Effects of Solids Accumulation on Greenhouse Gas Emissions, Substrate, Plant Growth and Performance of a Mediterranean Horizontal Flow Treatment Wetland

Abstract

In treatment wetlands (TWs), solids accumulation can result in hydraulic malfunction, reducing the operation life, and it could enhance biological activity by favoring biofilm development. It is still unknown whether the solids accumulation can affect greenhouse gas (GHG) emissions. This study aims to evaluate the solid concentration along a horizontal flow (HF) TW, and its role in GHG emissions, hydraulics, treatment performance, and vegetation development (Phragmites australis (Cav.) Trin. ex Steud.). The study was carried out in an eight-year-old full-scale HF-TW located in the Mediterranean region (Sicily, Italy). To collect data inside the HF unit, nine observation points (besides the inlet and the outlet) along three 8.5-m-long transects (T1, T2, and T3) were identified. The first transect (close to the inlet zone) showed a hydraulic conductivity (Ks) reduction approximately one order of magnitude higher than the other two. Results highlighted that GHG emissions increased during the summer, when the temperature and solar radiation were higher than in the rest of the year, matching the macrophyte growth rate. Theoretical methane (CH4) emissions followed the trend of volatile solids (VS), which was around 3.5 and 4 times in T1 to T2 and T3. Pore clogging affected carbon dioxide (CO2) emissions, which decreased from T1 to T3, with maximum monthly values in T1 (21.4 g CO2·m−2·d−1) being approximately double with respect to T2 (12.6 g CO2·m−2·d−1) and T3 (10.7 g CO2·m−2·d−1) observed in July. The same trend for chemical oxygen demand (COD) removal efficiency, decreasing from T1 to T3, was observed. Notwithstanding this behavior, the final effluent quality was very satisfactory, with an average value of COD removal efficiency above 90%.