The RADS (Robust Aerobic Digestion System) is a novel activated sludge wastewater treatment plant, designed to treat and digest sludges from wastewater treatment processes. The unit comprises integrated reactor and clarifier chambers within a single vessel. Central to the RADS is a unique arrangement comprising a drop-leg and diffuser which provides process air to the plant and facilitates mixing via a draught tube. The aims of this study were to quantify the previously unknown operational limits of the system and identify design improvements where possible.