High-performance size exclusion chromatography with a multi-wavelength absorbance detector study on dissolved organic matter characterisation along a water distribution system

Abstract

This study examined the associations between dissolved organic matter (DOM) characteristics and potential nitrification occurrence in the presence of chloramine along a drinking water distribution system. High-performance size exclusion chromatography (HPSEC) coupled with a multiple wavelength detector (200–280 nm) was employed to characterise DOM by molecular weight distribution, bacterial activity was analysed using flow cytometry, and a package of simple analytical tools, such as dissolved organic carbon, absorbance at 254 nm, nitrate, nitrite, ammonia and total disinfectant residual were also applied and their applicability to indicate water quality changes in distribution systems were also evaluated. Results showed that multi-wavelength HPSEC analysis was useful to provide information about DOM character while changes in molecule weight profiles at wavelengths less than 230 nm were also able to be related to other water quality parameters. Correct selection of the UV wavelengths can be an important factor for providing appropriate indicators associated with different DOM compositions. DOM molecular weight in the range of 0.2–0.5 kDa measured at 210 nm correlated positively with oxidised nitrogen concentration (r = 0.99), and the concentrations of active bacterial cells in the distribution system (r = 0.85). Our study also showed that the changes of DOM character and bacterial cells were significant in those sampling points that had decreases in total disinfectant residual. HPSEC-UV measured at 210 nm and flow cytometry can detect the changes of low molecular weight of DOM and bacterial levels, respectively, when nitrification occurred within the chloraminated distribution system.

Introduction

The character of dissolved organic matter (DOM) is a key index used in drinking water quality management (Chow, C.W., et al., 2008, Korshin, G., et al., 2009). DOM is a complex molecular mixture variable in composition and functionality which depends on its source. The presence of DOM in waters can be one of the principal causes of microbes' growth. A decrease in disinfectant residual has also been suggested to be associated with interaction between disinfectant and DOM (Wilczak, A., et al., 2003, Kristiana, I., et al., 2013). The properties of DOM, particularly that of molecular weight, have been demonstrated to strongly affect natural organic matter reactivity in natural systems as well as different stages of the treatment process (Chow, C.W., et al., 2008, Korshin, G., et al., 2009, Kristiana, I., et al., 2010). Hence, an effective determination of the molecular weight distribution of DOM is critical not only for improving the water treatment process in terms of removal but also for understanding the disinfection process downstream in the distribution system.

Although promoting various benefits and improvements for drinking water treatment processes and distribution system, the application of chloramine as secondary disinfectant is associated with a number of adverse effects on water quality (Lipponen, M.T., et al., 2002, Motzko, S., et al., 2009, Zhang, Y., et al., 2009, Bai, X., et al., 2015). Nitrification management has drawn the most attention. A decrease in disinfectant residual and an increase in oxidised nitrogen concentrations (either nitrite or nitrate) as well as an increase in microbial risks are the common adverse effects owing to the occurrence of nitrification. Microorganism growth and regrowth and nitrite concentration increase have serious potential health risks (Lipponen, M.T., et al., 2002, Krishna, K.B., et al., 2013, Bai, X., et al., 2015). Management of chloramine decay, maintenance of adequate disinfectant residual and determination of several operational, chemical and microbiological parameters throughout the water treatment process and distribution system are essential to ensure the supply of safe and high quality potable water to all consumers.

Nitrification is a two-step process. Ammonia is initially oxidised to nitrite, and nitrite is further oxidised to nitrate (Lipponen et al., 2002). These processes are associated with nitrifying microorganism occurrences and activity. An increased amount of ammonia in chloraminated waters is linked to nitrification in the drinking water distribution system. Increase in either nitrate or nitrite concentration is normally considered to be positively correlated with nitrification (Odell et al., 1996). Many researchers have also indicated that the level of microbes in pipe water is inversely related to the content of total disinfectant (Lipponen, M.T., et al., 2002, Zhang, Y., et al., 2009). Monitoring and understanding treated water quality in the distribution system is hence required to effectively manage distribution system performance.

High performance size exclusion chromatography coupled with ultraviolet (UV) detection (HPSEC-UV) has been successfully and widely used for determining the performance of drinking water treatment processes, such as DOM removal evaluation, potential disinfection by-product (DBP) formation prediction and treated drinking water distribution management (Fabris, R., et al., 2008, Chow, C.W., et al., 2008, Xing, L., et al., 2012, Kristiana, I., et al., 2013). These previous studies have demonstrated that the low molecular weight DOM compounds are recalcitrant to removal by conventional coagulation treatment. Some studies have also indicated that these low molecular weight fractions remaining in treated water are particularly associated with water quality degradation in distribution systems (Kristiana, I., et al., 2010, Kristiana, I., et al., 2013). In most cases, such measurements have generally focused on aromatic components by detection at a single wavelength in the UV range of 250–280 nm. The non-aromatic groups of DOM associated with low molecular weight fractions do, however, not absorb strongly at this range of wavelengths but can potentially impact water quality. HPSEC coupled with a multiple wavelength detector may further characterise DOM molecular weight distribution and yield more insight into the key information that is of importance in water quality investigations.

Additionally, flow cytometry in combination with a fluorescence staining method has been utilised to characterise microbes present in distribution system. Due to the ability to provide rapid, accurate and high throughput, this advanced analytical method has been frequently used to characterise bacterial removal throughout water treatment processes and distribution systems (Hoefel, D., et al., 2005, Hammes, F., et al., 2008, Ho, L., et al., 2012).

In this paper, HPSEC coupled with a multiple UV wavelength detector, from 200 nm to 280 nm, was employed to characterise DOM as an indirect assessment tool for potential nitrification occurrence along an operating distribution system. This multi-wavelength HPSEC approach would be useful to investigate the impact of different fractions of DOM on nitrification, including those fractions of relatively lower molecular weight, less aromatic character and with a weak absorbance response in the 250–280 nm wavelength range. A key focus was on developing an organic characterisation tool for understanding how DOM impacts on water quality in chloraminated distribution systems. This study covers both aspects of general water quality parameter changes associated with nitrification occurrence in the distribution system and their associations with changes in the DOM molecular weight profile. Due to the nature of an operating system with possible changes in both environmental and operational conditions along the distribution system, the study was designed to compare the interrelationship between samples in different sections of a distribution system supplied from the same water treatment plant.