Nitrogen transformations and removal mechanisms in algal and duckweed stabilisation ponds

This thesis describes the results of a comparative study of the performance of algae-based ponds (ABPs) and duckweed-based ponds (DBPs) for wastewater treatment, with emphasis on nitrogen transformations and removal mechanisms.

Batch experiments simulating algae and duckweed ( Lemna gibba ) stabilisation ponds for domestic wastewater treatment were conducted to quantify the importance of various nitrogen removal mechanisms under controlled conditions of pH and DO. N-removal in both systems by different mechanisms are more dependent on the pH variations than on the oxygen variations. Significantly higher N-removal efficiency in the duckweed system (26-33%) than in the algae system (14-24%) was found at lower pH range of 5 to 7. At high pH values of 7 to9, the increase in N-removal by sedimentation and volatilisation in the algae system and the decrease in N-uptake by duckweed in the duckweed system resulted in significantly higher N-removal efficiency in the algae system (45-60%) than the duckweed system (38-41%).

Further research was carried out in pilot scale algae-based and duckweed-based systems that consisting of 4 similar ponds in series for each system, fed with wastewater with hydraulic retention time of 7 days in each pond.

Tracer experiments showed that the hydraulic characteristics were similar in both ABPs and DBPs. Actual retention times were observed to be higher than the theoretical retention times due to the spurious tracer curves resulting in negative dead spaces. This suggests that the higher density of LiCl solution compared to pond wastewater density, probably caused LiCl to pass onto the bottom of the pond and slowly dissolved into pond water where it leached out at a much longer period and thus giving spurious tracer curves. The hydraulic behaviour of the ponds was neither plug-flow nor completely mixed, but rather showed a dispersed flow. A tracer experiment in larger scale algae and duckweed-based ponds in Colombia showed lesser short-circuiting and more plug flow conditions in the duckweed pond than the algae pond. For larger surface areas the presence of duckweed cover improved the hydraulic performance of pond. The better hydraulic behaviour of the pond with duckweed cover may be explained by reduced wind-induced short circuiting and reduced mixing caused by the absence of algae biomass activities.

Higher BOD and TSS removal efficiencies were achieved in DBPs compared to ABPs. In both systems, the removal of BOD and TSS did not differ significantly during the different seasons of warm and cold weather. Total-P was more effectively reduced in DBPs than in ABPs, irrespective of the season due to phosphorus uptake of duckweed and subsequent removal from the system via harvesting.

Increase in organic loading resulted in an increase in BOD and TSS removal rates in both ABPs and DBPs. Phosphorous removal was similar during the two experimental periods in ABPs. DBPs behaved likewise. Removal of FC in ABPs was higher than in DBPs. FC removal in the ABPs and in the DBPs was significantly higher during low organic loading period compared to high organic loading period. FC removal in ABPs during the low and high organic loading periods was 3.8 and 3.4 log units, respectively. Corresponding values for DBPs were significantly lower at 2.2 and 1.8 log units. Lower FC removal was found during the cold period in both systems.

During the low organic loading period at warm temperature, nitrogen removal efficiency was higher in ABPs (80%) than in DBPs (55%) despite the fact that approximately one third of the influent nitrogen to the DBPs is removed via duckweed harvesting. Lower N-removal in both systems was obtained during cold season but similar removal was found during periods of low and high organic loading.

Quantification of N-fluxes in both systems showed that the major fluxes of nitrogen in the ABPs were sedimentation (33-40%) and denitrification (14-24%). Sedimentation and denitrification in DBPs were of equal importance except during the warm season and low organic loading operation, when sedimentation was low. In DBPs, 30-33% and 15% of the total nitrogen was recovered into biomass and removed from the system via duckweed harvesting during the summer and winter period, respectively. During the high organic loading period, nitrogen recovered via duckweed was 19%. Ammonia volatilisation in both treatment systems was found to be a minor N-removal mechanism responsible for less than 1.1 % of total influent nitrogen. Nitrification and denitrification occurred in the aerobic water phase and anaerobic sediment, respectively. Higher DO concentrations in ABPs, especially during the warm season, favoured higher nitrification in ABPs as compared to DBPs. Predictive models for nitrogen removal in ABPs and DBPs were proposed. They presented a good reflection of nitrogen fluxes on overall nitrogen balance under the prevailing experimental conditions. Validation of the models with reported data from literature gave poor results for shallower ponds, while better agreement was obtained using data for deeper ponds. Further elaboration and validation of the model to accommodate pond design and environmental parameters that dictate pond performance is required.