Performance evaluation of a completely stirred anaerobic reactor treating pig manure at a low range of mesophilic conditions Jianbin Guo a,⇑ , Renjie Dong b , Joachim Clemens c , Wei Wang a a Department of Environmental Engineering, Tsinghua University, Beijing 100084, PR China b College of Engineering, China Agricultural University, P.O. Box 184, Beijing 100083, PR China c Institute of Crop Science and Resource Reservation (INRES), University of Bonn, Karlrobert-Kreiten-Strasse 13, 53115 Bonn, Germany article info Article history: Received 16 July 2012 Accepted 11 June 2013 Available online 8 July 2013 Keywords: Anaerobic digestion Organic loading rate Temperature Pig manure Sludge yield abstract Many Chinese biogas plants run in the lower range of mesophilic conditions. This study evaluated the performance of a completely stirred anaerobic reactor treating pig manure at different temperatures (20, 28 and 38 °C). The start-up phase of the reactor at 20 °C was very long and extremely poor perfor- mance was observed with increasing organic loading rate (OLR). At an OLR of 4.3 g ODM L 1 d 1 , methane production at 28 °C was comparable (3% less) with that at 38 °C, but the risk of acidification was high at 28 °C. At low OLR (1.3 g ODM L 1 d 1 ), the biogas process appeared stable at 28 °C and gave same meth- ane yields as compared to the reactor operating at 38 °C. The estimated sludge yield at 28 °C was 0.065 g VSS g 1 COD removed, which was higher than that at 38 °C (0.016 g VSS g 1 COD removed ). Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction In China, more than 30,000 medium (300–500 m 3 ) and large scale (>500 m 3 ) biogas plants were in operation in 2010 (Ministry of Agriculture, 2011). They are widely used to reduce the environ- mental pollution from animal wastes and to produce biogas (Zhang et al., 2009). However, many of them run either at low organic loading rates (OLRs) or in the lower range of mesophilic conditions because of lack of insulation and insufficient heating which results in low volumetric biogas production (Guo et al., 2012). Temperature is one of the most important factors affecting the anaerobic digestion process. Anaerobic digestion can proceed un- der psychrophilic (<25 °C), mesophilic (25–40 °C) and thermophilic (>45 °C) conditions (El-Mashad et al., 2004). However, at psychrophilic temperatures below 20 °C, the start-up procedure is long and tedious (Bouallagui et al., 2003). Therefore, most reactors operate at either mesophilic (35 °C) or thermophilic (55 °C) temperatures (Guo et al., 2012; Johansen et al., 2013; Marañón et al., 2012; Ward et al., 2008). The energy requirement for maintaining the reactor at the opti- mum temperature is one of the most critical factors for economi- cally viable reactor operation. Thermophilic digestion is normally considered not to be economically viable due to the large energy input and operational difficulties. In the case of protein-rich substrates, Hansen et al. (1998) studied the effect of temperature on anaerobic digestion of swine manures in a CSTR system at 37, 45, 55 and 60 °C and found the methane yield to be 188, 141, 67 and 22 mL CH 4 g 1 VS, respectively, with the yield inhibited by increasing free ammonia concentration with increasing tempera- ture. Similar results were reported by Angelidaki and Ahring (1994). Mesophilic temperatures (35–37 °C) are commonly considered the optimum conditions for maintaining the stability of the bacte- rial activity and biogas production from animal manure (Sakar et al., 2009). Sánchez et al. (2001) studied the effect of temperature on the anaerobic digestion of piggery waste in batch, completely mixed reactors. They concluded that the biogas process was more stable at 35 °C than at ambient temperatures (16.8–29.5 °C) and that chem- ical oxygen demand (COD) removal rates at ambient temperatures were significantly lower than those at mesophilic temperatures. Some conflicting results were obtained by Chae et al. (2008) when comparing the biogas yield at 25 °C, 30 °C and 35 °C in the batch anaerobic digestion of pig manure. Although there was obvi- ously faster degradation at high temperatures, biogas production from the reactor at 30 °C was only 3% less than that at 35 °C within 20 days. Therefore, those authors suggest that the net energy bal- ance must be considered when deciding the most economical digestion temperature. However, the microbial processes occurring in batch reactors are in an unsteady state, in contrast to those full-scale, semi-con- tinuous biogas plants, fed at least once daily, such as completely stirred tank reactors (CSTR). For such a continuous system, it is 0956-053X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2013.06.015 ⇑ Corresponding author. Tel./fax: +86 10 62782910. E-mail address: jianbinguo@gmail.com (J. Guo). Waste Management 33 (2013) 2219–2224 Contents lists available at SciVerse ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman