Influence of the photoperiod on carbon dioxide, methane and nitrous oxide emissions from two pilot-scale stabilization ponds

74Greenhouse gas emissions (CO₂, CH₄, and N₂O) from pilot-scale algae facultative and duckweed-based ponds (AFP and DBP) were measured using the static chamber methodology. Daytime and night-time variations of GHG and wastewater characteristics e.g. COD, and pH were determined via sampling campaigns during the midday (12:30–15:30) and middle of the night (00:30–03:30). The results showed that under daytime conditions in the AFP median emissions were −232 mg CO₂ m^-2 d^-1, 9.9 mg CH₄ m^-2 d^-1, and 6.9 mg N₂O m^-2 d^-1, and in the DBP median emissions were −1,654.5 mg CO₂ m^-2 d^-1 and 71.4 mg CH₄ m^-2 d^-1, and 8.5 mg N₂O m^-2 d^-1 respectively. During night-time conditions the AFP median emissions were 3,949.9 mg CO₂ m^-2 d^-1, 12.7 mg CH₄ m^-2 d^-1, and 5.5 mg N₂O m^-2 d^-1 whereas the DBP median emissions were 5,116 mg CO₂ m^-2 d^-1, 195.2 mg CH₄ m^-2 d^-1, and 2 mg N₂O m^-2 d^-1 respectively. Once data measured during the daytime were averaged together with night-time data the median emissions for the AFP were 1,566.8 mg CO₂ m^-2 d^-1, 72.1 mg CH₄ m^-2 d^-1, and 9.5 mg N₂O.m^-2.d^-1 whilst for the DBP they were 3,016.9 mg CO₂ m^-2 d^-1, 178.9 mg CH₄ m^-2 d^-1, and 8.6 mg N₂O.m^-2.d^-1. These figures suggest that there were significant differences between CO₂ emissions measured during daytime and night-time periods (p<0.05) signifying a sink-like behaviour for both the AFP and DBP in the presence of solar light, which indicates the influence of photosynthesis in the CO₂ emissions. Overall, according to the compound average (daytime and night-time) both AFP and DBP systems might be considered as net sources of GHG.