Greenhouse gas (GHG) emissions (CO2, CH4) from pilot-scale algal and duckweed-based ponds (ABP and DBP) were measured using the static chamber methodology. Daylight and nocturnal variations of GHG and wastewater characteristics (e.g. chemical oxygen demand (COD), pH) were determined via sampling campaigns during midday (12:30–15:30) and midnight (00:30–03:30) periods. The results showed that under daylight conditions in ABP median emissions were −232 mg CO2 m−2 d−1 and 9.9 mg CH4 m−2 d−1, and in DBP median emissions were −1,654.5 mg CO2 m−2 d−1 and 71.4 mg CH4 m−2 d−1, respectively. During nocturnal conditions ABP median emissions were 3,949.9 mg CO2 m−2 d−1, 12.7 mg CH4 m−2 d−1, and DBP median emissions were 5,116 mg CO2 m−2 d−1, 195.2 mg CH4 m−2 d−1, respectively. Once data measured during daylight were averaged together with nocturnal data the median emissions for ABP were 1,566.8 mg CO2 m−2 d−1 and 72.1 mg CH4 m−2 d−1, whilst for DBP they were 3,016.9 mg CO2 m−2 d and 178.9 mg CH4 m−2 d−1, respectively. These figures suggest that there were significant differences between CO2 emissions measured during daylight and nocturnal periods (p < 0.05). This shows a sink-like behaviour for both ABP and DBP in the presence of solar light, which indicates the influence of photosynthesis in CO2 emissions. On the other hand, the fluxes of CH4 indicated that DBP and ABP behave as net sources of CH4 during day and night, although higher emissions were observed from DBP. Overall, according to the compound average (daylight and nocturnal emissions) both ABP and DBP systems might be considered as net sources of GHG.

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