Enhancing sulfur-based denitrification with natural zeolite: performance under autotrophic and mixotrophic operation

In this study, the influence of natural zeolite on sulfur-based denitrification was evaluated through batch assays and parallel up-flow packed-bed biofilters operated under autotrophic and mixotrophic conditions. Reactors containing elemental sulfur alone and sulfur co-packed with natural zeolite were operated at nitrogen loading rates ranging from 1.08 to 12.87 mg N L⁻¹ h⁻¹. During the mixotrophic stages, methanol was added as the organic carbon source to both up-flow packed-bed biofilters, one packed with elemental sulfur (S⁰) alone and the other with a mixture of S⁰ and natural zeolite, to achieve C/N ratios ranging from 0.9 to 2.85. In batch experiments, the addition of zeolite enhanced autotrophic denitrification kinetics, increasing specific nitrate removal rates from 27–32 to 31–39 mg NO₃⁻-N g⁻¹ VSS d⁻¹, while significantly reducing nitrite accumulation. In continuous reactors, zeolite showed limited influence at moderate nitrogen loading rates (NLRs) but substantially improved reactor performance at higher NLRs, maintaining nitrate removal efficiencies up to 87.3% at 12.87 mg N L⁻¹ h⁻¹, compared to 71.9% in the sulfur-only reactor. Under mixotrophic conditions, the sulfur-zeolite reactor consistently achieved higher nitrate removal efficiencies (94.5–99.3%) and lower effluent nitrate and nitrite concentrations than the control system. These results demonstrate that natural zeolite enhanced the system’s robustness under high NLR and improved performance under mixotrophic conditions.