Younghyun Park, Seonghwan Park, Van Khanh Nguyen, Jaecheul Yu, César I. Torres, Bruce E. Rittmann, Taeho Lee. Complete nitrogen removal by simultaneous nitrification and denitrification in flat-panel air-cathode microbial fuel cells treating domestic wastewater. Chemical Engineering Journal. Volume 316, 15 May 2017, Pages 673-679. ISSN 1385-8947. DOI:https://doi.org/10.1016/j.cej.2017.02.005.
South Korean researchers have tested a prototype of a small device called a microbial fuel cell for its potential to generate electricity while treating wastewater in a wastewater treatment plant. This microbial fuel cell technology could radically reduce the energy requirements for large-scale wastewater treatment plants in two important ways: (1) the technology removes harmful wastewater components, specifically, nitrogen, without needing expensive and energy-intensive fans or blowers to introduce oxygen, and (2) the microbial fuel cells produce a small amount of electric current while removing those wastewater components. This means that wastewater treatment could become significantly “greener” in the future, both in terms of environmental impact and cost-effectiveness.
Wastewater Treatment – COD & N removal
In developed and urbanized areas of the world, anything flushed down a toilet or rinsed down a drain or sink ends up in a wastewater treatment facility. At the wastewater treatment plant, harmful chemicals and components are removed so that fairly clean water can leave the treatment facility and enter a waterbody. Aside from toxic chemicals (like lead, zinc or chromium), wastewater treatment plants must remove as much phosphorus, nitrogen and organic matter (think solids that get flushed…) as possible, to protect the waterbody the facility discharges into. For example, high levels of nitrogen entering a marine ecosystem can wreak havoc, causing harmful algal blooms and fish kills.
To remove nitrogen from the incoming wastewater, the ammonia must first be nitrified to nitrate, and then denitrified to harmless nitrogen gas (which makes up about 80% of our atmosphere; check out this video which explains these processes). Nitrogen gas escapes from the wastewater and is therefore removed from the system. Currently, many wastewater treatment plants try to accomplish this by sending the wastewater through alternating aerobic (with oxygen; blowers add oxygen to the wastewater) and anaerobic (no oxygen) compartments. These conditions are designed to be favorable for successive nitrification and denitrification, but the aeration step is very costly in terms of energy (up to half of a wastewater treatment plant’s energy use can go to aeration!).
Microbial Fuel Cells
The microbial fuel cells tested in this research are capable of simultaneous nitrification and denitrification without the need for expensive aeration. To understand this, we need to look at what a microbial fuel cell is, and how it works… A microbial fuel cell is simply a device that harnesses the power of microbes (bacteria and other microscopic single-celled organisms) and their normal metabolic activities (breaking down “food” molecules and releasing “waste” products). Usually, there is an anaerobic (oxygen-free; “anode”) area and an aerobic (oxygen-rich; “cathode”) area, separated by a membrane or soil. Microbes in each area produce electrons that are passed to the anode or cathode, and as these electrons move, an electric current is produced. This technology is sometimes used in environmental science to power small electrical devices (like sensors that collect data), since microbial fuel cells behave as inexhaustible batteries.
In this study, Park et al. connected five small microbial fuel cell prototypes in series. They pumped wastewater through the successive fuel cells and measured how much nitrogen and organic matter was removed, and how much electric current was generated. Wastewater spent a total of 2.5 hours in the fuel cell series – much shorter than the 6-8 hour residence time in large-scale wastewater treatment plants. Over the eight-month experiment, organic matter and nitrogen removal improved (removing up to 85% organic matter and 94% nitrogen), while the current generated decreased (from 16.7 to 6.3 W/m3). Though the power output was minimal, the fuel cells were surprisingly effective at cleaning up the wastewater (removing nitrogen) without the need for energy-intensive aeration processes. Additionally, the wastewater leaving the fuel cell met Korean discharge regulations in only 2.5 hours, indicating that this technology could treat more wastewater in less time if it were scaled up successfully to wastewater treatment plant size. The researchers report a trade-off between nutrient removal and energy generation, and suggest that scaling up the experiment and further optimizing performance could improve both processes. However, the experimental fuel system was dismantled for analysis at the end of the eight month experiment, so the potential total lifetime of the fuel cell is hard to predict. A further draw-back of this experimental fuel cell, is that the electrodes were made of platinum, and so would be incredibly expensive to implement on a large scale. If engineers manage to find lower-cost components and optimize this technology to serve a full-scale wastewater treatment facility, this could substantially improve the cost and energy efficiency of wastewater treatment facilities.