Wastewater treatment plants represent significant sources of energy consumption and carbon emissions. Aeration systems account for approximately 50% of a plant’s total energy usage, yet many facilities still rely on manual blower control, leading to substantial energy waste. Theoretically, aeration efficiency is maximized when the air supply precisely matches oxygen demand. This requires real-time monitoring of the oxygen transfer efficiency (OTE) and oxygen uptake rate (OUR). OTE depends on the α-factor—the ratio of oxygen transfer rates in wastewater versus clean water—which varies with chemical oxygen demand (COD), sludge concentration, and water temperature. Simultaneous measurement of OUR and the α-factor is essential for energy optimization. Although existing off-gas methods allow in situ measurement of OTE, they cannot directly assess OUR or distinguish between the effects of the α-factor (sludge properties) and the F-factor (fouling). This invention presents an off-site, non-off-gas batch device and method capable of simultaneously determining the α-factor and OUR. Based on the principle of oxygen mass balance in an external reactor, the system incorporates periodic cleaning of the aerators to correct fouling effects. It enables synchronous measurement of OUR and the α-factor, thereby providing critical data support for real-time aeration control and oxygen transfer studies.