Issue (Who cares and why?)
Mining brings to the surface rock that has been buried for millions of years, exposing it to air and weather. This weathering causes the rock to oxidize, which forms sulfuric acids that lower the pH of water running over the rock to toxic levels. The acidic water then leaches metals out of the rock and poisons the waterways.

Bioreactors, like a sewage treatment plant, collects the toxic water before it flows into nearby streams and adds certain bacteria (sulphur-reducing bacteria). These bacteria are typically fed ethanol to power a series of biological processes. The bacteria turn the sulfuric acid back into sulfides, which combine with the leached toxic metals and pull them out of the water. This produces sludge at the bottom of the bioreactor ponds, which is later hauled away, while the clean water flows freely over the top.

The only problem with this system is that ethanol can costs up to $4 per gallon. This project investigated alternative solutions to feed the bacteria.

What has been done?
Researchers investigated the use of biodiesel waste as a food source for sulfate reducing bioreactors. To determine the operational response of the bioreactor to supplementation with different alcohol substrates, the original ethanol was removed and replaced with biodiesel waste fluid. Researchers found that waste fluid remaining after biodiesel fuel production contains primarily methanol and glycerol, alcohols utilized by sulfate-reducing bacteria, and the leftover potassium hydroxide neutralizes acidic water.

The bioreactor successfully transitioned to biodiesel waste over a 55-day pilot test. Sulfate reduction was maintained at 10-12% and metals were successfully removed below regulatory limits under normal operating conditions.

During the transition, the microbial community profile was tracked. The profiles indicate an unchanging community in the acclimated ethanol-fed bioreactor and a changing community upon exposure to bio-diesel waste. The profile technique may offer an inexpensive method for tracking changes in bioreactor microbial populations.

A laboratory-scale, sulfate-reducing bioreactor column study was used to investigate removal of arsenic, selenium, and sulfate in neutral to alkaline simulated mine water.

Impact
Bently Nevada, a Gardnerville corporation, manufactures about 300,000 gallons of biodiesel fuel each year by recycling used french fry oil. This process leaves about 20,000 gallons of waste. This waste, which was originally food-grade oil with potassium hydroxide and methanol added to convert it to biodiesel fuel, appears to be as good as ethanol as a food for the bacteria and also raises the pH.

Professor Glenn Miller states, “If Bently Nevada can ultimately be paid for its waste product, the company could make biodiesel fuel more financially attractive. It’s a win-win-win.”

“While all of these carbon sources do the job, we feel that biodiesel waste has the most promise, because it is not only inexpensive, but it works well and is readily available as a waste product.”

Researchers have also developed methods to assess the acclimation and health status of the bioreactors using molecular methods, and found methods that show promise for quickly evaluating the status of the bioreactors. Professor Miller claims that, “This system is cost-effective and is very likely the method of choice for treating acid mine drainage in remote locations where power is limited.”

Given the potential for serious environmental damage and burdensome reclamation costs, it is practical to seek long-term, cost effective treatments for acid mine drainage. The potential advantages of passive treatment are lower costs, fewer site visits required, ability to work in remote areas, opportunities to use recycled or waste materials, and more natural appearance.