Biologic removal of sulfate from acid mine drainage in an anaerobic sequencing batch reactor: Temperature and inoculum origin effects on the startup operation

Highlights

• Methanogenic and sulfidogenic inoculum at different temperatures were studied.

• Operating temperature and inoculum origin influence sulfate removal in a bioreactor.

• High sulfate removal rate can be achieved with a sludge of methanogenic origin.

• Species were selected at 55 °C, resulting in a decrease in the SRBs population.

Abstract

Acid mine drainage represents a major cause of environmental impact associated with the activity of mineral extractive industries. In this study, we evaluated the treatment of this wastewater through an anaerobic sequencing batch reactor at 15, 30, and 55 °C with a sludge of sulfidogenic and methanogenic origin. The operation at 30 °C with sulfidogenic sludge showed the highest average removal of sulfate (80 %), while the highest average removal of COD (92 %) occurred at 30 °C with methanogenic sludge. The sulfidogenic phases showed lower apparent kinetic constants (K^ap), compared at the same temperature, with the methanogenic phases, according to the first-order kinetic adjustment, considering residual concentration. The sulfate-reducing bacteria reproduced at 15 °C, according to growth enumeration by the most probable number method, which demonstrates its contribution to the growth of communities present in the inoculum due to the environmental conditions.

Introduction

Acid mine drainage (AMD) is known as the acid solution generated from the contact between minerals containing sulfides and climatic agents, such as water and atmospheric air, besides to microbial activity. Its generation occurs naturally and due to the environmental impact, it represents a worldwide challenge for the mining industry [1,2]. AMD conventional treatment consists of physical-chemical neutralization processes. However, it has high costs related to reagents, operation, and maintenance, which requires electricity power [3,4]. In contrast, the biological treatment by sulfate-reducing bacteria (SRB), presented relevant results concerning the costs and efficiency of removing sulfate and metals [5].

Thus, sulfidogenic reactors of different configurations are required to the AMD treatment [[6], [7], [8], [9]], however, the sulfate and metals removal efficiency from AMD during the startup of biological reactors is highly influenced by the inoculum origin. The adaptation or startup phase is often overlooked or referred to as the first phase in studies involving bioreactors [10]. This period generally presents data variation and the initial sulfate concentration has a great influence on the selection of the microbial consortium and on the removal efficiency, according to the studies with concentrations of 500 [11], 1000 [10] and 1500 mg L⁻¹ [9] which obtained sulfate removals of 38.5 ± 3.7, 43 ± 13 and 39.1 ± 22.2 %, respectively.

The time between the adaptation phase to the wastewater and the stabilization phase, or maximum sulfate removal efficiency, may be reduced if the inoculum presents species adapted to the pollutants and the environmental conditions. However, there are difficulties in obtaining inoculants previously selected for the required operating conditions. The operating temperature of biological reactors, intended for the AMD treatment, may affect the sulfate and metals removal efficiency since it is a determining factor in the speed of chemical and enzymatic reactions, and may cause irreversible damage to cellular components. The kinetic parameters, such as the maximum consumption rate, may also be modified according to operational variations.

SRBs have representatives in psychrophilic, mesophilic, thermophilic, and hyperthermophilic groups [12]. Although, in general, the favorable environment for the growth of SRB is the same for mesophilic species, the obtained results of species in environments psychrophiles or thermophiles should not be underestimated. Knoblauch et al. [13] worked with 19 strains of psychrophilic SRB species isolated from Arctic sediments. The results indicated that psychrophilic SRB had higher specific metabolic rates for sulfate consumption than mesophilic species at low temperatures. To the authors’ knowledge, the influence of the temperature and inoculum origin has not been discussed in the anaerobic sequencing batch reactor (ASBR) startup, especially treating AMD.

In this way, aiming at the reproduction of different environments for the AMD treatment or even, the technological application in the treatment of sulfate-rich industrial wastewater, we proposed to evaluate the startup of an ASBR treating AMD at 15, 30 and 55 °C, with inoculants of sulfidogenic (adapted) and methanogenic (non-adapted) origin. Chemical oxygen demand (COD) and sulfate removal kinetics were evaluated by first-order kinetic adjustment, considering the residual concentration. The most probable number (MPN) method enumerated statistically the growth of microorganisms in the SRB groups.