Evaluation of Biogas Production Potential by Dry Anaerobic Digestion of Switchgrass���Animal Manure Mixtures H. K. Ahn & M. C. Smith & S. L. Kondrad & J. W. White Received: 17 November 2008 /Accepted: 23 March 2009 / Published online: 22 May 2009 # Humana Press 2009 Abstract Anaerobic digestion is a biological method used to convert organic wastes into a stable product for land application with reduced environmental impacts. The biogas produced can be used as an alternative renewable energy source. Dry anaerobic digestion [15% total solid (TS)] has an advantage over wet digestion (10% TS) because it allows for the use of a smaller volume of reactor and because it reduces wastewater production. In addition, it produces a fertilizer that is easier to transport. Performance of anaerobic digestion of animal manure���switchgrass mixture was evaluated under dry (15% TS) and thermophilic conditions (55 ��C). Three different mixtures of animal manure (swine, poultry, and dairy) and switchgrass were digested using batch-operated 1-L reactors. The swine manure test units showed 52.9% volatile solids (VS) removal during the 62-day trial, while dairy and poultry manure test units showed 9.3% and 20.2%, respectively. Over the 62 day digestion, the swine manure test units yielded the highest amount of methane 0.337 L CH4 /g VS, while the dairy and poultry manure test units showed very poor methane yield 0.028 L CH4/g VS and 0.002 L CH4/g VS, respectively. Although dairy and poultry manure performed poorly, they may still have high potential as biomass for dry anaerobic digestion if appropriate designs are developed to prevent significant volatile fatty acid (VFA) accumulation and pH drop. Keywords Anaerobic . Digestion . Animal manure . Switchgrass . Biogas . Renewable energy Introduction Recent increases in fossil fuel prices have increased the demand for biofuel production from crops. The diversion of crops for biofuel production resulted in increased food price and food security concerns. Therefore, research developing alternative biomass for bioenergy has become increasingly important. Animal wastes are good sources of biomass because Appl Biochem Biotechnol (2010) 160:965���975 DOI 10.1007/s12010-009-8624-x H. K. Ahn (*) : M. C. Smith : S. L. Kondrad : J. W. White Environmental Management and Byproduct Utilization Laboratory, USDA-ARS, Bldg 306, BARC-East, Beltsville, MD 20705, USA e-mail: heekwon.ahn@ars.usda.gov

they contain an abundance of organic matter and nutrients. Using animal wastes as biomass offers many advantages for livestock operations by minimizing waste disposal costs and also reducing odors and contaminants [1]. Anaerobic digestion is a biological method used to convert organic wastes into a stable product for application to land without adverse environmental effects. In addition, the biogas produced can be used as an alternative renewable energy source. Dry anaerobic digestion (15% TS) has benefits over conventional anaerobic liquid digestion (10% TS) because it reduces the volume of the reactor and wastewater, as well as producing a more easily transportable fertilizer [2]. Switchgrass (Panicum virgatum) has received much attention in recent years and has been a focus of bioenergy research for over a decade [3]. It is a very productive North American native, perennial warm season grass suitable for growth on marginal land [4, 5]. As such, it has great potential as sustainable bioenergy crop. Most past research has focused on switchgrass utilization through combustion, thermochemical conversion, or cellulosic ethanol production [6���9]. However, switchgrass may also have potential for use as a co- digestion substrate in anaerobic digestion by supplementing manure biomass resources and potentially increasing biogas production. Switchgrass cell walls are primarily composed of cellulose and hemicellulose, for example, Dien et al. [9] report switchgrass cellulose concentrations ranging from 273 to 322 g kg-1 dry matter (DM) and hemicellulose concentrations ranging from 235 to 279 g kg-1 DM depending on the age of the plant material. Hydrolysis of these cell wall components yield sugars which are readily converted to methane during anaerobic digestion. Cellulose hydrolysis has been identified as the rate- limiting step during anaerobic digestion [10], and research is needed to assess the effect of manure and switchgrass co-digestion on biogas production. In order to develop a suitable dry anaerobic digestion system, appropriate pretreatment and operating strategies need to be employed based on the characteristics of the feedstock. Biogas production depends on many parameters including feedstock composition, operating temperature, and organic loading rate. Although dry anaerobic digestion has many potential advantages, further investigations have been limited. Thus, the aim of this research is to evaluate the biogas production potential of animal manure mixtures with switchgrass during batch operating dry anaerobic digestion. The purpose was to understand the influence of animal manure type (dairy, swine, and poultry manure) on dry anaerobic digestion performance of manure���switchgrass mixtures by investigating the biogas production, composition, substrate removal efficiency, and leachate characteristics. Materials and Methods Digester System Experiments were carried out using laboratory scale digestion bags made of air- impermeable plastic with effective volume of about 1 L (Fig. 1). A fitting that combined a hose barb and septum was mounted on each digestion bag. The hose was used for biogas collection, and the septum was used for leachate sampling. Nine anaerobic digestion bags were filled with manure���switchgrass mixtures, sealed, and placed in a 55 ��C incubator to maintain thermophilic conditions. An initial leachate sample was collected for character- ization prior to the start of the digestion period. The temperature of the digestion bags was measured using type-T thermocouples. The volume of biogas production was measured with a wet-tip gas meter which is connected to each digestion bag. All temperature and 966 Appl Biochem Biotechnol (2010) 160:965���975