Turbidity, as a measure of cloudiness or haze in water, has many useful applications for industrial processes, pharmaceutical manufacturing, environmental monitoring, and utility applications. Unlike general commercial applications, however, the use of turbidity readings in municipal drinking water treatment comes with unique demands and considerations related to regulatory compliance.
For the purposes of this article devoted to water treatment plant (WTP) applications, the primary focus is on meeting U.S. EPA drinking water standards based on the EPA turbidity provisions guidance document. Many of the following application considerations, principles of operation, and selection criteria, however, can also be relevant to other turbidimeter applications in and beyond WTP operations.
It is important to note that the EPA does not approve equipment, only methods, and some of those are for measuring turbidity. Since 1993, subsequent alternative methods submitted by manufacturers have been approved as being compatible with the original Method 180.1 and providing equal or better performance. Instruments approved as compatible to standards other than those set by Method 180.1 might produce slightly different results.
1. Application-Specific Designs
While all turbidimeters operate on a similar concept of measuring light interactions with a fluid, different designs are suited to different applications. Different designs are required for different applications because variations in the size, number, shape, and color of particles suspended in that fluid can affect the readings provided.
2. On-Line Vs. Laboratory Instruments
On-line instruments are the favored choice for EPA compliance monitoring of post-filtration water flow, because they provide continuous readings in real time to offer the quickest notification of changing trends or process upsets. Lab devices can be used for occasional grab samples and emergency backup.
Historically, WTP personnel have considered laboratory instrument readings to be more accurate than process equipment readings when they saw differences between the two units. In actuality, there are multiple factors about sample handling that can affect differences in laboratory instrument readings — settling of particles or introduction of air bubbles in the time it takes to transfer samples, other human-induced errors, the introduction of a glass vial, or simply differences in calibration, etc.
New nephelometers that standardize sensing technology to provide identical readings from both on-line and laboratory units and use the same exact calibrant can eliminate that confusion and instill greater confidence in all readings.
One of the most important advancements in the accuracy of turbidity measurement since publication of the original EPA Method 180.1 document in 1993 relates to light sources. Today, LED and laser diode light sources capable of providing far better performance have been approved as acceptable alternatives to the original tungsten light bulb specified by the original method. In fact, they provide a stable, more controllable light that avoids the changes in the power and geometry experienced with tungsten bulbs over their lifetime of operation. The enhanced performance made possible by these light sources has led to immense improvements in the stability, accuracy, and repeatability of turbidimeter readings. Instrumentation with built-in software that complements quality assurance and quality control requirements can improve operating integrity and boost end user confidence in turbidity readings.
4. Speed Of Response
Newer turbidimeter designs can detect changes in rising turbidity values in a matter of seconds, not minutes — going from zero to full-scale reading in as little as 5 seconds. In WTPs using membrane filtration, that speed of response can be critical to indicating potential membrane deterioration or catching actual breakthroughs immediately after they occur, allowing enough time to prevent the process from going out of compliance. Slow-response devices can mask short-duration turbidity events by averaging out skewed readings over a longer period. Speed of response also helps minimize backwash cycle times by detecting when the backwashed flow from filter media starts running clean.
5. Digital Vs. Analog
While any nephelometer that satisfies the EPA 180.1 Method requirements should provide satisfactory results, the advantages of digital nephelometers lend themselves to applications where data is automatically captured and managed by digital information systems. First, there is no danger of skewed data readings due to analog interference. Second, instant data access across a range of devices, made practical through cloud connectivity, provides an added degree of convenience and comfort for end users. Digital monitoring and recording also makes it easier to identify process upset conditions and then pinpoint and analyze the cause of problems in the process. Analog loops require additional periodic electronic verification and calibration verification not required by digital connections.
6. Ease Of Maintenance
Maintenance efforts mandated by the EPA or necessitated by frequent sensor cleaning due to surface fouling can vary drastically from one nephelometer design to another. Consider all aspects of routine calibration/verification and ancillary maintenance costs along with spare-part and contract-service availability.
7. Total Cost Of Ownership
The best instrument choice is the one that can provide the overall best results at the most efficient total cost — including unit purchase price; cost of consumables; labor and material costs for operation, calibration, and maintenance, etc. Take into account how much calibration standard will be required every three months — a liter, 100 ml, or 10 ml. Units that minimize and/or simplify maintenance, calibration, and verification procedures will be more cost-effective in providing consistently reliable results over the long run.
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Image credit: "Beaker," KP © 2009, used under an Attribution 2.0 Generic license: https://creativecommons.org/licensses/by/2.0/