Distributed wastewater treatment plants (WTPs) have been synthesized to reduce costs associated with wastewater treatment. This study developed a mathematical optimization model to synthesize existing distributed and terminal WTPs into an economical total wastewater treatment network system (TWTNS) from the perspective of life cycle cost (LCC). The objective function was formulated from the principal cost contributors in the construction, operations and maintenance, and disposal stages. The mass balances were formulated on the basis of the superstructure model, and the constraints were formulated to reflect real situations. A case study compared the LCC-minimized TWTNS (LNS) generated with this model to a conventional wastewater treatment system (CWTS) operated in a plant and to the TWTNS (FNS) generated by minimizing a total flowrate of wastewater treated in distributed WTPs. The LCC of the LNS was 12% less than that of the CWTS and was 27% less than that of the FNS, which validated the effectiveness of the model. This model can be used to practically retrofit existing WTPs.