“Microbe Herder.” This phrase probably has some wondering why anyone would want to herd microbes and how to go about herding something that cannot be seen without a microscope. The answer comes from the fact that microbes are critical to many wastewater treatment processes, such as municipal sewage plants. And while this is reason enough to “herd” microbes, researchers at Mines have been discovering other novel uses for microbes, such as treating mine drainage. The problem is how to translate an interesting observation to an engineered system that can be built on a site. This is where Dr. Linda Figueroa’s research comes in.
The initial discovery that microbes could be used to treat mine drainage was made by a research team at Mines and was the subject of the previous article “The Manure Connoisseur: Tackling Acid Mine Drainage.” This team, composed of Dr. Ronald Cohen, Dr. Ronald Klusman, and Dr. Thomas Wildeman originally set out to develop a model of why wetlands were removing some metals from mine drainage. Through experimentation, the team discovered that the metal removal had nothing to do with the wetland plants and was instead caused by microbes living in the decomposing organic matter.
The team quickly started experimenting with organic materials that were not as far along in the decay process, earning Cohen the distinction of being “the manure connoisseur.” The team then achieved its goal of figuring out what was happening in the wetlands and demonstrated the feasibility of using microbes to treat acid mine drainage. Unfortunately, there is a big difference between knowing that sulfur-reducing bacteria are responsible for a process and implementing them in effective treatment facilities. This is where the original team passed the baton off to Figueroa.
Figueroa’s goal is “to get to a more engineered system” by developing a detailed model that can be used to construct microbial treatment systems for mine drainage in a similar manner to a regular municipal wastewater plant. For this to work, engineers need to know what sort of behavior they can expect from a system with a specific set of conditions. As Figueroa put it, “How do you tell people how to make the system out of local materials?”
According to Figueroa, there are two primary groups of microbes at work, the sulfate reducing bacteria (SRB) and the microbes that produce the food for the SRB. The SRB are the ones that are actually responsible for removing the metals. The SRB take sulfate and reduce it to sulfide. Since sulfate is responsible for the low pH, its removal helps raise the pH of the acid mine drainage some. The sulfide reacts with the metals in the water, making metal sulfides, which precipitate out of the water. Cohen noted this is a much less harmful form. The other population eats the food source and produces a food that the SRB can eat.
The problem is that this process is more difficult to cultivate than it sounds. There are two primary microbe groups that must be kept happy (alive and performing the right functions). One of the challenges brought up by Figueroa is that SRB have a bad energy handicap. Sulphate provides them with far less energy than other electron receptors, like oxygen and nitrates, and provides competing species that are useless for the intended process. This means that part of the process must take place in low oxygen conditions. Another nuance Figueroa discovered is that while SRB are hearty and can stand high toxicity from metals, the microbe population that feeds the SRB is more easily poisoned by metals.
Under Figueroa, the research has moved towards design parameters and the impact of variables like temperature on the system. Figueroa has tested the impact of different foods and different sized materials on the process. Composted manure has been replaced with materials that are less decayed. Figueroa notes that these have more food value, though a little composted manure is used to jump start the process by introducing live microbes much like a one time activated sludge system. The size of the feed impacts the reaction rate. Small particle sizes yield a higher reaction rate. A good example of this is a pile of shredded newspaper burning much more rapidly than a solid chunk of wood of a similar mass. Figueroa has also been experimenting with different mixes of particle sizes in order to provide a good ‘home’ in the reactor for the microbes as well as adjusting the reaction rate. For microbes, temperature affects reaction rates just as it does in chemistry.
The other critical part of the question is, as Figueroa stated, “Who’s there?” Figueroa collaborated with Dr. Amy Pruder, then of the University of Colorado, to figure out what specific microbes were in the picture. Though the application of molecular tools tools proved difficult initially, some have since been shown to work for characterizing the microbes.
Figueroa hopes that the this research will help “push forward what tools work for characterizing microbes, the physical characteristics, and chemical characteristics of the feed.” The existence of a detailed theoretical model, like what Figueroa is developing, will likely make microbial treatment of acid mine drainage a much more practical option than it presently is.