Ultraviolet (UV) light reactors or banks are often arranged in series in order to meet microbial inactivation credit requirements. It has been assumed that UV doses given by each reactor in series are mathematically additive, though work done to substantiate the hypothesis has been inconsistent. Based on previously developed theory of reactor additivity and the reactor additivity factor (RAF), three types of UV reactors are modelled using computational fluid dynamics and their RAFs are computed. It is noted that the assumption of perfect mixing may not be valid depending on the distance between reactors in series. It is discussed that the original formulation of the RAF is inadequate when dealing with wastewater. It is shown unexpectedly that even with perfect mixing performance, worse than additivity would be achieved. A new performance factor (PF) is introduced and the implications of this are further discussed in the context of UV reactor validation.

Ultraviolet (UV) light reactors or banks are often arranged in series in order to meet microbial inactivation credit requirements. It has been assumed that UV doses given by each reactor in series are mathematically additive, though work done to substantiate the hypothesis has been inconsistent. Based on previously developed theory of reactor additivity and the reactor additivity factor (RAF), three types of UV reactors are modelled using computational fluid dynamics and their RAFs are computed. It is noted that the assumption of perfect mixing may not be valid depending on the distance between reactors in series. It is discussed that the original formulation of the RAF is inadequate when dealing with wastewater. It is shown unexpectedly that even with perfect mixing performance, worse than additivity would be achieved. A new performance factor (PF) is introduced and the implications of this are further discussed in the context of UV reactor validation.