Using discrete and online ATP measurements to evaluate regrowth potential following ozonation and (non)biological drinking water treatment.

Water utilities must control microbial regrowth in the distribution system to protect public health. In this study, an adenosine triphosphate (ATP)-based biomass production potential test using indigenous bacterial communities were used to evaluate regrowth potential following ozonation with either biofiltration (BF) or sustained chlorination (SCl2). Two full-scale water treatment plants with different upstream processes (i.e., WTP-BF: ozonation, coagulation/flocculation, biofiltration, UV irradiation, chlorination; and WTP-SCl2: ozonation, chlorination, coagulation/flocculation, filtration, chlorination) were compared. Characterization of indigenous bacteria using 16S rRNA gene sequencing, qPCR, and cellular ATP (cATP) showed microbial diversity changes across treatment, biomass sloughing from biofilters (effluent cATP = 30 ± 1 ng/L), and disinfection by chlorine (cATP < 1 ng/L). For both WTPs, 14-day cumulative biomass production (CBPt =  ∑t=0tATP(t)×Δt) was highest for ozonated water samples (CBP14 = 1.2 × 103-3.0 × 103 d ngATP/L). CBP further increased with increasing ozone dose due to production of more biodegradable carbon. Growth promotion by carbon was confirmed from the consumption of ozonation byproducts (carboxylic acids, aldehydes) and the increase in CBP (9.5 × 102-2.9 × 103 d ngATP/L) after addition of 50-300 μgC/L acetate. Ozone followed by sustained chlorination (WTP-SCl2) effectively controlled biomass growth across the treatment process (CBP14 <10 d ngATP/L). In contrast, ozone followed by biofiltration (WTP-BF) reduced regrowth potential by 30% (biofilter influent CBP14 = 1.3 × 103 d ngATP/L; biofilter effluent CBP14 = 9.3 × 102 d ngATP/L). After adding chlorine to the biofilter effluent, CBP14 was reduced to <10 d ngATP/L. Lastly, online ATP measurements confirmed the discrete measurements and improved identification of the cATP peak and growth phases of indigenous bacteria.