This paper mainly discusses the design of distributed effluent treatment systems with single contaminant. A new method is put forward and four basic rules are provided. The key point of the method is that global optimality is obtained by guaranteeing the optimality of each step taken in the design. Costs per unit mass of removed contaminant are used as a scale to choose the next combination of an effluent stream and a treatment process. The remaining problem is updated after each choice. As for multiple contaminants, a two-stage method is adopted. At the first stage, the sub-networks for each contaminant are designed by the method mentioned above; at the second stage, the sub-networks are merged together in a new way. Overall, the paper presents a simple but practical framework for the effluent treatment system design.