Removal of carbonaceous and nitrogenous disinfection by-product precursors in biological activated carbon process of drinking water: Is service life a pivotal factor?

Biological activated carbon (BAC) process is commonly used for drinking water treatment. Granular activated carbon (GAC) serving as the initial filter media can be converted to BAC with biofilm development during extending service life. Consequently, GAC/BAC effectiveness for disinfection by-product (DBP) precursors removal varies over service life. Effluents of full-scale ozonation and BACs operated for 1-3 months (1-month GAC), 4-6 months (4-month BAC), and about 7 years (7-year BAC) were analyzed. All BACs were efficient in fluorescence components removal, with decreasing efficiency in the order of 1-month GAC > 4-month BAC > 7year BAC. Accordingly, reduction ratios of total DBPs concentration formed in post-chlorination dropped from 39% to 21% and 8% by BACs with different ages. Due to varied toxic potencies of DBPs, toxicities reduction was not completely related to DBPs concentration reduction. Haloacetonitriles (HANs) group accounted for small proportion of DBPs concentration but drove both of cytotoxicity and genotoxicity. Hence, the precursor removals of carbonaceous (C-) DBPs and nitrogenous (N-) DBPs were further compared in detail. 1-month GAC exhibited relatively higher effectiveness for protein-like components removal than humic-like component removal, for HANs formation reduction than trihalomethanes (THMs) formation reduction, and for HANs cytotoxicity reduction than THMs cytotoxicity reduction. A similar pattern was observed for 4-month BAC, while 7-year BAC exhibited the opposite pattern. Given the complexity of real water, this full-scale study provides valuable insights into the differences in precursor removals of C-DBPs and N-DBPs by BACs with different ages, which is critical for advancing our understanding of BAC process.