Effect of storage, aqueous ammonia soaking and hydrothermal pretreatments in anaerobic digestion of Sparganium erectum biomass.

Sparganium erectum is a macrophyte whose use is very recent and offers a promising feedstock for biogas production. This study evaluates its methane yield after applying low and high-energy demand pretreatments (solar drying or ensiling, and aqueous ammonia soaking or hydrothermal treatment). These treatments are relatively easy to implement in existing biogas plants by using waste heat from the biogas engine or an aqueous ammonia solution recovered from the digestate. All pretreated biomasses exhibited higher methane yields than the untreated biomass (103 NmLCH4/gCOD), reaching the hydrothermal carbonization at 200 °C the greatest increase (46 %). Drying altered slightly the biomass composition and structure, while alkali and hydrothermal pretreatments produced notable effects. Although all pretreatments enhanced the methane yield, less than 50 % of COD was converted, suggesting room for biodegradability improvement. At the microbiological level, only the alkaline pretreatment with ammonium caused a significant change in the population involved in anaerobic digestion, as well as a decrease in the abundance of taxa. These findings highlight the importance of pretreatments for the anaerobic digestion of macrophytes, where simple processes such as drying prove to be the most viable given its net energy gain obtained (474 %) compared to the obtained with untreated biomass (231 kWh).