Electrical conductivity was measured by a WTV Cond330i con- ductivity metre. Potassium concentration was determined by atomic absorption spectrometry (wavelength: 766.5 nm), using a SpectrAA-250 Plus from Varian, equipped with an air-acetylene burner. Total polyphenols were determined according to the colorimetric method (APHA, 1995) by using the Folin-Ciocalteau reagent (Box, 1983; Morita, 1980). Absorbance was measured at 760 nm by a Varian Cary UV-visible spectrophotometer. Calibration was performed using gallic acid as a standard, and polyphenols were reported as mg L 1 of gallic acid. Nitrates were measured by ion chromatography using a Dionex AS14A 4 mm 250 mm col- umn and UV detection (220 nm). Isocratic elution was carried out at a flow rate of 1.0 mL min 1 with an aqueous solution of 10 mM NaHCO3 and 10 mM Na2CO3. For the characterization of olive mill residues, total organic carbon and total nitrogen were measured by means of a TOC-V CSH carbon analyser equipped with a total ni- trogen measuring unit (model TNM-1) by Shimadzu Co, Japan. Ammonium was determined by the direct Nessler method by means of spectrophotometric detection.