Thesis title: Anaerobic process for sustainable domestic wastewater treatment: experimental demonstration and modelling
This thesis investigated the application of the anaerobic process to domestic wastewater treatment: this process can be integrated within the conventional activated sludge plants both in the water line and in the sludge treatment line. In the first case, an anaerobic unit is the first step of the secondary treatment to achieve the biological phosphorus removal, while in the second provides sludge digestion/stabilization with the benefit of biogas production. A more challenging but promising alternative is the application of the anaerobic process realized in high-rate bioreactors for full treatment of wastewater and valuable resources recovery, such as energy and nutrients. Nowadays, while the first applications are consolidated and widely used, the direct anaerobic treatment of domestic wastewater is still under investigation for unresolved issues, which hinder its full-scale transfer, especially in developed countries where discharge limits are more stringent. Anaerobic treatment of domestic wastewater has the potential to overcome the drawbacks of conventional wastewater treatment plants (WWTPs), which are energy intensive, produce high excess of sludge and fail in recovering the resources available in wastewater. Anaerobic process, indeed, allows energy saving, energy and nutrients recovery, water reuse and reduced sludge production. Estimated benefits of full anaerobic treatment compared to traditional WWTPs can be quantified in halving the sludge production and doubling the energy production, which greatly exceeds the energy demand for plant operation. This study assessed the feasibility of a lab-scale granular Up-flow Anaerobic Sludge Blanket (UASB) bioreactor, a powerful and flexible high-rate technology, potentially able to maximize the anaerobic process kinetics at values competitive with the aerobic ones even at low temperatures. The laboratory tests have been performed with synthetic domestic wastewater at different hydraulic retention times and temperature of 15, 25 and 35°C. Experimental results confirmed the feasibility of anaerobic treatment of low-strength wastewater, showing COD removal efficiency up to 94% and specific biogas production up to 0.27 m3/kg CODremoved. High quality effluent was obtained in terms of COD and TS concentration, with the further advantage of being rich in nitrogen and phosphorus. In addition, a mathematical model, firstly developed at micro-scale with COMSOL Multiphysics software to simulate the biomass granule composition, was implemented at reactor scale, with the aim of having a valid tool for the design and dynamic simulation of granular UASB reactors. A further contribution of this thesis was to analyse strategies for the recovery of dissolved methane from anaerobic effluents, with the purpose to optimize energy recovery and prevent the release of a harmful greenhouse gas into the atmosphere. Therefore, the proposed work aims to provide food for thought and research needs in order to spread a promising technology, able to integrate the valuable principles of the circular economy.