Is Biomethane Production from Common Reed Biomass Influenced by the Hydraulic Parameters of Treatment Wetlands?

Abstract

Treatment wetlands (TWs) are Nature-Based Solutions which have been increasingly used worldwide for wastewater (WW) treatment as they are able to remove mineral and organic pollutants through both physical and biochemical processes. Besides the reusable effluent, the TWs produce, as their main output, plant biomass that needs to be harvested and disposed of at least once a year with significant management costs and causing the TW to be temporarily out of service. This study aims (i) to evaluate the potential of TWs’ biomass for local energy production and (ii) to understand the effects of TWs’ hydraulic conductivity (Ks) on the biomass biomethane yield. Specifically, this was addressed by determining the Biochemical Methane Potential of common reed (CR) (Phragmites australis) samples collected at three harvest times from the 10-year-old horizontal subsurface treatment wetland (HSTW) used as a secondary WW treatment system for the IKEA® store situated in Catania (Eastern Sicily, Italy). Furthermore, the falling-head test was conducted to assess the hydraulic conductivity (Ks) variation in the hydraulic conductivity (Ks) of the HSTW, in order to understand its influence on the CR biomethane production. The average methane content values were 130.57 Nm3CH4/tVS (±24.29), 212.70 Nm3CH4/tVS (±50.62) and 72.83 Nm3CH4/tVS (±23.19) in August, September, October 2022, respectively. Ks was correlated with both dry matter (R2 = 0.58) and fiber content (R2 = 0.74) and, consequently, affected the biomethane yield, which increased as the Ks increased (R2 = 0.30 in August; R2 = 0.57 in September). In the framework of a circular economy, the results showed the successful possibility of integrating bioenergy production into TWs. The research could contribute (i) to encouraging plant operators to reuse biomass from TWs for local energy production and (ii) to help plant operators to understand Ks effects on the biomass biomethane yield in order to increase the sustainability of the system and to reduce the maintenance costs.