Water/Wastewater / Water/steam

Wastewater Treatment Method Saves Energy

Researchers develop a purification process that’s carbon-negative and energy-positive.

By Seán Ottewell, Editor at Large

Engineers at the University of Colorado Boulder have developed a wastewater treatment process that both mitigates carbon dioxide emissions and actively captures greenhouse gases.

Method could potentially contain huge benefits for a number of emission-heavy industries.

Known as microbial electrolytic carbon capture (MECC), the new process purifies wastewater in an environmentally friendly fashion by using an electrochemical reaction that absorbs more carbon dioxide than it releases while at the same time creating renewable energy.

Wastewater, the heart of the process, is used as an electrolyte for microbially assisted electrolytic production of hydrogen gas and OH– ions at the cathode, and protons at the anode. The acidity dissolves silicate and liberates metal ions that balance the OH– ions, producing metal hydroxide, which transforms carbon dioxide in situ into bicarbonate.

Results from different industrial wastewaters show that 80–93% of the carbon dioxide derived from the organic oxidation — making the overall process carbon-negative. High rates and yields of hydrogen gas were produced with 91–95% recovery efficiency, resulting in a net energy gain of 57–62 kJ/mole of carbon dioxide captured. The pH remained stable without buffer addition and no toxic chlorine-containing compounds were detected.

“This energy-positive, carbon-negative method could potentially contain huge benefits for a number of emission-heavy industries,” says Zhiyong Jason Ren, an associate professor of civil, environmental, and architectural engineering at the university and senior author of the new study, which appears in the June issue of the journal Environmental Science and Technology.

Wastewater treatment typically produces carbon dioxide emissions in two ways: from the fossil fuels burned to power the machinery, and the decomposition of organic material within the wastewater itself. Existing wastewater treatment technologies also consume high amounts of energy. Public utilities in the United States treat an estimated 12 trillion gallons of municipal wastewater each year and consume approximately 3% of the nation’s grid energy, say the authors.

Current carbon capture technologies are energy-intensive and often entail costly transportation and storage procedures. MECC uses the natural conductivity of saline wastewater to facilitate an electrochemical reaction designed to absorb carbon dioxide from both the water and the air. The process transforms carbon dioxide into stable mineral carbonates and bicarbonates that can serve as raw materials for the construction industry, as a chemical buffer in the wastewater treatment cycle itself or to counter acidity downstream from the process, such as in the ocean. The reaction also yields excess hydrogen gas, which can be stored and harnessed as energy in a fuel cell.

The findings offer the possibility that wastewater could be treated effectively on-site without the risks or costs typically associated with disposal. Further research is needed to determine the optimal MECC system design and assess the potential for scalability.

“The results should be viewed as a proof-of-concept with promising implications for a wide range of industries,” says Ren.

One he particularly points out is power; with the U.S. Environmental Protection Agency’s Clean Power Plan expected to take full effect in 2020, strategies to reduce carbon dioxide emissions are coming into sharp focus.

The technology also may have positive long-term implications for the world’s oceans, he believes. Approximately 25% of carbon dioxide emissions are subsequently absorbed by the sea, which lowers pH, alters ocean chemistry and, hence, threatens marine organisms, especially coral reefs and shellfish. However, dissolved carbonates and bicarbonates produced via MECC could act to chemically counter these effects if added to the ocean.

Many wastewater treatment plants are located on coastlines, raising the possibility that future MECC implementation in these facilities could couple both carbon dioxide and ocean acidity mitigation.

The MECC work is part of Ren’s broader investigations into the use of microbial and electrochemical systems to directly convert biodegradable materials, such as wastewater and biomass into hydrogen gas, electricity and other value-added commodity chemicals. Here, his team uses molecular microbiology tools and electrochemical analyses to understand the fundamental determinant factors of those systems to enhance design, operation and monitoring — in concert with traditional approaches.

The team’s microbial electrochemical technologies (METs) have two main focuses: bioenergy and commodity chemical production from wastewater and biomass; and the development and characterization of microbial fuel cells, microbial electrolysis cells and microbial desalination cells as part of an overall bioelectrochemical system development strategy.

Their other main focus is developing sustainable water desalination systems. Here, they are exploring technologies such as novel membranes and capacitive deionization to deal with oil and gas flow back water and produced water management, and in seawater and groundwater desalination.