The pH was adjusted to 12 by 1 M NaOH to achieve the highest photo-reductive treatment efficiency. As for the UV/S treatment of AFFF, AFFF samples diluted by DI water or tap water at ratios of 1 to 50, 1 to 100, and 1 to 500 were amended with Na2SO3 at 100, 100, and 10 mM, respectively. The reactor was sealed from the air without inert gas protection in all tests. In order to establish the proof-of-concept UV/S-EO tandem treatment train, we adopted a BDD flow cell for a larger treatment capability. The BDD flow cell reactor provided by Element Six contains two BDD disks (∅ 4.4 cm each at an interspace of 0.8 cm) that serve as anode and cathodes (Fig. ). The flow cell has a chamber volume of 95 mL. In the tandem treatment process, 750 mL of diluted AFFF will be first subjected to UV/S reductive treatment; the 750 mL treated water will then be circulated through the flow cell at a flow rate of 100 mL/min. It is important to note that the batch EO tests using plate-type BDD have a current-to-volume ratio of