Overcoming the Obstacles of Operating a Biochemical Reactor and Aerobic Polishing Cell Year Round in Central Montana

Abstract

A demonstration-scale passive treatment system (PTS) including a biochemical reactor (BCR) and an aerobic polishing cell (APC) has been constructed at a historic gold mine in Central Montana. This site provided the challenge of being located where the frost depth is over four feet and temperatures dropped to negative 40 F in the winter of 2007-2008. The demonstration-scale PTS, including the APC, was operated through the winter. This paper presents the results of the testing as well as the special consideration and precautions taken to ensure the PTS could function properly year round. The BCR uses a mixture of organic components including wood chips, sawdust, hay, limestone, manure, and crushed basalt to remove constituents of concern (COC), including thallium, selenium, zinc, and nitrate from the mining influenced water (MIW) at the site. As the MIW passes through the BCR, a suite of biological and chemical reactions (biological reduction, metal sulfide precipitation, metal hydroxide precipitation, adsorption, etc.) combine to reduce the COC concentrations in the effluent. Over the first 14 months of BCR operation thallium was removed at >99%, selenium was removed at >99% until BCR maintenance disturbed the substrate in the fall of 2008, and zinc and nitrate were removed to non detect levels throughout. The BCR has operated successfully through two winters where temperatures reached levels as low as -40 F for up to weeks at a time. Due to the natural degradation of the organic substrate mixture used in the BCR, the water exiting the cell has elevated levels of organic matter as well as manganese, iron, and arsenic. These constituents are removed in an APC, which is comprised of a series of ponds that contain vegetation and large surface area to promote aeration of the water. As oxygen is introduced to the water, the biochemical oxygen demand, manganese, iron, and arsenic are changed from dissolved to particulate form and either settle or are filtered out by the vegetation in the APC. The APC has yet to operate through a full winter without retrofits and upgrades. Additional