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Performance investigation of pilot scale constructed wetlands treating olive mill wastewater

Douzi Anna

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Summary

In the context of this diploma thesis, the performance of pilot-scale constructed wetlands, of both horizontal and vertical subsurface flow, was investigated and evaluated. The main objective was the treatment of wastewater from a three-phase olive mill, focusing on the reduction of the organic load and the removal of nutrients and other pollutants, in order to draw conclusions regarding the design validity and the overall efficiency of the systems. The experimental setup, installed outdoors at the School of Chemical and Environmental Engineering of the Technical University of Crete in Chania, comprised six units: four with horizontal flow and two with vertical flow. Different configurations of filter media were tested in each tank, including zeolite, lightweight expanded clay aggregate (LECA), biochar, and recycled plastic (RHDPE), both with and without vegetation (P. Australis and A. Donax). The first unit (C) served as an unplanted control unit, with a substrate of zeolite and LECA. The next three horizontal flow units (ZL, PB, P) were planted with P. Australis and A. Donax and featured different filter media: ZL had the same composition as C (zeolite and LECA), PB contained RHDPE and biochar, and P contained only RHDPE. The two vertical flow units, L and LPB—also planted with P. Australis and A. Donax—consisted of LECA alone (L) and a combination of LECA, biochar, and RHDPE (LPB). Over a seven-month period (November 2023 - May 2024), the tanks were fed every two days with diluted olive mill wastewater, sourced from a three-phase olive mill in the Perivolia area of Chania. The wastewater was diluted with water at a 1:8 ratio before being fed to the units. Regular sampling and analysis of the tank effluents, the diluted influent, and the raw wastewater were conducted to evaluate physicochemical parameters such as pH, electrical conductivity (EC), total suspended solids (TSS), color, biochemical and chemical oxygen demand (BOD₅, COD), total nitrogen (TN), ammoniacal nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃⁻-N), total phosphorus (TP), phosphates (PO₄³⁻-P), and total phenols. Additionally, after harvesting, the plant biomass was analyzed to determine its content of total nitrogen (TN), total phosphorus (TP), metals, and metalloids. The experimental results indicated that the average pollutant removal efficiency varied significantly, depending on the composition, the flow type of each system, and the local environmental conditions. Specifically, unit C demonstrated exceptionally high removal rates, achieving 91.4 ± 3.9% for COD, 97 ± 1.8% for BOD₅, 90.8 ± 5.6% for TP, 95.4 ± 6.0% for PO₄³⁻-P, and 83 ± 8.2% for total phenols, along with a significant reduction in TSS. Unit ZL, with the same substrate but vegetated, showed similarly high performance, with removal rates of 89.8 ± 4.7% for COD, 97.7 ± 2.0% for BOD₅, 87.6 ± 11.9% for TP, 83.2 ± 14.8% for PO₄³⁻-P, and 83 ± 8.2% for total phenols. Conversely, units PB and P, which were based on recycled plastic, exhibited comparatively lower performance. PB recorded removal efficiencies of 75.7 ± 14.3% (COD), 94.8 ± 2.5% (BOD₅), 71.2 ± 28.9% (TP), 48.5 ± 22.2% (PO₄³⁻-P), and 59.1 ± 20.5% (phenols), while P showed values of 76.1 ± 11.4% (COD), 93.3 ± 4.2% (BOD₅), 38 ± 15.3% (TP), 39.6 ± 14.7% (PO₄³⁻-P), and 60 ± 18.9% (phenols). The vertical flow units also demonstrated satisfactory performance, with unit L achieving 82.5 ± 7.8% (COD), 95.3 ± 1.9% (BOD₅), 61.9 ± 21.1% (TP), 63.9 ± 21.7% (PO₄³⁻-P), and 76.7 ± 9.6% (phenols), while unit LPB recorded removals of 81.8 ± 9.8% (COD), 97.6 ± 1.3% (BOD₅), 51.5 ± 18.4% (TP), 53.4 ± 19.7% (PO₄³⁻-P), and 69.1 ± 16.1% (phenols). In conclusion, the pilot units containing zeolite and LECA as filter media demonstrated the highest efficiency in removing organic and nutrient pollutants, with the presence of vegetation not being a decisive factor. This confirms that constructed wetlands represent an efficient, environmentally friendly, and sustainable solution for the management of olive mill wastewater.

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