Purdue Biomakers

Microbial Desalination Cells

One of the UN’s top sustainable developmental goals is universal access to clean water and sanitation. But with a rising world population and freshwater accounting for only 3% of the world’s supply, there are many challenges still left to overcome in order to meet that goal. Barring the discovery of large underground aquifers or other improbable events, the most likely way to meet this goal is through water desalination.

Current Methods

The challenges posed by water desalination are in no way new and until recent years have been met with many methods equally ‘historic’ in nature. More recent methods however (deionization/ion exchange and membrane processes) are capable of desalinating water sources on a much larger scale, but carry with them significant energy requirements…

8-20 MW to desalinate a 62ML (50,000 AF) volume of brackish water (3,000 - 5,000 g/L)
20 - 35 MW to desalinate the same volume of saline water (5,000 - 35,000 g/L)
For comparison 30 MW will power about 20,000 homes (about the size of Boston University’s student population)

As well as environmental, regulatory, and growth factors to consider. Considering the prevalence of coal and gas fired power plants, it is not hard to see how many of these problems go hand-in-hand.

But what if there was another way?

Microbial desalination cells (MDCs) combine the technologies used in microbial fuel cell (MFCs) with those from electrodialysis systems to create low-cost, low-maintenance, and environmentally-friendly alternatives to current industrial desalination procedures. Unlike these current methods, the ‘hands-off’ operation of MDCs with their free energy requirements make them accessible to both developed and developing countries alike.

What’s more is that MDCs can not only be used to desalinate water, but to produce electricity as well, economizing land use all the while (potentially) turning a profit.

Returning to the example above, linearly extrapolating on findings from a small proof-of-concept study on MDCs, by passing that same volume of water (approximately 1/8th the size of Sydney Harbor) through an MDC one could potentially produce energy on the gigawatt scale, all the while removing 90% of salt content in a single run-through.

Though MDCs (like MFCs) typically rely on anodophilic and exoelectrogenic bacteria, by expressing shewanella-derived nanowires in our E. coli chassis, we are able to both greatly improve E. coli’s aptitude for desalination and open the door for further study with the model organism.

Microbial Fuel Cells

As briefly mentioned on our page for MDCs, clean water supply is one of the UN’s top sustainable development goals. Another one of these goals is to “ensure access to affordable, reliable, sustainable and modern energy for all”. Although scientists and engineers are currently exploring many roads to meet that end -- be they solar, wind, nuclear, geothermal or otherwise -- one we believe that has yet to be studied to its full potential is microbial fuel cell (MFC) technology. While there has been significant engineering involved in the electrode design and physical construction of MFCs, preciously little has been done on the biological side. Because of this, we wanted to provide a platform for better MFC (and ergo transitively MDC) efficiency.

So, what exactly are they?

MIT’s professor Bruce Logan defines MFCs as “devices that use bacteria as [...] catalysts to oxidize organic and inorganic matter and generate current”. That is to say that even with the introduction of unconventional fuels such as wastewater, discarded foodstuffs, or compost, MFCs are capable of producing energy.

Are they effective?

While MFCs used to have a reputation for inefficiency, this came largely in part from the misconception that chemical mediators were absolutely necessary to transfer electrons from bacteria to the anodes of fuel cells. Since the 1970s however, the use of electrochemically active proteins, like seen in the Shewanella and Geobacter genera has removed the need for mediators and led to improved efficiency and net positive energy production.

Additionally, whereas other energy sources (sustainable or not) tend to require some degree of complex oversight (consider the litany of redundant protocols involved in nuclear power production) MFCs are largely passive, with even the most “high-maintenance” models requiring little more than a continuous inflow of fuel (think wastewater piping).

In terms of raw power output, MFC power production values have been cited to range from 15.5 to 51.0 W/m3 based on fuel source, and are capable of efficiencies higher than those seen in typical combustion processes.

Building on the Past

In 2013, Bielefeld’s iGEM team conducted a project on MFCs, also looking at their efficiency and the lack of engineering on the biological side to the technology. They had originally intended to look at nanowires but ultimately decided to focus on the other numerous, monumental tasks they were exploring. They did work on increasing E. coli digestive efficiency though, which would be a great addition to our final prototype. Additionally, they designed (but did not test) one of the cytochrome complexes required for nanowire development. TU Delft-Leiden improved and characterized this part for use in landmine detection, but we plan to characterize and improve the part by looking at its function and structure in MFCs/MDCs. To test our system, we will use Bielefeld’s 3-D printable MFC design and will attempt to improve upon the design if necessary. We will also use Bielefeld’s results as one of the benchmarks by which we will measure the success of our engineered E. coli.