Anammox start-up can be limited by the availability of seeding biomass in some areas. Previous studies have listed suitable alternative seeding sludge for anammox start-up such as anaerobic digestion sludge and conventional activated sludge (CAS), the ecological reasons behind has long been ignored. In this study, the inherent ecological factors that trigger a prompt start-up of anammox were identified, focusing on the initial relative abundance and concentration of anammox bacteria. An external membrane bioreactor was utilized as an enriching tool due to its suitability of retaining cells. Results revealed that a high initial concentration of anammox bacteria benefitted the start-up, meanwhile an even community seeding sludge (Gini coefficient < 0.25) gained a more than three-time higher anammox activity compared to the uneven one (Gini coefficient > 0.5). The discovery reminds to select the seeding sludge that is ecologically appropriate rather than to only care for the type of sludge in general. (C) 2013 Elsevier Ltd. All rights reserved.