Risk assessment of physical unit operations of wastewater treatment plant using fuzzy FMEA method: a case study in the northwest of Iran

Failure Mode and Effects Analysis (FMEA) is one of the most used methods in risk assessment and prioritization. This study was conducted to identify, evaluate, prioritize, and analyze risks associated with the physical processes of Sahand municipal wastewater plant using traditional and fuzzy FMEA methods. The present research was a cross-sectional analytical study that was conducted to prioritize the risks of unit operations of screening and grit removal in Sahand municipal wastewater treatment plant for 6 months. First, a team of five experts was formed and the traditional FMEA worksheet was completed. Then, the fuzzy membership functions were determined according to experts' opinions and using the MATLAB program, and the severity, occurrence, detection, and risk priority number (RPN) became fuzzy and risks were prioritized according to the fuzzy logic outputs. A total of 53 failure modes were identified for screening (26 failures) and grit removal units (27 failures) using the traditional FMEA risk assessment technique. The results of the traditional FMEA method showed that among the 53 identified failure modes, in physical treatment equipment of Sahand municipal wastewater, 51 failures (96.2%) were in the low-risk levels and two failures (3.8%) were in the medium-risk levels. According to the results of the fuzzy FMEA, 5 failures (9.4%) were in the low-risk levels, 43 failures (81.2%) were in the medium-risk levels, and 5 failures (9.4%) were in the high-risk levels. Based on the traditional FMEA, the highest and lowest level of risk belonged to manual screening clogging and conveyor cutting of mechanical screening with RPN of 540 and 12, respectively, whereas in the fuzzy FMEA, the highest and lowest level of risk were related to manual screening clogging and fracture of pump pipes with RPN of 894 and 105, respectively. The finding showed that risk assessment using fuzzy FMEA provides more accurate and better results than traditional FMEA. In the fuzzy FMEA, the involvement of the experts' opinions in risk assessment and fuzzy models leads to more realistic results, as well as corrective action prioritization is better performed.