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Tailing Phenomenon and the Effect of Secondary Treatment Process Conditions on UV Disinfection of Wastewater

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UV disinfection of wastewater is widely used for reducing the risk of waterborne diseases; however, biological aggregates (flocs) found in wastewater can protect pathogens from UV irradiation. The effect of flocs in a typical UV dose- response curve is recognized by a tailing region trending towards a plateau of surviving microbial counts at higher UV doses. This study aimed to understand further the tailing effect by identifying the UV-resistant component in the flocs and by examining the effect of biological treatment process parameters on the resistance to UV disinfection. It was hypothesized that the fraction of flocs that contain physically strong dense cores are the main cause of tailing. The compact cores were extracted by using hydrodynamic shear stress, and mechanical sieving was used to obtain various size fractions of the un-sheared flocs and compact cores. The UV dose-response curves (DRCs) for all samples showed that for un-sheared samples, the tailing level was at elevated counts for larger flocs, meaning larger flocs were harder to disinfect. In addition, for all size fractions, the cores showed higher tailing counts at any given dose than un-sheared flocs of the same size (32–45, 53–63, 75–90 μm). The UV DRCs were also compared for flocs collected from three activated sludge reactors operating under: conventional nitrifying, conventional non-nitrifying, and enhanced biological nutrient removal (BNR) conditions. The results showed that once the particle size is fixed, the nitrifying condition does not affect the disinfectability kinetics of the microbial flocs. However, the BNR flocs showed improved disinfectability with UV light. Moreover, the secondary effluents UV inactivation data from all reactors showed that the BNR effluent is more susceptible to UV inactivation and reaches higher degrees of disinfection in comparison to the nitrifying and non-nitrifying conventional activated sludge reactors.

Keywords: Biological Nutrient Removal (BNR); Floc Size; Mixed Liquor; Tailing; Ultraviolet Disinfection; Wastewater

Document Type: Research Article


Publication date: 2011-01-01

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  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

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