Turbidity is the clarity or haziness of water, which depends on the presence of suspended insoluble particles. Sometimes the particles are not even visible to the naked eye, but high turbidity might affect the appearance of the water. Measuring turbidity determines how much the passage of light is restricted by the suspended solid matter. In this article we will be examining the value of monitoring turbidity in industrial and beverage applications.
The cloudy causes
The causes of turbidity vary depending on the water source and treatment systems. Open sources, such as lakes, rivers and wells, might be turbid due to the presence of phytoplankton, which are oxygen-producing photosynthetic organisms, while soil and microorganisms contribute to turbidity levels, too. Water in these sources might also be turbid due to runoff storm water that brings with it agricultural, industrial and residential deposits.
In potable water, turbidity is frequently due to the presence of oxygen or excessive amounts of calcium, which are not harmful. However, it could also be caused by suspended matter, such as mud, silt, clay or sediment. Excessive turbidity, or cloudiness, in drinking water is not only aesthetically unappealing — it may also represent a health concern in certain situations. The consumer is not necessarily informed of the cause of a cloudy or colored appearance in the water that is coming out of the faucet, and is not always able to judge whether there is a likelihood of contamination. In such cases, the alarmed end user is likely to prefer boiling the water before drinking or cooking with it, or switching to bottled water until it runs clear again.
For these reasons, turbidity is commonly measured for environmental monitoring and in municipal drinking water systems. Following U.S. drinking water standards, water suppliers are required to measure turbidity of water from each filter. There are also low cost devices for checking grab samples. The U.S. has a treatment requirement to achieve 0.3 nephelometric turbidity units (NTU) in filtered surface water supplies to protect against Cryptosporidium entering the drinking water from surface sources. Groundwater sources are usually not required to filter unless they are under the influence of surface water. Inorganic turbidity, such as from groundwater or post precipitation of calcium carbonate, is usually not a concern.
By carefully implementing a smart system of high-precision water analyzers that include turbidity meters, accompanied by effective methods of chemical dosing and filtration as well as a sophisticated alert system, municipal water treatment systems are able to correct the water quality and make it safe for drinking and other uses. In some cases of particularly high turbidity registered in the water, a decision might be made to halt pumping from the river or lake source, not to treat water that does not meet minimal standards or to terminate treatment that is not cost effective.
Impact on industrial systems
In the case of industrial water treatment, although monitoring turbidity can be crucial, it might be overlooked. Industries use water-based processes in a variety of applications and for different needs. These processes may include filling, heating, cooling, processing, cleaning or rinsing. Industries frequently use water treatment to improve process efficiency and reduce operating costs and risks. High-precision water treatment is applied in those industries that handle quality-sensitive products that are either in contact with water, such as semiconductors, or contain water, such as food, beverages and pharmaceuticals.
In the food and beverage industry, for example, water used in production, bottling and cleaning processes is generally monitored for a range of parameters that include pH, total chlorine, free chlorine, conductivity, temperature and flow. However, applying a turbidity measurement standard to detect low turbidity in such processes is the most reliable indicator of insoluble and unwanted matter in the water, which cannot be detected by the other quality parameters, and also contributes to an improved maintenance schedule.
Measuring turbidity will indicate the presence of pathogens or particles from the pipes or tanks, which would most likely be the result of a fault in the process’ filtration system. Should this go undetected at any stage, bacteria might attach to the suspended particles and further contaminate the water. Free chlorine treatment might be sufficient to treat the bacteria at a given point and measured levels of free chlorine might show that the correct balance is in place, yet the levels of turbidity can still be over a prescribed limit and continue to present a risk for contamination further along the processing line. It is important to point out that an abundance of particles in the water can actually shield contaminants and thereby interfere with chlorine disinfection and balance in the treatment process. In the food and beverage industry in particular, just as with drinking water, higher turbidity levels — even in the low turbidity range — increase the risk of undetected contamination and ultimately pose a risk to public health.
As such, and particularly in industries that require low levels of free and total chlorine, it is recommended that monitoring and control systems that are set up to manage disinfection should be accompanied by turbidity monitors.
Accuracy on a budget
Manufacturers that rely on water for production and quality processes must invest in meeting the highest standards of water quality and precision. However, tight budgets require efficient and cost effective solutions to maintain those standards.
Monitoring turbidity in industrial applications often requires extremely low turbidity detection, or even zero (absence) of chlorine, which can only be achieved with a very high precision meter. A sensor that uses the multi-angle nephelometry turbidity measurement is a reliable form of monitoring low turbidity (0-1.0 NTU). A system that can also remove bubbles and clean the measurement path (the window that the light passes through for measurement) is a significant advantage for continuous precision and ongoing maintenance.
The World Health Organization (WHO) recommends "measuring the turbidity of the water before and after each part of the system," in both domestic and industrial water treatment, as this indicates where maintenance is needed. Upon detecting turbidity above the level programmed into the analyzer, which is based on the individual needs of the user, the monitor signals the system to shut down so that the source of the turbidity can be located and resolved.
When faced with budgetary considerations, industries that are not required to test for turbidity might opt to forgo the more expensive option of installing an analyzer for that purpose. That, however, is not likely to be the wisest compromise, nor is it necessary. Keeping a close eye on filtration systems and detecting those otherwise unseen contaminants that compromise quality and introduce health risks does not essentially mean installing separate turbidity meters, which incur high purchase, installation and maintenance costs. Instead, managers can improve precision while cutting costs and manpower by looking out for multiple-parameter solutions that have entered the market.
As suggested above, industrial operations that measure chlorine, as part of a disinfection process to clean the water or for other purposes, can achieve a much more reliable measurement by measuring turbidity at the same locations and at the same time.
For potable water being used in industrial and municipal processes, a measurement of chlorine and turbidity on the same water sample will facilitate faster and better decisions regarding the necessity of treating the water with chlorination or, if turbidity is above the permissible level (which varies in different countries), stopping the treatment process to service a filter. For water that is being treated before bottling, for example, measuring zero chlorine and turbidity on the same water sample can greatly reduce maintenance frequency and also serves as a better gatekeeper to guarantee the highest standard of water quality.
In general, excess turbidity in filtered water usually indicates that the filtration is not as efficient as it should be relative to the specification that is determined by the application.
The benefits of measuring turbidity levels at critical points in industrial water treatment systems simultaneously with other parameters should be considered both in the planning stage for a new facility and for an operational system that seeks to maintain its commitment to safer water. Selecting and installing equipment that can carry out the measurements accurately while also supporting cost efficiency is a sure way to support the most successful and precise treatment processes.
Stela Diamant is chief technical officer at Blue I Water Technologies, a global provider of water quality analyzers and controllers. Prior to joining Blue I, Stela served as director of advanced process control solutions for MKS Instruments Inc. Previous positions include technical staff member for Applied Materials Inc. and head of R&D process and applications for Sagitta, Ltd. Stela holds patents in plasma sources, optical components process and automated process control. She is a graduate of the Technion-Israel Institute of Technology with a PhD in Physical Chemistry. She can be contacted at [email protected].