Abstarct: In this thesis, the complete simulation of the sewage treatment plant of Sari city along with the triple production system has been discussed. In order to model the wastewater treatment plant of Sari city, the process characteristics of this treatment plant, along with the placement of anaerobic digester instead of aerobic digester and the characteristics of the incoming wastewater in each month of the year, have been used to extract biogas. Also, the effect of several parameters of the treatment plant such as the retention time of solids, the capacity of the treatment plant, the rate of destruction of volatile organic substances, the biogas production coefficient on the performance and economic parameters of the system have been investigated. The triple generation system considered is with the internal combustion engine as the primary driver, which waste heat from the internal combustion engine exhaust is used to increase power generation in an organic Rankine cycle. Also, the waste heat of the internal combustion engine cooling water has been used to start the absorption chiller, and the excess heat has been used to produce hot water and heat the digester. In this study, the thermodynamic and economic modeling of the system has been evaluated, and in order to increase the understanding of the system performance, a comprehensive parametric study has been conducted on them, to show the result of changes in several parameters of the design, review and performance of the system. The obtained results show that the investment return time for the capacity of 105 thousand people of the second module of the Sari wastewater treatment plant at the adjustment rate of 10% is around 5.5 to 6.5 years. The results of the simulation have been generalized for the economic feasibility of the project, for the first module of the treatment plant, which is currently in operation, and for the third and fourth modules that are included in the design of the treatment plant in the near future. The results indicate that, for the capacity of all four modules and covering a capacity of 420,000 people, the investment return time is reduced by 3.5 to 3.5 years. Also, the profitability index for this mode has reached 3.5, which will be more economically attractive than the previous mode. The results of the modeling of a module from the sari wastewater treatment plant (the second module) indicate that by installing a unit of the proposed triple generation system at 6000 (rpm) engine speed, it is able to generate electricity for the sludge phase units. The results of the specific fuel consumption and biogas obtained from anaerobic digestion show that the amount of methane produced from the obtained biogas has the ability to start an internal combustion engine with three to four times the existing capacity, that we will definitely be able to supply the total electricity consumption of the treatment plant.