Achieving a Novel Polysulfide-Involved Sulfur-Based Autotrophic DenitrificationProcess for High-Rate Nitrogen Removal in Elemental Sulfur-Packed Bed Reactors

Elemental
sulfur-driven autotrophic denitrification (SADN) process
is becoming a promising and cost-effective process for treating low
C/N ratio wastewaters. However, the low bioavailability of sulfur
particles is a key technical barrier of the SADN process, resulting
in a low-nitrogen removal rate and restricting process upscaling.
Given that polysulfuration from the abiotic reaction between sulfur
and sulfide can significantly improve sulfur bioavailability, we proposed
a novel polysulfide-involved SADN (PiSADN) process to achieve high-rate
autotrophic nitrate removal. In the two sulfur-packed bed reactors
(S0-PBRs) operated in parallel, we realized the PiSADN
process to accelerate nitrate removal without the addition of exogenous
chemicals (i.e., sulfide and organics). The results showed that high
nitrate removal rates (1.46–1.65 kg N/m3-d) were
obtained in the S0-PBRs during the long-term trials, which
were substantially higher than those previously reported in SADN systems
under similar conditions. Sulfur disproportionation (SD) was found
to be the key biochemical reaction for the establishment of a self-driven
PiSADN process because SD could produce sulfide and stimulate polysulfuration.
16S rRNA gene amplicon sequencing and genome-centric metagenomics
analysis provided evidence for the presence of sulfur-disproportionating
bacteria in the bioreactors (e.g., Dissulfurimicrobium sp.). Increasing initial bicarbonate concentration and/or pH level
could enhance the SD process and polysulfuration and then facilitate
the PiSADN process. These findings suggested that the PiSADN process
could achieve highly efficient autotrophic denitrification with SD
being a probable key factor for maintaining the self-driven PiSADN
process.