Start-up of a free water surface constructed wetland for treating olive mill wastewater

An olive mill's existing evaporation pond was separated into five cells and transformed into a free water surface constructed wetland. The constructed wetland was used as a post-treatment stage for olive mill wastewater (OMW). Wastewater was previously treated by an aerobic trickling filter. The influent concentrations in the constructed wetland were 27400, 4800, 105 and 770 mg.L-1 for COD, phenols, ortho-phosphate and TKN, respectively. Despite the rather high influent concentrations, the performance of the constructed wetland was very good since after the 60-day start-up operation period it achieved removal rates of about 94, 95, 95 and 98% for COD, phenols, ortho-phosphate and TKN, respectively. The major pollutant removal processes can be attributed to both biological processes occurring in the wetland and photo-oxidation. Laboratory-scale experiments with OMW from fifth cell of the wetland revealed that the net contribution of photo-oxidation after 112 h of simulated solar radiation at 765 W/m(2) (i.e., about 38 days of sunlight irradiation) was 18 and 31% removal for COD and phenols, respectively. In the constructed wetland, the total removal reached 81 and 86% for COD and phenols, respectively, for the same time period (38 days).