Anaerobic ammonium oxidation (ANAMMOX) is an innovative process for the treatment of ammonia-contaminated waters. ANNAMOX is usually preceded by a nitrifying step in which ammonia is partially oxidized to nitrite. The effectiveness of the overall process depends on control of the nitrification and creation of a suitable influent for ANAMMOX. In this work, impacts of ammonia concentration and loading rate on partial nitrification and composition of the resulting effluent were investigated in continuous stirred tank (CSTR) and biofilm reactors fed with various ammonia concentrations (17.6-61.5 mM; 299-1045 ppm). Regardless of ammonia concentration, loading rates from 3.1 to 5.4mM/h in the CSTR and 6.4-12.1 mM/h in the biofilm reactor generated effluents with nitrite to ammonia ratios of 1.2 +/- 0.3 (suitable ANAMMOX influent). Under these conditions, the highest ammonia loading and nitrite production rates in the CSTR and biofilm reactors were 5.4 and 2.5 mM/h (HRT: 3.7 h) and 12.1 and 6.5 mM/h (HRT: 1.6 h), respectively. Results reveal that ammonia loading rate can be used effectively to achieve suitable ANAMMOX influent without the need for precise control of dissolved oxygen (DO). Considering the difficulty in regulating DO in large-scale systems and the need for the nitrifying process to be flexible with respect to various ammonia concentrations, the loading rate appears to be a practical option to control partial nitrification. Verifying the range of ammonia loading rates that generate ANAMMOX influent allows operation of the nitrifying step with any level of ammonia in the feed, with the proper loading rate achieved through adjustment of hydraulic residence time.