WATER QUALITY SENSOR
Water quality sensors provide information on the composition of the
water and/or the water’s toxicity. The sensors are typically installed
in the raw water, at the treatment plant and in the distribution
system. When sensors are installed at the critical points and inlets
of the distribution system, they show full visibility of individual pressure
zones and, when combined with hydraulic models, allow tracing of water
quality and identification of sources of contamination. Furthermore, water quality sensors can be used to evaluate the impact of mixing various water sources and management of the water quality in the network.
Many types of water quality sensors are available. For use in smart water networks, it is important that the instruments are compatible with installation and operation in a water distribution environment. Sensor placement tools (i.e. USEPA model - Threat Ensemble Vulnerability Assessment – Sensor Placement Optimization Tool) can be used to find the optimal location for sensors in the distribution network.
Water quality sensors can be divided into the following types:
Single Online Sensors – Generic
Single online sensors (generic) are able to detect a wide range of water contamination with a single sensor. Water quality parameters include pH, conductivity, and refractive index. Having a single sensor to detect the full spectrum of (chemical) contamination would be ideal, as it would make it economically viable to deploy a higher density of sensors into the water distribution network.
Single Online Sensors – Specific
Single online sensors (specific) measure specific process parameters, which may also be of interest for event detection (e.g. chlorine, total organic carbon, conductivity). These sensors are also used to monitor indicator parameters for substances with shared chemical or physical properties (e.g. Total Organic Carbon, pH, etc.). However, they provide limited coverage of the contamination spectrum and can therefore miss critical incidents. They are typically only used in the treatment process and at a limited number of strategic locations.
Multi Parameter Arrays
Multi-parameter arrays use a combination of sensors, each sensor covering part of the contamination spectrum. This system can take the form of a single multi-parameter probe, or of a water quality monitoring panel onto which a group of individual instruments is mounted. In some cases, high costs limit application to strategic locations, but newer and more affordable technologies are being brought to market.
Effect Based Monitors – Generic
Effect based monitoring (generic) use "biomonitors" or live organisms as a sensing element. These systems detect changes in organism metabolism or behaviour as a response to the presence of substances toxic to the test organism. Effect-based monitors are highly sensitive but very expensive to install and maintain, and therefore are usually limited to strategic locations. These are typically placed in raw and treated water at water treatment facilities where staff are present and can easily service the equipment.
Fingerprint sensors are instruments that measure spectral information (e.g. UV/Vis spectroscopy, electrochemical fingerprinting, electronic noise, etc.) and use advanced signal processing to deduce water quality information from multidimensional raw data. These sensors are expensive and therefore typically deployed only at a limited number of strategic locations.
Water Security-Related Evaluations of Sensors Combinations
Work done by the US EPA Water Security Division has demonstrated that combining the results of TOC, chlorine residual, and conductivity provides early identification of 10 of the 12 contaminant classes identified by the US EPA. Similar work in the Netherlands for non-chlorinated distribution systems has found that the combination of refractive index, conductivity and TOC is capable of detection of all contaminant classes.
Benefits of using online water quality sensors include:
(Near) real-time detection of variations in water quality
Early warning of water quality changes
Continuous real-time water quality information that can be used as input for process control and network management/operations
Continuous real-time water quality information provides input for daily network operations and inspection / maintenance planning
Support for compliance monitoring and reporting
When combined with disparate data streams (e.g. Consumer Calls, Public Health Events, Security alarms) and spatially displayed, sensors enhance the decision-making power of the water quality information (EPA Water Security Initiative)
A typical water quality sensor set-up consists of a sensor, either a submersible probe or a flow-through panel and a controller, which manages the measurements and provides the human-machine display as well as communications options (e.g. analog and digital out, MODBUS/PROFIBUS, GSM/GPRS, 3G, 4G LTE, etc.)
For use in the distribution network, sensors need to be compact, robust and low maintenance. Furthermore, infrastructure requirement should be minimal (e.g. wireless data transmission is preferred). Most sensors require mains power as well as a sampling point and a drain for operations in a flow-through configuration. A few systems are available for in-pipe installation and/or battery powered operation. Typical maintenance requirements for current water quality sensors is approximately once per month. Frequency can be much lower, closer to once per month, for solid state sensors (e.g. optical sensor systems) that do not need consumables and have no aging components (e.g. selective membranes, electrolytes, etc.).
Sensors are offered both as integrated and stand-alone systems. Integrated systems are complete turnkey water quality sensor solutions. Vendors that sell these systems provide all the components including sensors, controller, communications unit, monitoring station and data processing software in one complete package. This has the advantage that the end-user only needs to provide basic infrastructure. Integrated systems interface with SCADA, data analysis software, Enterprise Resource Planning software etc., using standardised communications protocols. This provides the user with full flexibility as to how to collect, process and display the data from online water quality sensors.
Some vendors will allow for other vendor water quality sensors to be integrated into their data processing controller. This allows utilities to select the highest performing sensors and integrate them into a single package.
The stand-alone products can be used in any combination of components. This gives the utility or the system integrator the flexibility to develop a network that meets specific requirements. However, this requires more engineering effort on the part of the end-user for system design and integration. Stand-alone sensors are interfaced with commercial, off-the-shelf components such as PLCs, SCADA or wireless modems, either directly (e.g. smart digital sensors with all data processing on-board) or through a local controller.