A dropping nitrification reactor was proposed as a low-cost and energy-saving option for the removal of NH₄⁺-N from contaminated groundwater. The objectives of this study were to investigate NH₄⁺-N removal performance and the nitrogen removal pathway and to characterize the microbial communities in the reactor. Polyolefin sponge cubes (10 mm × 10 mm × 10 mm) were connected diagonally in a nylon thread to produce 1 m long dropping nitrification units. Synthetic groundwater containing 50 mg L⁻¹ NH₄⁺-N was added from the top of the hanging units at a flow rate of 4.32 L day⁻¹ for 56 days. Nitrogen-oxidizing microorganisms in the reactor removed 50.8–68.7% of the NH₄⁺-N in the groundwater, which was aerated with atmospheric oxygen as it flowed downwards through the sponge units. Nitrogen transformation and the functional bacteria contributing to it were stratified in the sponge units. Nitrosomonadales-like AOB predominated and transformed NH₄⁺-N to NO₂⁻-N in the upper part of the reactor. Nitrospirales-like NOB predominated and transformed NO₂⁻-N to NO₃⁻-N in the lower part of the reactor. The dropping nitrification reactor could be a promising technology for oxidizing NH₄⁺-N in groundwater and other similar contaminated wastewaters.

Constructed wetlands (CWs) are an effective technology to remove organic compounds and nitrogen (N) from wastewaters and contaminated environmental waters. However, the feasibility of CWs for ammonium-N (NH₄⁺-N)-contaminated groundwater treatment is unclear. In this study, zeolite-based laboratory-scale CW was operated as a tidal flow CW with a cycle consisting of 21-h flooded and 3-h rest, and used to treat NH₄⁺-N (30 mg L⁻¹) contaminated groundwater. In addition to NH₄⁺-N, nitrite (NO₂⁻-N) and nitrate (NO₃⁻-N) were also not detected in the effluents from the tidal flow CW. The N removal constant remained high for a longer period of time compared to the continuous flow CW. The higher and more sustainable N removal of the tidal flow CW was due to the in-situ biological regeneration of zeolite NH₄⁺-N adsorption capacity. Vegetation of common reeds in tidal flow zeolite-based CW enhanced nitrification and heterotrophic denitrification activities, and increased the functional genes of nitrification (AOB-amoA and nxrA) and denitrification (narG, nirK, nirS, and nosZ) by 2‒3 orders of magnitude, compared to CW without vegetation. The results suggest that the combination of zeolite substrate, tidal flow, and vegetation is key for the highly efficient and sustainable N removal from NH₄⁺-N contaminated groundwater.