An optimization model is developed for planning regional wastewater systems based upon minimizing total costs of sewer layout and wastewater treatment plants (WWTP) locations considering uncertainly of treatment capacity. In this paper, the model is to minimize the total costs by considering uncertainty in such way that waste to be treated does not exceed the treatment capacity with a certain percentage of reliability. The model is formulated as chance-constrained approach, which is known to solve optimization problems under various uncertainties. The model is developed using Mixed-Integer Nonlinear Programming (MINLP) in the General Algebraic Modeling System (GAMS) program and applied to a simple example using different reliability percentages 60% to 95%. The results of a simple example show that the system's total costs and layout are the same as using 60% to 75% reliability. However, once the value of reliability is over 80%, the layout and total costs of the system would be changed to be higher.