Depurative performance and methane emission of a pilot hybrid constructed wetland plant treating biogas digestate liquid fraction

Constructed wetlands (CWs) are an efficient system to treat wastewater, but only few information are available on CWs performance when biogas digestate liquid fraction (DLF) is treated. The aim of this research was to gain information of an hybrid CW pilot plant treating DLF in North-East Italy both on depurative performance and CH4 emission (before and after plant cutting). The system consists of two vertical subsurface flow (VSSF) beds vegetated with Phragmites australis and Arundo donax and an horizontal one (HSSF) vegetated with P. australis. The DLF main value pollutants input concentrations were: Total Nitrogen (TN) 427.3±58.7 mg L-1, COD 4955.5±1017.9 mg L- 1 and turbidity 2639.1±731.9 NTU. The hybrid system abated TN by 49%, COD by 41.1% and turbidity by 53.9%. Regarding plant management, mean higher CH4 emissions were registered after plant cutting (378.9±337.9 mg m-2 d-1) whereas with plants the average emissions was 68.5±115.2 mg m-2 d-1. HSSF bed showed lower CH4 emission, both before and after cutting (28.5±49.3 and 110.3±51.8 mg m-2 d-1 respectively), than VSSF ones (88.6±137.2 before and 513.25±341.58 mg m-2 d-1 after cutting). VSSF beds vegetated with A. donax emitted higher CH4 than P. australis, both before (177.1±153.4 vs 0 mg m-2 d-1 respectively) and after cutting (716.8±350.6 vs 309.8±211.1 mg m-2 d-1 respectively).