NEW WATER TURBIDITY
MEASUREMENT TECHNOLOGY— The amount of insoluble matter present in drinking water is an
essential quality indicator. Silt, sand, bacteria, spores, and chemical precipitates all contribute to the cloudiness or turbidity of water.
Drinking water, which is highly turbid, can be unpalatable and unsafe. Consumption of even low concentrations of certain bacteria and other microorganisms can cause serious health effects. Consequently, an
accurate and sensitive measurement of turbidity is vital for ensuring that drinking water is free of these contaminants.
Public health and safety organizations throughout the world have recognized the importance of measuring drinking water quality through turbidity. The EU Drinking Water Directive identifi es turbidity as one of the nine fi xed monitoring parameters which must be measured for all water intended for human consumption. The US EPA requires turbidity monitoring for all produced drinking waters. The WHO recommends monitoring turbidity frequently and at multiple points throughout the treatment process. While regulatory limits vary across national borders, there is widespread agreement that reliable turbidity monitoring is an essential component of drinking water production.
Turbidity can be measured with online, benchtop, or fi eld instrumentation. Online measurement allows drinking water producers to constantly monitor their operations, ensuring that production is functioning properly. Laboratory benchtop instruments are frequently used for regulatory reporting purposes, and to verify process instrument results. Both instrument platforms should produce the same accurate results. In addition, the optimal process turbidity measurement should be fast. Fast response ensures a prompt re- sponse to potential fi lter breakthrough and other turbidity events.
Figure 1: 360° x 90° Measurement System
New Technology Hach®
has developed a new turbidity technology to address these
requirements. The TU5000 series turbidimeters utilize a 360° x 90° measurement system (see Figure 1) to provide the fastest and most accurate turbidimetric measurements possible. Rather than measuring a single 90° light beam refl ection, the new turbidimeters collect an array of 90° measurements from 360° around the sample cell. Collecting the refl ected light in the full circle allows signifi cantly increase signal-to-noise (S/N) ratio that lays the foundation for better precision of the turbidity measurement, especially at the low end of the measurement range.
At the same time, the TU5 series turbidimeters employ a small 10 mL measurement cell. This small cell reduces sample residence time for process analyzers. Lower residence time leads to a signifi cant decrease in event detection time which eliminates minutes of delayed response time. The measurement systems are the same for both the process and laboratory instruments. This design maximizes matching between the two instruments. Both process and laboratory turbidimeters also incorporate an optional RFID system to facilitate reliable sample tracking and data comparison.
Response Test
The TU5400 process turbidimeter was tested against the extremely sensitive FT660 laser nephelometer to measure the response time of both instruments to a turbidity spike as might be seen during a fi lter breakthrough event. The chart in Figure 2 illustrates the performance of these two process turbidimeters in this application.
An extremely accurate amount of Formazin standard was spiked into fi lter effl uent stream, which was fed to both instruments. Flow rates to both instruments were tightly controlled. The data logging intervals were set to 5 s.
The TU5400 reached maximum peak height within 28 s, and the FT660 gradually reached maximum peak height after 7:12 min. After each spike TU5400 also returned to baseline much quicker. The vastly minimized response time, 15 times faster, allows operators to respond to turbidity events such as fi lter breakthrough much earlier.
Conventional Filtration: Comparative Turbidimeter Performance – Spike Study
TU5400, NTU (SAVG=OFF) FT660, NTU (SAVG=30s)
TU5400: SAVG=30s (trendline)
Average Response Time, min T95
Ratio
Conventional Filtration, ~0.5NTU spike FT660sc 07:12
15
TU5400sc 00:28
1
Figure 2: TU5400 vs FT660 response to 0.5FNU Formazin spike
NTU
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