Bed-immersion-ratio variation as an efficient strategy to regulate denitrification efficiency directionally in elemental sulfur packed-bed reactors

Autotrophic denitrification in sulfur packed-bed reactors (S⁰PBR) has been widely employed for treating municipal secondary effluent. However, the fixed volume of packed sulfur in S⁰PBR restricts the ability to adjust denitrification efficiency in response to fluctuating influent nitrate levels, leading to either effluent standard exceedances or unnecessary sulfur consumption. Here, we proposed a novel method for directionally regulating nitrate removal efficiency (NRE) in S⁰PBR by adjusting the bed-immersion-ratio (BIR). The results demonstrated that the NRE could be effectively controlled through adjustments of BIR, with the maximum NRE achieved when BIR was increased to 1.00. Notably, a non-zero minimum NRE was observed when BIR decreased to 0.00. This could be associated with actual hydraulic retention time (aHRT), with a strong correlation observed between aHRT and BIR. Based on these findings, a kinetic model was developed that integrated both exposure and immersion parts, expressed as R_j=(C_in-0^1/2-C_e^1/2)Q2.54AH_t-0.65. This model proved effective for describing the nitrate reduction kinetics in the S⁰PBR under BIR variation. The effluent concentration achieved a more stable designed nitrate level under BIR variation guided by our developed model, compared to the effluent nitrate concentration under full immersion. Besides, sulfur consumption could be reduced by 7.8 % to 31.6 % under BIR variation due to the reduced nitrate removal. Overall, this study proposed a novel approach for achieving stable nitrate effluent guided by our developed model under fluctuating influent nitrate loadings, which was both efficient and economical.