Wastewater treatment is both an energy and carbon intensive process. Algal fuel cells are devices that implement the use of algae, in order to harness the chemical energy in wastewater to generate electricity.
Current wastewater treatment methods, which aim to remove contaminants from wastewater in order to be able to return it to the water cycle, are highly energy intensive. In the United States wastewater treatment takes up an entire 3% of the
electricity produced by the country.
Wastewater filtration ponds. | Ivan Bandura / Unsplash.
Wastewater, which is any form of contaminated water output from industrial, domestic, or farming applications, contains ‘chemical’ energy. Chemical energy is the energy which arises when the chemical bonds of the pollutants that are found
in wastewater are broken.
This energy can be harnessed and turned into electricity, using what are known as algal fuel cells. These cells innovatively make use of living microorganisms, as well as the chemical and physical properties of several other components, to
create a circuit that produces electricity.
So, how is electricity generated via this technology, and where do the algae come in? Firstly, electricity is defined as the flow of electrons. Thus, in order to generate it, electrons need to be directed to move through a wire. Algal fuel
cells do just that.
Within the cell, there are two chambers: an anode chamber where electrons are generated, and a cathode chamber where the electrons are received, constituting a circuit. At the anode, specialised microbes break down the pollutants in the
wastewater, resulting in the release of electrons, which then flow through a wire to generate electricity. These electrons are then accepted at the cathode, with the aid of microalgae.
‘This energy can be harnessed, and turned into electricity, using what are known as algal fuel cells.’
Ideally, this process would provide the electricity needed to treat the wastewater, offsetting the electricity inputs of wastewater treatment facilities, and removing the need to burn fossil fuels. However, there are many other benefits to
this technology.
Algal fuel cells have the potential to be integrated into a much larger, circular green process. Firstly, they do not only generate electricity, but they also ‘treat’ the wastewater where the electrons are sourced from in the first place,
leaving clean water in its wake.
This is due to microbes at the anode being able to break down organic, carbon-based pollutants, and the algae at the cathode have the ability to assimilate the remaining pollutants, particularly those containing nitrogen and phosphorus, as
these are essential for a functioning algal metabolism.
Secondly, microalgae are the largest group of carbon-capturers in the world. Alone, marine phytoplankton account for half of the global primary productivity, which is the net difference between carbon dioxide absorbed from the atmosphere or
ocean versus how much is released during respiration. Algae can also capture up to fifty times more carbon dioxide than terrestrial plants. Therefore, running this technology alone would absorb carbon dioxide from the atmosphere.
Further carbon dioxide can be absorbed by the additional input of flue gases, which are waste gases from industrial processes and also have the potential to be cleaned during this process. This would therefore reduce further greenhouse gas
emissions, instead redirecting them to enhance the electricity output of algal fuel cells.
An algal bloom in the Gulf of Alaska can be seen from a NASA satellite. | Stuart Rankin / Flickr
A final advantage of incorporating algal fuel cells into industrial processes is the refinery of the algal biomass into further added-value products. These may include biofuels, but more broadly may also be fertilisers, medicinal products,
food and animal feed.
The potential applications of algal fuel cells do not stop at wastewater treatment. Instead of only using algae at the cathode chamber, as part of the central process, algae could also be added to the anode in order to feed the microbes.
This has already been successfully carried out using algae that have been sourced from harmful algal blooms.
Harmful algal blooms occur in eutrophic lakes and take place when an excess of nutrients leads to the overgrowth of algae. These often result in the death of many other organisms within the ecosystem, as they are depleted of the oxygen that
the overwhelming number of algae consume.
Often, these blooms are removed manually, but then the biomass has traditionally been disposed of in landfills, contributing to greenhouse gas emissions as it decomposes. Algal fuel cells offer a new perspective on the recovery of eutrophic
lakes.
Algal fuel cells can also be used to treat contaminated sediments at the bottom of urban water bodies, where harmful pollutants may deposit as wastewater flows through them. Here, the sediments themselves act as the electron donors, as the
pollutants within them are broken down.
‘Algal fuel cells offer a new perspective on the recovery of eutrophic lakes.’
Despite all of these apparent benefits from this technology, the question remains: does it actually work? Hundreds of laboratory studies have now been conducted at a highly reduced scale, which have extensively proved that electricity can
be in fact generated from wastewater using algae.
Recently, researchers from the Indian Institute of Technology Jodhpur constructed a ten litre algal fuel cell that could generate electricity outdoors. This was the first proof-of-concept study conducted outside of the lab using cheap
materials. However, much like other previous studies, the power output was so low that it could only light LED lights.
Current industrial requirements for electricity average around 300 to 900 Watts per metre-squared of anode area, whereas the most effective algal fuel cells resulted in an output of only 0.6 Watts per metre-squared.
As idyllic as they sound, algal fuel cell technologies are far from being able to scale-up to power whole wastewater treatment facilities. Thus, the dream of green, self-sustaining wastewater recycling remains just that—a dream.
Featured Image: Liz Harrell | Flickr
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