Nitrate limitation and biogenic sulfide formation in sulfur autotrophic denitrification reactors: Mechanisms, restart risks, and an idle-tolerance framework

Sulfur autotrophic denitrification (SAD) removes nitrate without organic carbon, but intermittent operation leaves elemental sulfur and biofilm in reactors after nitrate supply stops. This review examines sulfide and ammonium generation in SAD reactors under nitrate limitation and nitrate deprivation, the consequences at restart, and management options. Evidence was compiled through a structured search of Web of Science, Scopus and PubMed and graded on a five-level scale according to how directly each finding was measured under nitrate-free conditions; only three experimental conditions reached the highest level, and none involved a flooded idle beyond seven days. Microbial sulfur disproportionation is exergonic under operating conditions and requires only retained sulfur and water, whereas sulfate reduction is limited by finite endogenous electron donors, and dissimilatory nitrate reduction to ammonium is confined to transition and restart windows. Continuous nitrate-scarce operation produced 5.5–43.9 mg/L effluent sulfide, depending on contact time, whereas a seven-day nitrate-free shutdown reached 94.5 mg-S/L within two days, releasing ammonium from biomass decay and headspace H₂S that poses an occupational hazard; yet denitrification recovered immediately. Accumulation appears bounded at about 100–120 mg-S/L by H₂S inhibition of disproportionating bacteria rather than by thermodynamics, and iron minerals raise this bound by scavenging sulfide. The proposed idle-tolerance framework spans reactor design, operation, microbial management and monitoring, is organized by idle duration, and includes temperature-indexed maintenance nitrate loadings and downstream, not composited, iron capture. The framework is offered as a set of testable design hypotheses derived from this evidence map, not as validated practice.