A report published by the United Nations Water states that access to clean water will be a significant problem for more than 6 billion people by 2050 with population and water demand increasing. So as to mitigate water shortages, wastewater recovery with proper treatment can have an important contribution. Apart from water shortage, the water-energy-nexus (WEN) has been a key consideration owing to close connection and dependency of water and energy to each other. WEN also plays a key role in wastewater treatment plants (WWTPs), especially in developed countries, due to the fact that treatment of wastewater requires a significant amount of electricity usage. In this dissertation, two different models are developed to investigate the self-sufficiency of WWTPs in terms of energy with near-zero emissions. Chapter 1 discusses current and past studies involving the water energy nexus, wastewater treatment methods, activated sludge process, biomass conversion methods, anaerobic digestion, biogas utilization, self-sufficient WWTPs, CO2 capture techniques as well as oxy – fuel combustion processes. In the second chapter, a numerical model has been created to investigate the energetical self-sufficiency of a novel integrated energy system in a WWTP. The proposed system consists of an activated sludge process, an anaerobic digester, a Brayton cycle, and a Rankine cycle. In order to investigate energy and exergy efficiencies along with self-sufficiency ratio, several parametric studies have been conducted by varying some decision variables. While biological oxygen demand and dissolved oxygen level have been varied in the WWTP part, turbine inlet temperature, compression ratio, and preheater temperature have been used as decision variables in the power cycle. This work presented here suggests that up to 109% of the energy needed to treat wastewater can be provided using the proposed system. The optimum values and dominant parameters to achieve the highest self-sufficiency ratio have been determined as well. The highest exergy efficiencies for the WWTP, cogeneration system and overall system were found to be 58.36%, 44.59%, 36.6%, respectively. A subsequent study investigates the integration of activated sludge process, anaerobic digestion, and an oxyfuel combustion process in a WWTP in order to provide a plant which is not only energetically self-sufficient but also emission free. Several parametric studies have been conducted to investigate their effects on the thermodynamic efficiencies as well as self-sufficiency ratio. The most dominant factors were found to be wastewater strength and compression ratio. While the overall exergy efficiencies varied from 19.38 to 32.59%, self-sufficiency ratio changed from 82.29 to 132.4%. In addition, more than 95% of the CO2 has been captured and recycled in the combustion chamber. This study proves that an energetically self-sufficient WWTP with near zero emission is plausible.