Integrated biological sulfidogenic process: Synergizing sulfate and elemental sulfur reduction for metal-laden...

Biological sulfidogenic processes (BSPs), involving either sulfate reduction process or sulfur reduction processes, are promising technologies for treating metal-laden wastewater. However, the extreme acidity and oligotrophic nature characteristic of metal-laden wastewater frequently limit the efficiency and cost-effectiveness of conventional BSPs. This study developed an integrated biological sulfidogenic process that couples sulfate reduction with elemental sulfur reduction to circumvent the limitations imposed by low pH and limited carbon availability. Over 210 days continuous operation, the integrated system demonstrated robust acid neutralization, primarily attributed to the significant buffering capacity of the sulfate-reducing bioreactor. Enriched with acid-tolerant sulfate-reducing bacteria and fermentative bacteria, the sulfate-reducing bioreactor remained stable even as the influent pH decreased from 6.13 to 3.21, consistently raising the pH to approximately 7.0. Notably, even under extreme acidity and nutrient-deficient conditions, a modest extent of sulfate reduction (31.8 mg S/L) was sufficient to elevate the influent pH from 3.45 to a near-neutral 6.37. This acidity neutralization protected the subsequent sulfur reduction process from acid inhibition, enabling high-rate biogenic sulfide production under near-neutral pH conditions. The sulfur-reducing bioreactor produced sulfide concentrations over 1000 mg S/L with a low carbon-to-sulfide (C/S) ratio of 0.30, demonstrating high carbon utilization efficiency. As a result, the integrated process achieved 100% removal of Zn, Cu, and Pb, while reducing chemical reagent costs by 38%-72% compared to single-stage systems. Overall, this integrated biological sulfidogenic process establishes a positive feedback synergy among acidity neutralization, biogenic sulfide production, and heavy metal removal, offering a cost-efficient and reliable solution for treating acidic and oligotrophic metal-laden wastewater.