14 WATER / WASTEWATER
Two ways to measure TOC levels in water: UV persulphate NDIR versus UV correlation
Plant engineers responsible for water and wastewater treatment in municipal and industrial plants will find that Electro-Chemical Devices (ECD) now offers them two different highly accurate and reliable solutions to measuring total organic carbon (TOC) levels to protect water quality.
Organic compounds are found in almost all types of water – from natural and treated drinking water to process water, cooling water, and water used in pharmaceuticals and food production. Too much organic contamination in the water interferes with many municipal and industrial processes. For example, when disinfecting drinking water an excess of organic matter can foster microbiological growth or indicate the presence of other undesirable byproducts.
Measuring TOC levels continuously with online analysers is a highly effective, dependable way to monitor water and wastewater for hazardous contaminants. Depending on the plant, the process and the water quality requirements, plant engineers typically select one of the industry’s two most popular sensing techniques to measure TOC. The ECD Model 3S TOC Analyser measures TOC using the UV persulphate oxidation method with carbon dioxide detection by nondispersive infrared absorption (NDIR). In comparison, the ECD Model UV-6 TOC Analyser measures TOC levels with a UV absorption correlation technique. Both ECD analyser models provide excellent TOC measurement accuracy and repeatability to ensure compliance with water quality standards.
ECD’s Model 3S TOC Analyser measures TOC in liquid samples ranging from 0–5 mg/L to 20,000 mg/L. The UV persulphate oxidation method conforms to EPA, DIN, CE, ASTM, and NAMUR regulations as well as meeting the requirements of ISO and EN directives.
The highly dependable Model 3S TOC Analyser features a fast-loop reservoir with a floating level sensor. If no sample reaches the reservoir for more than a pre-set time, the analyser switches automatically to standby mode. As soon as the sample flow re-starts, the analyser switches back to the analysis cycle automatically. Air bubbles are removed in the reservoir before the sample enters the analyser.
The sample first is acidified and then sparged to remove inorganic carbon. The remaining liquid is mixed with sodium persulfate and digested by two high-performance reactors. The resulting CO2 is then stripped from the liquid and, after drying, its concentration is measured by a NDIR analyser to determine TOC levels.
An alternative TOC analytical method is the UV correlation technique performed by the ECD UV-6 Analyser. The analyser is factory configured by ECD to measure TOC as correlated with the 254 nm wavelength and the final correlated calibration is done by matching the TOC measured to the on-site grab sample TOC value.
The Model UV-6 Analysers are a family of on-line sampling analysers that use UV absorption to perform an analysis. The analysers are configured to perform analysis over a wide range of values for each parameter measured, nitrate, colour, or the correlation at 254 nm wavelength for TOC, COD or BOD.
This technique measures TOC based on the measurement of UV absorption in the sample. The absorbance of the solution or gas is measured though a quartz flow cell at the chosen parameter’s specific wavelength using a long-life xenon light source and photo-detectors. The absorbance level is related to the sample concentration according to the ‘Beer-Lambert Law’.
The UV6 Analyser features an easy-to-use reagent-less design. The UV spectroscopy measuring principle requires no chemical reagent resulting in very low operating and maintenance costs. There are no reagent chemicals to order, store, track and dispose at the end of their useful life, simplifying the entire process.
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Low maintenance UV nitrate sensor
Responding to global demand for improved monitoring of nitrate in freshwater, OTT Hydromet has launched an optical nitrate sensor, the OTT ecoN, specifically for use in environmental surface and groundwater applications. “This is fantastic news for scientists, catchment managers, water companies, regulators and environmental consultants,” says Peter Schaeffeler, Product Lifeycle Manager at OTT HydroMet. “In the past, customers have been caught between low-cost/limited-performance sensors and high-end technologies, for which purchase costs prevent large scale deployment. The OTT ecoN neatly fills that gap; as a continuous nitrate monitor, with minimal maintenance requirements, it can be left to monitor in remote locations, and an optional wiper makes it suitable for extended deployments.”
In addition to a low purchase cost, the OTT ecoN also lowers the cost of ownership because it is factory calibrated for life. With automatic drift compensation, zero baseline checks with ultrapure water are
quick and easy to perform and are only required as an occasional test of instrument function.
The OTT ecoN measures nitrate by the absorption of UV light at 212 nm and removes the possible effects of organics and turbidity by similar measurements at 254 nm and 360 nm respectively. As an optical sensor, the OTT ecoN provides high accuracy and low resolution without the drift and interferences of ion-selective electrodes, or the ongoing reagent and maintenance costs of wet- chemistry analysers.
The OTT ecoN can transmit data via an SDI-12 interface due to the new OTT ecoN Modbus to SDI-12 Converter. The converter translates the Modbus protocol for the OTT ecoN nitrate sensor to allow for easy and continuous remote data access. Remote configuration can be achieved as well, to export data and configure the OTT ecoN via a web interface.
No specialised software is necessary to communicate with the OTT ecoN – a simple, intuitive browser-based user interface is accessed by connecting a small interface unit to the sensor.
Measurements can be stored internally within the sensor, but as a specialist in sensing, datalogging and telemetry, OTT HydroMet can supply the most appropriate monitoring system for every application. This includes the choice of sensor, because the OTT ecoN is available with different cell pathlengths to ensure optimum performance across all measurement ranges. However, OTT HydroMet is also able to conduct a retrospective change of optical path length when customers need to change application, such as moving it from very turbid surface water to groundwater for example.
Summarising, Schaeffeler says: “Nitrate is an extremely important contaminant of fresh surface and groundwater; causing health issues in humans and playing a major role in eutrophication and the proliferation of algae. It is therefore essential that the role of nutrients is fully understood and that water professionals are able to access accurate, reliable data on nutrient sources and loading, as well as being provided with alarms when pollution events take place.
“The OTT ecoN has proven extremely accurate and reliable in the field, and has been priced for extensive deployment, so we are very excited with the prospects for this technology.”
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IoT-ready flow measurement for the water industry
Nivus offers the NivuFlow devices including a mobile phone modem with worldwide approval. The manufacturer’s IoT concept allows the operators to transmit data automatically in combination with many options for visualisation and logging. Readings and system data as well as error messages or alarms can be transmitted directly via mobile networks. Due to this extension, Nivus are able to provide faster service for devices. Remote maintenance, remote diagnostics and remote commissioning open new possibilities to reduce time- consuming journeys for service works to a minimum.
By using Nivus Connectivity, the Multiroam-SIM-Chip automatically uses the most powerful mobile network available. A benefit for operators is predictable costs since providers do not invoice roaming fees here. Data is provided on the Nivus WebPortal and can be transmitted to other systems if required. The data transmission complies with the IT security standards by DWA, DVGW and BSI.
NivuFlow units are compact flow measurement devices for all areas of applications in the water and wastewater sector. With NivuFlow measurements can be carried out even in Ex areas. For these purposes, the manufacturer provides versions based on the ultrasonic transit time principle, the patented ultrasonic cross correlation method as well as on radar technology. A wide range of sensors for contactless metering as well as for measurements in the medium complete the measurement systems. Accessories that are adjusted to the systems allow for quick and reliable installation.
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