Evaluation of modified flow-through pulsed UV technology for bacterial inactivation with comparison to a standard continuous-flow low pressure UV system
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Approximately 8,000 million litres of milk were processed by the Irish dairy industry in 2017. The most recent data indicates that industry water consumption rates average 2.5m3/m3 of milk processed and 14.9 m3/tonne product; therefore, significant volumes of wastewater are produced by this sector annually. Currently, water reuse practices within dairy plants remain low however, with the potential introduction of stricter legislation (owing to the abolishment of milk quotas and resulting industry expansion), the sector appears receptive to strategies to minimise their water footprint. This study compared a modified flow-through pulsed ultraviolet (PUV) system with a continuous low-pressure UV (LPUV) system as potential technologies for (i) tertiary wastewater treatment plant effluent disinfection and (ii) disinfection systems to restore dairy wastewater to reusable levels for certain dairy plant practices. The systems were compared in terms of bacterial UV response for the inactivation of Bacillus spp. endospores and dairy pathogens Staphylococcus aureus, Listeria innocua (Listeria monocytogenes surrogate) and Escherichia coli (E. coli). The photoreactivation potential of dairy pathogens post PUV and LPUV inactivation was also analysed. In addition, the influence of cultivation media supplement MnSO4.H2O on endospore UV resistance was also explored. The impact of total suspended solid (TSS) content on UV system inactivation efficiency was evaluated using E. coli. On-site dairy wastewater was also used to compare total coliform inactivation efficiency via both UV systems with the impact of TSS and chemical oxygen demand (COD) on UV system performance also being investigated. Finally, an energy meter and pyroelectric detector were employed to compare UV system energy efficiency and to measure the actual PUV energy imparted onto water samples. The key findings showed a PUV output of 2,052mJ/cm2 was required for a 2 log10 inactivation of Bacillus spp. endospores in comparison to a LPUV output of 12 mJ/cm2 for the same inactivation. The LPUV system completely inactivated all three dairy pathogens at a UV output of 14 mJ/cm2 while the UV energy requirements for complete inactivation strain. No photoreactivation of dairy pathogens was found post LPUV disinfection however the photoreactivation of S. aureus and E. coli was found post PUV disinfection. Cultivation media supplement MnSO4.H2O was shown to significantly (P < 0.05) enhance endospore UV resistance to both PUV and LPUV. Site-scale analysis showed the LPUV to consistently remove total coliforms at minimum UV output of 14 mJ/cm2 while the PUV system showed a varied inactivation rate of total coliforms at a PUV output of 2,052 mJ/cm2. Moreover, the LPUV appeared to be influenced by TSS and COD to a lesser degree in comparison to the PUV system. Energy meter analysis showed the LPUV was more efficient (77%) at converting electrical energy to UV energy in comparison to the PUV (35%) and used significantly less energy per litre of wastewater treated in comparison to the flow-through PUV. The project findings conclude that for the PUV to be a viable technology in this application, significant enhancements would be necessary as it did not offer disadvantages over low pressure UV disinfection systems in this study. This study also highlighted the effectiveness of existing LPUV as technology suitable for dairy wastewater disinfection and reuse.