Characterizing Dissolved Organic Matter in Reclaimed Urban City Water

It’s becoming increasingly popular to use reclaimed water in urban cities, where water consumption and pollution are elevated. As well as being a sustainable water strategy, using reclaimed water has the added benefit of lessening wastewater discharge to natural water sources. One of the problems in using reclaimed water, however, is that it contains dissolved organic matter (DOM), which is difficult to characterize due to its complexity. Phungsai et al. show how this problem can be overcome by using Orbitrap mass spectrometry to characterize DOM.¹

The team measured water from a water reclamation plant in Japan that receives secondary effluent from a municipal wastewater treatment plant. During water reclamation, the plant treated the water by biofiltration, ozonation and chlorination.

They used “unknown” screening analysis using electrospray ionization (ESI) Orbitrap mass spectrometry with an Exactive Orbitrap mass spectrometer (Thermo Scientific). They used ESI since it is well-known to produce singly charged and mostly unfragmented molecules. The team set the m/z range to 100–1,000 and analyzed six replicates of samples and an extraction blank for reproducibility.

Phungsai et al. processed the raw data using Sieve software (Thermo Scientific). They kept peaks with intensities significantly higher than the extraction blank and with a signal-to-noise ratio greater than three for further analysis. Most detected ions were in the m/z range of 100 to 450. In total, the team assigned 2,412 formulae with various heteroatoms. Formulae with carbon (C), hydrogen (H), oxygen (O) and sulfur (S) were the most abundant species.

After converting elemental composition to ratios—of H/C and O/C, for example—the scientists used the ratios to plot van Krevelen diagrams. The van Krevelen diagrams of molecular formulae showed that CHOS-class molecules were relatively saturated, while CHO-class molecules were unsaturated carbon, lignin-like or black carbon–like molecules. The diagrams also showed that the compounds changed during the biofiltration treatment. For example, the reclamation process removed CHO-class molecules that had high H/C values. This shows simultaneously that biofiltration preferentially removes saturated molecules and that unsaturated samples are more resistant to biodegradation.

Ozonation had the largest effect on low–molecular weight DOM. The process removed a wide range of compounds and produced many oxidized molecules. Phungsai et al. were able to trace these back to potential organic precursors, which originated from secondary effluent as well as the other reclamation processes of biofiltration and ozonation. Furthermore, the team showed that the chlorination process created disinfection by-products. Over 100 chlorine-containing molecular formulae were detected after the chlorination treatment.

DOM can cause many problems both during and after the water reclamation process: causing organic fouling of membranes used in the process, increasing the formation of disinfection by-products and promoting microbial growth. By measuring DOM using orbitrap mass spectrometry, Phungsai et al. have taken an important step in tackling these problems.  

Reference
1. Phungsai, P., et al. (2016) “Molecular characterization of low molecular weight dissolved organic matter in water reclamation processes using Orbitrap mass spectrometry,” Water Research, 100 (pp. 526–536), doi: 10.1016/j.watres.2016.05.047.

Post Author: Kathryn Loydall. Kathryn is a science and medical writer with a background in protein chemistry, biochemistry and applied biology. She enjoys making science accessible to a lay audience through writing, illustrations and media. Originally from the UK, she moved to Vancouver Island in 2008 to complete her PhD focussed on sepsis research and x-ray crystallography of monoclonal antibodies. In 2012, Kathryn left the lab behind to start freelancing as a science and medical writer and editor, and hasn’t looked back since.