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Study of pilot constructed wetland treating olive mill wastewater

Ntalakogeorgou Athanasia

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Summary

This thesis focuses on the monitoring and evaluation of the performance of pilot-scale constructed wetlands, operating under both horizontal and vertical subsurface flow conditions. The primary aim is the reduction of organic load and the removal of nutrients and other pollutants from the wastewater of a three-phase olive mill, as well as the extraction of conclusions regarding the design reliability and treatment efficiency of the systems. The experimental setup consisted of six constructed wetland units: four horizontal subsurface flow systems and two vertical subsurface flow systems, installed outdoors at the Technical University of Crete in Chania. The first unit (C) was unplanted and filled with zeolite and lightweight expanded clay aggregate (LECA). The following three units (ZL, PB, and P) were planted with Phragmites australis and used different substrate materials: zeolite and LECA in unit ZL; recycled plastic and biochar in unit PB; and only plastic in unit P. The two vertical flow units, L and LPB, were filled with LECA and a combination of LECA, biochar, and plastic, respectively. The wastewater originated from a three-phase olive mill located in the area of Perivolia, Chania, and was diluted with water at a 1:8 ratio before being fed into the systems. Physicochemical analyses included measurements of pH, electrical conductivity (EC), biochemical oxygen demand (BOD₅), chemical oxygen demand (COD), nitrate nitrogen (NO₃⁻-N), ammonium nitrogen (NH₄⁺-N), total nitrogen (TN), orthophosphate (PO₄³⁻-P), total phosphorus (TP), total phenols, and total suspended solids (TSS). According to the experimental data, the average removal performance of the constructed wetland units varied depending on the substrate composition, the flow configuration, and the environmental conditions at the site. Unit C, which was unplanted and filled with zeolite and LECA, demonstrated high removal efficiencies: 93.4 ± 2.6% for COD, 98.4% for BOD₅, 91.6% for both TP and PO₄³⁻-P, and 85% for TSS. It also showed significant removal of total phenols. Similarly, unit ZL, which had the same substrate but was planted with P. australis and Arundo donax, achieved comparable performance, with 89.9% COD, 97.8% BOD₅, 92% TP and PO₄³⁻-P, and 87% TSS removal. On the other hand, units PB and P, which used recycled plastic and biochar (PB) or plastic alone (P), demonstrated relatively lower removal efficiencies. In unit PB, removal rates were 78% for COD, 83% for BOD₅, 52% for both TP and PO₄³⁻-P, and 74% for TSS, while unit P achieved 75% COD, 80% BOD₅, 50% TP and PO₄³⁻-P, and 70% TSS removal. The vertical flow units also demonstrated notable performance. Unit L, which was filled solely with LECA, achieved 82% COD, 88% BOD₅, 76% TP and PO₄³⁻-P, and 78% TSS removal. Unit LPB, which combined LECA, biochar, and plastic, performed even better, reaching 90.2% COD, 95% BOD₅, 85% TP and PO₄³⁻-P, and 83% TSS removal. In conclusion, the systems using zeolite and LECA regardless of the presence of reeds demonstrated the highest efficiency in removing organic and nutrient pollutants. Constructed wetlands proved to be an effective and environmentally friendly solution for the treatment of olive mill wastewater.

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