Potentials of using duckweed (Lemna gibba) for treatment of drainage water for reuse in irrigation purposes

The potential use of duckweed (Lemna gibba) for the treatment of drainage water was investigated. Three continuous flow duckweed-based treatment systems (one-pond, two-pond, and three-pond) were used. Removal efficiencies of CODtotal and ammonia in the two-pond system were significantly higher (60.2 +/- 6.1% and 80.2 +/- 1.4%) than that found for single-pond system (30.6 +/- 7.9% and 56.8 +/- 3.3%), respectively, at a total hydraulic retention time (HRT) of 14d. Performance of three-pond system connected in series was evaluated at different HRTs of 21, 14, and 7d. Results showed that increasing the HRT and area of duckweed pond to pond depth (A(duckweed)/d(pond)) ratio from 7 to 14d and from 63.83 to 127.66 substantially increased the removal efficiency of CODtotal from 59.7 +/- 3.29 to 88.34 +/- 1.82%, respectively, resulting an effluent quality of 13.6 +/- 2.3 mg COD/L in the treated effluent. However, the removal efficiency of CODtotal remained almost constant when increasing the HRT from 14 to 21d and A(duckweed)/d(pond) from 127.66 to 191.49. This was not the case for nitrification efficiency, where ammonia removal increased from 32.6 +/- 7.95 to 71.75 +/- 6.1% and from 71.75 +/- 6.1 to 85.6 +/- 4.6% when increasing the HRT from 7 to 14d and from 14 to 21d, respectively.