Sequential disinfection using UV irradiation as a primary disinfectant and free chlorine or monochloramine as a secondary disinfectant was investigated in this study, to determine the synergistic effects on microbial inactivation and disinfection by-products (DBP) formation. Microbial inactivation kinetics was investigated using MS2 bacteriophage as the surrogate, and trihalomethanes (THMs) and haloacetic acids (HAAs) formation was measured. Residual consumption during sequential exposures was also investigated....[ Read more ]

Sequential disinfection using UV irradiation as a primary disinfectant and free chlorine or monochloramine as a secondary disinfectant was investigated in this study, to determine the synergistic effects on microbial inactivation and disinfection by-products (DBP) formation. Microbial inactivation kinetics was investigated using MS2 bacteriophage as the surrogate, and trihalomethanes (THMs) and haloacetic acids (HAAs) formation was measured. Residual consumption during sequential exposures was also investigated.

Two types of synergistic effects were observed in microbial inactivation. UV pre-treatment or co-treatment in the primary disinfection stage with either low-pressure (LP) or medium-pressure (MP) UV irradiation resulted in the enhancement in the rate of secondary inactivation when either free chlorine or monochloramine was used. The secondary inactivation rates were 1.28-2.74 (free chlorine) and 2-5 (monochloramine) times faster than those data obtained without pre-UV or co-UV irradiation. The enhancement was also found to increase with increasing UV dose. A synergistic effect in MS2 coliphage inactivation was also observed by simultaneous exposure to both disinfectants during the primary disinfection stage. The gross kill under such simultaneous exposure were 0.04-2.38 (chlorine) and 0.63-2.7 (monochloramine) log larger than the sum of log inactivation achieved by each of the two disinfectants at the same doses. Similar trends were observed when using different UV sources (LP and MP lamps) and different secondary disinfectants (free chlorine or monochloramine).

In DBP formation, DBP formation potential (DBPFP) and changes in UV absorbance spectra were also measured. In all cases, UV pre-irradiation led to an increase in free chlorine consumption but a beneficial decrease in monochloramine consumption. While rapid decomposition of both chlorine and monochloramine were observed under co-exposure with UV irradiation. Exposure to LPUV irradiation in all approaches induced substantial changes in humic acid composition and increased the time-dependent DBP formation compared to sole chlorination or monochloramination, with highest DBP formation in co-exposure approaches. However, only the NH₂CI/MPUV approach resulted in a similar manner as LPUV, while other approaches under exposure to MPUV irradiation generally reduced the DBP formation instead. The yields of DBPs were higher using free chlorine compared to monochloramine in all cases. DBPFP generally followed the similar manner as time-dependent DBP formation in all cases, with unexpected low DBPFP in HOCI/MPUV and MPUV/NH₂CI approaches. In addition, coagulated humic acid solutions resulted in reduction in DBP formation in all sequential approaches compared to that of raw humic acid solutions using MPUV irradiation.