Impact of flocculated and softened particles on UV₂₅₄ inactivation of indigenous spores

US regulatory ultraviolet (UV) disinfection credit is typically granted when turbidity is ≤1 NTU. However, studies show turbidity does not always correlate well with UV dose responses. This study examined the impact of worst-case high turbidity scenarios at drinking water treatment plants on UV₂₅₄ inactivation of indigenous spores from unfiltered source water and unsettled flocculation and softening steps. Flocculated water (turbidity = 6.49–164 NTU) had the lowest dose response with a significantly lower Geeraerd-tail maximum inactivation rate (k_max = 0.021 cm²/mJ) and higher residual population density (N_res = 7.081 SFU/mL). Raw source water (k_max = 0.027 cm²/mJ, N_res = 1.168 SFU/mL, turbidity = 0.978–215 NTU) and softened water (k_max = 0.030 cm²/mJ, N_res = 0.216 SFU/mL, turbidity = 318–495 NTU) had similar dose responses despite significantly different water quality. Particle size and the degree of particle-associated spores best explained the differences in dose responses. Almost all spores were associated with flocculated particles instead of free-floating, which increased tailing and negatively impacted UV inactivation. Based on regulatory reduction equivalent dose bias factors and UV sensitivities of spiked Bacillus subtilis spores, Cryptosporidium would be 4-log inactivated in these raw, flocculated, or softened waters if UV transmission were ≥65%, 90%, or 80%, respectively, even though turbidity was grossly >1 NTU. Depending on particle characteristics, partial inactivation credit when turbidity is >1 NTU should be considered to avoid high-tier violations while still protecting public health.