In mixed-acid fermentation, carbon and nitrogen are critical nutrients for cell synthesis, growth, and metabolism. To study the effect of C/N ratio on the yield of carboxylic acids, wastewater sludge was co-digested with pretreated bagasse; the amount of sludge was varied from 0% to 100% (dry weight basis). Fermentation was performed at 55 degrees C at a solids concentration of 50 g dry solids/L, and lodoform was used to inhibit methane formation. It was observed that C/N ratio significantly affects yield, especially at extreme ratios. The highest carboxylic acid yield (0.36 g acids/g VS fed) was obtained for C/N ratios ranging from 13 to 25 g C/g N. C/N ratio also affected the composition profile of carboxylic acids. In all mixtures, acetic acid was the major fraction, followed by butyric acid. However, i-butyric. valeric acid, and i-valeric acid increased with increasing sludge content, which likely resulted from protein degradation. (C) 2012 Elsevier Ltd. All rights reserved.