Power Production of Limiting Nutrients in Microbial Fuel Cells

Abstract

This summer I worked with a wonderful group of people in efforts to clear the horrible smell that came with Lake Menomin. While, of course, we did not eradicate the issue, we all did continue to make strides towards this final goal. Below the surface of the lake there is an accumulation of phosphorus rich sediment that we call the “legacy phosphorus”. It has been built up for such a long time that even if we eliminated the agricultural and domestic runoff of the phosphorus levels into the lake, this legacy phosphorus would still be high for some time. This is something that needs to be recognized and investigated more.

This summer my goal was to take the first steps towards bringing technology into the solution. I first studied the varying sediment composition of lake sediment in devices that use bacterial metabolism to produce an electric potential. These devices are called microbial fuel cells or MFCs. MFCs are interesting cells that can be used to improve the water quality but also to create the electric potential. In my research I looked at the creation of the electric potential; future studies using larger cells could be used to improve water quality on a smaller scale.

The amount of power produced by each cell was compared over a three-week period. Using five groups of MFCs, I used lake sediment spiked with four different solutions and a control set-up. Some of the surrounding bedrock in the Red Cedar Watershed has large compositions of varying limiting nutrients. The glauconite formation contains iron and the apatite formation contains phosphorus. We then used a mixture of potassium nitrate and potassium phosphate to test nitrogen as a limiting nutrient. Five groups were set-up: a control, a glauconite mixture, an apatite mixture, a glauconite and apatite mixture, and finally the potassium nitrate and potassium phosphate.

For a total of three weeks, power output measurements for each of the MFCs were recorded. Something interesting about MFCs is that you can visually see a difference in the sediment over time. The longer the MFCs are hooked up to a circuit gives the microbes or bacteria more time to continue metabolizing. This changes the color of the top layer of sediment. The longer the color change typically means the more electric potential created. The results of the set-up, however, did not show this. The MFC with the longest color change in sediment was the glauconite set-up, but the apatite recorded the most power. A possible reason for this is that something inside the glauconite mixture was leaching electrons away from the circuit and acting as an electron acceptor. This could either be air bubbles in the sediment or other bacteria that may have been living on the glauconite rock before it was mixed in with the lake sediment.

The potassium nitrate and potassium phosphate set-up never showed a display of power over the three weeks. It either created too little to even be acknowledged or it just never started emitting power at all. To understand more about this, I filled another MFC that contained composted cow manure with potassium nitrate. Nitrate seemed to be the issue in this case because the apatite formation and the control both contained phosphorus and showed high indication of power output. The MFC pre-nitrate spike was emitting 50 microwatts of power. Over the course of three days the power dropped to nearly 0 microwatts. From this, I assumed that the potassium nitrate was acting as an electron acceptor and creating various nitrogen-based compounds in the sediment as a result. To understand this on a greater level, more research should be done.

While all the data was being collected for the MFCs, I also began working on an electronic circuit using various sensors and Arduino micro-controllers. I wanted to leave a lasting impression on the community by taking the first steps to creating a website/phone app called Menomi-net. Menomi-net is a seed idea from Dr. Matthew Kutcha in the geology department of UW-Stout. It will provide water quality data at various locations on Lake Menomin. Using the Arduinos, it will receive information from the GPS locations for those who frequently visit the lake as well as researchers interested in studying more about the lake. Due to a time crunch this summer, Menomi-net is still a seed idea. However, I was able to create a code, circuit, and prototype that can be useful for Menomi-net. At the moment the prototype can only test water temperature as well as measure the sediment voltage when attached to an MFC. But I have hopes that it will be able to provide much more water information. The prototype can be found in the UW-Stout's Dirt Lab in a tank called Mini-nomin.

This project can be expanded in many different groundbreaking ways. Microbial fuel cells are interesting devices that have a lot of uses. While these specific cells were too small to create power that is significant enough to use as a power source, they are a good reminder about what is going on under the surface of the lake and that the legacy phosphorus should not be ignored. On a slightly larger scale MFCs can create hundreds of microwatts of power. If harvested properly , these can ultimately be used as power source for small scale projects, like powering an AA battery or an LED. Menomi-net, will hopefully be available in the upcoming years. It can be used to display more water quality data. It is important for the community members to be aware of the issues going on around them, and Menomi-net would be a useful way to do that.