Optimizing plant biomass recovery from constructed wetlands within the water-energy-food nexus: Influence of harvesting time on biogas production

Constructed wetlands (CWs) are increasingly recognized as multifunctional Nature-based Solutions capable of simultaneously addressing wastewater treatment and biomass valorization challenges within the Water-Energy-Food nexus. However, the seasonal variability of plant biomass composition, which is intrinsically linked to species-specific phenological stages, remains poorly investigated in relation to its impact on anaerobic digestion (AD) performance. This study evaluates the biochemical methane potential (BMP) of four common CW macrophytes (Iris pseudacorus, Phragmites australis, Carex spp. and Typha latifolia) harvested from a full-scale CW at three phenological stages (spring, summer, autumn), to assess the influence of harvesting time on biogas production potential and quality. Physico-chemical characterization revealed significant seasonal shifts in dry matter content, C/N ratio, and biochemical properties of the biomasses, which translated into marked differences in methane yields and biogas composition. Autumn-harvested feedstocks exhibited the highest cumulative methane yields (up to 357 mL CH₄/gVS), attributed to a favourable balance of moisture content, moderate lignification, and optimal C/N ratios. Furthermore, autumn samples showed the highest methane enrichment (up to 61% CH₄), suggesting improved process stability and microbial conversion efficiency. These findings demonstrate that the timing of CW biomass harvesting is a critical parameter in optimizing AD-based energy recovery, and supports the integration of CWs within circular bioresource management strategies aimed at enhancing system multifunctionality and resource-use efficiency.