Effect of the method of falling water aeration-reflux on nitrogen removal and applicability in a novel upflow microaerobic sludge reactor treating low carbon-to-nitrogen ratio wastewater.

• A novel upflow microaerobic sludge reactor was constructed. • The removal efficiencies of NH 4 +-N and TN reached 90.53% and 80.77%, respectively. • AOB, NOB, anammox, autotrophic and heterotrophic denitrifiers cooperated in UMSR. • Partial denitrification/anammox was dominant nitrogen conversion pathway. • The small height waterfall was applicable to treat low strength NH 4 +-N wastewater. A novel falling water aeration-external reflux upflow microaerobic sludge reactor (UMSR) was designed to treat wastewater with the low chemical oxygen demand (COD) to total nitrogen (TN) ratio. The result showed the concentration of dissolved oxygen (DO) in the reactor could be accurately controlled by adjusting the reflux ratio of oxygenated water. The higher aeration efficiency in pollutant removal could be obtained by the reoxygenation mode of the small height falling water. At the reflux ratio of 5:1, the ammonium, nitrite and nitrate nitrogen concentrations in the effluent of UMSR were 6.0, 0.4 and 6.1 mg/L on average, respectively. The removal efficiency of ammonium nitrogen and total nitrogen reached 90.53% and 80.77%, respectively with the influent COD/TN as being 1.0. The structure of the microbial community confirmed the existence of partial-denitrification/anaerobic ammonium oxidation (anammox) bacteria, autotrophic and heterotrophic denitrifiers contributed to nitrogen and carbon removal in UMSR. [ABSTRACT FROM AUTHOR]