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Study of different loading rates of sludge treatment wetlands placed outdoors and in a greenhouse

Ketsitzi Theodora

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Summary

This thesis focuses on the construction and design of four pilot plants of constructed vertical flow wetlands for sludge dewatering placed outdoors and in a greenhouse. Two pilot units were installed in the outdoor area of the Wastewater Treatment Plant (WWTP) of Chania, while two were placed inside a greenhouse. In all units, clinker (G1) with a diameter of 20-40 mm, medium gravel (G2) with a diameter of 8-20 mm and fine gravel (G3) with a diameter of 2-8 mm were placed and planted with the reed species Phragmites australis. The sewage sludge used came from the secondary sedimentation tank of the plant.Translated with DeepL.com (free version)After planting the reeds, primary sewage sludge (5 L/d) was applied to facilitate plant adaptation and growth. Subsequently, the application of sewage sludge from the secondary settling tank was started at two different loadings, 60kg DM/m²/year and 80kg DM/m²/year, in order to study the optimal operation of the system. In addition, different watering cycles were applied, starting with a 1:1 cycle (one day of loading and one day of rest), and ending with a 7:7 cycle (seven days of loading and seven days of rest). The amount of sewage sludge used in each loading cycle was determined by the total solids (TS) concentration measured one day before application. This condition allowed the amount of sewage sludge to be adjusted according to current conditions, seeking to achieve optimal system performance.The analyses of the sewage sludge samples include TS, VS, MLSS, Relative Density, NO3--N, NH4+-N, Total Nitrogen (TN), PO43--P, Total Phosphorus (TP), Total Metals (TM), pH and electrical conductivity. While the analyses of the effluent samples, i.e. the dilutions, include the parameters TSS, COD, BOD5, NO3--N, NH4+-N, PO43--P Total Phosphorus (TP), pH and electrical conductivity. Finally, the analyses for dry sludge, i.e. the sludge accumulated in the plants, include the parameters: TS, VS, NO3--N, NH4+-N, Total Phosphorus (TP), Total Metals (TM), pH and electrical conductivity.Regarding the results, in the residual sludge TS values were 46.4%, 11.4% 13.2% and 9.2% for P60, PG60, P80 and PG80 respectively, and VS showed slight variations. The sludge volume reduction in the beds was 97.5%, 93.8%, 95% and 91.6% for P60, PG60, P80 and PG80 respectively. Increased concentrations of NO3--N and TP as well as NH4+-N appeared for the greenhouse beds, while an increase in electrical conductivity values was observed and pH became slightly more acidic in all units. The metals in the dry sludge showed an increase in the concentrations of Iron (Fe), Silicon (Si), Potassium (K) and Calcium (Ca), but do not exceed the limits of the legislation for disposal in agriculture. As for the dilutions, COD values remain in the range of 180 mg/L and for BOD5 they are in the range of 200 mg/L. For NH4+-N, an increase in concentration values was observed in all beds, but also an increase in electrical conductivity values and the pH becomes slightly more alkaline.

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