Changes in the organic composition of wastewater during biological treatment as studied by NMR and IR spectroscopies

Despite its importance, relatively little is known about the composition and fate of wastewater organic matter (OM) in treatment plants. Monitoring the chemical changes in OM during activated sludge treatment can improve our knowledge of the processes involved in the biological elimination of OM. Direct chemical analyses of treated water OM typically account for about 20% of the OM, and structural information was obtained in this study using Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared (FTIR) spectroscopic techniques. Distinct changes in the OM during wastewater biological treatment were underlined. ¹³C and ¹H NMR showed that aromatic carbons were minor constituents of the samples. Alkyl chains exhibited a more highly branched character in treated water, as compared to long chain aliphatic carbons present in wastewater. Carboxyl signals in the ¹³C NMR spectrum of wastewater could be due to peptide bonds in proteins, whereas in the treated water spectrum, this signal could be related to the presence of non-proteinaceous nitrogen. Besides the non-degraded compounds, treated water OM could contain recondensation products of simple molecules. Their refractory character probably derives from their complex structures rather than from particular chemical functions, as suggested by the lack of fundamental differences in the chemical structures of wastewater and treated water OM.