Sulfate Removal in Biochemical Reactors and Scrubbers Treating Neutral Low-Metal Concentration Miw

Abstract

Abstract. Sulfate and metals are commonly found in mining influenced water (MIW). A biochemical reactor (BCR) is an established technology that can remove sulfate and metals. Three organic mixtures were bench-tested for approximately six months to decrease sulfate concentration in a circum-neutral pH MIW containing low metal concentrations. Organic materials included wood pellets, oat straw, biochar, and manure as an inoculum. These were blended with limestonedolomite sand. Additionally, sulfide polishing units (SPUs), which were charged with native soil from the site, zero valent iron (ZVI), or magnetite, were evaluated for removal of dissolved sulfide discharged from each of the BCRs. Median MIW influent contained about 3000 mg/L of sulfate and very low concentrations of metals. The flow rates varied from 144 to 1,231 mL/day. Among all the BCRs tested, the hydraulic retention times varied from 5 to 75 days. All BCRs demonstrated similar removal rates of about 1.3 (BCR 1), 1.4 (BCR 2), and 1.6 (BCR 3) mol SO4 -2 /m 3 -day during the last week of testing. While the SPUs removed dissolved sulfide from the BCR effluents as expected, they removed sulfate as well. Dissolved organic carbon in the BCR effluents promoted sulfatereducing microbial activity in the SPUs where the inorganic materials functioned as a solid support for the microbial community. The magnetite was not an effective medium for post-BCR sulfate removal. Sulfate removal efficiencies in the BCRs were 55% (BCR 1), 57% (BCR 2), and 67% (BCR 3) during the final week of the bench-scale testing. Sulfate removal in the SPUs (from the BCRs effluents) was 35% and 37%, for SPU 1 and SPU 2, respectively. Novel reactor charging configurations in single units may therefore be much more effective and efficient than approaches exclusively using lignocellulosic or inert supports. Sulfate reducing microbial populations were still increasing when the test was concluded.

The Evolving Nature of Semi-passive Mine Water Treatment

“…One possible option is to develop passive and semi-passive treatment systems that rely on the anaerobic transformation of sulfate for its removal as hydrogen sulfide or metal sulfides. Most anaerobic passive treatment systems designed for metal removal only remove 5-15% of incoming sulfate, but bioreactors that were designed to remove sulfate have achieved higher removal rates, as high as 2.2 mol SO 4 −2 / m 3 -day (Fattore et al 2017;Walker 2017). Higher sulfate removal rates have been observed in permeable reactive barriers, where retention times are much higher (Benner et al 2002).…”