Targeted pretreatment and inoculation strategies for horse manure fermentation: Impact on metabolites and microbial community composition.

Horse manure is a lignocellulosic biomass found in significant quantities with a vast indigenous flora, not yet fully valorized apart from anaerobic digestion. Its use in the fermentation process can lead to the production of higher-value metabolites. This study investigates three inoculation strategies coupled with five pretreatment conditions for horse manure fermentation. Two microwave pretreatments (200W and 1000W) were compared with a conventional thermal pretreatment, a thermo-acid pretreatment, and an unpretreated condition. The sole horse manure indigenous microorganisms were used in fermentation and compared with two inoculation strategies using external inoculum, which was i) thermally treated or ii) pretreated simultaneously with manure. A statistically similar total metabolite production (0.088 ± 0.010 gCOD/gVS) was observed, with more than 50 % of acetate produced for all the pretreated conditions. When no pretreatment was performed (Ctrl), methane was produced as a major metabolite. The metabolic profile of the thermo-acid pretreatment condition using solely indigenous microorganisms was different from the other conditions, with ethanol (0.015 ± 0.004 gCOD/gVS) and hydrogen (0.009 ± 0.002 gCOD/gVS) production. This was related to the Klebsiella genus abundance increase recorded for this condition. Both microwave pretreatments shared similar metabolite results and microbial composition with the conventional thermal pretreatment. However, a heat shock is needed to inhibit methane production from archaea and can be performed by microwave or conventional thermal pretreatment. To conclude, indigenous horse manure microorganisms are suitable for fermentation with equivalent yields compared to an external inoculum from a wastewater treatment plant whatever the pretreatment applied. However, a heat shock is needed to inhibit methane production from archaea and can be performed via microwave or conventional thermal pretreatment.