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Many waterworks that apply conventional treatment of surface water, by flocculation and filtration, have to deal with seasonal changes of the raw water quality, increasing concentrations of natural organic matter (NOM), and the necessity to improve particle removal. A pilot plant was operated to closely resemble the full-scale treatment at a surface water treatment plant in Göteborg, Sweden. Treatment consisted of flocculation, sedimentation and rapid granular activated carbon (GAC) filtration. To assess the effects of biological pre-filtration, the feed water to the pilot plant was switched weekly between surface water and biofiltered water which had passed through adsorptively exhausted GAC at an empty bed contact time (EBCT) of 34 minutes. The processes were investigated with regard to NOM, the bacterial re-growth potential, as well as their function as a barrier for suspended particles that originate from the raw water. Biological pre-filtration improved particle removal and made it less dependent on the post-sedimentation GAC rapid filter, thereby improving robustness. Episodically elevated concentrations of earthy-musty odour compounds, which are not reliably removed by flocculation and filtration, were reduced by the biofilters. The process combination may be of particular interest for waterworks with variable raw water quality. [ABSTRACT FROM AUTHOR]