1Pre-Treatment Technologies for Increasing the Biogas Potential of Agricultural Wastes T.A. Shepherd 15 November 2006 Abstract Anaerobic digestion (AD) is a common wastewater treatment method employed in municipal, industrial, and agricultural systems. AD requires a receiving stream with low total solids (TS) content, typically less than 8%. Low TS influent facilitates optimal conditions for microbial degradation and reduces operational cost that is associated with increased energy consumption for wastewater with high TS. Animal manures, especially beef manure, generally contain higher levels of solids that municipal and industrial wastewaters, this can limit the application of anaerobic digestion as a treatment option. Pre-treatment strategies have been employed in municipal and industrial systems to disrupt conglomerated solids and breakdown cell structures; this facilitates the reduction of TS content and has been shown to improve AD performance and increase the bio-methane potential. Investigations of pre-treatment methods have focused primarily on municipal and industrial systems; their application to agricultural systems may provide producers with an improved waste treatment system that is capable of handling higher solid influents. This paper will discuss maceration, thermal hydrolysis, chemical amendment, liquefaction, sonication, and ozonation as pre-treatment methods for optimizing anaerobic digestion and investigate their application within agricultural systems. Key Words Anaerobic Digestion, Agricultural Wastewater, Pre-treatment, Maceration, Thermal Hydrolysis, Chemical Treatment, Liquefaction, Sonication, Ozonation Introduction Conservation and management of nutrients and resources is becoming an essential factor of sustainable agriculture today. Growing concerns of water quality and land management have introduced new factors for the treatment and distribution of agricultural wastes. Government regulations on land application of animal manures, present and pending, will have a large impact on the production practices that are used today. Furthermore, energy concerns have become a high priority topic, and focus on the development and optimization of environmentally friendly bio-renewable energy sources. Technologies that are capable of achieving multiple ecologically sound goals such as conserving nutrients and producing renewable energy sources provide producers with an incentive to implement these practices into their operations. Anaerobic digestion (AD) has long been implemented as a wastewater treatment practice in municipal, industrial, and agricultural sectors. It is the most applied technique for sewage sludge stabilization, reducing volatile solids while producing methane biogas (Tiehm et al. 2001). AD is capable of conserving nutrients with sludge stabilization while simultaneously producing renewable energy in the form of methane which can be used for heating or electrical generation. AD is generally defined by a three step enzymatic and microbial process that converts substrate into methane, carbon dioxide, and stabilized biomass sludge. The primary substrates within the process are based on soluble and insoluble organics within the wastewater. The first step in the process is the hydrolysis of insoluble substrate and occurs through biological and enzymatic reactions which produce soluble substrate. Soluble substrate can then be consumed by acidogenic bacteria which produce volatile fatty acids (VFA) or anaerobically oxidized to form hydrogen. VFA and hydrogen are consumed by group specific acetogenic bacteria, producing acetate. The final step in AD is methane generation. Methanogenic bacteria consume acetate or oxidizing hydrogen to produce methane and carbon dioxide. Figure 1 provides a visual process flow of methane production from AD.2 Figure 1. Anaerobic Digestion Microbial and Enzymatic Process Diagram The hydrolysis of insoluble organic substrate has been identified as the rate-limiting step of the AD process (Eastman and Ferguson, 1981; Shimizu et al. 1993). Applying a pre-treatment process that hydrolyzes insoluble organics prior to AD has been shown to improve process performance and shorten the required treatment time. Animal feeding operations often develop manures that are high in fibrous materials. The high fiber contents are mainly attributed to animal bedding and feed that is carried in the manure waste. The lignocellulose and hemicellulose within plant fibers creates the most difficulties for AD. Lignin is very resistant to enzyme hydrolysis and microbial degradation, and it creates a physical barrier preventing efficient enzyme hydrolysis of the cellulose (Angelidaki and Ahring 2000). Some pretreatment strategies are capable of separating the lignin from the readably degradable cellulose fibers, allowing for greater AD efficiency. Energy efficiency within the AD is paramount in providing a sustainable and profitable treatment system. Losses in AD efficiency are associated with high total solid (TS) content of the influent. High TS increases the viscosity, raising the heating, mixing, and pumping requirements (Richard et al. 1991). Typical operating limits of TS are set at 5%. Depending on operation management practices, agricultural manure slurries have a TS range of 2-10%, limiting the application of tradition AD systems. Agricultural operations most commonly utilize lagoon systems which develop a natural anaerobic zone for treatment. Anaerobic lagoons are generally operated for solids reduction and stabilization, and in most cases do not collect the methane gases that are produced. Anaerobic lagoons are most common in the south and have substantially lower energy demands than a traditional AD system. In Iowa and other northern states, seasonal temperatures often restrict the use of anaerobic lagoons, in this case traditional AD must be incorporated to sustain treatment. Many pre-treatment systems are capable of reducing the viscosity associated with high TS manures, making AD feasible. Coupled with the benefits of from increased solubility of insoluble organics, pre-treatment of high TS manure may create an economically beneficial AD system. Pre-treatment technologies that are capable of reducing the time requirements for the hydrolysis of insoluble organics, increasing the availability of readily degradable organic material, and decreasing viscosity are outlined below: • Maceration • Chemical Treatment •