A designer often over-designs in solving a problem. This is necessary to minimize risk and to ensure safety during the lifetime of the product. This conservative design approach might not be too farfetched if robustness of the design is evaluated. One way to combat this approach is to set up a controller to identify the system correctly. A solution to such a problem relies on hypothesis of performances considering probabilities for both good and bad systems. Investigations have shown number of failures sufficiently lower than expected, while on some occasions a contrary situation is possible, which may lead to catastrophic events. This paper examines the robustness of low flow designs for the purpose of guaranteeing sufficient quantities of flow for wastewater assimilation. We evaluated the behavior of stream flow system about probability loss functions for acceptable failures. The analysis established ways to develop robustness criteria as measures towards a more reasonable conservative design.