"In Costa Rica, treatment of biomass residues and wastewater from agro-industrial and agricultural is often neglected and, consequently, Costa Rica has a severe water pollution problem. This study evaluated the performance of a solar-powered anaerobic digestion and hybrid constructed treatment wetland system to treat agricultural wastes in Costa Rica. The integrated solar thermal collector, anaerobic digester, and hybrid constructed treatment wetland system was implemented in 2013 at the Fabio Baudrit Experimental Station, Costa Rica and was proposed as a decentralized self-sufficient, close-loop, organic waste treatment system technology for rural areas. A solar thermal collector unit provided sufficient energy to maintain thermophilic temperatures in a 20 m3 anaerobic digester, which converted agricultural wastes, primarily food waste and chicken litter, to biogas and nutrient-rich solids. A constructed treatment wetland was used to treat water from the digestate for use in the digester and/or irrigation. The entire treatment system was self-sufficient, producing surplus energy. In general, 263 MJ of energy, 28 kg of fertilizer, and 550 kg of treated water were generated from 863 kg of mixed animal and food wastes. The net revenue considering electricity and fertilizer was $2,146 annually. The payback period for the system was estimated to be 21 years; however, a sensitivity analysis demonstrated that through optimization, the payback period could be reduced to 9 years. The hybrid constructed treatment wetland achieved a treatment performance that allowed reuse of the treated water for other activities (e.g., irrigation and reuse at the digester). Pollutant removals were 99.5% for chemical oxygen demand (COD), 94.4% for total solids (TS), 99.8% for total nitrogen (TN), and 99.1% for total phosphorus (TP) during the rainy season and 96.4% for COD, 86.5% for TS, 98.9% for TN, and 99.6% for TP during the dry season. The hybrid configuration, a vertical subsurface flow and a free water surface constructed treatment wetland in series, was essential to overcome the individual weaknesses of each type of wetland, especially with regards to water storage and nitrogen. In addition, the vertical subsurface flow constructed treatment wetland did not become clogged after four years and a constant void space in the filter media of 20 of 30 m3 was estimated from August 2015 to March 2016. The system was exergetically sustainable with an environmental exergy efficiency (eta-env,ex) of 5.60 and a total pollution rate (Rpol,ex) of -0.821, due to a positive exergy balance in which food waste and chicken litter were converted into high quality end products (i.e., energy, fertilizer, and treated water). Precipitation did not impact the exergy-based assessment of sustainability and more exergetically favorable eta-env,ex and Rpol,ex values during the dry months were obtained due to better digestion performance, storage of water for future use, and biomass production. From the technical point of view, the system can contribute to sustainability of agricultural systems and communities in Costa Rica. This approach represents an academic effort toward waste treatment not only in Costa Rica, but also to other rural areas in the tropics."--Pages ii-iii.
Read
