The production of NO2 --N from NH4 + -N via partial nitrification is necessary for anaerobic ammonium oxidation (anammox). This study investigated partial nitrification using a dropping reactor that comprised a hanging 10 × 10 × 10 mm polyolefin sponge cube with an effective length of 30 cm. In this present study, a dropping reactor was proposed for the partial nitrification. This configuration simulated a down-flow hanging sponge reactor. The synthetic NH4 + -N contaminated groundwater (40 mg/L) was fed at a 1.0-5.0 mL/min flow rate for 287 d. We found that the efficiency of the partial nitrification method increased with an increase in the synthetic groundwater flow rate. The partial nitrification percentage (NO 2 --N in effluent) was 11.0-59.5% at 4.0-5.0 mL/min. The amoA gene of ammonia-oxidizing bacteria (AOB) and nxrA gene of nitrite-oxidizing bacteria (NOB) and 16S rRNA genes of Nitrosomonas (AOB) Nitrobacter and Nitrospira (NOB) were detected in the sponge material of the partial nitrification reactor. The AOB and NOB might have contributed to partial and complete nitrification. The results suggested that the proposed partial nitrification method effectively produced NO2from NH4 + in the contaminated groundwater without the high cost of aeration. Therefore, this new method could costeffectively provide the NO2required for anammox.