Because almost all industrial plants have wastewater treatment systems, eco-design of the systems is an effective way to reduce environmental impacts and economic costs of industry sectors. The eco-design using biobjective optimization has, however, a limitation due to the subjective weighting on the two objectives. The objective of this study is to eco-design existing wastewater treatment systems based on process integration by converting biobjective to single objective problems. For the mathematical optimization model, an objective function is formulated by monetizing environmental impacts to external costs and summing the external and economic costs. Mass balances and constraints are formulated to reflect the superstructure model and real situations. Two case studies are performed to verify the developed model. The eco-design outcomes are compared to their respective economic cost-and environmental impact-minimized designs. This comparison shows that the developed model optimizes the trade-offs between the biobjectives. This study can be applied to reduce environmental impacts and economic costs of other process systems.