Fig 1.
Schematic overview of the experimental setup.
Cascades consisted of container 1 (1), filled with effluent and a sludge compartment (1.2), connected with a pipe to container 2 (2) which was also filled with effluent. A peristaltic pump (P) pumped the water from container 2 back into container 1. Chironomus larvae and egg ropes were added to container 1 of the MC and MP treatments, and Azolla was added to container 2 of the CP and MP treatments. To assess microbial sludge degradation, nutrient dynamics and GHG emissions, a control treatment without Chironomus and Azolla (CC) was also included in the setup.
Fig 2.
Sludge dry weight (a), N content (b), P content (c) and C content (d) of the initial sludge, and at the end of the 26-day experiment for the CC, CP, MC and MP treatments. Boxes show interquartile ranges, bold lines represent the median, whiskers indicate the lowest and highest values within a 1.5x interquartile range from the box, dots represent outliers.
Fig 3.
Dissolved nitrogen (a), dissolved phosphorus (b) and dissolved organic carbon (c) concentrations in the overlying water (μmol L-1) during the 26-day experiment for the CC, CP, MC and MP treatments. Boxes show interquartile ranges, bold lines represent the median, whiskers indicate the lowest and highest values within a 1.5x interquartile range from the box, dots represent outliers. White circles and triangles represent the average concentrations in respectively container 1 and container 2.
Fig 4.
CH4 (a) and CO2 (b) fluxes for all treatments during the 26-day experimental period. Note the numbers given above some CH4 fluxes, which correspond to the number of flux measurements unusable due to ebullition in container 1. Boxes show interquartile ranges, bold lines represent the median, whiskers indicate the lowest and highest values within a 1.5x interquartile range from the box, dots represent outliers. White circles and triangles represent the average concentrations in respectively container 1 and container 2.
Fig 5.
Mass balance of the treatment cascades for N (a), P (b) and C (c) in mmol at the start and the end of the 26-day experiment. Compartments include leftover material (grey), Azolla biomass (green), Chironomus biomass (red), overlying water (blue) and sludge mass (brown).