharged into rivers and streams.

The group includes Harun Bilirgen, principal investigator and ERC principal research scientist, Bruce Hargreaves and Donald P. Morris, associate professors of EES, and Ebru Akkaya, ERC visiting research scholar and associate professor of environmental engineering at Yildiz Technical University in Turkey.

The project, which began in early 2011, is funded by the U.S. Department of Energy through the Ben Franklin Technology Partners of Northeast Pennsylvania. Partners include the Lehigh County Authority and the Odyssey Green and Renewable Energy Company.

The advantages of algae

Scientists have endeavored since the 1970s to grow algae that can be converted into fuels. Microalgae have advantages over corn-based ethanol and other biofuels. They grow and reproduce quickly because of small cell size and do not consume agricultural land or require freshwater if produced in enclosed PBRs.

Algae biomass can be used as a fertilizer or in pharmaceutical products, and can be converted into methane, biodiesel, ethanol and other fuels. Especially attractive are algae species like Nannochloropsis that accumulate oil instead of starch under conditions where excess energy is stored inside their cells.

“By growing algae for fuel,” says Bilirgen, a mechanical engineer who specializes in fluid flow behavior and combustion systems, “we can turn waste products into valuable products. By contrast, growing corn for ethanol requires expensive potable water and chemical-based fertilizer.”

Algae can be grown in ponds or in PBRs. Ponds are less expensive to build and operate but they consume more land and water (because of evaporation) and are susceptible to contamination. PBRs, as closed systems, allow scientists to control the variables governing algae growth and, potentially, to achieve greater productivity.

To grow, says Hargreaves, an ecologist specializing in aquatic ecosystems, algae require water, a light source, CO₂ and nutrients such as phosphates and nitrates. CO₂ is present in the flue gas emitted by power plants, while phosphates and nitrates are found in wastewater from water-treatment plants.

The Lehigh group is measuring the metabolism and growth rate of Nannochloropsis and characterizing its physiology and biochemistry. By controlling these factors, says Morris, who studies the biogeochemistry of aquatic ecosystems, researchers can maximize the rate at which the algae consume CO₂ and wastewater nutrients.

In prototype PBR columns, the researchers are conducting short-term photosynthesis experiments that correlate metabolism with growth rates of Nannochloropsis using varying levels of CO₂, nutrients and lighting to determine which conditions produce the fastest growth rates and highest oil yields.

“We have seen that more nutrients result in faster growth while lower nutrient levels result in more oil production,” says Hargreaves. “We’re trying to find an optimum balance for fast growth with high oil content.”