Constructed wetlands with free water surface for treatment of aquaculture effluents

The aim of the study was to determine the reduction of the overall environmental load (in terms of organic and nutrient load) in effluents of a flow-through trout farm. Effluents of a flow-through system for rainbow trout (Oncorhynchus mykiss) production passed through constructed wetlands with free water surface. Removal of nutrients was determined in three wetlands of 350 m(2) each at hydraulic residence times (HRTs) of 3.5, 5.5 and 11 h. The areal load of total suspended solids (TSS), chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN) varied in terms of HRTs from 12.3-36.8 g m(-2) day(-1), 21.7-65.2 g m(-2) day(-1), 0.23-0.70 g m(-2) day(-1), and 1.46-4.37 g m(-2) day(-1). Values for reduction of suspended solids, COD, TP, and TN were 67-72%, 30-31%, 41-53% ,and 19-30%, respectively. Significantly lower nutrient concentrations in the effluent among the wetlands were only found for nitrogen parameters: TN and ammonia concentrations were lower in the wetlands with a HRT of 5.5 h (0.89 mg L(-1), 0.11 mg L(-1)) and 11 h (0.81 mg L(-1), 0.11 mg L(-1)) compared with the one with 3.5 h (0.96 mg L(-1), 0.16 mg L(-1)).