Performance analysis of a full-scale duckweed-covered sewage lagoon

A sewage lagoon for 2000–3000 capita (0.6 ha) has been operated successfully with a duckweed cover for over four years. The cover suppressed algal growth; the effluent turbidity was always below 12 Ntu. Because of inappropriate construction, one fifth of the inflow is lost by percolation and seepage during the dry season; during the wet season the loss is limited. During a detailed sampling period in the dry season actual hydraulic retention time was 20.4 d, and surface loading rate was 48–60 kgBOD5/ha • d. Concentration reduction was 90–97% for COD, 95–99% for BOD5, and 74–77% for Kjeldahl-N and total P. Effluent contained 2.7 mg Kjeldahl-N/l and 0.4 mg total P/l. The water column remained aerobic. At two-thirds of retention time the plants had absorbed virtually all NH+4 and ortho-PO3−4 from the water column. The duckweed harvest would remove in a watertight lagoon 60–80% of the N and P load, or 0.26 gN/m2 • d and 0.05 gP/m2 • d (in the first three-quarters of retention time). The results during this period were representative for the 4-year operation so far. Corrected for the leakage, plant productivity under these fertilised and managed conditions was sustained for several years at the level of 58–105 kg(dw)/ha • d, or 715–1200 kg/ha • d (over full lagoon surface) in the dry and wet season, respectively. We suggest that the microbial hydrolysis of the more complex organic N and P into NH+4 and ortho-PO3−4 is the limiting step for enhanced biomass production.