Miniaturised SAC Measurement for Continuous Monitoring of Water Quality

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1 Miniaturised SAC Measurement for Continuous Monitoring of Water Quality White Paper September 2015

2 Miniaturised SAC Measurement for Continuous Monitoring of Water Quality Andreas Ulsperger, Product Manager Online Analysis System Dr. Martin K. Garbos, R & D Microfluidics & Optics In the entire chain, from design through installation and commissioning, continuous operation and all the way to maintenance, many specialists are involved in assuring the continuous monitoring of water quality. They include the engineering company, the plant manufacturer, the operator, the service and maintenance technicians and the programmers of the SCADA systems. In the development of online analysis systems, specialists from diverse disciplines must likewise work together. If one seeks the best supplier for each single measurement, the result will certainly be high investment costs. This is especially true if one insists on having measurements that are precise as possible but loses sight of the initial goal. If an operator or plant manufacturer consistently proceeds in this manner, in the end there will be measuring instruments in the field from several manufacturers, each one being operated differently. It must also be taken into account that training will be necessary for each single device, that maintenance will be required by different specialised companies and that it may be necessary to program different interfaces to the SCADA system. Before everything can work properly, many hoses must be installed between measurements or from separate sampling points and the measuring hardware must be connected. The alternative is to assemble a measurement system that can measure all parameters with the required precision and that performs all the functions of separate measuring devices, however on a single platform. This eliminates all of the disadvantages mentioned above: only one system has to be configured, only one interface to the SCADA has to be defined, only minimal installation is required and the operator and the service and maintenance technicians only Miniaturisierte SAK-Messung 2

3 have to be trained on one device. For the manufacturer of such a system, this means: electronics engineers, software developers, experts in the fields of mechanics, optics, fluidics and water have to cooperate closely. Microbiologists and chemical specialists may also be involved, in order to combine all of the functions required of an online analysis system. Modular online analysis system A modular system that has to combine the functions of several conventional devices first needs a platform for the modular visualisation of all functions. The Type 8905 online analysis system from Bürkert is based on a platform with three pillars: The fluidic, mechanical platform: All available sensors can be connected and disconnected also during operation on a standardised backplane. This means that all sensors developed for the platform must fit into the defined modular grid and that the dimensions of the sensors are largely pre-defined. The electrical platform: All available sensors and control modules, as well as analogue and digital I/O modules, Industrial Ethernet modules, etc. can be combined on a standardised backplane. The software platform: With standard communication conventions, the two platforms described above would be ineffective. All modules within the online analysis system are connected to a bus system, which also makes it possible to detect new modules and integrate their functions in the system, without additional configuration requirements. Design and functions The requirement of a modular system requires compliance with all three platform standards. The complete online analysis system is shown in Fig. 1 (left). The water sample enters the system through fluidic interconnects and flows through all embedded sensor cubes in the bottom housing. The top part contains the electronic components for the analysed data from the sensor and its display on the 7" touch display. Much like a tablet PC, this display allows the user to change parameters, display measurements as values or trend diagrams and to conduct all control processes in the system. The bottom part contains the fluidic backplane, which hosts up to six sensors. When a sensor is inserted, the fluidic connection is unlocked and the water sample can flow through the cube. All sensors are hot swappable and interchangeable according to the user s choice of measuring parameters. An indicator light bar on the system and LEDs on each inserted module indicate the status of each sensor. A white light bar signals to the user that the overall system is operating correctly. Miniaturisierte SAK-Messung 3

Fig. 1 Left: View of the online analysis system with the control unit at top (7 display) and the fluidic compartment at bottom, where the single sensors can be inserted.

4 Fig. 1 Left: View of the online analysis system with the control unit at top (7 display) and the fluidic compartment at bottom, where the single sensors can be inserted. The water sample flows through the entire fluidic component, the backplane and each embedded sensor. Right: Close-up view of the SAC sensor module with the dimensions 44 mm x 100 mm x 158 mm. SAC measurement In the following, the module for measuring the spectral absorption coefficient (SAC) at 254 nm is chosen (Fig. 1, right) to describe the effect of the fluidic and mechanical aspects on the optics. In the end, the central goal is to build up a compact online analysis system with the described benefits. The measuring principle of the SAC sensor is based on the German standard for water, waste water and sludge analysis. It defines the process that makes it possible to determine the quantity of dissolved organic compounds, even in the case of high turbidity. The measurement is based on the attenuation coefficient µ at 254 nm and 550 nm. The attenuation coefficient for a given wavelength λ is defined as: μ (λ) = α (λ) + s (λ) Where α is the absorption coefficient and s is the scattering coefficient, i.e. light scattered by particles in the sample. Organic compounds strongly absorb in the UV range (e.g. at 254 nm). Therefore, the spectral absorption coefficient SAC at 254 nm (α (254 nm)) provides a direct measure for the quantity of organic material in the sample. However, only the attenuation coefficient µ (254 nm) can be measured. There can also be additional attenuation due to scattering by small particles s (254 nm). This is important primarily in the case of turbid samples. To account for this effect, a turbidity compensation is performed by measuring the attenuation at 550 nm (µ (550 nm)). At this wavelength, the absorption coefficient of pure water (α (550 nm)) is practically zero (even if contaminated with organic substances) and one can measure the scattering coefficient: μ (550 nm) = α (550 nm) + s (550 nm) s (550 nm) s (254 nm). Miniaturisierte SAK-Messung 4

Therefore, the turbidity compensation is performed as follows: μ (254 nm) - μ (550 nm) α (254 nm) Result shows the spectral absorption coefficient α (254 nm) as an indirect measure for the quantity

5 Therefore, the turbidity compensation is performed as follows: μ (254 nm) - μ (550 nm) α (254 nm) Result shows the spectral absorption coefficient α (254 nm) as an indirect measure for the quantity of organic contamination in the water sample. In practice, this principle was implemented as an optofluidic measurement in a miniaturised format (Fig. 2). The sensor measures only 44 mm x 100 mm x 158 mm and consists of a lamp module (left) and a spectroscope (right). The cross section shown from the left side in Fig. 2 illustrates the optical path of the sensor. The electronic components are located on the right side of the module (underneath as shown in Fig. 2); the integrated software autarkically induces measuring cycles as soon as the sensor is connected. All calculations, averaging and corrections are performed in the sensor cube and only the result, the SAC value, is transmitted to the platform. Fig. 2 Cross section of the SAC module: (a) Xenon flash lamp and entrance slit to spectrometer, (b) cuvette with two quartz windows containing a continuously flowing 60 mm fluid sample, (c) collimating lens; creates a collimated light beam, (d) aperture; reduces the beam waist in the Fourier space and improves the resolution, (e) reflective grating, optimised for 250 nm, (f) focusing lens; focuses the image onto the detector, (g) linear detector array. A Xenon flash lamp (Fig. 2 (a)) generates the µs light impulse as a light arc with a characteristic broadband spectrum that ranges from deep ultraviolet (190 nm) to infrared. The spherically emitted light passes through the entrance slit at a 90 angle and enters the cuvette (Fig. 2 (b)), which consist of two sapphire windows and one fluid passage with the continuously flowing sample fluid. A lens (Fig. 2 (c)) creates a collimated light beam, which is narrowed down by an aperture (Fig. 2 (d)), therefore reducing the possible angles in the Fourier space. The narrow beam is incident on a blazed optical grating (Fig. 2 (e)) and dispersed into its spectral components. A focusing lens (Fig. 2 (f)) images the light arc dispersed on the 1.5 mm detector array, corresponding to the spectrum of the Xenon lamp altered by the properties of the fluid sample. Due to the aperture in the Fourier space (Fig. 2 (d)) the possible angles for each wavelength are reduced, resulting in an increased wavelength resolution. Miniaturisierte SAK-Messung 5

6 The utilised setup requires no direct imaging of the light arc from the Xenon lamp. The jitter from flash to flash causes a spectral shift of the spectra on the array of 2-3 nm. Through calibration of the sensor, the software calculates for each flash the correct correlation of the wavelength per pixel for each single spectrum. This is done by comparing the position of a distinct Xenon peak to a calibration spectrum in the database of the sensor. This shows the feasibility of an ultra-compact sensor with a fully equipped micro-spectrometer. The special challenge was the miniaturised design, especially the choice of the light source, as well as the analysis of minute quantities of organic contamination in a cuvette with a short length. Overview of the most important parameters In addition to the SAC sensor cube, the overall online analysis system can be equipped with the following sensor cubes: - ph value (ISFET technology) - Chlorine (MEMS, amperometric, membrane covered) - ORP / redox (MEMS, Ag/AgCl) - Conductivity (MEMS, graphite, c = 1) - Turbidity (scattered light acc. to ISO 7027 or EPA) - Iron (flow injection analysis) - Development of the series will continue In addition to the electric modules, which are available for control, operation and connection to the process as well as for control engineering, an automatic cleaning and calibration system, likewise based on the platform technology, is also available for the sensor cubes. Monitoring of water quality is a central component within the water safety plan in the Guidelines for Drinking Water Quality (GDWG) of the WHO and is also dealt with in the publication Water Safety in Distribution Systems issue It can be assumed that the number of parameters to be analysed in accordance with WHO guidelines will increase. This is due in part to the increasing contamination of waters and water resources. The WHO guidelines describe and define the principles of the lab processes. Water Safety in Distribution Systems discusses the processes and potentialities that can be applied in the network. They can therefore be applied to the waterworks and water stations on the way to the consumer and in the end also substantiate the requirement for miniaturisation. Driven by miniaturisation and the implementation of the latest technologies and combining expertise from diverse disciplines, compact and integrated online analysis systems can help to provide transparency of the analysed values in the field. The modularity of the system makes it possible to select the suitable measurement parameters and functions of the overall online analysis system for every specific monitoring requirement. Literature Peter H. Gleick, Dirty Water: Estimated Deaths from Water Related Diseases , Pacific Institute for Studies in Development, Environment, and Security, Research Report (2002). Miniaturisierte SAK-Messung 6

7 German standard methods for the examination of water, waste water and sludge Physical and physical-chemical parameters (group C) Part 3: Determination of absorption in the range of the ultraviolet radiation, Spectral absorption coefficient (C 3), DIN Deutsches Institut für Normung e.v., Berlin, DIN : (2005). Richard A. Dobbs, Robert H. Wise, Robert B. Dean, The use of ultra-violet absorbance for monitoring the total organic carbon content of water and wastewater, U.S. Environmental Protection Agency, National Environmental Research Center, Water Res, Vol. 6 10, Global Drinking Water Guidelines, WHO Library Cataloging-in-Publication Data Guidelines for drinking-water quality - 4th ed. - ISBN (2011) Martin K. Garbos, Philipp Hartmann, Anne März, Georg Moll, Christian P.M. Oberndorfer, Christoph Scholl, Raoul Schroeder Conference Paper: Modular optical sensor system for fluidic online analysis applications (2015) Miniaturisierte SAK-Messung 7

8 Contacts Do you have questions or can we show you our newest controlling technology? Just contact: Bürkert Fluid Control Systems Andreas Ulsperger Product Manager Online Analysis System Phone: +49 (0) Christian-Bürkert-Straße Ingelfingen Website: Dr. Martin K. Garbos R & D Microfluidics & Optics Phone: +49 (0) martin.garbos@burkert.com Christian-Bürkert-Straße Ingelfingen Website: Miniaturisierte SAK-Messung 8

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