Monochloramine penetration in biofilms and nitrification detection in a model chloraminated water distribution system.

摘要

Water quality deterioration due to nitrification occurrence in distribution systems has been challenging water utilities using chloramination for disinfection. The growth of nitrifying microorganisms embedded in biofilms in chloraminated water distribution systems (CWDSs) has been blamed for this phenomenon. The objective of this dissertation was to study NH2Cl penetration in biofilms and nitrification detection in CWDSs by using microelectrodes.;Based on voltammetric experiments, microelectrodes applicable to NH 2Cl measurements were developed. Under polarization potential at -90 mV (vs Ag/AgCl) and pH at neutral, the microelectrodes displayed linear responses to changes of NH2Cl concentration within ranges (from 0 to 28.0 mg/L as Cl2) tested. NH2Cl diffusion coefficient was determined to be 1.7 x 10-6 cm2/s, which, later in combination with use of NH2Cl microelectrode, helped make estimations on NH2Cl flux into biofilms.;Inactivation of NH2Cl against biofilm microorganisms was investigated using the NH2Cl microelectrode with focus on the penetration process of NH2Cl in biofilms grown on different substratum materials including concrete, polyvinylchloride and polycarbonate. It was found that contact time, NH2Cl concentration and substratum material all could affect NH 2Cl penetration process in biofilms, and thus in turn affected its inactivation process against biofilm microorganisms.;Biofilm tests using microelectrodes to measure oxygen, ammonium, nitrate and pH in biofilms were implemented. This approach was found promising in providing a potential application in nitrification detection in CWDSs. Actively ongoing nitrification was detected by this approach when water nitrite was as low as 0.02 mg-N/L. This indicates that, compared with 0.05 mg NO 2--N/L, the often adopted threshold for confirmation of nitrification occurrence in CWDSs, a lower nitrite threshold probably should be proposed in terms of its use in early detection of nitrification in CWDSs.;Microscopic observations on artificial biofilm and oxygen microelectrode measurements on natural biofilms both disclosed that microelectrode tip penetration facilitated mass diffusion in biofilms. This explains why oxygen concentrations in biofilms are sometimes overestimated using microelectrodes in comparison with noninvasive techniques. This study indicates that precaution is needed in dealing with microelectrode fabrication, experimental design, as well as collection, alignment and interpretation of data from microelectrode measurements in biofilms.