The EBPR has become an important process in wastewater treatment. A method has been developed to detect, by a simple stain, the presence of polyphosphate (poly-P) in a sludge, that is an evidence of the presence of polyphosphate accumulating organisms (PAO). Using the appropiate software is possible to do an image analysis to quantify this PAO, and monitorize the evolution of the poly-P content in a sludge.