Anaerobic ammonium oxidation (anammox) is a promising biotechnology suitable for the removal of nitrogen from wastewaters. A proper choice of the seeding sludge and operational conditions is critical when dealing with the start-up of a new anammox reac tor. The objective of this research was to evaluate the feasibility of enriching anammox bacteria using two different sources of activated sludge. Both sludges were obtained from full-scale treatment plants targeting N-removal from high strength wastewater. The monitoring of the reactors was carried out according to N-compositional variables and also by polymerase chain reaction (PCR) detection of the functional gene encoding for the hydrazine-oxidizing enzyme (hzo), which is specific of anammox bacteria. The combined analysis of both macroscopic and microscopic data demonstrated that anammox bacteria could be enriched using activated sludge from a treatment plant processing pig slurry in Catalonia