To determine the optimal scenario for mixed-acid fermentations, an array of batch fermentations were performed that independently varied the C/N ratio (adjusted using urea) and the blend of carbohydrate (office paper) and nutrient (wet chicken manure (CM)). A blend of 93% paper and 7% wet CM (dry basis) with a C/N ratio of 37 g C(NA)/g N had the highest culture yield (0.21 g acid(produced)/g NAVS(initial)), total acid productivity (0.84 g acid(produced)/(L(liq) d)), and conversion (0.43 g NAVS(consumed)/g NAVS(initial)). Compared to a control blend of 80% paper and 20% wet CM (dry basis, no urea) the culture yield, productivity, and conversion improved 53%, 44%, and 70%, respectively. Selectivity was a strong function of C/N ratio and varied from 0.167 to 0.667 acid(produced)/g NAVS(consumed). Nitrogen supplementation with urea improves performance and reduces costs of (1) capital, (2) nutrients, and (3) downstream processing by reducing impurities from nutrient-rich wastes (e.g., manures, sewage sludge). (C) 2011 Elsevier Ltd. All rights reserved.