The paper focuses on the improvements to engineered features of a passive technology that has been used for remediation of acid rock drainage (ARD). This passive remedial technology, a sulfate-reducing bacteria (SRB) bioreactor, takes advantage of the ability of SRB that, if supplied with a source of organic carbon, can increase pH and alkalinity of the water and immobilize metals by precipitating them as metal sulfides or hydroxides.The remoteness of ARD sites and their abundance require that the design of an SRB bioreactor be simple and inexpensive. Therefore, bioreactors need to be designed to a size that allows for transportation using primitive roads. To satisfy these requirements a design for a modular treatment system was developed using reactive cartridges (RC) that are prefabricated as 2.44-meter diameter vessels. The RC has been designed so it supports the prime functional aspects of a bioreactor such as high permeability, ample supply of organic carbon, ability to maintain anaerobic conditions, and capacity to accumulate precipitated metals and means for their periodical removal, as needed. In addition, the configuration of the RC allows for an easy replacement of the organic carbon. The RCs can be transported to an ARD site and assembled into a treatment system with a number of modules as required by the ARD flow rate and the metals load. A bioreactor system consisting of four RCs will be installed at an abandoned mine site with ARD of pH 5 or lower and a significant load of metals. The process of site selection is in progress.The RC design was developed by the Mine Waste Technology Program (MWTP) at MSE Technology Applications (MSE), Butte, Montana, USA.

The US Environmental Protection Agency's Mine Waste Technology Program (MWTP) has emphasized the development of biogeochemically-based treatment technologies for mitigation of acid rock drainage (ARD). Progressive technology demonstrations by the MWTP over the past 15 years have resulted in improved operation of sulfate-reducing bacteria (SRB) bioreactors. Although using SRB to treat ARD is now fairly widespread, it was uncommon in the early 1990s when the MWTP used this innovative biotechnology. The first and longest running demonstration was an in situ bioreactor installed within the flooded subsurface workings of the Lilly/Orphan Boy Mine in 1994. The second project, at the Calliope Mine, compared the performance of several SRB bioreactor configurations and operational attributes, including lime pretreatment and reactor temperature. The third demonstration, at the Golden Sunlight Mine, consisted of two treatment steps with a recycle stream. The fourth project was an investigation of existing bioreactor designs and resulted in an improved bioreactor configuration. Significant findings included: (1) a mineshaft could be used as a long-term, in situ bioreactor, (2) SRB thrive in temperature extremes, (3) sulfide recycle effectively avoids contact of ARD with bacterial populations, and (4) ideal bioreactor substrate provides short-term and long-term nutrients, good support matrix, and enhanced permeability.

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Demonstrations and technology development conducted under the Mine Waste Technology Program (MWTP) have lead to significant improvements of sulfate-reducing bacteria (SRB) bioreactors used to treat acid rock drainage (ARD). Benefits of this technology include the reduction of dissolved metal ions to insoluble metal sulfides and the neutralization of the acid rock drainage resulting from the production of bicarbonate from the oxidation of organic nutrients by the sulfate-reducing bacteria. Over the past 15 years, MSE has conducted many bench-scale and design tests along with several pilot-scale field demonstrations. This paper addresses engineering design criteria including the selection of organic media, maintenance of system permeability, and strategies for mitigation of ARD at remote sites with relatively small discharge. Specific applications and results from field SRB bioreactors and associated research are presented. They include: (1) An in situ bioreactor in flooded subsurface mine workings where the bioreactor was operated from 1993 to 2005. (2) A set of field bioreactors that allowed various operational attributes to be evaluated including pretreatment and operational temperature. This demonstration was conducted over a three-year period. (3) A set of both anaerobic and aerobic field bioreactors that operated in staged fashion to show the comprehensiveness of bioreactor applications for acid rock drainage treatment. (4) A bench-test study that focused on the effectiveness of SRB to reduce dissolved sulfate and heavy metals. (5) A bench test and field verification of the advantages of horizontal flow configuration in SRB bioreactors. (6) A reactive cartridge, a component of a modular SRB bioreactor that can be easily transported to remote sites.

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