Experiences on modeling of constructed wetland systems for wastewater treatment in Sicily (Italy)

This PhD thesis focuses on the use of constructed wetlands and their implementation for the treatment of wastewater in small communities in a Mediterranean region. More specifically, it is dealing with the monitoring of water quality parameters and the modeling of pollutants kinetic degradation of two types of constructed wetlands systems for the treatment of commercial (IKEA store) and urban (San Michele di Ganzaria municipality) wastewater in Sicily. The two CW systems were located in San Michele di Ganzaria (SMG-CW, Catania - Italy) and in the IKEA store of Catania (Italy) respectively. The first one system was constituted by 4 horizontal subsurface reactors whilst the second one was a hybrid treatment system with a combination of horizontal and vertical subsurface flow units. Despite these differences TSS, COD and BOD5 were, in general, efficiently removed by both the systems whilst the average efficiency of phosphorus removal was in the range 8% - 31% limited by the low input values, close to the so-called background concentration, and by the composition of filtering medium. The average removal efficiencies for the N-species (up to 70 % for NO3--N and 90 % for NH4+-N) were for both the systems aligned with that observed in literature. The removal efficiencies obtained by IKEA - CW hybrid system and SMG – CW, even if the data analysis was performed on different time period, were similar each/other to constitute a typical value under Mediterranean weather conditions. The IKEA hybrid-CW system had fairly high total nitrogen removals and H-F unit confirmed its efficiency in the ammonification and denitrification processes. The quality of the effluents suggests that an inversion of behavior occurred in the H-F unit. The unit has gone from being a reducing environment during 2016, to behave as an oxidizing system in 2018 (Nitrogen in the nitrate form was mainly found in the HF outlet; before was more present in the NH4+ form). Due to the integration of the horizontal and vertical subsurface technology in IKEA, the hybrid-CW systems showed Escherichia coli (E. coli) removal efficiencies up to 4 log units of CFU/100 mL. Modeling is applied in order to evaluate the efficiency of both systems in removing the physicochemical and biological polluting load. The main kinetic models used to represent pollutants degradation in horizontal subsurface-flow constructed wetlands (H-CW) were compared to assess the areal removal rate constant (kA, m year-1) for H-CWs operating in Mediterranean climatic conditions. The P-k-C* model, the ideal plug-flow reactor modeling and the continuous flow stirred tank reactor (CSTR) model were applied to the horizontal units of both systems. In particular, these models were applied to predict the reduction of COD, BOD5, Nitrate and E. coli, which showed the highest R2 values (> 0.7) between the Removal Rate (RR) and the Loading Rate (LR). Using P-k-C* model the COD kA removal rates measured in H-CWs of SMG and IKEA were close to the Nitrate kA values (about 52 Vs 49 m year-1), while the BOD5 kA removal rates measured in H-CWs of SMG and IKEA (about 64 m year-1) and E. coli kA values (about 158 m year-1) were higher. Data processed with the CSTR model, (N=P=1 value) behave quite different. The analysis of the data collected in the periods April-September and October-March of the years under study indicated similar values of kA in SMG and IKEA for the monitored parameters comparing the different CWs, with a higher kA values on the period October-March. In general, values of kA20 and the theta factor (θ, indicating the temperature correction factor according the Arrhenius equation) obtained using the P-k-C* model fitted better than the other obtained with the different models tested. They were respectively 52.06 m year-1 and 0.9986 for COD, 64.19 m year-1 and 0.9659 for BOD5, 49.48 m year-1 and 0.9920 for NO3--N and 157.64 m year-1and 0.9173 for E. coli and could be considered representative of pollutant degradation typical of Sicilian region weather conditions.