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An old saying goes: ‘If you don’t monitor it; you can’t manage it’, and this has never been truer than when applied to the management of Water.
States, industry sectors and individual processes all have water quality objectives and monitoring is essential in order to be able to measure the effects of improvement initiatives and to demonstrate compliance with regulations. Historically, water quality monitoring has relied on sampling or spot checking, however, the trend has been for process operators to move towards continuous monitoring.
This, in turn, has created a need for sensors to be developed that are more robust and reliable and require less maintenance and ongoing calibration. At the same time, data collection and communication technologies have advanced considerably and it is now possible to view real- time water quality data at any time from anywhere. This article will outline some of the latest sensor technologies and examine the ways in which they are now able to resolve the problems traditionally associated with continuous monitoring.
An important water quality indicator for almost any use is the presence of dispersed, suspended solids particles not in true solution and often including silt, clay, algae and other microorganisms, organic matter and other minute particles. The extent to which suspended solids can be tolerated varies widely, as do the levels at which they exist. Industrial cooling water, for example, can tolerate relatively high levels of suspended solids without significant problems.
In modern high pressure boilers, however, water must be virtually free of all impurities. Solids in drinking water can support growth of harmful microorganisms and reduce effectiveness of chlorination, resulting in health hazards. In almost all water supplies, high levels of suspended matter are unacceptable for aesthetic reasons and can interfere with chemical and biological tests.
Suspended solids obstruct the transmittance of light through a water sample and impart a qualitative characteristic, known as turbidity, to water. Turbidity can be interpreted as a measure of the relative clarity of water. Turbidity is not a direct measure of suspended particles in water but, instead, a measure of the scattering effect such particles have on light.
Theory of light scattering
Very simply, the optical property expressed as turbidity is the interaction between light and suspended particles in water. A directed beam of light remains relatively undisturbed when transmitted through absolutely pure water, but even the molecules in a pure fluid will scatter light to a certain degree. Therefore, no solution will have a zero turbidity. In samples containing suspended solids, the manner in which the sample interferes with light transmittance is related to the size, shape and composition of the particles in the solution and to the wavelength (colour) of the incident light.
A minute particle interacts with incident light by absorbing the light energy and then, as if a point light source itself, re-radiating the light energy in all directions. This omnidirectional re-radiation constitutes the “scattering” of the incident light. The spatial distribution of scattered light depends on the ratio of particle size to wavelength of incident light. Particles much smaller than the wavelength of incident light exhibit a fairly symmetrical scattering distribution with approximately equal amounts of light scattered both forward and backward. As particle sizes increase in relation to wavelength, light scattered from different points of the sample particle create interference patterns that are additive in the forward direction.
This constructive interference results in forward-scattered light of a higher intensity than light scattered in other directions. In addition, smaller particles scatter shorter (blue) wavelengths more intensely while having little effect on longer (red) wavelengths. Conversely, larger particles scatter long wavelengths more readily than they scatter short wavelengths of light.
Particle shape and refractive index also affect scatter distribution and intensity. Spherical particles exhibit a larger forward-to-back scatter ratio than coiled or rodshaped particles. The refractive index of a particle is a measure of how it redirects light passing through it from another medium such as the suspending fluid. The particle’s refractive index must be different than the refractive index of the sample fluid in order for scattering to occur. As the difference between the refractive indices of suspended particle and suspending fluid increases, scattering becomes more intense.
The colour of suspended solids and sample fluid are significant in scattered-light detection. A coloured substance absorbs light energy in certain bands of the visible spectrum, changing the character of both transmitted light and scattered light and preventing a certain portion of the scattered light from reaching the detection system.
Light scattering intensifies as particle concentration increases. But as scattered light strikes more and more particles, multiple scattering occurs and absorption of light increases. When particulate concentration exceeds a certain point, detectable levels of both scattered and transmitted light drop rapidly, marking the upper limit of measurable turbidity. Decreasing the path length of light through the sample reduces the number of particles between the light source and the light detector and extends the upper limit of turbidity measurement.
Modern instruments
Today, many methods exist for the determination of water contaminants, yet turbidity measurement is still important because it is a simple and undeniable indicator of water quality change. A sudden change in turbidity may indicate an additional pollution source (biological, organic or inorganic) or may signal a problem in the water treatment process. Modern instruments are required to measure both extremely high and extremely low turbidity levels over an extreme range of sample particulate sizes and composition. An instrument’s capability to measure a wide turbidity range is dependent on the instrument’s design.
Dissolved oxygen
Dissolved oxygen (DO) is one of the most important of the dissolved gases found in water. In natural waters, DO exists in a dynamic equilibrium controlled by biochemical depletion and oxygenation through atmospheric diffusion, aeration, and photosynthesis. As a result, bacterial populations proliferate and provide a key input up the food chain. However, dissolved oxygen is subject to detrimental fluxes when a catastrophic event occurs such as the discharge of organic waste into natural waters. Depending on the severity of the insult, DO may be depleted to the point where higher trophic organisms such as macro invertebrates and fish are killed off.
In wastewater treatment, organic-based sewage is degraded under controlled aerobic conditions. Failure to maintain adequate supplies of DO result in anaerobic conditions that lead to offensive and corrosive sulfides. Excessive aeration, on the other hand, is wasteful and drives up unnecessary operational costs. Prudent monitoring of DO is essential for assessing environmental risk in natural waters and for optimal wastewater treatment performance and regulatory compliance. As a result, precision and accuracy of the DO measurement becomes a critical issue of interest – not only for estimating the degree of water quality or purification, but in calculating industry discharge loading costs from public owned treatment work facilities.
Example of application – Eurotunnel
Natural groundwater collected around the tunnels operated by Eurotunnel is continuously monitored before being pumped to the coast and discharged into the sea. Six multiparameter water quality monitoring systems ensure that there is no potential for harmful water to enter the pipelines. Each monitoring system is connected to a sophisticated data collection and alarm system that is able to divert water into vast underground sumps if alarm conditions occur. To-date, no such emergency has taken place.
The Channel Tunnel is 50km long, with the 39km undersea section making it the longest undersea tunnel in the world. The Eurotunnel system actually consists of three separate tunnels: two rail tunnels through which the trains travel, and a central service tunnel. This “safe haven” is used for maintenance and evacuation, and is linked to the rail tunnels every 375 metres. On average, the tunnels lie 40 metres below the seabed of the English Channel.
The water that seeps down to the tunnels is a mixture of groundwater and seawater. It is collected at six drainage stations and is continuously monitored. The main purpose of the monitoring system is to protect the enormous pumps (capable of almost 1000 m 3 /hr) and pipes from corrosive attack. It also serves to ensure that water discharged to the sea is not harmful to the environment. The early versions of the monitoring system suffered from a number of problems that largely resulted from blockages in the small pipes that passed water to the sensors.
Water quality monitoring
A flow-through holding tank resolves potential problems with blockages, large bore pipes are employed and sediment can be removed easily. In addition, the latest sensor technology means that the requirement for recalibration is much lower.
Each of the six flow-through tanks contain sensors for conductivity, turbidity, dissolved oxygen, pH, Redox and temperature and data is transferred to a PLC that is programmed to raise alarms when pre-specified conditions occur. If an alarm is raised all water is immediately passed to an underground storage sump and remains in quarantine until tested and passed as fit to be allowed into the pipeline. The monitoring system returns to normal once water quality levels leave the alarm condition. Any quarantined water can then be removed by bowsers.
In the early years, water was passed though a wastewater treatment works near Dover, however, the water quality was found to be consistently of good quality so the treatment works was decommissioned and water is now passed directly to the sea under a discharge consent from the Environment Agency. Naturally, the monitoring system prevents the discharge of any water outside the consent conditions.
Superior monitoring and controling system
The water quality monitoring sensors are connected to controllers which are ‘plug and play’ – once the serial number is input into the controller it starts to monitor correctly automatically. The reliability of the system means that false alarms are no longer experienced, which is a major benefit; there are strict procedures in place before a mechanical and electrical team can enter the tunnel to investigate an alarm, coupled with the amount of time it takes to drive to the monitoring equipment, false alarms can be very costly. It is estimated that this system requires a quarter of the maintenance that was previously necessary, which saves a great deal of time and money.
One of the reasons for this is the new dissolved oxygen sensor, the LDO TM (Luminescence Dissolved Oxygen), which employs an optical monitoring technology that does not require recalibration – the only requirement is simply to change the sensor cap every year. In order to prevent sensor fouling, the sensor is fitted with a compressed air system that automatically cleans the sensor heads.
For more than fifty years galvanic and polarographic sensors have been used to measure dissolved oxygen. These sensors employ membranes, anodes, cathodes, and electrolyte solutions that generally require a high degree of maintenance. The sensors also suffer from drift, and as a result have to be recalibrated frequently.
Historically, there have been a number of problems associated with galvanic and polarographic sensors. The membranes are relatively delicate, and can become contaminated or damaged, in which case it would be necessary to replace the internal electrolyte. The sensor’s anode is consumed over a period of time and will require replacement, or it may need replacement if it, or the electrolyte, becomes poisoned by gases such as hydrogen sulphide.
There are other factors that can affect the accuracy of these traditional sensors, including variations in pH or the presence of chemicals that induce voltage, such as iron and aluminium salts, and polymers.
The LDO TM sensor is coated with a luminescent material, called luminophore, which is excited by blue light from an internal LED. As the luminescent material relaxes it emits red light, and this luminescence is proportional to the dissolved oxygen present. The luminescence is measured both in terms of its maximum intensity and its decay time. An internal red LED provides a reference measurement before every reading to ensure that the sensor’s accuracy is maintained.
Sensors
Turbidity is measured with a ‘Solitax’ sensor which uses a dual-beam infrared scattered light photometer and receptors to monitor water quality. The instrument is available as a turbidity-only analyser, and as an analyser that can measure both turbidity and suspended solids using an additional sensor photoreceptor.
An LED (light-emitting diode) light source in the analyser’s sensor transmits a beam of infrared light into the sample stream at an angle of 45° to the sensor face and a pair of photoreceptors in the sensor face detect light scattered at 90° to the transmitted beam. In models that measure suspended solids, a back-scatter photoreceptor positioned at 140° to the transmitted beam detects light scattered in high-solids sample streams.
Conductivity is measured by an inductive (electrodeless) sensor capable of measuring from zero to 20,000 mS.
pH is measured with a ‘pHD’ sensor employing the Differential Electrode Measurement Technique. This field-proven technique uses three electrodes instead of the two normally used in conventional pH sensors. Process and reference electrodes measure the pH differentially with respect to a third ground electrode. The end result is unsurpassed measurement accuracy, reduced reference junction problems, and elimination of sensor ground loops. These sensors provide greater reliability, resulting in less downtime and maintenance.
ORP is measured by the same sensor. These combination sensors are designed for specialty applications for immersion or in-line mounting. The body is molded from chemically-resistant Ryton® or PVDF, and the reference junction is coaxial porous Teflon®. All sensors are rated 0 to 105oC up to 100 psig, and have integral 4.5 m (15 ft.) cables with tinned leads. The PC-series (for pH) and RCseries (for ORP) combination sensors are ideal for measuring both mild and aggressive media.
References
• Turbidity Science Technical Information Series – Booklet No. 11 Michael J. Sadar, Hach Company©
• Report on the Validation of Proposed Method 360.3 (Luminescence) for the Measurement of Dissolved Oxygen in Water and Wastewater, Hach Company©
Published: 01st Mar 2007 in AWE International
