Performance of duckweed-covered sewage lagoons—I. Oxygen balance and COD removal

Abstract

Abstract Laboratory scale experiments were performed in a non-continuous batch reactor system with 0.8–41.2 l domestic sewage exposed to constant light intensity, temperature and humidity. The treatment performance of duckweed ( L. gibba )-covered sewage lagoons (DSL) was studied within a COD total range of 200–500 mg/l (113–294 mg COD filt /l), in 10, 30, 70 and 95 cm deep reactors, and liquid mixing intensity (power dissipation) of 0, 0.3, 1.0, 2.3 and 34.1 W/m 3 . The duration of each experiment was 20 days with biomass harvesting every 5 days. COD removal at extreme depths and extreme mixing intensities was compared with that in covered control reactors without duckweed. Removal of COD filt did not differ in duckweed-covered and control reactors. The role of duckweed cover was marginal in changing the redox potential or the DO. COD removal λ r (kg COD filt /ha 20 days) correlated strongly with initial surface load λ s (kg COD filt /ha). Concentration removal (as mg COD filt /l) was also proportional to initial COD filt concentration. For a given COD filt mass input, increasing depth up to 1 m affected DSL performance only by increasing surface load, and not by hampering oxygen transfer. Mixing (up to 2.3 W/m 3 ) raised COD filt removal. Therefore, at depths beyond 70 cm, moderate mixing is recommended. The first-order kinetic removal rate coefficient for COD filt was 0.04–0.06 d −1 .