Molecular Evidence of Internal Carbon-Driven Partial Denitrification Annamox (PdNA) in a mainstream Pilot A-B System Coupled with Side-stream EBPR treating municipal wastewater

Achieving mainstream short-cut nitrogen removal via nitrite has been a carbon and energy-efficient goal which wastewater engineers are dedicated to explore. This study applied a novel pilot-scale A-B-S2EBPR system process integrated with sidestream enhanced biological phosphorus removal) to achieve the nitrite accumulation and downstream anammox for treating municipal wastewater. Nitrite accumulated to 5.5 ± 0.3 mg N/L in the intermittently aerated tanks of B-stage with the nitrite accumulation ratio (NAR) of 79.1 ± 6.5%. The final effluent concentration and removal efficiency of total inorganic nitrogen (TIN) were 4.6 ± 1.8 mg N/L and 84.9 ± 5.6%, respectively. Batch nitrification/denitrification activity tests and functional gene abundance of ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) suggested that the nitrite accumulation was mostly caused by partial denitrification without NOB-selection. The unique features of S2EBPR (longer anaerobic HRT/SRT, lower ORPs, high and more complex VFAs etc.) seemed to impact the nitrogen microbial communities: the conventional AOB kept at a very low level of 0.13 ± 0.13% during the operation period, and the dominant candidate internal carbon-accumulating heterotrophic genera of Acinetobacter (17.8 ± 15.5)% and Comamonadaceae (6.7 ± 3.4%) were highly enriched. Furthermore, the single-cell Raman spectroscopy-based intracellular polymer analysis revealed the dominate microorganisms that could utilize polyhydroxyalkanoates (PHA) as the potential internal carbon source to drive partial denitrification. This study provides insights and a new direction for implementing the mainstream PdNA short-cut nitrogen removal via incorporating S2EBPR into sustainable A-B process. ### Competing Interest Statement The authors have declared no competing interest.