Biodegradable dissolved organic material and ammonia present problems for conventional water treatment processes and may contribute to biological instability in the treated water. One solution may be to use a biological process upstream of the regular water treatment process. Biofiltration may be cost-effective in removing ammonia and the precursors of trihalomethanes but the characteristics of the biotreated effluent may affect to the subsequent coagulation process. A continuous flow biological filter packed with reticulated polyurethane foam markedly altered the particle size distribution and the charge density of the mixed liquor, shifting the granulometric distribution toward larger sizes. The mean and median diameter of the particles increased from 9.7 and 5.9 mu m to 97.6 and 37.1 mu m, respectively. The average charge density of the biofilter effluent (7.6 meq dm(-3)) was much lower than that of the raw water (12.7 meq dm(-3)). The optimum coagulant dosage for the subsequent coagulation was reduced substantially from 10 mg dm(-3) to 1 mg dm(-3) as Al due to the lowered charge density of the mixed liquor and the enhanced cation bridging of the extracellular polymers on the bioparticle surface. (c) 2005 Society of Chemical Industry.