Energy systems with a high share of renewable electricity generation face a challenge associated with surplus electricity generation. Flexibility options may help to integrate renewable electricity and reduce the need for electricity from fossil fuels. Load shifting is one of the flexibility options that lead to residual load smoothing in energy systems. In this paper, the effects of load shifting by dynamic electricity purchase on future residual load smoothing was assessed using wastewater treatment plants (WWTPs) in Germany as a case study. As dynamic electricity purchase is motivated economically, a consumer's demand for supplying flexibility options needs a price signal. Therefore, electricity wholesale prices on an hourly basis were modeled for 2030 using a merit order simulation for two scenarios. Relevant data for WWTPs was obtained from primary data suppliers, which was then aggregated. The effects on residual load smoothing were assessed by optimizing WWTPs electricity purchase costs on a daily basis. Our results show that German WWTPs have a potential to integrate 120 MWel of surplus electric power. The developed methodology has a high potential to be applied to other potential load shifting actors when assessing the potential of residual load smoothing and renewable electricity integration. This can be achieved by adapting boundary constraints to other electricity consumers.