About 27.6 million tons of used bedding materials are generated every year in the U.S. Handling these wastes is a technical and economic challenge. Solid-state anaerobic digestion (SS-AD) of used horse bedding materials for bioenergy production could be an appropriate solution to this problem. This fact sheet discusses the feasibility of using SS-AD to convert used bedding materials from horse stalls into biogas, a source of methane.
Wheat straw (WS) is commonly used as a bedding material in the horse industry. Fresh WS is usually clean with a golden yellow color and has long, straight, smooth and intact fibers. After bedding, the residuals, or spent wheat straw (SWS), become slightly darker and soil-like, and the fibers are rough with serrated edges. Compared to WS, SWS has a lower pH (6.3 vs. 7.4) value and lower cellulose (34.2% vs. 37.9%) and hemicellulose (19.1% vs. 21.8%) contents, but a much higher volatile fatty acid (VFA) content (11.4% vs. 3.16%).
Process of SS-AD
Anaerobic digestion is a collection of processes by which microorganisms break down organic materials such as carbohydrates, fats and proteins, in the absence of oxygen and produce biogas. Methane is a major component of the produced biogas. A typical anaerobic digestion process includes four steps: hydrolysis, acidogenesis, acetogenesis and methanogenesis. SS-AD is capable of handling feedstocks with 15–40% total solids (TS) compared to liquid-phase AD, which requires <15% TS. Therefore, SS-AD is suitable for treating lignocellulosic biomass such as wheat straw. Compared to liquid-phase AD, SS-AD requires less energy input, has higher volumetric biogas productivity and generates a stackable byproduct.
SS-AD of Spent Wheat Straw
SS-AD is affected by a variety of factors such as inoculum and TS content. The inoculum provides digestion microorganisms. Effluent from an existing liquid anaerobic digester can be a good choice as an SS-AD inoculum. The mixing ratio of feedstock to inoculum (F/I) is critical, and an F/I of 2–4 (based on volatile solids) is preferred in most cases, while an F/I higher than 6.0 may be problematic. The TS content for SS-AD ranges from 15–40%. In this study, 22% TS was used. The digestion tests were carried out in 1-L reactors at 37 °C for 30 days.
Changes of Feedstock Properties
A feedstock experiences intensive biochemical reactions during anaerobic digestion and can be substantially changed in terms of chemical composition and morphology. Typically, the carbon to nitrogen (C/N) ratio decreases as a result of the carbon reduction during biogas production. In this study, the C/N ratio of SWS decreased from 17.8 to 14.8 (F/I=2.0). In contrast, the content of ammonia-N (NH3-N) increased from 3.6% to 4.4%. An increase of NH3-N may inhibit digestion and reduce biogas yield. After the 30-d digestion, the highly-ordered and intact fiber structures of WS and SWS fibers were largely deconstructed, and micro lateral fractures could be seen.
Degradation of Cellulose and Hemicellulose
Lignocellulosic biomass such as WS is composed of cellulose, hemicellulose and lignin. Degradation of cellulose and hemicellulose is closely related to methane production. In horse stalls, activities such as hoof movement and microorganisms from feces help pretreat these organic components in the WS bedding, thus improving degradation during anaerobic digestion. As shown in Figure 1, about 10–30% higher degradation rates were observed for SWS compared to WS at F/I ratios of 2.0 and 4.0. No significant difference in degradation rates were measured at an F/I ratio of 6.0.
Production of Methane
Largely due to different degradation rates, methane production from the SWS was over 40% higher than that from WS (Figure 2). Digestion of SWS also had a relatively shorter lag phase (1–2 d) compared to the digestion of WS (4–8 d), which may have been caused by its higher initial VFA content, as VFAs can be easily converted to methane. For SWS at F/I ratios of 2.0 and 4.0, the cumulative methane yield reached about 150 L/kg-VS (4,500 ft3/ton-TS), which is equivalent to 4.63 MMBtu/ton-TS. Little methane was produced at an F/I=6.0, due to low degradation rates (Figure 2) and possible ammonia inhibition. For WS at F/I ratios of 2.0 and 4.0, the methane yields were about 95 L/kg-VS (2,850 ft3/ton-TS), which is equivalent to 3.14 MMBtu/ton-TS. Generally, methane content in biogas ranges from 40–70%. In this study, the methane content for digested SWS and WS was 50–60% and 45–50%, respectively.
The success of converting SWS to methane offers a feasible alternative for the treatment of horse stall wastes. This study showed that horse activities facilitated degradation of cellulose and hemicellulose in SWS, and eventually improved methane production. If an F/I ratio of 2–4 is selected and assuming an average methane yield of 4,500 ft3/ton-TS (or energy yield of 4.63 MMBtu/ton-TS), SS-AD has the potential to generate roughly 8.95X1010ft3 methane, or 1.15X108 MMBtu, every year if all the used bedding materials in the U.S. are taken into account. The obtained methane can be used for heating livestock facilities or generating electricity, or can be upgraded to other energy carriers.
|Figure 1. Degradation of cellulose and hemicellulose in SS-AD at different F/I ratios.|
|Figure 2. Production of methane in SS-AD with SWS and WS.|
This fact sheet is based on a publication by Z. Cui, C. J. Shi and Y. Li titled, "Solid-State Anaerobic Digestion of Spent Wheat Straw from Horse Stall," published in Bioresource Technology 102:9432–9437.