Evaluation of sewage treatment performance and duckweed biomass production efficiency in a primary sedimentation basin + duckweed pond + downflow hanging sponge (PSB + DWP + DHS) system

A duckweed cultivation process was introduced into a sewage treatment system to remove nitrogen and phosphorus and to produce duckweed biomass. The system, consisting of a primary sedimentation basin followed by a duckweed pond (DWP) and a downflow hanging sponge (DHS), was installed in a sewage treatment plant in Japan and operated for a long time. The effluent quality was comparable to that of the conventional activated sludge (CAS) process at the sewage treatment plant. Total nitrogen and total phosphorus were mainly removed in the DWP, demonstrating that nutrients were removed during the production of duckweed biomass. In addition, the coliform removal rates were superior to those of the CAS process. Water temperature significantly affected the effluent water quality of the system and duckweed biomass production. The results also demonstrated the importance of properly managing biomass density in DWPs to increase biomass production. A change in hydraulic retention time (HRT) did not significantly affect the treatment performance of the system. It is proposed that DWPs with a longer HRT and DHSs with a shorter HRT be designed to maximize biomass production and sewage treatment performance.