Fe(Ⅱ/Ⅲ) cycling-based buffering strategy to mitigate sulfide and nitrate overflow in sulfur autotrophic denitrification process under wastewater quality fluctuations.

The sulfur autotrophic denitrification (SAD) process has been widely applied in wastewater treatment plants and constructed wetlands due to its advantages in minimizing sludge production and avoiding COD exceedance. However, under fluctuating water quality conditions, the SAD process is prone to sulfide and nitrate overflow, potentially leading to severe ecological consequences. In this study, we propose a novel strategy by adding an Fe-based buffer packed-bed (FB) system downstream of the SAD process to buffer sulfide and nitrate overflow through Fe(Ⅱ/Ⅲ) cyclic redox reactions on the filler surface. This strategy demonstrated sustained sulfide and nitrate removal for over 15 days with only trace iron losses under extreme water quality variations, confirming its effective and sustainable buffering capacity. The FB system stored excess sulfide from the SAD process under low-nitrate conditions and utilized it for denitrification during periods of nitrate overload. This electron storage capability significantly reduced sulfur loss caused by sulfur disproportionation. The buffering effect and electron storage were mediated by FeO and (S, Fe(Ⅱ)-S, and Fe(Ⅱ)-O) adsorbed onto the magnetite surface. These reactive media were immobilized through the biofilm on the surface of the fillers in the FB system, and there was a significant correlation between the biofilm content and the buffering capacity. This work provides an efficient, seamless, and sustainable safeguard strategy for the application of SAD systems in wastewater treatment systems.