Could hydraulic parameters variation affect the vegetation development in treatment wetlands?

Abstract: Treatment wetlands (TWs) are complex ecosystems due to variable conditions of hydrology, soil hydraulics, plants and microbiological species diversity and mutual interactions. On the one hand, hydraulics plays a vital role on the treatment performance and on the life cycle of TWs, on the other hand, the vegetation substantially contributes to remove and to retain pollutants. As well known, the unavoidable and progressive clogging phenomenon in TWs affects their hydraulics. A lack of knowledge still remains to what extend hydraulic parameters variation can affect the vegetation developments in TWs. To answer to this question, the Phragmites australis development in comparison with hydraulic characteristics was monitored in a 8 years old - horizontal flow (HF) TW located in Mediterranean area (Eastern Sicily, Italy). Data were collected in nine observation points equally distributed along three transects established at 8.5 m (T1), at 17 m (T2) and at 25.5 m (T3) from the inlet. The falling head (FH) test was conducted to assess the hydraulic conductivity (K_s) variation in the HF-unit. Residence time distribution (RTD) analysis was performed to evaluate the real hydraulic retention time (HRT) and the hydraulic efficiency parameter (λ). Finally, the saturation method was applied for substrates porosity (φ) determination. In the HF-TW a morphological and chemical characterization of Phragmites australis above-ground biomass was carried out in 2022. In particular, plants density (in terms of culms number) and height (m) were measured at the end of the growing season (July). In each transect of the HF-TW, fresh weight (g), dry matter (DM, %), ash (%), volatile solids (VS, %), pH, Total Kjeldahl Nitrogen (TKN, % of DM) and fiber content (cellulose, hemicellulose and lignin) were estimated. Preliminary results showed a strong positive regression between DM and both K_s (R² = 0.78) and porosity values (R² = 0.97) observed in the HF-TW. This study could contribute to help plant operators to understand hydraulic characteristics effects on the biomass, to improve TWs treatment efficiency, system management and lifespan.

Keywords: Wastewater treatment, Phragmites australis, plants growth, hydraulic characteristics, substrate.

Acknowledgments: This research was funded by the University of Catania-PIAno di inCEntivi per la RIcerca di Ateneo 2020/2022—Linea di Intervento 3 “Starting Grant” and the PhD Course in Agricultural, Food and Environmental Science (Di3A, University of Catania).