The manure connoisseur: Tackling acid mine drainage

Dr. Ronald Cohen came to Mines to work on cleaning up nuclear weapons sites and later received a certificate of special recognition from Congress, among other research accomplishments. Then something went horribly wrong. As Cohen put it, “Imagine me in a hardhat and rubber boots standing on top of a dump truck load of manure and I’m shoveling it into wheelbarrows as my graduate students are taking it into a mine.” Cohen continued, “That triggers thoughts of my mother telling me that if I get an education, I could use my head instead of my back, and what am I doing? I’m shoveling cow manure in a mine, where have I gone astray?”

Cohen went astray when he joined a team of professors trying to solve a particularly vexing problem, acid mine drainage. The problem is that it is impossible to simply plug the mineshaft because the pressure head is often enough to force water through fractured rock. This requires continual treatment of the mine drainage, which is expensive with traditional technologies. The goal of the research was to develop a method of treating acid mine drainage that was considerably less costly in terms of initial capital and operations costs. Researchers noticed, however, that wetlands were able to achieve a low level of treatment.

The initial researchers in this area tried constructing wetlands to treat acid mine drainage using peat and wetland plants. The dream was to design a perpetual system. Cohen’s view was that, “[It was] an admirable dream, but that’s all it was. Nobody knew what to do because nobody knew how it worked.” The wetlands lost their effectiveness after a month or two. This is where the Mines researchers started. Cohen joined Dr. Ronald Klusman, Dr. Thomas Wildeman, and later Dr. Linda Figueroa, who set out to understand the process. Their approach was, as Cohen put it, “Not to look at it as a black box, but to address how it works.”

In order to figure out what was happening, the team conducted experiments to isolate what processes were at work. Plants were not found to be doing anything. They discovered that it was sulfate reducing bacteria (SRB) taking the metals out of the water. The SRB were taking the sulfate, which was responsible for the low pH, and reducing it to sulfide. The sulfide was then reacting with the metals to make metal sulfides which were precipitating out of the water. The transformation to metal sulfides brought the metals back to the form that they had been in underground.

With an understanding of the processes at work, the researchers were then able to start designing an engineered system. According to Cohen, “The key was making microbes happy.” SRBs need a low or no oxygen environment to thrive. The first attempt was using peat like the earlier wetland experiments but it fizzled out just as quickly. The reason is because peat is the end of the decay cycle and is not a particularly good home for microbes. Other materials were tried, like hops waste. The winners turned out to be wood chips and livestock manure.

While the research failed to achieve the dream of a perpetual system, it made significant strides in terms of cost effective treatment. The manure does not last forever, but it can make it for four to six years before it needs to be replenished. It proved that microbiological can work well for low to moderate acid mine drainage flows, but not high flows. The SRB systems turned out to be between one tenth and one hundredth of the cost of a traditional system. The only frequent maintenance it needs is to have somebody check the system to make sure it is not plugged every few weeks and change the food out every three to five years.

Klusman and Wildman have retired since the successful demonstration of the SRB-based treatment systems. Figueroa has continued the study of what makes SRB “happy.” A better understanding of the needs of SRB will likely improve the effectiveness of processes in the future. Cohen has branched out into other methods of mitigating the impact of mining and is now teaching a class on the topic.