Digestion of cattle manure: Thermogravimetric kinetic analysis for the evaluation of organic matter conversion M. Otero a , A. Lobato b , M.J. Cuetos b , M.E. Sánchez b , X. Gómez b,⇑ a CESAM & Department of Chemistry, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal b Chemical Engineering Department, University of Leon, IRENA-ESTIA, Avda. de Portugal 41, León 24071, Spain article info Article history: Received 22 July 2010 Received in revised form 6 October 2010 Accepted 6 October 2010 Available online 12 October 2010 Keywords: Anaerobic digestion Manure Thermogravimetry Kinetic analysis abstract Anaerobic digestion of cattle manure was studied under thermophilic and mesophilic conditions with the purpose of evaluating the effect of temperature on the quality of the final digestate. Non-isothermal ther- mogravimetric kinetic analysis was applied for assessing organic matter conversion of biological stabil- ization. The mathematical approximation proves to be a useful tool for evaluating the differences attained during biological degradation. The anaerobic digestion of the organic substrate resulted in a reduction of the activation energy value obtained from the different applied kinetic models. Results obtained from thermal kinetic analysis were in accordance with those from the monitoring of the anaer- obic digestion process. The higher values of methane yield reported for the mesophilic digestion in com- parison to that of the thermophilic indicated a greater capability of the former process in the utilization of substrate and thus a higher conversion of organic matter which can be quantified by the activation energy value. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Biological treatments of wastes are capable of degrading organ- ic matter resulting in a stabilized product with a low putrefaction potential. As the concerns about economic waste disposal and recycling of valuable materials are increasing, the aerobic or anaer- obic degradation (composting–digestion) of organic waste is gain- ing popularity as a way of ecologically sustainable recycling (Klammer et al., 2008). In the case of wastes from livestock farms, due to their nutrient value, a traditional practice is their direct application to crop lands (Jackson et al., 2006) so their impact on the environment has become an issue of concern. The use of man- ure as organic fertilizer can benefit agriculture and can be, poten- tially, an inexpensive way for society to protect the environment and to conserve natural resources (Moral et al., 2005). Further- more, animal manures are rich in organic matter, which is a major reservoir of essential plant nutrients (especially N, P, and S) in soils (Laird et al., 2008). However, the degradation of raw animal manure in the soil leads to higher CO 2 production than that of composted animal manure, which could cause anaerobic and reducing conditions in the soil, due to the decreased O 2 level (Bernal et al. 1998). In the European Union, the Nitrate Directive (EEC, 1991) has been the main driving force to develop and apply management methods adopting adequate fertilization plans, adapted to soils and crops needs. By the establishment of Good Agricultural Prac- tices in each country, farmers have been prompted to make deci- sions, and to design and use nutrient management planning (Flotats et al., 2009). Among the possible strategies on nutrient management planning, minimization of the volumes of produced manures and nutrients plays a relevant role, being anaerobic diges- tion the most suitable option since it allows the valorization of the waste by the production of energy from the biogas obtained (Insam and Wett, 2008). The choice of an anaerobic treatment system strongly depends on the substrate characteristics, the simplicity of the design and the operation and on economical and technical aspects (Lettinga, 2001). The composition of cattle waste depends on the conditions under which animals are kept on the farm (dairy cows, breeding cows and calves). Cattle manure extracted from cow-houses contains residues of food and bedding (straw, sand, sawdust, etc.) in addition to solid and liquid wastes. Many of these solids are either not biodegradable or only slowly so (Rico et al., 2007). One aspect of major importance is the determination of the transformation undergone by organic matter when the stabil- ization of waste is intended for posterior land application to crops. Composting was believed to be the only way to build up humic substances in a technological process thus providing additional environmental benefits by carbon sequestration (Smidt and Tintner, 2007). However, several authors have recently reported that the 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.10.016 ⇑ Corresponding author. Tel.: +34 987 291841; fax: +34 987 291839. E-mail address: xagomb@unileon.es (X. Gómez). Bioresource Technology 102 (2011) 3404–3410 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech