Advanced characterisation of dissolved organic nitrogen in drinking water sources impacted by algae and the potential formation of nitrogenous disinfection by-products

Algal organic matter (AOM) includes dissolved organic nitrogen (DON) which can impair water quality by acting as precursors to form nitrogenous disinfection by-products (N-DBPs) during drinking water disinfection. N-DBPs are believed to show a greater toxicity than the regulated carbon DBPs. So it is important to understand how algal DON might form N-DBPs during algal blooms. However, algal DON has not been well characterised and its relationship with N-DBP formation has not been comprehensively investigated.In this study, the physicochemical properties of AOM from green algae (Chlorella vulgaris) and cyanobacteria (Microcystis aeruginosa, Dolichospermum circinale and Cylindrospermopsis raciborskii) were comprehensively characterised throughout their growth phases using multiple techniques, which included: AOM concentration; molecular size distribution of AOM using both size exclusion chromatography with organic carbon and nitrogen detectors and ultrafiltration; protein profiling using the Bradford assay and sodium dodecyl sulfate-polyacrylamide gel electrophoresis; and spectroscopic methods including 15N-nuclear magnetic resonance (NMR) spectroscopy and fluorescence excitation and emission matrix (F-EEM) spectroscopy. Additionally, the N-DBP formation potential for three haloacetonitriles (HANs), two halonitromethanes, two haloacetamides and eight N-nitrosamines were measured on bulk and fractionated AOM during chlorination and chloramination. Formation of N-DBPs after chloramination was also studied with 15N labelled algal-DON to investigate the roles of AOM and monochloramine as N-DBP precursors.When assuming an algal cell population of bloom proportions, 1x105-1x106 cells mL-1, the average algal DON for all species was in the range of 0.02-0.2 mg N L-1 (measured in the stationary phase). High molecular weight DON (HMW, >1 kDa) dominated total algal DON, becoming increasingly dominant with age, reaching above 90% for all species, with the exception of C. raciborskii. The biopolymer algal DON was mainly distributed around 70~120 kDa and 1-2 kDa while algal protein mostly accumulated around 15-25, >50 and >100 kDa for all species. 15N-NMR spectroscopy indicated that 2° amides were the predominant form in algal DON. F-EEM spectroscopy showed the presence of protein-like Peak T was dominant in the biopolymer fraction in most cases. N-DBP formation during chlorination was limited and restricted to HANs, with dichloroacetonitrile predominating. The HMW algal DON fraction was found to form more N-DBPs than the LMW fraction on both a C and N basis for all species. The high 14N-labelled DCAN formation during the chloramination for the species tested indicated that DCAN formation was mainly incorporated from monochloramine. The high 14N,14N-labelled NDMA suggested the possibility of incomplete 15N labelling of the AOM possibly due to N2 assimilation during algae culture. On the basis of these results, an estimation of algal-derived N- DBP concentrations that might be experienced during algal blooms indicates that the formation of N-DBPs from all species during the entire growth phase could be expected to reach a maximum of 1 μg L-1 for DCAN (C. vulgaris) and 12 ng L-1 for NDMA (C. raciborskii), which are likely to be compliant with the current drinking water regulations from USEPA of 6 μg L-1 for DCAN, or guidelines from WHO and Australia of 20 μg L-1 for DCAN and 100 ng L-1 for NDMA