This paper uses countercurrent fermentation to anaerobically convert paper fines and industrial biosludge to carboxylate salts using a mixed culture of acid-forming microorganisms. Using the MixAlco process, the carboxylate salts can be thermally converted to ketones and hydrogenated into mixed alcohol fuels. Continuum particle distribution modeling (CPDM) correlated batch fermentation data to countercurrent fermentation data, allowing the prediction of product concentrations and conversions over a wide range of solid loading rates and liquid residence times. For 80% paper/20% biosludge, the predicted product concentrations agreed with the data within 7.7%. The predicted conversion agreed with the actual conversion within 27.8%. By correcting for varying selectivity, the predicted conversion agreed with the actual conversions within 15.2%. For 40% paper/60% biosludge, the predicted product concentrations agreed with the data within 9.6%. The predicted conversion agreed with the actual conversion within 28.3%. By correcting for varying selectivity, the predicted conversion agreed with the actual conversions within 15.4%. For both the 80/20 and 40/60 cases, CPDM predicts that 90% conversion is possible with a 20 g/l product concentration, 300 g/l substrate concentration, 16 day liquid residence time, and 2.5 g/(l d) solids loading rate. Before proceeding to an industrial plant, these predictions must be verified in a pilot plant. (C) 2003 Elsevier Ltd. All rights reserved.