An integrated O/A two-stage packed-bed reactor (INT-PBR) for total nitrogen removal from low organic carbon wastewater

- A novel integrated O/A PBR reactor was developed for rural sewage under low COD/N.
- No return sludge and internal recycle, in contrast to conventional A/O process.
- TN removal efficiency reached >90% using organic-free synthetic wastewater.
- Autotrophic, heterotrophic & mixotrophic denitrifiers were combined in HAD process.

Total nitrogen (TN) removal from low organic carbon wastewater is challenging. Traditional heterotrophic denitrification requires additional organics as electron donors, potentially increasing the operational challenges and costs of wastewater treatment. Especially for decentralized domestic sewage in rural areas, it is essential to develop an economical and efficient integrated reactor to satisfy high TN removal demand. This study presents an integrated O/A two-stage packed-bed reactor (INT-PBR) composed of two concentric zones, with total effective volume of 30.6 L. Influent is pumped into the inner oxic (O) zone for nitrification, then flows through the outer anoxic (A) zone for denitrification. Iron-carbon micro-electrolysis carriers (IC-MECs), which provide H2/[H] and Fe2+ as inorganic electron donors for autotrophic denitrifiers, and luffa cylindrical sponge (LCS), which provides slow-release organic carbon for heterotrophic denitrifiers, were placed into the A zone to achieve combined heterotrophic and autotrophic denitrification (HAD) without requiring organic carbon in influent. Thus, the INT-PBR involves no return sludge and internal recycle. Using organic-free synthetic wastewater with 39.36 ± 0.74 mg/L NH4+-N, approximately 98% of ammonia in influent was transferred to nitrate in the O zone, and approximately 93% of TN was removed by HAD process at 30 ± 2 °C in the A zone. Microbial community analysis showed that autotrophic denitrifiers (e.g., Thiobacillus) and heterotrophic/mixotrophic denitrifiers (e.g., Hydrogenophaga) were combined in the reactor, consistent with the performance findings. The results suggest a promising technology for total nitrogen removal from wastewater with low organic carbon.

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