Effect of introducing aerobic zones into a series of duckweed stabilization ponds on nitrification and denitrification

Duckweed pond technology presents a low cost alternative for many wastewater treatment applications. Although its potential for removing carbonaceous and suspended material from wastewater has been demonstrated, duckweed systems could be further optimized for nitrogen removal. Nitrogen removal was studied in a pilot plant consisting of seven small duckweed ponds in series. The feed of the duckweed pond system consisted of the effluent of a real scale UASB reactor, which treated domestic wastewater from Ginebra, a small town in Colombia. This experiment was run in two consecutive phases. During the first phase, the seven ponds of the pilot plant were fully covered with duckweed ( Spirodela polyrrhiza ). Before the start of the second phase, the duckweed cover was removed from ponds 1 and 3. The system was operated with a continuous flow to produce a hydraulic retention time (HRT) of3 days per pond and a total HRT of21 days. The duration of each phase was three months and the system was monitored for pH, temperature, oxygen and all fluxes of nitro gen. Effluent total nitrogen was significantly different in the two phases, with 13.8± 2.9 mg TN l -1 (63 % removal) and 3.7±1.5 mg TN l -1 (90%) for first and second phase, respectively. Denitrification was the most important removal mechanism during phases, and amounted to 43.5 % and 76.2 % of influent nitrogen, in first and second phase, respectively. Ammonia volatilization and sedimentation were insignificant processes for nitrogen removal in both phases. Nitrification played an important role in nitrogen transformations in the duckweed systems and it was favored by the introduction of aerobic zones. Denitrification also played a key role in nitrogen transformations and removal. Despite the presence of oxygen in the water column, denitrification occurred, probably due to the anaerobic microenvironment of system biofilms. This research showed that higher nitrogen removal might be obtained in duckweed pond systems through the introduction of aerobic zones in early stages of the system. Where effluents can not be reused for crop irrigation, strict nitrogen effluent criteria can be met using hybrid duckweed-algal ponds at considerably shorter hydraulic retention time compared to fully covered systems.