In the single-sludge reactor system treating high-strength nitrogen wastewater (similar to anaerobically pretreated piggery wastewater), the NH4+-N removal efficiencies (98-82%) are higher than total nitrogen removal efficiencies (71-43%). The mixed liquor recycle ratio only imposes a slight effect on total nitrogen removal efficiency. The alkalinity change data could be used for monitoring and control of the reactor system. To evaluate the performance of the single-sludge reactor system, a simplified nitrification-denitrification model (with nitrification capacity, denitrification capacity, and denitrification potential concepts) and a graphically analytical technique are proposed. It turns out that ammonia nitrification and total nitrogen removal efficiencies are strongly dependent on the process load and reactor configuration, and an optimal operating condition requires a proper match between nitrification and denitrification.