Design and operation insights concerning a pilot-scale S0-driven autotrophic denitrification packed-bed process

Elemental sulfur autotrophic denitrification (S0AD) is viewed as a promising alternative to conventional heterotrophic denitrification due to lower running costs, zero carbon dioxide emission, and minimum excess sludge production. However, its scale-up capability and robustness in treating real-life wastewater have not been convincedly demonstrated. In this study, a pilot-scale S0AD packed-bed with over 1000 m3/d of actual wastewater treatment capacity was operated for 197 days. The S0AD packed-bed could effectively remove nitrate to below 12 mg-NO3−-N/L that is 20% stricter than China's national standard (15 mg-TN/L). The temperature effect coefficient Q10 was calculated as 1.01, illustrating that denitrification efficiency could be doubled when the temperature increased every 10 °C. Mass balance calculations indicated that 85% of removed nitrate was contributed by S0AD process, and the rest 15% was by heterotrophic and assimilative processes. A total of 2684 kg sulfur was consumed during the course of the experiment which was attributed to 16.1% DO oxidation and 83.9% denitrification. Nitrogen gas produced through denitrification, could be trapped in the S0 packed-bed that was the primary causality of clogging. Daily gas venting instead of conventional back washing, could effectively recover the packed-bed flux and promote denitrification efficiency. In addition, a weekly thorough back washing was deemed necessary to deeply clean the trapped SS and sloughed overgrowing biofilm. The predominant S0AD bacteria belonged to the genus Thiobacillus, was enriched throughout the packed-bed providing robust and stable denitrification. Overall, we provided some guidance to the design and operation of S0AD packed bed in practical engineering.