Drinking Water

Chlorine dioxide (ClO₂) is commonly used as an alternative disinfectant tochlorine in drinking water treatment because it produces limited concentrations of halogenatedorganic disinfection byproducts. During drinking water treatment, the primary ClO₂ byproductsare the chlorite (50−70%) and the chlorate ions (0−30%). However, a significant portion of theClO₂ remains unaccounted for. This study demonstrates that when ClO₂ was reacting withphenol, one mole of free available chlorine (FAC) was produced per two moles of consumed ClO₂. The in situ formed FACcompleted the mass balance on Cl for inorganic ClO₂ byproducts (FAC + ClO₂¯+ ClO₃¯). When reacting with organic matterextracts at near neutral conditions (pH 6.5−8.1), ClO₂ also yielded a significant amount of FAC (up to 25%). Up to 27% of thisin situ formed FAC was incorporated in organic matter forming adsorbable organic chlorine, which accounted for up to 7% ofthe initial ClO₂ dose. Only low concentrations of regulated trihalomethanes were produced because of an efficient mitigation oftheir precursors by ClO₂ oxidation. Conversely, dichloroacetonitrile formation from ClO₂-induced generation of FAC washigher than from addition of FAC in absence of ClO₂. Overall, these findings provide important information on the formationof FAC and disinfection byproducts during drinking water treatment with ClO₂.

Increasing transmembrane pressure (TMP) can compress and increase the hydraulic resistance of membrane biofilms. The purpose of the present study is to evaluate how compression of membrane biofilms occurs and how structural rearrangement can affect hydraulic resistance. Biofilms with heterogeneous and homogeneous physical structures were grown in membrane fouling simulators (MFS) in dead-end mode for 20 days with either (i) a nutrient enriched condition with a nutrient ratio of 100:30:10 (C: N: P), (ii) a phosphorus limitation (C: N: P ratio: 100:30:0), or (iii) river water (C: N: P ratio: ca. 100:10:1). The structural and hydraulic response of membrane biofilms to (a) changes in transmembrane pressures (0.06-0.1-0.5-0.1-0.06 bar) and (b) changes in permeate flux (10-15-20-15-10 L/m²/h) were investigated. Optical coherence tomography (OCT) was used to monitor biofilm structural response, and OCT images were processed to quantify changes in the mean biofilm thickness and relative roughness. Nutrient enriched and river water biofilms had heterogeneous physical structures with greater surface roughness (Ra' > 0.2) than homogeneous P limiting biofilms (Ra' < 0.2). Compression of biofilms with rough heterogeneous structures (Ra' > 0.2) was irreversible, indicated by irreversible decrease in surface roughness, partial relaxation in mean biofilm thickness and irreversible increase in hydraulic resistance. Compression of homogeneous biofilm (Ra' < 0.2) was on the other hand reversible, indicated by full relaxation of the biofilms structure and restoration of initial hydraulic resistance. Hydraulic response (i.e., change in the specific biofilm resistance) did not correspond with the change in physical structure of heterogeneous biofilms. The presented study provides a fundamental understanding of how biofilm physical structure can affect the biofilm's response to a change in TMP, with practical relevance for the operation of GDM filtration systems.

Detecting disturbances in microbial communities is an important aspect of managing natural and engineered microbial communities. Here, we implemented a custom-built continuous staining device in combination with real-time flow cytometry (RT-FCM) data acquisition, which, combined with advanced FCM fingerprinting methods, presents a powerful new approach to track and quantify disturbances in aquatic microbial communities. Through this new approach we were able to resolve various natural community and single-species microbial contaminations in a flow-through drinking water reactor. Next to conventional FCM metrics, we applied metrics from a recently developed fingerprinting technique in order to gain additional insight into the microbial dynamics during these contamination events. Importantly, we found that multiple community FCM metrics based on different statistical approaches were required to fully characterize all contaminations. Furthermore we found that for accurate cell concentration measurements and accurate inference from the FCM metrics (coefficient of variation ≤ 5%), at least 1000 cells should be measured, which makes the achievable temporal resolution a function of the prevalent bacterial concentration in the system-of-interest. The integrated RT-FCM acquisition and analysis approach presented herein provides a considerable improvement in the temporal resolution by which microbial disturbances can be observed and simultaneously provides a multi-faceted toolset to characterize such disturbances.