Solid substrate cultivation of thermophilic, anaerobic bacteria offers an alternative production method for many bio-based chemicals; however the process must be optimized for each substrate-organism fermentation. The effects of initial substrate moisture content (SMC, 30%, 50% and 70% wet-basis), supplemental nutrient concentration (SNC, 12%, 50% and 100%) and duration of cultivation time (6, 10 and 14 days), on product formation (lactate, ethanol and acetate) by Clostridium thermocellum 27405 were examined during growth on paper pulp sludge. Water activities at moisture contents above 30% wet-basis were essentially identical (similar to 0.99), yet the water contents differed significantly, and affected the metabolic activity of C. thermocellum. Increases in initial substrate moisture content from 50% to 70% for cultures supplemented with 50% or 100% nutrients resulted in a 75-145 mM increase in total end products. At 70% SMC, the addition of 100% SNC generated a 56% increase in product formation above the addition of 50% nutrient supplementation. Increases in the quantity of free water present in the solid substrate cultivation system up to the water holding capacity of the paper pulp sludge led to improved performance of this anaerobic bacterium. While nutrient supplementation is common in the form of salts for many aerobic microorganisms, efficient metabolism for anaerobic C thermocellum grown in SSC was highly dependent on added salts, vitamins and reducing agents. Further studies are needed to determine if this is a general effect for other anaerobes grown in solid substrate cultures. (c) 2006 Elsevier Ltd. All rights reserved.