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Wastewater, Algae Have Untapped Potential for Clean Water, Energy Generation

The U.S. Department of Energy (DOE) is funding a project to cultivate microalgae as biomass for fuels and an array of consumer products. The microscopic single-cell organisms could unlock an affordable way to generate energy using only sunlight and carbon dioxide (CO2).

The U.S. Department of Energy (DOE) is funding a project to cultivate microalgae as biomass for fuels and an array of consumer products. The microscopic single-cell organisms could unlock an affordable way to generate energy using only sunlight and carbon dioxide (CO2).

“Our goal is to develop systems to make growing microalgae more affordable and sustainable and to produce it on scales large enough to meet growing demands in the United States and globally,” Bruce Rittmann, one of the Arizona State University (ASU) engineers leading the research, said.

Microalgae typically grows best in sunny, warm climates, but need a source of concentrated CO2 to grow. Power plant emissions are a potentially cost-effective source, but are often not located near good growing locations. The researchers hope to capture and deliver CO2 from the air, which presents a significant opportunity since it would not only reduce air pollution but would also make microalgae production feasible in any sunny climate or environmental condition.

“The current atmospheric levels of CO2 are too low to produce high rates of microalgae growth. We want to feed the microalgae with CO2 concentration significantly higher than in the atmosphere to enable the microalgae to grow much faster,” Rittman added.

His colleague, a physicist named Kris Lackner, developed an approach that uses ‘moisture swing sorption,’ or a special material that, when dry, absorbs CO2, and when wet, releases CO2 that is at least 100 times more concentrated that it was in the air.

“My part of the project is about a novel way to deliver that additional CO2 at very high efficiency,” Rittman explained. The membrane material he developed has no pores, so bubbles are not formed and do not allow CO2 to escape before it can be ‘inhaled’ by the algae. The researchers are using a method of membrane carbonation to connect their developments into one solution.

The DOE awarded ASU a three-year, $1 million grant to fund the Atmospheric Carbon Dioxide Capture and Membrane Delivery project.

Others are also exploring the power of algae: ASU professor Peter Lammers and colleagues at New Mexico State University reported progress on an energy-positive wastewater treatment method using algae in August 2015, and Nevada-based Algae Systems claims its technology makes algae-base biofuel profitable by transforming raw sewage into fuel and clean drinking water.

Besides biofuel production, microalgae and algal oils are being used for a variety of applications, such as in soap and as a source of protein. Algae is also being researched as a replacement for petroleum in plastics and palm oil.

Meanwhile, ASU engineer Bruce Rittman is also working with researchers from the Chinese Academy of Science & Technology on more cost-effective wastewater treatment.

In a recent contribution to Nature, Rittman, Wen-Wei Li and Han-Qing Yu argue that the costs of domestic and industrial wastewater treatment could be recouped if valuable chemicals, such as useful forms of carbon, nitrogen, and phosphorus, were captured in the process. They also claim that the main reason wastewater resources are not currently common practice is a high level of uncertainty about which techniques are the most useful and how to combine them into an effective process – a problem which they aim to remedy.

In their article, they discuss the two main ways that domestic wastewater has been treated – the aerobic ‘activated-sludge process’ and anaerobic digestion – as well as promising replacement systems. They suggest that anaerobic membrane bioreactors (AnMBR) and microbial electrochemical cells (MXCs) could be used to generate energy during domestic wastewater treatment. AnMBRs retain and concentrate solid waste to prolong their degradation time and increase methane production; the methane can be collected and processed to generate energy. The biggest associated challenge is cleaning the membranes and keeping them from clogging. MXCs can produce energy-rich chemicals such as hydrogen gas in microbial electrolysis cells or can generate electrical power directly in microbial fuel cells. These products are more valuable and readily usable than methane, but the reactions involved take several days and MXCs have performed poorly on large scales.

The researchers also discuss two emerging technologies for capturing and concentrating phosphorus and nitrogen: ion exchange and electrodialysis. Both are still being tested on small scales, but if successful, the technology could present opportunities to use drastically less energy in nitrogen fertilizer production. Atmospheric nitrogen gas is currently recovered to produce fertilizer, but the process involved accounts for a few percent of the world’s annual energy use. The researchers claim that substituting just 5 percent of the existing production would save more than 50 terawatt-hours of energy, or 1.5 percent of China’s annual energy consumption.

Biosolids and water present further opportunities. The researchers suggest that in combination, emerging technologies could yield millions in revenues and reduce the costly nature of wastewater treatment. They conclude their contribution with a call to governments to establish regulatory frameworks that consider waste disposal and emissions costs, and support efforts to develop wastewater-resource factories.