1 Influence of Water Quality in the Efficiency of Retention Aids Systems for the Paper Industry Ruth Ordóñez, Daphne Hermosilla, Elena de la Fuente and Ángeles Blanco* Department of Chemical Engineering. Universidad Complutense de Madrid. Avda. Complutense, s/n. 28040 Madrid (Spain) * Corresponding author: Tel.: +34 91 394 4247; fax: +34 91 394 4243. E-mail address: ablanco@quim.ucm.es (Ángeles Blanco).