Constructed wetlands as a treatment method for effluents from intensive trout farms

This study examined the effects of different hydraulic loading rates on the treatment efficiency of subsurface flow (SSF) constructed wetlands treating effluents from trout farming over a period of 6 months. Six identical wetland cells with a pre-sedimentation zone of 9.6 m² and a root zone of 23.6 m² were loaded with effluents from intensive trout farming (> 2.1 kg feeding stuff per L/s and day). The total runoff of 13.2 L/s was treated in the wetland cells, where two duplicate cells received equal hydraulic loads of 3.9, 1.8 and 0.9 L/s. All examined wetland cells had significant treatment effects on the nutrient fractions containing particulate matter total nitrogen (TN), total phosphorous (TP), biological oxygen demand in 5 days (BOD₅), chemical oxygen demand (COD), and total suspended solids (TSS)].

Efficiency was between 5.5% for TN and 90.1% for TSS. The SSF wetland also had a high treatment effect on total ammonia nitrogen (TAN), with efficiencies of 61.2 to 87.8%. Nitrate nitrogen (NO₃–N) and phosphate phosphorous (PO₄–P) showed a significant increase in the wetland effluent by 8.4 to 209%. Nitrite nitrogen (NO₂–N), had no significant, or significant effluent increase depending on the inflow rate. Treatment efficiency for particulate nutrients and TAN increased with decreasing hydraulic load, while the differences between 1.8 and 0.9 L/s were not significant. The treatment efficiency for TP was constant for all cells, at around 40%. The wetland receiving 3.9 L/s was over-flooded after 10 to 12 weeks due to colmatation. Nevertheless, the wetland still showed high treatment efficiencies. For commercial trout farms, SSF wetlands are a highly effective method of effluent treatment. A hydraulic load of 1 L/s on 13.3 m² wetland area (1.8 L/s on the examined wetland) seems most suitable. Higher loads lead to accelerated wetland colmatation, while lower loads waste space.