humic acids, fulvic acids, proteins, various aromatic substances) absorb UV light,
this signal can be used as a surrogate measure of NOM.
These early devices had serious limitations; apart from the limited lifetime of the
mercury lamp, the use of a single wavelength makes the measurement sensitive to
cross-interference; a change in the composition of the matrix, such as caused by
heavy rainfall, the influx of industrial wastewater or the daily and seasonal changes
in composition can cause strong changes in the relationship between UV254 and the
true parameter of interest. For improved correlation, devices using multiple wavelengths were developed. The following types of instruments are now widely
available:
• Dual wavelength, with a second wavelength used to compensate for turbidity and
suspended matter.
• Multiple discrete wavelengths using LEDs with emission spectra at various
wavelengths. Specificity is achieved through basic algorithms.
• Full spectral instruments, measuring the entire UV (200–400 nm) or UV/Vis
(200–700 nm) range with nanometre resolution allowing for advanced
algorithms.
UV/Vis spectrometers are offered both as submersible in situ probes and flowthrough devices that can be used to monitor a sidestream. The sensitivity of the
instrument depends on the length of its measurement compartment; a longer path
length gives higher sensitivity but also a reduced maximum concentration level at
which the instrument can operate. Therefore, a device with an optical path that fits
the application needs to be used. Typical path lengths available are in the order of
0.5–100 mm.
5.1.1 Sum Organic Parameters
Natural water as well as domestic and industrial wastewater consists of a mixture of
various organic substances. Using UV/Vis spectroscopy, the sum of all the absorption signals in the mixture is measured (Eq. 3). The recorded spectra are typically
broad and lack characteristic features, because they consist of overlapping spectra of
the individual components in the sample. Determination of individual substances is
possible only in few applications, with substances with highly characteristic signals
in areas of the spectrum where absorption by other components is low. It is,
however, possible to accurately calculate sum organic parameters from absorption
spectra, even when individual components cannot be identified. Parameters that can
reliably be derived from the spectrum include total organic carbon (TOC), chemical
oxygen demand (COD) and biological oxygen demand (BOD). As the traditional
analytical methods for these parameters are time-consuming, e.g. the 5-day BOD
test, or require toxic chemicals such as perchromate, a purely optical method to
obtain an accurate indication of their concentration levels in real-time is a powerful
tool. Using mathematical turbidity compensation, e.g. based on the description of the
optical properties of turbidity and suspended matter [14], it is possible to filter out the
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this signal can be used as a surrogate measure of NOM.
These early devices had serious limitations; apart from the limited lifetime of the
mercury lamp, the use of a single wavelength makes the measurement sensitive to
cross-interference; a change in the composition of the matrix, such as caused by
heavy rainfall, the influx of industrial wastewater or the daily and seasonal changes
in composition can cause strong changes in the relationship between UV254 and the
true parameter of interest. For improved correlation, devices using multiple wavelengths were developed. The following types of instruments are now widely
available:
• Dual wavelength, with a second wavelength used to compensate for turbidity and
suspended matter.
• Multiple discrete wavelengths using LEDs with emission spectra at various
wavelengths. Specificity is achieved through basic algorithms.
• Full spectral instruments, measuring the entire UV (200–400 nm) or UV/Vis
(200–700 nm) range with nanometre resolution allowing for advanced
algorithms.
UV/Vis spectrometers are offered both as submersible in situ probes and flowthrough devices that can be used to monitor a sidestream. The sensitivity of the
instrument depends on the length of its measurement compartment; a longer path
length gives higher sensitivity but also a reduced maximum concentration level at
which the instrument can operate. Therefore, a device with an optical path that fits
the application needs to be used. Typical path lengths available are in the order of
0.5–100 mm.
5.1.1 Sum Organic Parameters
Natural water as well as domestic and industrial wastewater consists of a mixture of
various organic substances. Using UV/Vis spectroscopy, the sum of all the absorption signals in the mixture is measured (Eq. 3). The recorded spectra are typically
broad and lack characteristic features, because they consist of overlapping spectra of
the individual components in the sample. Determination of individual substances is
possible only in few applications, with substances with highly characteristic signals
in areas of the spectrum where absorption by other components is low. It is,
however, possible to accurately calculate sum organic parameters from absorption
spectra, even when individual components cannot be identified. Parameters that can
reliably be derived from the spectrum include total organic carbon (TOC), chemical
oxygen demand (COD) and biological oxygen demand (BOD). As the traditional
analytical methods for these parameters are time-consuming, e.g. the 5-day BOD
test, or require toxic chemicals such as perchromate, a purely optical method to
obtain an accurate indication of their concentration levels in real-time is a powerful
tool. Using mathematical turbidity compensation, e.g. based on the description of the
optical properties of turbidity and suspended matter [14], it is possible to filter out the
294
J. van den Broeke and T. Koster