Thermic Model to Predict Biogas Production in Unheated Fixed- Dome Digesters Buried in the Ground Georgina Terradas-Ill, † Cuong H. Pham, ‡ Jin M. Triolo,* ,† Jaime Martí-Herrero, § and Sven G. Sommer † † Institute of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark ‡ Ministry of Agriculture and Rural Development, National Institute of Animal Science, Thuyphuong, Tuliem, Hanoi, Vietnam § Centre Internacional de Me ̀ todes Numè rics en Enginyeria (CIMNE), Building Energy and Environment Group, Edifici GAIA (TR14), C/Rambla Sant Nebridi 22, 08222 Terrassa, Barcelona, Spain * S Supporting Information ABSTRACT: In many developing countries, simple biogas digesters are used to produce energy for domestic purposes from anaerobic digestion of animal manure. We developed a simple, one-dimensional (1-D), thermal model with easily available input data for unheated, unstirred, uninsulated, fixed- dome digesters buried in the soil to study heat transfer between biogas digester and its surroundings. The predicted temperatures in the dome, biogas, and slurry inside the digester and the resulting biogas production are presented and validated. The model was well able to estimate digester temperature (linear slope nearly 1, R 2 = 0.96). Model validation for methane production gave root-mean-square error (RMSE) of 54.4 L CH 4 digester -1 day -1 and relative-root-mean-square errors (rRMSEP(%)) of 35.4%. The validation result was considerably improved if only using winter data (RMSE = 26.1 L CH 4 digester -1 day -1 ; rRMSEP(%) = 17.7%). The model performed satisfactorily in light of the uncertainties attached to it. Since unheated digesters suffer critically low methane production during the winter, the model could be particularly useful for assessing methane production and for improving the ability of unheated digesters to provide sufficient energy during cold periods. ■ INTRODUCTION Production of biogas in anaerobic digesters is one of the most efficient technologies for supplying clean and renewable energy from biomass with high water content. In developing countries, cheap and simple digesters are particularly important as a national energy infrastructure may be weak or nonexisting. 1 In addition, biogas production from animal manure is useful for recycling nitrogen (N) and phosphorus (P), while also reducing greenhouse gas emissions from manure manage- ment. 1-3 Because of their advantages for poor households, to date millions of simple biogas digesters have been constructed on small and medium-sized animal farms in developing countries. In Asia, more than 30 million biogas digesters are now in use, the most popular designs being the fixed-dome digester and the floating-drum digester, 4 while the low-cost tubular digester are most popular in South America, 5,6 with some models being adapted to cold climate through addition of insulation and greenhouses. 7 Most of the biogas digesters are buried underground to ensure a constant temperature and are not heated, stirred, or insulated (simple biogas digesters). These simple digesters are fully functional in tropical or subtropical climate zones, but temperatures below 20 °C during winter can lead to low biogas production. 8 This is because of slow growth of the microorganisms responsible for the digestion of organic matter at low temperatures. 9,10 As a consequence, biogas production cannot cover the energy demand during winter periods. 10 There is an urgent need to develop a new design of a simple and reliable biogas digester that produces the required energy throughout the year. The focus must be on increasing digester temperature, which can be achieved by better insulation, solar heating by means of a greenhouse or heat exchanger, or using some of the gas produced to heat the digester. 10-12 When designing technology to solve the problem, environmental conditions, availability of biomass, and the amount of heating required need to be taken into account. This information can be obtained by quantifying and modeling heat exchange between the biogas digester and its surroundings. The average temperature in a fixed-dome digester can be described by a simple algorithm, assuming that the soil temperature is similar to groundwater temperature. 11 For lagoon digesters, a heat transfer model has been developed to predict the energy required to operate the digester at a specific temperature. 13 Heat losses from lagoon digesters can be reduced by designing the construction with the help of a 3-D model that simulates heat transfer. 14 Heat transfer models can Received: July 19, 2013 Revised: February 11, 2014 Accepted: February 11, 2014 Published: February 11, 2014 Article pubs.acs.org/est © 2014 American Chemical Society 3253 dx.doi.org/10.1021/es403215w | Environ. Sci. Technol. 2014, 48, 3253-3262