Food industry waste can become clean energy

Researchers are developing a method to produce hydrogen using wastewater from the processing of oranges.

By Karina Toledo

Agência FAPESP – Researchers at the Araraquara campus of São Paulo State University (UNESP), Brazil, are studying the feasibility of using wastewater from the orange juice industry to produce hydrogen, a renewable, inexhaustible and non-polluting source of energy.

The research project, supported by FAPESP, is in progress at the Fuel, Biofuel, Petroleum & Petroleum Product Quality Monitoring & Research Center (Cempeqc), which is part of UNESP’s Chemistry Institute.

“The advantage of producing hydrogen from wastewater is that you can make sustainable use of a carbon source that’s currently being thrown away,” said Sandra Imaculada Maintinguer, a researcher at Cempeqc.

The proposal, according to Maintinguer, is to reuse the energy generated locally in the industry itself to power the pumps in biological treatment systems, for example.

“The method could benefit not only the citrus industry but also the sugar and ethanol industry, the soft drinks industry, brewers and manufacturers of other foods,” Maintinguer said.

Hydrogen, she explained, is almost three times more energetic than hydrocarbons or methane and four times more energetic than ethanol. However, because the cost of storing and transporting hydrogen is still high, it is not yet be feasible to use the gas, for example, as a substitute for hydroelectric power, which is inexpensive in Brazil.

The group of researchers at Cempeqc are studying three different types of waste from orange processing, obtained free of charge from a juice manufacturer based in Matão, São Paulo State: molasses, vinasse and wastewater.

Although molasses and vinasse have a higher sugar content (40-150 grams of glucose per liter), preliminary tests suggest that wastewater (12 g glucose/L) is more suitable for biological production of hydrogen.

“If the substrate concentration is very high, the growth of the microorganisms that break sugars down into smaller molecules, such as organic acids and hydrogen, may be inhibited. There’s an ideal zone, which appears to be that of wastewater,” Maintinguer said.

In addition to glucose, the researchers have also found other sources of carbon in wastewater, such as fructose and organic acids, as well as impurities such as oil and detergent used in industrial processes.

“We performed tests using wastewater with all the impurities and, even so, the results were very promising. We managed to convert approximately 65% of this waste into hydrogen. You never get to 100% because the microorganisms first use the nutrients to grow and multiply,” she said.

Methanogenic archaea

Bench-scale tests were performed in anaerobic reactors (hermetically sealed glass vials) because contact with oxygen would inhibit the production of hydrogenase, an enzyme that plays a highly important role in biological hydrogen production.

The wastewater was inoculated with microorganisms of different classes collected from biological sewage treatment plants. The inoculum can also be obtained from the sludge in industrial waste treatment systems, according to Maintinguer.

However, pretreatment is required to eliminate microorganisms called methanogenic archaea, which would otherwise convert the hydrogen produced into methane, undesirable in this case.

“The anaerobic biological process comprises several stages, and a different class of microorganisms is key to each stage. Carbohydrates are broken down into sugars, organic acids, acetate, hydrogen and, if the process isn’t interrupted, methane,” she said.

To prevent this from happening, the inoculum is submitted to heat shock, and the pH of the medium is reduced to 5.5. Pretreatment causes elimination of the methanogenic archaea, while useful bacteria for the process enter a vegetative state, resuming multiplication only when the conditions become favorable.

“It’s a cheap and easy method and has to be performed only once. I can then reapply the inoculum to a different sample when the reactor substrate is used up. For now, we’re using only a reactor batch configuration [vials with a limited volume in which the reaction proceeds until the substrate is depleted, after which they have to be refilled]. The next step is to test the method in a continuous flow reactor,” Maintinguer said.

In addition to hydrogen, the process produces volatile fatty acids, such as butyric acid and acetic acid, which can also be converted into hydrogen by photoheterotrophic bacteria.

“They consume these acids in the presence of light and release more hydrogen, thereby increasing the yield,” she explained.

In Maintinguer’s view, Brazil has considerable potential to become a benchmark in hydrogen technology and enjoys the advantage of being a tropical country with annual average temperatures in the range of 25°C, favorable to the growth of bacteria.

“In countries like the Netherlands or Germany, the reactors have to be heated for the process to be successful,” she noted.

The Mining & Energy Ministry has plans to introduce hydrogen into Brazil’s energy mix by 2025, including its use as automotive fuel. One of the Brazilian government’s targets is to ensure that hydrogen is produced solely from renewable sources after 2020.