The relative importance of Lemna gibba L., bacteria and algae for the nitrogen and phosphorus removal in duckweed-covered domestic wastewater

To arrive at detailed nutrient balances for duckweed-covered wastewater tre atment systems, five laboratory-scale experiments were carried out in shall ow (3.3 cm), 11 batch systems to assess separately the contributions of duc kweed itself, attached and suspended bacteria as well as algae to N- and P- removal in domestic wastewater. Depending on the initial concentrations, ou r duckweed-covered systems removed 120-590 mg N m(-2) d(-1) (73-97% of the initial Kjeldahl-nitrogen) and 14-74 mg P m(-2) d(-1) (63-99% of the initia l total phosphorus) in 3 days. Duckweed (Lemna gibba L.) itself was directl y responsible for 30-47% of the total N-loss by uptake of ammonium and, pro bably dependent on the initial P-concentrations, for up to 52% of the total P-loss. The indirect contribution of duckweed to the total nutrient remova l was also considerable and included the uptake (and adsorption) of ammoniu m and ortho-phosphate by algae and bacteria in the attached biofilm and the removal of N through nitrification/denitrification by bacteria attached to the duckweed. Together these accounted for 35-46 and 31-71% of the total N - and P-loss, respectively. Therefore, approximately 3/4 of the total N- an d P-loss could be attributed to the duckweed mat. The remaining quarter is due to non-duckweed related components: uptake and nitrification/denitrific ation by algae and bacteria attached to the walls and the sediment of the s ystem (including sedimentation). Other processes, like NH3-volatilisation, N-fixation and nutrient uptake as well as nitrification/denitrification by suspended microorganisms did not influence the N- and P-balance of our syst ems, but could become important with increasing water depths and retention times. (C) 1998 Elsevier Science Ltd. All rights reserved.