Repurposing Used Wheat Straw from Horse Stalls into Methane Gas
In the U.S., approximately 27.6 million tons of used bedding materials are generated annually from horse stalls, primarily from wheat straw (WS). A study published in Bioresource Technology suggests that solid-state anaerobic digestion (SS-AD) could offer a feasible and sustainable solution for handling this organic waste and producing renewable energy.
SS-AD is a process in which microorganisms break down organic materials, such as spent wheat straw (SWS), to produce biogas, with methane as a major component. This method, which treats feedstocks with high total solids content (15-40%), is particularly suitable for lignocellulosic biomass like wheat straw.
The study, titled "Solid-State Anaerobic Digestion of Spent Wheat Straw from Horse Stall," was conducted by Z. Cui, C. J. Shi, and Y. Li. The research focused on the digestion of SWS in 1-L reactors at 37 °C for 30 days.
Key factors affecting the SS-AD process include digestion conditions and startup, degradation rates, and methane yield. High total solids substrates like SWS often cause a long startup time and incomplete fermentation due to limited microbial access to the organic matter. However, pretreatments and co-digestion strategies can improve microbial activity and digestion kinetics.
Lignocellulosic materials like SWS decompose slowly, necessitating pretreatment methods to break down lignin and enhance the biodegradability and degradation rate of the straw during AD. Studies show that fine hydrochar and hydrolysates generated from pretreatment can promote electron transfer between microbes and methanogens, facilitating higher methane yields.
In the horse industry, SWS is a considerable organic waste source. Implementing SS-AD could reduce waste volume and generate renewable energy. The degradation rates of SWS in horse stalls are about 10-30% higher than those of WS during anaerobic digestion, particularly at F/I ratios of 2.0 and 4.0. Methane production from SWS is over 40% higher than that from WS, largely due to different degradation rates.
If an F/I ratio of 2-4 is selected and all the used bedding materials in the U.S. are taken into account, SS-AD has the potential to generate roughly 8.95X10ft methane, or 1.15X10 MMBtu, every year. For WS at F/I ratios of 2.0 and 4.0, the methane yields are about 95 L/kg-VS (2,850 ft/ton-TS), which is equivalent to 3.14 MMBtu/ton-TS. The methane content for digested SWS and WS is 50-60% and 45-50%, respectively.
Horse activities facilitate degradation of cellulose and hemicellulose in SWS, eventually improving methane production. The success of converting SWS to methane offers a feasible alternative for the treatment of horse stall wastes.
In conclusion, solid-state anaerobic digestion of spent wheat straw is feasible with adequate pretreatment and process optimization to overcome challenges of high solids and lignocellulosic complexity. Effective hydrolysis and microbial electron transfer enhancement can improve degradation rates and methane yields, making the process a promising renewable energy option for the U.S. horse industry waste management.
- In the horse industry, spent wheat straw (SWS) is a substantial source of livestock-related organic waste.
- Environmental science and technology can potentially help mitigate climate-change issues by leveraging data-and-cloud-computing tools to optimize the solid-state anaerobic digestion (SS-AD) process for SWS, thereby producing renewable energy.
- The results from this study/research suggest that technology-driven advancements in SS-AD processes, particularly in the areas of pretreatments and co-digestion strategies, could significantly improve the degradation rates and methane yields of SWS, benefiting both waste management and renewable energy production in the U.S. environmental-science sector.
