The use of free water surface constructed wetland to treat the eutrophicated waters of lake L’Albufera de Valencia (Spain)

Three free water surface constructed wetlands (FWSCWs) have been built in the border of the Lake L’Albufera de Valencia (Valencia, Spain). This lake is the emblematic element of one of the most important wetlands in Spain, L’Albufera de Valencia Natural Park, and it is highly eutrophicated. The function of the set of CWs (9 ha) is treating the eutrophic water from the lake with the objective of reducing the phytoplankton population and nutrients. The treatment wetlands named as FG and fp are comprised of three basins in a series, while the last, F4, consists of a single cell. During the first 2 years of operation, the inflow from lake was gradually increased from 0.01 m3 s−1 (April 2009) to 0.13 m3 s−1 (December 2010) with the goal of establishing the maximum hydraulic loading rate (HLR) and finding the highest removal efficiency. Input concentrations of different water quality variables studied showed a high variability. The inflow contained 8.80–94 mg l−1 TSS, 0.16–1.13 mg l−1 TP, 1–17.30 mg l−1 TN, 0.13–13.10 mg N l−1 DIN, 0.10–11.50 mg NO3−-N l−1, and 3.34–257.03 μg l−1 Chl a. The best removal results for these parameters were obtained in the FG wetland, where the average mass removal efficiencies were 75% TSS, 65% TP, 52% TN, 61% DIN, 58% NO3−-N, and 46% Chl a. In the set of constructed wetlands the removal rates increased with the hydraulic load rate for TSS and TP but neither for nitrogen species nor Chl a; for these variables the input concentrations are a key factor in removal. For instance, mean removal rate for nitrate is 101 mg N m−2 d−1 but values higher than 400 mg N m−2 d−1 were obtained when input concentrations were about 7 mg NO3−-N l−1. Values of first-order constant aerial rate (kA) have been obtained for all variables. The corresponding values for nitrogen species and total phosphorus are higher than obtained in previous studies but the value kA value for TSS is low (94.9 m year−1) owing to the eutrophic characteristics of water and a kA value for phytoplankton-Chl a of 65.1 m year−1 is introduced. The management of CWs implies the harvest of vegetation, not for remove nitrogen because nitrification–denitrification processes reduce the 83.3% of TN that enters, but for the phosphorus, limiting nutrient in the Lake L’Albufera, that now is accumulated in plants and soils but could be sent back in several years.