The effect of halophyte planting density on the efficiency of constructed wetlands for the treatment of wastewater from marine aquaculture

The low volume batches of highly-concentrated wastewater discharged from land-based marine recirculating aquaculture systems are ideally suited for treatment by halophyte planted constructed wetlands. To evaluate the role of plants and the effect of planting density on yield and performance in small-scale saline constructed wetlands (CWs), NH4+ + NO3− + NO2− = total dissolved inorganic nitrogen (TDIN) and dissolved inorganic phosphorus (DIP) were measured at regular intervals over 24 h periods. CWs were planted with the halophyte Salicornia europaea at high- and low-densities and were compared to the performance of unplanted controls. S. europaea plants were cropped regularly to assess potential commercial yield at the two densities. There was no significant effect of planting density on performance or crop yields and planted beds consistently outperformed the control beds removing 62.0 ± 34.6 mmol N m−2 d−1 (34–73% of influent TDIN) compared to 23.0 ± 26.8 mmol N m−2 d−1 (−1% to 41% of influent TDIN) by control beds. Results for DIP were less clear, significant removal occurred only once, with reduction of 18.3 ± 5.0 mmol P m−2 d−1 by planted beds and 18.1 ± 2.6 mmol P m−2 d−1 by the unplanted controls. The results demonstrate the effectiveness of halophyte-planted CW in treatment of marine aquaculture wastewater.