By Peter S. Cartwright, P.E.
The concern about adequate supplies of safe drinking water is one of the world’s most pressing issues today. Regulations drive the water treatment industry and as new, health-related contaminants are becoming identified and/or the maximum allowable levels of previously regulated contaminants are lowered, the emphasis on monitoring the performance of treatment technologies becomes significantly greater.
Today’s instrumentation technologies are designed to provide real-time performance validation of water purification and wastewater treatment systems. For applications where water streams are being treated for health-related contaminants, instrumentation is mandatory.
In general, it is possible to separate contaminants into two classes: chemical and physical. Chemical classifications include such parameters as conductivity, pH, ORP (oxidation reduction potential), TDS (total dissolved solids), specific ions, etc. Physical parameters include flow, pressure, temperature, etc.
The specific contaminants in the feed water supply and the removal requirement associated with each contaminant determine the chemical parameters. For example, the US EPA Long Term 2 Enhanced Water Treatment Rule mandates that the turbidity of the treated water be less than one nephelometric turbidity unit (NTU). As a result, on-line turbidity instrumentation with the appropriate sensitivity must be utilized.
It stands to reason that the smaller the treatment system, the less instrumentation is included. Large industrial and municipal installations can economically justify extensive instrumentation; typically, residential and small commercial systems cannot. It is even possible to install computer controls that both troubleshoot the treatment system and make corrective adjustments and/or tell the operator what has to be done.
Certain water sources have extremely high concentrations of contaminants, which require more aggressive treatment technologies, along with the appropriate instrumentation. Seawater desalination typically treats feedwater containing 30,000 to 50,000 mg/L TDS, mostly sodium chloride. Wastewater streams are notorious for the wide variation in types of contaminants, thus requiring an equally wide variety of instrumentation. Contaminants found in wastewater streams include oils and greases, suspended solids, organic solvents, heavy metals, biological contaminants, etc. The increasing activity in seawater desalination, combined with burgeoning emphasis on water reuse with the huge resources of wastewater, will create a greater demand for instrumentation capable of operating in these environments.
In understanding instrumentation, it is important to recognize the difference between monitors and controls. Monitors simply read physical and chemical parameters, while controllers both read the parameter and control some action. A good example is pH. A pH controller will read (monitor) the pH and, based on a preset level, will add caustic or acid to reach and maintain this level. Monitors may record readings; however, controllers perform an additional function such as chemical change, increase or decrease motor speed, open or close valving, etc.
Technologies/applications
Just as the applications of water treatment technologies are numerous and varied, so are the requirements for instrumentation. Each treatment technology is employed to remove a certain class of (or a specific) contaminant. Instrumentation is used to monitor and possibly control a technology; whereas different applications often address the same classes of contaminants, such as suspended solids, dissolved solids, etc. This means that the same parameters can often be monitored regardless of the application. On the other hand, certain applications require very specific monitoring technologies. An example is in the area of ultrapure water. Here, although the contaminant class is not unusual, the degree of instrumentation sensitivity is. Dissolved ions, for example, must be typically removed to 18.3 megohm resistivity, which amounts to approximately 0.028 mg/L TDS. This required very sensitive instrumentation.
The bottom line, therefore, is that different classes of contaminants require different instrumentation, but the degree of contaminant removal will dictate the sensitivity of that instrumentation, which may then require a completely different device. Table 1 is a summary of treatment technologies and their effectiveness on the classes of contaminants.
The specific kind of instrument and its degree of sophistication are a function of:
- contaminant to be removed
- degree of removal required
- size (cost) of treatment system
- regulatory requirements
To illustrate and underscore the huge variety of applications and the concomitant instruments utilized in each, the following examples are offered.
Pools and spas
Pools and spas present challenges to water treatment system designers and operators: while raw water characteristics must be addressed, the contamination contributed by the occupants must also be considered.
As with most applications where humans come into contact with water, two (often unrelated) issues impact the quality of the water: aesthetics (odor, eye and skin irritation, hair discoloration, appearance, etc.) and health (pathogenic microorganisms, dangerous organic compounds, certain heavy metals, etc.).
Most pool and spa treatment technologies are designed to treat both issues, with the various treatment technology choices listed below.
Treatment technologies:
- Filtration (suspended solids removal)
- The dominant filtration technologies are diatomaceous earth (DE), granular media (sand) and cartridge/bag.
- Disinfection
Obviously, pool and spa occupants themselves are the major sources of microbiological contamination; therefore, water disinfection is extremely important, with chlorination a usual requirement. In spite of its disadvantages (health-related trihalomethane formation, discomfort to occupants, etc.), chlorine provides a disinfectant residual. As a result, public pools and spas in the US are required to use chlorine as the primary disinfectant technology.
The addition of chlorine, either as sodium hypochlorite (liquid) or calcium hypochlorite (solid), will raise the pH of the water; the pH is lowered by the addition of sodium bisulfite (NaHSO3), gaseous carbon dioxide or hydrochloric acid. Large pools may use onsite chlorine (sodium hypochlorite) generators, utilizing electrodes and salt (NaCI) water. Other disinfectants used in pools and spas include bromine, copper ion, silver ion, ozone and ultraviolet irradiation.
Other factors
Pools and spas must be monitored for other factors (See Table 2)
Temperature
The normal water temperature for pools ranges from 76 — 86 °F (25 – 30°C). The temperature range for spas is 96 – 104°F (36 – 40°C). For every 18°F (10°C) increase in temperature, the rate of chemical reactions in the water doubles. As a result and because of the much smaller volume of spas, the chemical addition, treatment and monitoring requirements are much more critical.
pH
As water evaporates from both pool and spa surfaces, makeup is required. Makeup water can affect both the chemistry and temperature of pools and spas. Although a greater concentration of the most effective form of chlorine (HOC1) is present at pH levels below five, filtration is much more effective at higher pH levels. Therefore, the recommended pH range is 7.2 to 7.8.
TDS
For pools the ideal TDS range is 1,000-2,000 mg/L, with a minimum of 300 and maximum of 3,000 mg/L. TDS increases as a result of usage (occupancy) and the addition of treatment chemicals. As an example of the effect of chemical addition on TDS, for every pound of dry chemicals added to a 15,000-gallon pool, TDS will increase by 8 ppm; for a 500-gallon spa, TDS increase amounts to approximately 250 ppm.
strong>
Generally, turbidimeters are not required to monitor filtration efficacy – visual inspection is usually sufficient.
ORP meters or equipment to measure the specific disinfection chemistry are used and in larger installations, controllers to maintain the appropriate concentration of disinfectant are employed. pH monitors/controllers are also employed and, in some cases, substitute for ORP meters. Thermometers, obviously, measure the temperature and are usually incorporated into thermostats to control the temperature. TDS is usually only measured with any regularity in larger or commercial pools and spas.
Seawater Desalination
The vast majority of seawater desalination activities are designed to produce potable water. The specific quality requirements established by US EPA vary only slightly from those of other regions of the world. The single parameter which distinguishes seawater and brackish water from drinking water supplies is TDS or conductivity. Interestingly, the TDS of seawater is not uniform throughout the world; it can range from less than 30,000 mg/L (parts per million) to above 50,000 mg/L, depending upon the location. Table 3 provides examples of seawater TDS concentrations from various locations:
Although not clearly defined, brackish water supplies may have TDS concentrations as low as 5,000 mg/L. Drinking water requirements generally do not allow a TDS above 1,000 mg/L. These differences are important because conductivity measurements will not only measure the ability of a technology to meet the TDS requirements, but also, when used on the raw water supply, will aid in determining the optimum desalination technology system design required.
Seawater monitoring
To ensure that any seawater desalination system is operating properly, i.e., producing an acceptable quality and quantity of water, it is critical that all water streams be consistently monitored with high quality, reliable and accurate instrumentation. The most important parameter to monitor is TDS; however, other water quality parameters such as ORP and pH may be required, depending on the application and water source characteristics.
Conductivity
TDS can be correlated to conductivity; one company has conducted extensive research relating these two parameters, resulting in instrumentation and calibration solutions that have become the standard of the RO industry. A sodium chloride standard solution is available for calibration and the instruments are capable of directly reading TDS.
ORP
ORP is an important parameter in measuring the concentration of a disinfectant such as chlorine, bromine or ozone. The activity of the disinfectant is critical to the microbiological ‘safety’ of drinking water and is usually a regulatory requirement.
pH
A measurement of the acidity and alkalinity of water, pH is very useful in predicting the
corrosivity of water streams, which can cause severe problems in tank, piping, pumps etc.
It also is an important parameter with regard to predicting the potential for scale formation during the desalination process.
Aguaculture management
With the rapid depletion of native fish throughout the world, the concept of fish farming (aquaculture) has developed into a thriving industry. One of the crucial parameters to ensure healthy fish populations is the dissolved oxygen content of the water.
Monitoring dissolved oxygen with immersed instrumentation presents challenges with regard to accuracy over time. Microbial activity produces bioflims that require frequent cleaning of the instruments. Some devices now on the market have automatic cleaning capabilities.
Water security monitoring
With the acute threat of terrorism a global problem, security of our drinking water supplies is of primary concern. Monitoring within the water distribution system is a difficult proposition. The shear number and diversity of potential threat agents that could be utilized in an attack against the system makes monitoring on an individual agent basis an effort that is doomed to failure from the start. Even this does not take into account the large number and diversity of compounds that may accidentally find their way into our drinking water. To counter and detect this unprecedented number and types of compounds, what is needed is a broad-spectrum analyzer that can respond to any likely threat: even unknown or unanticipated events.
The difficult mission of detecting such a wide variety of potential threats is not the only challenge confronting a monitoring system for the distribution system. The environment that any such sensor would be exposed to is extremely harsh. Extreme variability in water conditions is routinely encountered in the pipes. Much of the existing water supply infrastructure is aged and in poor condition. This means corrosion and scaling that may cause the fouling of sensors that are not robust enough to operate under such conditions on an extended deployment timeframe. Biofilms may also form on exposed surfaces, leading to sensor failure over time. What is needed is an extremely rugged sensor that is capable of withstanding long-term deployment and has the ability to respond to all types of threat agents.
Rather than attempting to develop individual sensors to detect contaminants or classes of contaminants, one company has created a sensor suite of commonly available off-the-shelf water quality monitors such as pH, conductivity, turbidity, chlorine residual and total organic carbon (TOC) linked together in an intelligent network. One of the difficulties encountered when designing such a system is that the normal fluctuations in these parameters found within the water can be quite pronounced. The question then is whether it is possible to differentiate between the changes that are seen as a result of the introduction of a contaminant and those that are a result of everyday system perturbation.
In the system as it is designed, signals from five separate orthogonal measurements of water quality (pH, Conductivity, Turbidity, Chlorine Residual, TOC) are processed from a five-parameter measure into a single scalar trigger signal in an event monitor computer system that contains the algorithms. The signal then goes through the crucial proprietary baseline estimator. A deviation of the signal from the established baseline is then derived. Then a gain matrix is applied that weights the various parameters based on experimental data for a wide variety of possible threat agents. The magnitude of the deviation signal is then compared to a preset threshold level. If the signal exceeds the threshold, the trigger is activated.
The deviation vector that is derived from the trigger algorithm is then used for further classification of the cause of the trigger. The direction of the deviation vector relates to the agents characteristics. Seeing that this is the case, laboratory agent data can be used to build a threat agent library of deviation vectors. A deviation vector from the monitor can be compared to agent vectors in the threat agent library to see if there is a match within a tolerance. This system can be used to classify what caused the trigger event. This system can also be very useful in developing a heuristic system for classifying normal operational events that may be significant enough in magnitude to activate the trigger. When such an event occurs the profile for the vector causing it is stored in a plant library that is named and categorized by the system operator. When the event trigger is set off, the library search begins.
The agent library, given priority, is searched first. If a match is made, the agent is classified. If no match is found, the plant library is then searched and the event is identified if it matches one of the vectors in the plant library. If no match is found, the event is classified as an unknown and can be named if an investigation determines its cause. This is very significant because no profile for a given event need be present in the libraries for the system to trigger. This gives the system the unique ability to trigger on unknown threats. Also, the existence of the plant library with its heuristic ability to learn plant events results in a substantial and rapid decrease in unknown alarms over time. The developed system has been subjected to strenuous testing in both laboratory and field scenarios.
The use of intelligent algorithms with standard bulk parameter monitoring equipment allows for a robust system that is capable of triggering on and classifying a wide diversity of threat agents including unknown events.
The Strange Case of the Chlorine Spikes
In one field deployment scenario, the system was very quiescent and rarely came anywhere close to causing a trigger alarm to go off, except, that every night at around midnight, the chlorine level would spike dramatically and cause an alarm. This was deemed very strange and extensive troubleshooting of the instruments and power supply revealed no abnormal conditions that could be causing the problem.
After a thorough investigation, the night operator at the treatment plant was queried about the strange chlorine response. His reply was that of course the system’s chlorine level spikes every night at midnight, because that is when he super-chlorinates the system just like he had been told to do.
Several years ago, when there was a pipe rupture in the system that may have allowed contamination to seep in, the night operator had been told to super chlorinate the system. Unfortunately, the operator was new at the time and the instructions were not explicit that the super-chlorination should take place that night only. Therefore, the operator had continued to perform the operation every night for years, resulting in a huge unnecessary cost in chlorine. This situation was remedied and should result in substantial chemical cost savings in the future.
The lesson learned here is that no matter how automated we become, if humans are involved somewhere along the way, personal communication cannot be completely replaced by a computer.
Conclusion
In the continuing quest to lower operating costs by minimizing human labor, monitoring and controlling water treatment technologies and systems are becoming hugely important. New developments are being introduced all the time – some very useful and others not.
About the author
Peter S. Cartwright, President of Cartwright Consulting Co., Minneapolis, is a registered Professional Engineer in Minnesota. He has been in the water treatment industry since 1974, has authored almost 100 articles, presented over 125 lectures in conferences around the world and has been awarded three patents. Cartwright has chaired several WQA committees and task forces and has received the organization’s Award of Merit. A member of the WC&P Technical Review Committee since 1996, his expertise includes such high technology separation processes as reverse osmosis, ultrafiltration, microfiltration, electrodialysis, deionization, carbon adsorption, ozonation and distillation. He can be reached at (952) 854-4911; fax (952) 854-6964; email: [email protected], or website: www.cartwright-consulting.com.