To investigate the applicability of ratio control strategy to other systems, a continuous floccular sludge reactor was used in this study. It was found that nitrite accumulation was barely detected throughout 70 days' investigation, being the average concentration in the effluent of 0.7 +/- 0.4 mg/L. Batch experi-ments indicated that low dissolved oxygen (DO < 0.3 mg center dot L (-1)) greatly repressed the ammonium oxidizing bacteria (AOB) but only slightly inhibited the nitrite oxidizing bacteria (NOB). However, high-throughput sequencing revealed that the ratio of abundance between Nitrospira and Nitrosomonas, being the domi-nant NOB and AOB respectively, was considerably low (1.2%/18.7%). The weak oxygen gradients in floc-cular sludge and the selectively enriched K-strategist NOB Nitrospira under oxygen-limited conditions were both contributed to the failure of achieving partial nitritation; therefore, the rapid start-up of partial nitritation process based on proposed ratio control strategy is not feasible for continuous floccular sludge systems treating low-strength wastewater.(C) 2016 Published by Elsevier Ltd.