This paper aimed to study the value of horse manure through anaerobic digestion. The study involved characterization of different components of horse waste and the evaluation of their biochemical composition, physicochemical characterization and the influence of the composition of horse waste on biochemical methane potential. More specifically, two bedding mixtures were studied: the first one was composed of wheat straw (WS), wood chips (WC) and horse manure (HM) with a volumetric composition of 85%, 14% and 1%, respectively; and the second one was a mixture of WS and HM with a volumetric composition of 99% and 1%, respectively. The analysis was carried out on the two bedding mixtures and on each substrate separately with 406 samples from May 2017 to October 2019. Biochemical methane potential tests conducted on these samples showed that the composition and structure of the substrate influenced the BMP. WS had the highest mono-digestion methane production with 176.1 NmL·g_VS⁻¹. The second bedding mixture (99% WS, 1% HM) showed a production of 189.4 NmL·g_VS⁻¹ compared to 127 NmL·g_VS⁻¹ by bedding mixture 1 (85% WS, 14% WC, 1% HM). The difference was due to a dilution effect on methane production caused by the presence of WC rich in lignin.

The effects of co-digesting sewage sludge (SS) and horse waste (HW), the composition of HW, and the ratio of HW:SS were studied using two semi-continuous digesters of 9.5 L of working volume. These digesters were operated in parallel with the mono-digestion of SS in digester 1 (D1) and the co-digestion of SS and HW in digester 2 (D2). In digester 2, there were two phases of digestion (durations of 40 and 43 weeks, respectively). The composition of HW in the first phase was 85% wheat straw (WS), 14% wood chips (WC), and 1% horse manure (HM), with 99% wheat straw (WS) and 1% horse manure (HM) in the second phase. Variable ratios of HW:SS were studied in the digesters. The co-digestion of sewage sludge (SS) and horse waste (HW) produced more biogas than the mono-digestion of SS alone, with a maximum of 15.8 L·d⁻¹, compared to 9 L·d⁻¹ at the end of the experiment. When comparing the results obtained in both phases, the production of methane in phase 2 was 18 NmL·g_VS⁻¹ higher than in phase 1. This slight increase in methane yield could be linked to the absence of wood chips (WC), which is considered to have a diluting effect on methane production. Therefore, this study shows that an organic loading rate (OLR) of 4.8 kg_VS·m⁻³·d⁻¹, a ratio of HW:SS of 3, and a composition of HW (99% WS, 1% HM) should be respected in the actual experimental conditions for a well-functioning anaerobic digestion.

Solid-state anaerobic digestion (SS-AD) is a promising technology for treating organic waste and producing renewable energy. This study explores the feasibility of using 3D printing to rapidly design cost-effective laboratory-scale digesters for optimization experiments. Batch reactors were designed using fused deposition modeling (FDM) with polylactic acid (PLA) and stereolithography (SLA) with High Temp V2 resin. PLA had a negligible impact on methane yields, while raw SLA resin positively influenced methanogenic potential, likely due to residual isopropanol used in post-processing, causing a 19% increase in CH4 yield. The performance of the 3D-printed reactors was compared to that of a conventionally machined PMMA reactor using cattle manure as a substrate, showing comparable methane yields and process stability. Three-dimensional printing technologies have demonstrated remarkable efficiency in designing laboratory-scale digesters, with a 70% cost reduction for SLA technology and an 80% reduction in design time compared to conventional reactors designed by plastics processing, while maintaining comparable biogas production. FDM technologies with PLA have shown that they are not suitable for these uses. This study demonstrates the potential of additive manufacturing to accelerate SS-AD research and development. However, care must be taken in material selection and post-processing to avoid introducing experimental bias.