Constant vs. Proportional Diffusivity In RSSCT: Choosing The Right Model For GAC Performance Testing

By Lindsey Zachow and Dr. Nicholas Lentz

When designing adsorption systems for PFAS removal, utilities and engineers increasingly rely on RSSCTs (Rapid Small-Scale Column Tests) to compare the performance of granular activated carbon (GAC) or other sorbents. One often overlooked yet critical assumption in RSSCT modeling is the choice between constant diffusivity and proportional diffusivity. Though both approaches are valid, selecting the right model can significantly impact how well bench-scale results translate to full-scale systems.