Thermodynamic Inhibition of Microbial Sulfur Disproportionation in a Multisubunit Designed Sulfur-Siderite Packed Bioreactor

Sulfur disproportionation ((SDP)-D-0) poses a challenge to the robust application of sulfur autotrophic denitrification due to unpredictable sulfide production, which risks the safety of downstream ecosystems. This study explored the (SDP)-D-0 occurrence boundaries with nitrate loading and temperature effects. The boundary values increased with the increase in temperature, exhibiting below 0.15 and 0.53 kg-N/m(3)/d of nitrate loading at 20 and 30 degree celsius, respectively. A pilot-scale sulfur-siderite packed bioreactor (150 m(3)/d treatment capacity) was optimally designed with multiple subunits to dynamically distribute the loading of sulfur-heterologous electron acceptors. Operating two active and one standby subunit achieved an effective denitrification rate of 0.31 kg-N/m(3)/d at 20 degree celsius. For the standby subunit, involving oxygen by aeration effectively transformed the facultative (SDP)-D-0 functional community from (SDP)-D-0 metabolism to aerobic respiration, but with enormous sulfur consumption resulting in ongoing sulfate production of over 3000 mg/L. Meanwhile, acidification by the sulfur oxidation process could reduce the pH to as low as 2.5, which evaluated the Gibbs free energy (Delta G) of the (SDP)-D-0 reaction to +2.56 kJ, thermodynamically suppressing the (SDP)-D-0 occurrence. Therefore, a multisubunit design along with (SDP)-D-0 inhibition strategies of short-term aeration and long-term acidification is suggested for managing (SDP)-D-0 in various practical sulfur-packed bioreactors.