Emissions of greenhouse gases carbon dioxide and methane from duckweed systems for stormwater treatment
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This study determined the greenhouse gas emission from lab-scale duckweed treatment systems that were used for stormwater treatment. By using the static chamber technique, the fluxes of CO2 emission from the duplicate duckweed systems were 1472 -- 721 and 626 -- 234 mg m-2 d-1, respectively. After the complete removal of duckweeds, CO2 emission from the systems decreased to 492 -- 281 and 395 -- 53 mg m-2 d-1, respectively. A thin-film model was successfully applied to predict the increasing CO2 concentrations approaching saturation in the static chamber. In contrast, the concentrations of methane in the closed chamber fluctuated a lot with time, which were attributed to complex methane production and consumption reactions at the soil-water interface. The CH4 flux from the two duckweed systems were 299 -- 74 mg m-2 d-1 and 180 -- 91 mg m-2 d-1, respectively. After the removal of duckweeds, the flux were 559 -- 215 mg m-2 d-1 and 328 -- 114 mg m-2 d-1, respectively. The higher CO2 emission in the duckweed systems was linked to more biomass debris formation on the soil surface due to duckweed growth and decay. As a result of duckweed growth, the duplicated duckweed systems removed 54 -- 13 % COD, 94 -- 4 % NH4 + -N, 87 -- 7 % NO3- -N, 34 -- 7 % PO4 3- -P at the hydraulic retention time of 10 days. When the duckweeds were removed, the nutrient removal efficiencies decreased significantly: 68 -- 3 % for NH4 + -N, 43 -- 7 % for NO3 - -N, 10 -- 6 % for PO4 3- -P. The COD removal efficiency without duckweeds was 47 -- 6 %, which did not change significantly.