levels, these vibrational states are discrete, and an IR spectrum reflects these discrete
energy transitions.
In the MIR the absorption bands are well defined, and it is possible to identify
specific atomic bonds. However, radiation in the MIR region does not penetrate
water well enough for direct measurement of water samples. The lower absorption of
NIR radiation by water allows for enough sample thickness to achieve sensitive
direct measurements. Absorption bands in the NIR represent the overtones of the
fundamental bands in MIR, and these overtones are relatively weak and not clearly
delineated; whereas the much sharper signals in the MIR range often allow identification of individual substances, NIR spectra often do not provide such detailed
information.
The applications of NIR are therefore found in situations where simple mixtures
are analysed. Examples include quality control in food and pharmaceutical industry,
where the expected NIR spectrum is known. A NIR sensor, monitoring deviations
between the measured spectrum and that of a pure product, is a tool in quality
control. Multivariate techniques such as principal component analysis (PCA) and
partial least squares (PLS) regression are used for development of calibration
models.
In the water industry, NIR has been used for various experimental studies, e.g. for
oil in water monitoring or studies on microalgae [25] and extracellular polymeric
substances in wastewater processes [13]. Commercial NIR systems for in situ
analysis of water samples primarily use reflection instead of transmission spectroscopy; in the transmission mode (e.g. as in UV/Vis spectroscopy), a beam of light
passes through a sample, and attenuation of the light by the sample is measured.
Reflection spectroscopy, however, analyses the light reflected by the top layer of the
matrix, where the makeup of the incident light is modified by the processes of
absorption and scattering. It is used for the analysis of contaminations that float on
the surface of water, e.g. oil slicks, and on media not transparent enough for
transmission spectroscopy. An example is the dewatered sludge from WWTPs
[13, 26]. Parameters measured in sludge include dry matter, ammonia and organic
matter. The application in sludge monitoring offers the possibility for smart process
control, allowing dosing control of polymers used in dewatering and safeguarding
optimal composition of sludge for subsequent digestion.
5.4 Further Optical Technologies with Potential for Online
Use in Smart Water Systems
UV/Vis absorbance and fluorescence are the most widely used spectroscopic
methods in the water industry. NIR also has a well-established place, especially in
industrial applications. Next to these technologies, there are optical methods with
potential for wider use in the near future. A selection is discussed in this section.
304
J. van den Broeke and T. Koster
energy transitions.
In the MIR the absorption bands are well defined, and it is possible to identify
specific atomic bonds. However, radiation in the MIR region does not penetrate
water well enough for direct measurement of water samples. The lower absorption of
NIR radiation by water allows for enough sample thickness to achieve sensitive
direct measurements. Absorption bands in the NIR represent the overtones of the
fundamental bands in MIR, and these overtones are relatively weak and not clearly
delineated; whereas the much sharper signals in the MIR range often allow identification of individual substances, NIR spectra often do not provide such detailed
information.
The applications of NIR are therefore found in situations where simple mixtures
are analysed. Examples include quality control in food and pharmaceutical industry,
where the expected NIR spectrum is known. A NIR sensor, monitoring deviations
between the measured spectrum and that of a pure product, is a tool in quality
control. Multivariate techniques such as principal component analysis (PCA) and
partial least squares (PLS) regression are used for development of calibration
models.
In the water industry, NIR has been used for various experimental studies, e.g. for
oil in water monitoring or studies on microalgae [25] and extracellular polymeric
substances in wastewater processes [13]. Commercial NIR systems for in situ
analysis of water samples primarily use reflection instead of transmission spectroscopy; in the transmission mode (e.g. as in UV/Vis spectroscopy), a beam of light
passes through a sample, and attenuation of the light by the sample is measured.
Reflection spectroscopy, however, analyses the light reflected by the top layer of the
matrix, where the makeup of the incident light is modified by the processes of
absorption and scattering. It is used for the analysis of contaminations that float on
the surface of water, e.g. oil slicks, and on media not transparent enough for
transmission spectroscopy. An example is the dewatered sludge from WWTPs
[13, 26]. Parameters measured in sludge include dry matter, ammonia and organic
matter. The application in sludge monitoring offers the possibility for smart process
control, allowing dosing control of polymers used in dewatering and safeguarding
optimal composition of sludge for subsequent digestion.
5.4 Further Optical Technologies with Potential for Online
Use in Smart Water Systems
UV/Vis absorbance and fluorescence are the most widely used spectroscopic
methods in the water industry. NIR also has a well-established place, especially in
industrial applications. Next to these technologies, there are optical methods with
potential for wider use in the near future. A selection is discussed in this section.
304
J. van den Broeke and T. Koster