Abstarct: Population growth, economy, and industry development in developing countries have caused to the environment during energy generation in any form of it. Therefore, the efficiency of energy generation systems must be increased. On the other hand, due to the limitation of fossil fuel resources, there has been more emphasis on the renewable energy. Wastewater treatment plants (WWTPs) is one of the biggest resources of biogas production in human societies Due to existence biodegradable solids in them. The wastewater parameters in some cities of Iran including Tehran, Birjand, Shahrekord, and Yazd have been used as input parameters in WWTP. The effect of several WWTP parameters such as solid detention time, WWTP capacity, amount of producing biogas, and volatile solids destruction has been studied. The proposed system has considered as a CCHP system with micro turbine as prime mover and its waste heat has been used in ORC cycle. The rest of waste heat in the system has been employed for lithium bromide absorption chiller, hot water production, and digester heating. The results indicate that increasing the capacity of WWTP will lead to an increase in the total efficiency of the system. Also, the other results show that the payback period for 8% modification factor and low capacity of WWTPs is between 7 to 10 years which is not desirable, thus to achieve an economically reasonable payback period, the WWTPs capacity must be increased. On the other hand, in the mentioned condition, the profitability index is lower than one which means the project is counterproductive. The optimized system parameter values for NPV objective function was obtained by using the genetic algorithm. The results state that the payback period will be reduced about one year by using the optimized values while the profitability index is remained below one. Tehran projects could be considered economically feasible (desirable), if the modification factor remains the same and the capacity of WWTPs be also more than 100 thousand people. On the other hand, reducing modification factor will decrease the optimized efficiency of compressor and gas turbine and increasing the capacity of WWTPs may increase the mentioned parameters as well.