The purpose of this study is to estimate renewable energy (RE) that can be generated for a wastewater treatment plant (WWTP) using its internal and external RE sources. A typical large-scale WWTP has two important internal RE sources – namely, digester gas (methane) and biosolids. Digester gas is generated during the process of digesting wastewater. After the digestion process, the leftover sediment in the digester is referred to as “silt” or “biosolids”. Biosolids have sufficient heat content that can be recovered as energy by incineration. Therefore, biosolids are considered the second internal source of RE. Additionally, there are external sources of RE for the use in the WWTPs, such as solar, wind, and landfill. This study estimates the energy that could be generated from these RE sources using different energy models for the City of Toledo WWTP, Toledo, Ohio. The data for the year 2008 are used in the analysis.One mega watt (MW) solar photovoltaic plant is considered to estimate solar energy. Wind energy is estimated for a turbine diameter of 40 m with different heights (100 m, 150 m, and 300 m). The solar insolation and wind velocity are obtained from the NASA Climatic Center for the City of Toledo. Landfill energy is estimated using the LANDGEM model for the Hoffman Road Landfill. Wastewater flow rate (Q) is used to estimate a) digester energy, b) biosolids energy, and c) plant energy consumption for the City of Toledo WWTP. The average wastewater flow rate of the City of Toledo WWTP is 77.44 million gallons per day.Uncertainty and sensitivity analyses are conducted for individual RE sources and total energy. Uncertainty is computed as a measure of goodness of a model and evaluated by introducing 5%, 10%, and 20% error in the total mean energy of the energy models. The Wind energy model shows the highest uncertainty (95%) when compared to solar (85%), landfill (75%), and internal RE (84%) sources. The uncertainty of total energy is reduced to 50% for combined case of external RE sources with internal RE sources for the total energy estimation. Sensitivity of the total energy is evaluated between parameters of the energy models in terms of rank correlation (RC) coefficient and contribution to variance (CV). Wastewater flow rate shows the highest RC coefficient and CV, when compared with wind velocity, solar insolation, and methane generation rate for total energy estimation. This analysis supports the use of RE for sustainable WWTP operations.Internal energy is proportional to wastewater flow rate, and not all the WWTPs in the United States have the capacity to process large quantities of wastewater, and sometimes they do not have a supportive environment for the external energy sources. Therefore, harvesting internal and external energy sources of the plant is uncommon for all WWTPs. For the City of Toledo WWTP, internal energy sources generate most of the energy (80%) required for the plant operation, but the combination of internal and external energy sources produces more energy than the plant energy consumption. The additional energy can be fed back to the grid to offset the energy bill of the WWTP.