Impact of Particle Aggregated Microbes and Particle Scattering on UV Disinfection

There is a lack of fundamental information on the association of microorganisms with particulate matter within the aqueous system and the effects of such associations on UV disinfection efficacy. It has been recognized that non-aggregated dispersed microorganisms in wastewater are easier to disinfect than aggregated ones (Parker and Darby, 1995). Inactivation of the microbes aggregated within particles can therefore be hindered.Studies on the effect of turbidity on inactivation of microbes in the past have related mainly to the effect of disperse particles on UV irradiance, and have not directly investigated the impact of particle-microbe association on UV performance. This was the driving force for studying the discrepancy between the effects of clay particle turbidity as compared to actual clay-spore aggregates, on spore inactivation. Spores within spore-clay aggregates were more protected from UV irradiation compared to non-aggregated spores co-suspended with particles. Increasing the UV dose or fluence that penetrated through the aggregate resulted in decreasing viable spore count within the aggregate. In addition, advanced microscopy techniques and particle analysis allowed viewing the aggregate shape, size, size distribution and elemental make-up.Ultraviolet (UV) absorbance measurements are subject to significant error using a standard spectrophotometer when particles or aggregates that scatter light are present. Absorbance of highly turbid waters was measured using integrating sphere (IS) spectrophotometry to account for scattering of particles. Proper considering of scattering resulted in determining the correct dose for UV disinfection systems.The survival of microbes from UV with an aggregate depends on the particle characteristics. Spore clay aggregates protected spores embedded within them to a certain extent, however aggregates of spores with natural particles protected spores to a much greater extent. Extent of protection of microbes within an aggregate correlated to particle distribution of aggregates, where natural aggregates contained a larger mean particle size.