A study of cleaning a Bulgarian UF 60 PAN spiral-wound module was carried out on an ultrafiltration unit (Millipore, USA). Chemical cleaning of the membrane with a 0.25% solution of sodium metabisulfite did not prevent biological fouling. When treated with a 1% solution of formaldehyde, membrane fouling was diminished and the ultrafiltrate obtained contained a significantly lower number of microorganisms and colloid formations including iron and humic acids. After treatment with the two cleaning agents, the membrane maintained its flux for the water studied within a range of 1.58 to 1.64 m(3)/m(2)/d. The effect on the contaminants to be removed was also found to be comparatively constant. Membrane selectivity remained constant at different permeate yields.