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Optimization of pilot scale constructed wetlands with recycled HDPE and biochar for leachate treatment from Chania landfill

Theodoraki Nikoletta

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Summary

In the context of the present thesis, the optimization of construction and operation parameters of constructed wetlands have been experimentally studied for the treatment of leachate from the landfill of Chania. The experiment concerns two horizontal subsurface flow constructed wetlands, which were placed in the outdoor area of the greenhouse of the School of Chemical and Environmental Engineering at the Technical University of Crete. Recycled HDPE plastic was used as filling material to both units, while in one unit a layer of biochar was also placed. The units were labelled P (Ρlastic) and PB (Ρlastic & Βiochar) and were planted with the common reed, Phragmited Australis.The inflow for the units was leachate, which was collected after the flocculation stage at the leachate treatment plant of the Chania landfill. Moreover, methanol was added to both units at certain time period. The operating volume of each unit was 188.3 L. Additionally, the average flow rate of volume for the units was 3.3L/day. The follow-up analyses concerned the inflow, outflow and the plant biomass of the units. More specifically, the inflow and outflows was analyzed for the determination of the following parameters: BOD5, COD, NO3-N, NO2-N, NH4-N, PO43--P, Electrical Conductivity, pH, metals and metalloids. The analysis of the plant biomass aimed at the determination of TN (total Nitrogen), TP (total Phosphorus), metals and metalloids.The results of the mean yields for the P and PB units are as follows. For COD the efficiency in unit P was 70.5±10.8% and in unit PB is 72.5±10.6%. For NO3-N the yield of unit P was equal to 81.2±10.5%, while for unit PB the yield reached 95.2±3.0%. For NO2-N the yield was 88.9±7.0% of the P unit and 95.0±4.8% of the PB unit. Regarding NH4-N, the unit P reached 71.3±12.4% and the unit PB 79.9±9.4%. Respectively, the yield for TSS was for unit P at 87.1±8.6% and 86.6±5.4% for PB. Moreover, the Electrical Conductivity seems to be almost at the same level as that of the inflow, while the pH is slightly alkaline. Also, BOD5 and PO43--P were almost undetectable in the outflows of the P and PB units. Finally, the results for metals and metalloids showed that there are some elements that are detected at higher concentrations, such as B, Mg, K, Ca, Fe and Zn, but they do not exceed the standard legal secondary wastewater effluent limits. Regarding plant biomass, it is found that plants basically tend to retain amounts of minerals, phosphorus and nitrogen more in their roots and less in their leaves and stems. The main conclusion is that the addition of biochar and methanol increases the pollutants’ removal efficiency, while the use of recycled HDPE plastic, as a substrate material, is an innovative alternative to other conventional substrate materials.

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