Verification of Full-Scale Ozone Contactor Inactivation Performance Using Biodosimetry

A biodosimetric technique was used to verify the concentration-contact time (CT) values [CT10, CT integrated disinfection design framework (CT-IDDF), CT segregated flow analysis (CT-SFA)] of the ozone contactors of the DesBaillets water treatment plant (Montreal), using indigenous aerobic spore formers (ASFs) as indicators of disinfection efficiency. ASF measurement in ozonated water was performed using a large water sample concentration method. Four assays, completed over a 6-week period, involved the implementation of biodosimetric calibration curves using an ozone pilot apparatus and followed by full-scale verifications. ASF inactivation kinetics were well described by a simple Chick–Watson model. The most accurate data also indicated that the CT10 underestimates the effective CT (by 1.2–1.9-fold), whereas the CT-IDDF and CT-SFA overestimate it (by 1.0–1.7-fold and 0.9–1.5-fold, respectively). Underestimation from CT10 was more pronounced with increased ozone dose while overestimation from CT-IDDF and CT-SFA is most likely due to the difficulty in obtaining a representative ozone residual profile within the contactor. The use of segregated flow analysis provided the best estimate of disinfection performance. Biodosimetry is useful in measuring the effective CT transferred, in verifying model predictions, and in determining the influence of water quality on microbial inactivation.