Anaerobic digestion of animal waste: Waste strength versus impact of mixing Khursheed Karim a , Rebecca Hoffmann a , Thomas Klasson b , M.H. Al-Dahhan a, * a Chemical Reaction Engineering Laboratory (CREL), Department of Chemical Engineering, Washington University, St. Louis, MO 63130, USA b Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 31 March 2003; received in revised form 10 January 2005; accepted 16 January 2005 Available online 2 April 2005 Abstract We studied the effect of mode of mixing (biogas recirculation, impeller mixing, and slurry recirculation) and waste strength on the performance of laboratory scale digesters. The digesters were fed with 5% and 10% manure slurry, at a constant energy supply per unit volume (8 W/m 3 ). The experiments were conducted in eight laboratory scale digesters, each having a working volume of 3.73 L, at a controlled temperature of 35 ± 2 °C. Hydraulic retention time (HRT) was kept constant at 16.2 days, resulting in a total solids (TS) loading rate of 3.08 g/L d and 6.2 g/L d for 5% and 10% manure slurry feeds, respectively. Results showed that the unmixed and mixed digesters performed quite similarly when fed with 5% manure slurry and produced biogas at a rate of 0.84–0.94 L/L d with a methane yield of 0.26–0.31 L CH 4 /g volatile solids (VS) loaded. This was possibly because of the low solids concentration in the case of 5% manure slurry, where mixing created by the naturally produced gas might be sufficient to provide adequate mixing. However, the effect of mixing and the mode of mixing became prominent in the case of the digesters fed with thicker manure slurry (10%). Digesters fed with 10% manure slurry and mixed by slurry recirculation, impeller, and biogas recirculation produced approximately 29%, 22% and 15% more biogas than unmixed digester, respectively. Deposition of solids inside the digesters was not observed in the case of 5% manure slurry, but it became significant in the case of 10% manure slurry. Therefore, mixing issue becomes more critical with thicker manure slurry. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Anaerobic; Biogas; Digestion; Draft tube; Manure; Mixing 1. Introduction Growth in livestock industries has resulted in large amounts of animal waste (cow manure) generation. In the United States over 100 million tons of dry matter is produced every year (Fontenot and Ross, 1980). This has brought in the requirement of safe waste manage- ment. Different types of waste management options may include technologies based on physical, chemical, or biological conversions. Examples are combustion/ incineration (gasification), chemical conversion (metha- nol) and biological conversion (anaerobic digestion). Combustion/incineration efficiently recovers the greatest amount of energy from manure, but the practicality of using the ash as a recycled material has yet to be proven. Moreover, self-sustaining incineration requires a waste of about 30% solids. Wetter manure with lower solids content requires supplemental fuel to sustain incinera- tion (OSU, 2000). The possibility of producing metha- nol production from animal wastes is promising, but there is no specific technology or research is available yet. Anaerobic Digestion is biological means of decom- position of manure in an oxygen-free environment, and has the advantage of producing a fuel gas (methane) and odor free residues rich in nutrients, which can be used as fertilizers. 0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.01.020 * Corresponding author. Tel.: +1 314 935 7187; fax: +1 314 935 7211.