Mitigating sulfur disproportionation during autotrophic denitrification in sulfur packed-bed reactors for groundwater nitrate removal.

Sulfur packed-bed bioreactor (S0-PBR) for sulfur autotrophic denitrification (SADN) process exhibit significant potential for groundwater nitrate remediation owing to their cost-effectiveness. However, sulfide accumulation resulting from the sulfur autotrophic disproportionation (SADP) significantly limits its engineering application. In this work, synergistic regulation of a S0-PBR was achieved by controlling sulfur particle size and empty bed contact time (EBCT), enabling highly efficient ex situ nitrate removal from groundwater while significantly mitigating sulfur disproportionation. The system attained an unprecedented nitrate removal rate (1.92 kg NO3--N/m3/d) by initiating with S0 particles at smaller size (i.e., 0.075-0.1 mm), which was 2.7-7.1 times superior to previously established S0-PBRs in groundwater treatment. However, low nitrate loading rates (0.08 kg NO3--N/m3/d) could promote the growth of sulfur-disproportionating bacteria (SDB) (relative abundance >2.5 %), producing sulfide at up to 40.12 mg/L. Nitrate shortage (i.e., NO3--N < 4.8 mg/L) was confirmed to be the primary factor inducing SADP process, which was effectively mitigated by reducing the EBCT from 6 h to 0.25 h, resulting in the enhancement in nitrate loading from 0.08 to 1.92 kg NO3--N/m3/d. The findings suggest that adequate nitrate loading will be necessary for limiting sulfur disproportionation during sulfur autotrophic denitrification processes when treating groundwater.