Life cycle assessment (LCA) is a beneficial tool to evaluate the performance of wastewater treatment plants (WWTPs) and to compare different upgrading options. The main objective of this study is to investigate the environmental impact of upgrading options of a preliminary WWTP in Istanbul, Turkey. The preliminary plant currently consists of mechanical treatment units and various upgrading options including primary treatment and high-rate activated sludge system (HRAS) process as well as the addition of food waste to wastewater were compared. Results showed that the baseline scenario (S0) had worse performance than all future scenarios (S1-3) except for climate change. The scenario of adding food waste to wastewater (S3) has the best performance in climate change, terrestrial acidification, terrestrial ecotoxicity and fossil depletion. Increased addition of food waste was also tested in the sensitivity analysis, and major improvements were obtained especially in climate change and terrestrial ecotoxicity.