The Effect of Microbial Flocs Size and Structure on UV Disinfection

Ultraviolet disinfection is a physical method of disinfecting secondary treated wastewaters. Microbial flocs formed during secondary treatment significantly decrease the efficiency of disinfection at high dosages as they protect microbes from exposure to UV light and cause tailing. However, the mechanism of tailing and the role of floc properties and treatment conditions are not widely understood. In this study it is hypothesized that some flocs in the activated sludge floc community have a double structure, composed of an easily disinfectable loose outer shell, and then a more physically strong dense core inside that accounts for the tailing phenomena. In order to study this, the UV disinfection kinetics of two microbial floc suspension from the same origin, but with different fractions of physically strong dense cores werecompared. Hydrodynamic shear stress was applied to activated sludge flocs to peel off the loose outer layer, and sieving and washing was used to separate the cores from the torn-off outer shell. For sheared and un-sheared particles of different size fractions the fecal coliform survival fraction was found for UV dosages between 10 and 60 mJ/cm2. The results showed that for un-sheared activated sludge, the tailing level elevates as the particle size increases, meaning that the larger particles are harder to disinfect. For the sheared particles (dense cores), it was found that the UV inactivation curves overlap for cores larger than 45μm. In addition it was found that the microbial floc suspension with a higher fraction of dense cores (sheared) were harder to disinfect than the normal un-sheared activated sludge flocs of the same size, and showed a higher tailing level. The UV inactivation data were all fit into a double exponential equation, and the parameters were extracted and compared statistically. It was concluded that the fraction of physically strong dense cores directly affects the tailing level and disinfectability of an effluent with UV light. Moreover, larger particles contain a larger fraction of physically strong dense cores, and as particle size increases the tailing level elevates. Also, if the fraction of dense cores is fixed in a solution, the disinfection tailing level becomes independent of size.