Performance of duckweed-covered sewage lagoons—II. Nitrogen and phosphorus balance and plant productivity

Laboratory scale experiments were performed in a non-continuous batch reactor system with 0.8 to 41.2 l with domestic sewage exposed to constant light intensity, temperature and humidity. The contribution of duckweed (L. gibba) to N and P removal in duckweed-covered sewage lagoons (DSL) was studied at NH+4 concentration of 25–96 mg/l with 10, 30, 70 and 95 cm deep reactors, and liquid mixing intensity of 0, 0.3, 1.0, 2.3 and 34.1 W/m3. Duration of each experiment was 20 d with biomass harvesting every 5 d. N and P removal at extreme depths and extreme mixing intensities was compared with that in covered control reactors without duckweed. For a given N input, depth as an independent variable would not affect overall N removal except through increase of surface loading, whereas mixing had a significant positive effect at high Kj–N concentration. TP removal was stronger correlated with depth than with surface loading λP. At low nitrogen surface loading λN of 183 kg N/ha, TN removal could be completely attributed to duckweed uptake, whereas at high λN (>300 kg N/ha), which correlated with high NH+4 concentration, N uptake was inhibited to less than 50% of total removal. TP removal increased from 8.25 kg P/ha 20 d at higher Kj–N concentration (102 mg N/l) to 18.66 kg P/ha 20 d at lower concentrations (26 mg N/l). P uptake increased from 2.54–6.90 kg P/ha 20 d by introducing mixing (2.3 W/m3). Relative growth rate decreased from 0.19 to 0.05 d−1 for initial concentrations of 25–96 mg NH+4-N/l, respectively. N and P removal followed first-order kinetics.