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Sulfate reducing passive bioreactors are increasingly being used to remove metals and raise the pH of acidic waste streams from abandoned mines. These systems commonly use a variety of organic substrates (i.e. manure, wood chips) for sulfate reduction. The effectiveness of these systems decreases as easily accessible reducing equivalents are consumed in the substrate through microbial activity. Using column studies at room temperature (23–26°C), we investigated the addition of lactate and methanol to a depleted manure substrate as a method to reactivate a bioreactor that had lost >95% of sulfate reduction activity. A preliminary experiment compared sulfate removal in gravity fed, flow through bioreactors in which similar masses of each substrate were added to the influent solution. Addition of 148 mg/l lactate resulted in a 69% reduction in sulfate concentration from 300 to 92 mg/l, while addition of 144 mg/l methanol resulted in an 88% reduction in sulfate concentration from 300 to 36 mg/l. Because methanol was found to be an effective sulfate reducing substrate, it was chosen for further experiments due to its inherent physical properties (cost, low freezing point and low viscosity liquid) that make it a superior substrate for remote, high elevation sites where freezing temperatures would hamper the use of aqueous solutions. In these column studies, water containing sulfate and ferrous iron was gravity-fed through the bioreactor columns, along with predetermined methanol concentrations containing reducing equivalents to remove 54% of the sulfate. Following an acclimation period for the columns, sulfate concentrations were reduced from of 900 mg/l in the influent to 454 mg/l in the effluent, that reflects a 93% efficiency of electrons from the donor to the terminal electron acceptor. Iron concentrations were reduced from 100 to 2 mg/l and the pH increased nearly 2 units.

The lifetime of traditional sulfate-reducing bacteria (SRB) bioreactors that utilize a source of reducing equivalents contained within the matrix (e.g. manure) is limited by the amount of readily available reducing equivalents within that matrix. In order to extend bioreactor lifetime indefinitely, the addition of known concentrations of alternative reducing equivalents (methanol and ethanol) to a depleted matrix was tested at low pH and low temperatures. Following acclimation, up to 100% efficiencies of reducing equivalents were directed toward sulfate reduction. Alcohol was added in stoichiometric concentrations to remove 50% of the added sulfate (900 mg/L), producing sufficient sulfide to precipitate all of the iron from solution. An average of 42% of the sulfate was removed following acclimation, reflecting 84% efficiency. An average of 93% of the iron was removed (93 mg/L). Bacteria acclimated to ethanol more rapidly than methanol, although both alcohols were effective as carbon sources. Efficient treatment was observed at the lowest temperatures (6 degrees C) and lowest pHs (pH=2.5) tested. The use of ethanol-fed, highly permeable bioreactor matrices of wood chip, pulverized plastic and rock was also examined to determine which of these porous matrices could be implemented in a field bioreactor. Results indicated that95% of the 100mg/L iron added was removed by all matrices. Sufficient reducing equivalents were added to remove 450 mg/L of sulfate, wood and rock matrices removed approximately 350 mg/L plastic removed approximately 225 mg/L. A study comparing rock size indicated that small rocks removed iron and sulfate more efficiently than medium- and large-size rocks. The results suggest that wood and rock in conjunction with ethanol are viable alternatives to traditional bioreactor matrices. These findings have direct application to semi-passive sustained operation of SRB bioreactors for treatment of acidic drainage at remote sites.