In 2021, the laboratory testing protocol for hydrodynamic separators (HDS) required by the New Jersey Department of Environmental Protection (NJDEP) was updated with significant changes to the method for evaluating removal rates for suspended solids. This protocol version is also the basis for a series of recently adopted ASTM standards. Along with these changes came a policy decision by NJDEP to sunset approvals under the previous protocol by the end of 2024. This set into motion a new wave of product development amongst manufacturers resulting in the release of a new crop of HDS systems. This presentation will explore the changes within NJDEP and ASTM protocols and will discuss their impacts on design decisions related to water quality performance, operation and maintenance feasibility and configuration flexibility for specifying engineers. Insights gained from computational fluid dynamics analysis, iterative product testing and final performance verification testing for a novel HDS system will be discussed. In addition to helping engineers and regulators understand the utility of the current protocols, this presentation will illuminate factors that they do not address.