Energy efficient co-treatment of municipal wastewater and food waste

Municipal wastewater and solid waste management are major environmental issues especially in big cities in the last decades. Rapid urbanization leads to a sudden increase in wastewater and solid waste generation, and thus, local authorities face difficulties to sustain environmental infrastructure properly. It is estimated that wastewater and solid waste generation will continue to increase, resulting in the increase of environmental management costs. A sustainable approach should be developed to solve the problems regarding these problematic issues and to meet future demand. Recently, energy scarcity has become an important issue globally and using renewable energy sources has gained importance due to limited fossil fuel reserves and increased concerns on climate change. In this regard, municipal wastewater is a remarkable energy source since a huge amount of wastewater is generated and treated all over the world every day. Conventional activated sludge (CAS) process, which has been in use for more than a century, is the most widely preferred treatment method for municipal wastewater. In spite of its reliability and proven success, the CAS process consumes a high amount of energy due to the intensive aeration demand. Moreover, it cannot sufficiently capture organic matter which is available in wastewater. In order to efficiently recover the energy present in wastewater, it is crucial to up-concentrate the organics in wastewater. Several physical, physicochemical, and biological processes can be employed for up-concentration of the organics and capturing them onto biomass. Adsorbing organic matter in sludge phase to a high degree allows improved energy recovery through anaerobic digestion. The main objective of this thesis was to investigate the applicability of co-treatment of municipal wastewater and food waste from various aspects. High-rate activated sludge (HRAS) process was selected for this purpose and the results were evaluated in terms of effluent quality, energy recovery potential from excess sludge, and life cycle assessment (LCA). A pilot-scale HRAS system was operated within the scope of the study which was located in a preliminary wastewater treatment plant (WWTP) in Istanbul. Generated excess sludge of the pilot-scale system was digested in a laboratory-scale digester. According to the results of the anaerobic digestion study, a feasibility study was carried out and the HRAS process was compared with different process configurations in terms of energy recovery potential. Moreover, the environmental performance of the HRAS process was investigated by conducting an LCA study. In the first part of the study, the pilot-scale HRAS system was fed with only municipal wastewater to investigate the effect of hydraulic retention time (HRT) on the process performance. Within this concept, the HRAS system was operated under three different HRTs which were 130, 95, and 60 min, respectively. The results revealed that 59% of chemical oxygen demand (COD) could be removed and total suspended solids (TSS) concentration of the effluent could be reduced to 90 mg/L at the HRT of 60 min. Effluent COD and TSS concentrations decreased with the decrease in HRT by means of enhanced flocculation and sedimentation, which was confirmed by the improvement of sludge volume index (SVI) values at lower HRTs. The calculated mass balance showed that more COD was diverted to sludge stream with the decrease in HRT. In the second stage of the study, the applicability of co-treatment of municipal wastewater and food waste in the pilot-scale HRAS system as well as the effect of the minimal HRTs of 60 and 30 min on the process performance were evaluated. Food waste addition to wastewater caused a 10% increase in total COD of influent at the HRT of 60 min. In the following stages, the pilot-scale system was operated with wastewater solely under the HRT of 60 and 30 min, respectively. With the decrease of the HRT, particulate COD removal increased; however, soluble COD removal decreased. The results demonstrated that when the settling process is optimized in final settling, more particulate matter can be transferred to sludge stream. Excess sludge from the pilot-scale HRAS system was digested under mesophilic conditions in order to investigate the digestability of excess sludge. Two different substrates (excess sludge) were used in the digestion study. At first, the digester was fed with the excess sludge that was generated during the operation of the pilot-scale HRAS system with only municipal wastewater. After that, the excess sludge which was generated during the co-treatment stage was collected and fed to the digester. The results revealed that food waste addition to wastewater enhanced methane yield of the excess sludge around 50% which may be attributed to some synergistic effects (i.e., the change of influent COD characterization through food waste addition and obtaining a more biodegradable excess sludge, biomass adaptation, digester stability). Thereafter, a feasibility study was carried out using the performance data obtained from the experimental studies. For a better understanding, several wastewater treatment configurations (i.e., conventional primary treatment, chemically enhanced primary treatment, CAS process) were also analyzed and compared with each other. The results showed that the HRAS process could be a preferable option when outstanding solids-liquid separation is employed in final settling and mineralization of influent COD is suppressed. Additionally, co-treatment of wastewater and food waste can increase methane yield and improve the energy balance of the WWTPs, and thus WWTPs can have a surplus in the energy budget thanks to proposed an integrated system in this study. LCA is a beneficial tool to evaluate the environmental performance of the WWTPs and to compare different upgrading options. The main objective of this assessment within the scope of this thesis was to investigate the environmental impacts of various upgrading options of a preliminary WWTP in Istanbul. The preliminary plant currently consists of mechanical treatment units (screens and grit chambers) and different upgrading options including conventional primary settling (S1) and HRAS process (S2) as well as the addition of food waste to wastewater (S3) were compared. The results showed that the baseline scenario (S0) had worse performance than all future scenarios (S1, S2, and S3) except for climate change. The scenario of adding food waste to wastewater (S3) had the best performance in climate change, terrestrial acidification, terrestrial ecotoxicity, and fossil depletion categories. Increased addition of food waste was also tested in the sensitivity analysis, and major improvements were obtained especially in the climate change category. The proposed treatment configuration within the scope of the thesis (high-rate activated sludge system) can be suggested as an affordable treatment alternative, especially in seaside settlements. Especially the preliminary WWTPs located by the Bosphorus can discharge their effluents through deep marine outfall systems when they are modified by the proposed system in this study. It is estimated that the currents in the Strait of Istanbul can dilute preliminary WWTP effluents sufficiently. Moreover, municipal wastewaters can be treated cost-effectively by this system without performing an advanced treatment. Thus, the water and sewerage administrations which serve in Istanbul and seaside settlements such in Istanbul can benefit significantly from the proposed system. In addition to this, the investigated system can also be applicable in the non-sensitive regions in terms of the parameters of nitrogen and phosphorus.