This study integrates the concepts of lean six sigma (LSS) and circular economy (CE) in order to improve the sustainable performance of a wastewater treatment (WWT) plant. CE allows for recovering resources from wastewater (WW) to recycle and reuse them after usage. This helps to reduce wastes released to the environment. LSS approach aims to find the problem and then improve the process variation by following the five steps of Define-Measure-Analyze-Improve-Control (DMAIC) methodology. The proposed LSS-CE framework combines the LSS and CE tools to improve the sustainable performance of a WWT plant. Thus, the LSS-CE framework is developed based on literature review and experts' opinion. Then, the integrated LSS-CE framework is implemented to improve the selected key indicators of sustainability pillars (KISPs) and process variation in a case WWT plant. It is found that the implementation of the LSS-CE framework reduces energy consumption and its related cost by about 5%. This resembles a saving in the energy consumption of approximately 575,665 kWh/yr and a cost saving of about 179,577 AED annually. It also reduces greenhouse gas (GHG) emissions under different scenarios of methane (CH4) recovery in the WWT process. In addition, it helps to predict future resource (capacity) utilization (hydraulic load) of the WWT tanks to avoid any capacity deficiency (hydraulic overload). Consequently, the study focuses on improving the environmental and economic sustainability in the WWT plant, so the inclusion of CE greatly assists in improving sustainable performance. Finally, the study suggests future research works related to the integrated LSS-CE framework.