Copper (Cu) is a toxic heavy metal which can cause inhibitory effects on nitrification process in biological wastewater treatment systems. However, few studies have done to dose accelerators into Cu-inhibited activated sludge system to promote bioactivity recovery of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). In this study, four ingredients of bio-accelerators were developed and dosed in five parallel SBRs. The effluent NH4+-N, NO2-N and NO3-N concentrations, specific oxygen uptake rate (SOUR), and Cu distribution were investigated to assess the bioactivity recovery effects. Results showed that, among biotin, L-aspartic acid and cytokinin, L-aspartic acid was the most effective for the fast bioactivity recovery of nitrifying bacteria. The combination of these three substances was optimum and Cu-inhibited activated sludge system could be 100% recovered in 8 days. The bio-accelerators proposed in this study can be potentially applied in the emergency treatment in biological wastewater treatment systems. (C) 2015 Elsevier Ltd. All rights reserved.