A novel wastewater treatment system promises to tackle several issues simultaneously by also generating electricity and removing ammonia from water.
The multipurpose system, created by Virginia Tech engineers, is a hybrid technology that uses both microbial electrochemical and forward osmosis cells, which means it is both a biological and electrochemical system.
This is the first time, they say, that ammonia-driven forward osmosis has been combined with an ammonia-generating microbial electrochemical cell.
The microbial electrochemical cell is based on bacteria that naturally complete chemical reactions during their routine metabolic processes. This system harnesses those reactions to make electricity, which researchers refer to as a bacterial battery. The ammonium ions are pushed to the other side of the cell in the process, which completes a circuit and produces an ammonium solution. Electricity produced by the electrochemical cell can be stored in a battery or passed to the grid.
The ammonium solution is then moved to a forward osmosis cell, which pushes the solution through a membrane that removes impurities. (Forward osmosis is not a new technology on its own. It has been studied by University of Puerto Rico and NASA for water recycling.)
In the Virginia Tech-developed system, the captured water will still have a high ammonium concentration, according to researchers. Other conventional water treatment processes — such as ion exchange or electrodeionization — can be used remove the positively charged ions (cations), creating clean water. The ammonia can be easily stored once it has been removed.
The system was designed by Mohan Qin, a second-year doctoral student in the College of Engineering, who told the Augusta Free Press:
It’s a hybrid system. […] We combine the two cells to get ammonia, clean water, and electricity from wastewater.
The ability of this system to address ammonia is very important. In large quantities, the pollutant contributes to nutrient-rich water. This can be the result of naturally-occurring nitrates or phosphates in the environment, as well as excessive nutrients introduced from fertilizer runoff or sewage discharge. These conditions create a breeding ground for toxic algal blooms, which can devastate aquatic ecosystems.
This type of system could prove extremely beneficial in those states or areas with significant agricultural enterprises. Areas with waterways polluted by farm runoff and animal manure would benefit from having clean water and ammonia-based fertilizers available for crops.
Jason He, an associate professor of civil and environmental engineering in the Virginia Tech College of Engineering, said:
We use so much energy to convert nitrogen gas into fertilizer, and then in wastewater treatment we consume a lot of energy to convert ammonia into nitrogen gas. […] A system that recovered the ammonia for later use, instead of just removing it, would be of great interest to sustainable wastewater treatment.
Demand for Fertilizer
The university says the system can be used to meet the high demand for ammonia-based fertilizers. It estimates that 1 percent of the world’s electricity consumption is used to produce ammonia from nitrogen gas. The system will not supplant conventional ammonia production, but will help provide a more sustainable method of ammonia production while also generating electricity and protecting aquatic ecosystems from algae blooms.
Researchers now have a bench-scale system running, but are seeking to construct a pilot-scale model plant that uses the combined microbial electrochemical and forward osmosis cells. Currently, they are working with local water treatment plants to establish a research facility. Jason He said:
It’s really hard to study a large-scale system in the lab. […] You can build it, but you can’t run it: there’s no wastewater. You need a place that can continuously supply wastewater.
A small-scale version of a different type of microbial fuel cell system developed by He and his colleagues is being used at the Pepper’s Ferry Regional Wastewater Treatment Authority in Radford, Virginia.
Qin won the 2015 Innovation Award for Best Technological Advancement from the International Society for Microbial Electrochemistry and Technology for her work.
Qin is now working on computer modeling studies designed to explain why the system works as it does. Computer modeling will also help the researchers discover possible changes or factors that might contribute to performance improvements.
Photo courtesy Virginia Tech.
Graphic by Ben Mills via Wikimedia Commons.