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Turbidity Measurement

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Optimize filtration with accurate turbidity measurements

Protect your filters and avoid costly mistakes by monitoring filter performance in municipal water treatment. Keep total output under control, optimizing the amount of water used for backwashing as well as the quality of water before disinfection. Having reliable turbidimeters at the outlet of each filter gives you confidence and peace of mind. Filter parameters include: Turbidity, Suspended Solids, Alkalinity, and Particle Counting.

Not only is monitoring turbidity effluent required for regulatory compliance, it helps to ensure the final product is safe for public consumption. In addition to meeting regulatory requirements, monitoring turbidity is also beneficial for optimizing filter performance, establishing filter backwash cycles, and detecting filter breakthrough.


What is Turbidity

Turbidity, a measure of cloudiness in liquids, has been recognized as a simple and basic indicator of water quality. It has been used for monitoring drinking water, including that produced by filtration for decades. Turbidity measurement involves the use of a light beam, with defined characteristics, to determine the semi-quantitative presence of particulate material present in the water or other fluid sample. The light beam is referred to as the incident light beam. Material present in the water causes the incident light beam to scatter and this scattered light is detected and quantified relative to a traceable calibration standard. The higher the quantity of the particulate material contained in a sample, the greater the scattering of the incident light beam and the higher the resulting turbidity.

Any particle within a sample that passes through a defined incident light source (often an incandescent lamp, light emitting diode (LED) or laser diode), can contribute to the overall turbidity in the sample. The goal of filtration is to eliminate particles from any given sample. When filtration systems are performing properly and monitored with a turbidimeter, turbidity of the effluent will be characterized by a low and stable measurement. Some turbidimeters become less effective on super-clean waters, where particle sizes and particle count levels are very low. For those turbidimeters that lack sensitivity at these low levels, turbidity changes that result from a filter breach can be so small that it becomes indistinguishable from the turbidity baseline noise of the instrument.

This baseline noise has several sources including the inherent instrument noise (electronic noise), instrument stray light, sample noise, and noise in the light source itself. These interferences are additive and they become the primary source of false positive turbidity responses and can adversely impact the instrument detection limit.


Ultra-High Turbidity Measurement

Ultra-high turbidity measurements are generally turbidity measurements where nephelometric light scatter can no longer be used to assess particle concentration in samples. In a sample with a measurement path length of 1-inch, nephelometric light-scatter signals begin to decrease at turbidities exceeding 2000 NTU. At this point, an increase in turbidity will result in a decrease in nephelometric signal.

In addition, color can be a major interference in ultra-high turbidity measurements. Because of the influence of sample color, the application of strict nephelometric turbidity has been limited, particularly in industrial processes that involve beverages, food products, cell cultures, and dispersed oil in water.

However, other measurements can be used to determine the turbidity of such samples. Three of these are transmitted, forward scatter, and back-scatter methods. Transmitted and forward-scatter signals are inversely proportional to increased turbidity and give good response to 4,000 NTU. Above 4,000 NTU (when using the standard 1-inch path), transmitted and forward-scatter signals are so low that instrument noise becomes a major interfering factor. On the other hand, back-scatter signals will increase proportionally with increases in turbidity. Back-scatter measurements have been determined to be highly effective at determining turbidity specifically in the range of 1,000 to 10,000 NTU (and higher). Below 1,000 NTU, back-scatter signal levels are very low, and instrument noise begins to interfere with the measurements. With a combination of detectors, turbidity can now be measured from ultra-low to very high levels.

This type of measurement is known as Ratio turbidimetry. The ratio turbidimeter’s optical configuration is the key to several performance characteristics. Among them are good stability, linearity, sensitivity, low stray light and color rejection. In a Ratio instrument, a large transmitted-light detector measures the light that passes through the sample. A neutral density filter attenuates the light incident on this detector and the combination is canted at 45 degrees to the incident light, so that reflections from the surface of the filter and detector do not enter the sample cell area. A forward-scatter detector measures the light scattered at 30 degrees from the transmitted direction. A detector at 90 degrees nominal to the forward direction measures light scattered from the sample normal to the incident beam. And a fourth, back-scatter detector measures the light scattered at 138 degrees nominal from the transmitted direction. This detector “sees” light scattered by very turbid samples when the other detectors no longer produce a linear signal. The signals from each of these detectors are then mathematically combined to calculate the turbidity of a sample.

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The use of ultra-high turbidity measurement has many applications. It is used in the monitoring of fat content in milk, paint resin constituents such as titanium dioxide, liquor solutions in pulp and paper processing mills, and ore slurries in milling operations.