Wastewaters from mining and mineral processing, generally known as acid mine drainage (AMD), are characterized by low pH and high concentrations of sulfate (SO₄^2-) and dissolved metals. These wastewaters may be toxic to many microorganisms and are a potential source of contamination for superficial water bodies and groundwater. Traditionally used techniques for the treatment of AMD are based on chemical neutralization and precipitation. The disadvantages of chemical treatment include a high cost of chemical reagents and the production of a bulky sludge, which must be disposed of. In view of this, efforts have been made to develop biological alternatives for the treatment of AMD. Sulfate reduction has become a suitable alternative for the treatment of wastewaters that contain oxidized sulfur compounds like AMD; sulfate reducing bacteria (SRB) reduce sulfate to hydrogen sulfide using organic compounds or hydrogen as electron donors. Hydrogen sulfide precipitates metals as sulfides and the alkalinity produced by the microbial metabolism allows the neutralization of AMD [1].

Sulfate reducing fluidized bed reactors are a promising alternative for AMD treatment. In this reactor configuration, bacteria form a biofilm on inert carrier material particles with large specific area that achieves higher biomass retention. Moreover, fluidification provides proper liquid mixing. In the inverse fluidized bed reactor (IFBR) the carrier material floats at the top, fluidification is achieved by a down-flow current of liquid, and the inverse flow allows the recuperation of particles at the bottom of the reactor. In addition the liquid and biogas are flowing in opposite direction, which helps for bed expansion [2]. The present work reports the performance of an IFBR for the anaerobic treatment of synthetic AMD.

The experimental work was carried out in a laboratory scale IFBR of 2.5 L. The IFBR was inoculated with sulfate reducing granular sludge equivalent to 1.63 grams of volatile suspended solids (VSS). As support, 600 mL of fine low density polyethylene particles were used, with an average diameter of 500 µm and an apparent density of 400 Kg/m³. The material was fluidized by recirculation flow (550 mL/min – 750 mL/min equivalent to a superficial liquid velocity of 13.6 and 18.9 m/h, respectively) maintaining 30 to 50% bed expansion (of the reactor volume). The system was fed with a synthetic influent constituted of mineral media (pH = 7), and ethanol/lactate or ethanol as electron donors. The organic loading rate varied from 0.5 to 2.5 g chemical oxygen demand (COD)/L-day; sulfate was added as electron acceptor an the sulfate loading rate varied from 1.5 to 3 g SO₄^2-/L-day (The COD/SO₄^2- ratio was fixed to 0.83, in order to promote the enrichment of sulfate reducing bacteria). Once stable sulfate reducing conditions were reached, Iron (FeCl₂•4H₂O) was fed into the reactor at a concentration of 100 mg/L, controlling the pH of the influent at 5.0 to ensure that the iron remained soluble. The performance of the reactor was evaluated through organic material and sulfate removal efficiencies, sulfide production and removal/precipitation of soluble Fe (as FeS). The biofilm was characterized by SEM, whereas the insoluble precipitate was characterized by EDX and XDR to determinate the composition and crystalline structures formed. The IFBR has been operated in continuous mode for more than 275 days at hydraulic retention times in the range of 48 to 24 hours and ambient temperature (18 to 30 °C).

The average efficiencies of COD removal and sulfate conversion were 50 and 38.8%, respectively, and the sulfide production reached values up to 436 mg/L indicating that the immobilization of SRB in the plastic support was achieved. The SRB immobilized in the biofilm did not completely oxidize the substrates as was evidenced by the acetate found in the effluent. The specific sulfate reducing activities with ethanol and lactate were 3.96 and 8.44 g COD-H₂S/g immobilized volatile solids (IVS)-day, respectively, and the immobilized solids reached values up to 1.4 g IVS/L_support. These results indicated that the biofilm had a higher sulfate reducing activity when comparing the values obtained with those of sulfidogenic granular sludge (0.45 to 2.1 g COD-H₂S/ g VSS-day) with volatile fatty acids as electron donors. After Iron was introduced into the reactor, the performance of the IFBR remained unaffected. Metal precipitation was 99% and effluent soluble Fe concentration was below 1 mg/L. Higher concentrations of Fe and other soluble metals (Zn, Cd, Cu) will be added to the system. This system has a potential application for the treatment of effluents that contain metals which react immediately with the produced sulfide and form insoluble metal sulfides that can be recovered easily at the bottom of the reactor.

[1] Kaksonen et al. 2004. Biotechnol Bioeng 86:332-343. [2] Celis-García et al. 2007. Biotechnol Bioeng. In press. DOI: 10.1002/bit.21288