When it comes to wastewater, what is considered “waste” may be a matter of perspective, as scientists and engineers seek to recover valuable resources from the treatment process.
Energy, fertilizer, and clean water are all valuable resources that can be generated from traditional wastewater treatment. Today, researchers are finding alternative ways of generating these valuable byproducts, and finding new materials in the process.
A Resource in The Wrong Location
There are several methods to extract energy from wastewater, many of which are successful in producing biogas or bioelectricity, but few can produce clean water in the process. At Washington University in St. Louis, however, researchers have developed a system that both filters wastewater and creates electricity using organic materials produced during the treatment process.
The lab of professor Zhen (Jason) He combined the filtration and energy production processes by integrating a filtration system into the anode electrode of a microbial electrochemical system.
“The system is set up like a typical microbial fuel cell, a bacterial battery that uses electrochemically active bacteria as a catalyst where a traditional fuel cell would use platinum,” He said. “In this type of system, the bacteria are attached to the electrode. When wastewater is pumped into the anode, the bacteria ‘eat’ the organic materials and release electrons, creating electricity.”
A permeable anode developed by He’s lab acts as a filter, producing clean water through the same process.
The anode, he said, is a dynamic membrane, made of conductive, carbon cloth. Together, the bacteria and membrane filter out 80 to 90 percent of organic materials resulting in water that can be further treated for non-potable uses.
He and his team used a mixed culture of bacteria that could survive in a zero-oxygen environment. This is because, in the presence of oxygen, He said, bacteria would just dump electrons to the oxygen and not the electrode.
The amount of electricity created with this approach should, theoretically be enough to help to offset the energy use for a typical U.S. water treatment plant.
But the primary goal of He’s system isn’t electricity production, it’s wastewater treatment and nutrient recovery.
“Bacteria can convert those organic materials into things we can use,” He said. “We can also recover nutrients like nitrogen or phosphorus for fertilizer. We can use it to feed plants. It’s only when we don’t use it, [that the byproducts of wastewater treatment] become waste.”
Nature’s ‘Bio-Refinery’
Researchers in Canada look to nature for inspiration when it comes to wastewater treatment, by filtering their municipal wastewater through the roots of willow trees. Experimenting with a plantation in Quebec, the scientists estimate that over 30 million liters of primary wastewater per hectare can be treated using this ‘bio-refinery’ each year.
“We’re still learning how these trees can tolerate and treat such high volumes of wastewater, but willows’ complex ‘phyto’-chemical toolkit is giving us exciting clues,” said Eszter Sas, lead author of the study and a PhD student at the University of Montréal (UofM).
In a press release, the UofM team say their technique can triple the biomass produced when compared to standard treatment methods. This biomass can then be harvested for renewable lignocellulosic biofuels, an alternative to fossil fuels.
Using advanced metabolomic (chemical) profiling technology, Sas and a Canadian-British team of crop scientists, biochemists, and chemical engineers from UofM and Imperial College London also identified new extractable ‘green’ chemicals produced by the trees. These chemicals have significant antioxidant, anticancer, anti-inflammatory, and anti-microbial properties.
“While most of the induced chemical compounds have not been seen before in willows, some have been observed in salt-tolerant plants such as licorice and mangroves and are known to be potent antioxidants,” Sas said.
“It’s amazing how much novel plant chemistry there is still to be discovered, even in willow trees, which have been around for thousands of years,” she added.
“One of the benefits of using natural solutions to address environmental challenges like wastewater treatment is that we can generate complementary bioproducts, such as renewable bioenergy and green chemistry,” senior author Frédéric Pitre said.
Microbe Powered
A research team at Washington State University have developed a wastewater treatment system that relies on electron-producing microbial communities to clean the water, generating electricity in the process.
In their work, the researchers used a unique microbial fuel cell system they developed as a substitute for external aeration. These fuel cells work by having microbes convert chemical energy to electricity. The fuel cell can fill the role that aeration and oxygen plays — accepting electrons that bacteria generate as a product of their metabolic work.
“It’s like we’re double dipping,” researcher Haluk Beyenal said. “We use the electrodes and then the electron acceptor to promote microbial growth. On the other hand, we gain a little bit of electricity for the pump and to aerate. With this approach, it is more powerful and can treat the wastewater faster.”
While the fuel cells have been used experimentally in wastewater treatment systems under ideal conditions, they often fail in real-world applications with varying conditions.
This is because the microbial fuel cells lack internal regulation controlling the potential of anodes and cathodes, and thus cell potential, which can cause system failure.
In the system the WSU team developed, the researchers added an extra electrode that allows additional control to their fuel cell system. According to a press release, the system is switchable. It can either work by itself as a microbial fuel cell, using no energy as it slowly cleans up waste, or it can be switched to one that uses a smaller amount of energy than aeration and that cleans more intensively.
The researchers were able to operate their system for a year in the laboratory without failure as well as at the pilot scale at a test wastewater treatment facility in Moscow, Idaho. The pilot scale treatment facility is owned and operated by University of Idaho Environmental Engineering Professor Erik R. Coats, who was a collaborator on the project. The system removed waste at comparable rates to aeration.
The system could potentially be used entirely independently from the power grid, and the researchers hope it could someday be used for small scale wastewater treatment facilities in rural areas.
“Over time, we have made a lot of progress,” researcher Abdelrhman Mohamed said. “There are still challenges that we need to overcome to see this as a real application, but it’s exciting to see the field moving significantly over a period of time.”
Modern challenges require modern solutions. Thanks to forward-thinking research, today’s wastewater treatment is about maximizing recovery for the most sustainable treatment possible, so the industry can ‘waste not.’ WW
About the Author: Alanna Maya is Chief Editor of WaterWorld magazine. Email her at [email protected].
