05 David Claridge 2019 - SWIG

05/14/2019

Products Solutions Services

Turbidity Measurement

Theory, technologies and practiceDavid Claridge – Endress+Hauser Ltd

Slide 1 David Claridge & Danny O'Connor

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What is turbidity?


Turbidity is an optical property of water used to gauge how opaque, dirty, murky or cloudy a water

sample is or isn’t.

Turbidity occurs when light is scattered at heterogeneous surfaces of particles.

Typically caused by particles suspended (not dissolved) in the water that can not be detected individually by the naked eye.

These particles can be items such as algae & dirt (organic material), minerals, proteins, oils or even bacteria.


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• As such, a variety of turbidity measurement units are used, depending on the method or standard being

referenced.

• Turbidity correlates with Total Suspended Solids (TSS) - but it is not the same measurement.

What is turbidity?

• Turbidity is a qualitative, rather than quantitative, measurement and has no inherent value.


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Simple measurement techniques:


• Secchi Disk:

• Secchi tube:


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Optical measurement techniques:

• Turbidity occurs when light is scattered at heterogeneous surfaces of particles.

Same basic principle for lab/hand held devices, falling stream devices and insertion sensors.


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Particle-size and its influence on turbidity

Scattering of light at spherical particles

n, k

Iinc: initial LightIsc: scattered light: scattering angle: wavelengthd: diameter particlen: refractive index particlek: grainsize particle


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Particle-size and its influence on turbidity

• Scattering of light at spherical particles with different size and shape

Symmetric • Smaller than 1/10th the wavelength of light

Scattering concentrated in Forward direction • Approximately 1/4 the wavelength of light

Extreme concentration of scattering in Forward direction • Larger than the wavelength of light


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Impact of manufacturing tolerances:


• Consequence:Measured result is not only defined by properties of the particle, but also by properties of the measurement set-up. Therefore different methods of measurement can display different results for the same sample

• Best case:• Parallel light, only a point as contact surface

• Real case:• No real parallel light, opening angle < 0


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Measurement standard for turbidity– ISO 7027:


• This standard provides a reference to which systems should be manufactured in terms of the following:

• Monochromatic light source – 860nm +/- 30nm (red – compensates for colour)• Primary detector angle – 90o +/- 2.5o – Nephelometric method

• Acceptance angle 20 – 30o

• Allowance for additional detectors within prescribed limits


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Reference:


• Remember – qualitative not quantitative!

• Well defined reference material is needed for• Comparison of results• Comparison of instruments• Adjustment in production• Calibration in the field

• Formazine produced in accordance with ISO 7027 – reproducible anywhere but unstable and not traceable (inherent inaccuracies in production).

• A nephelometric measurement device will measure in NTU’s –Nephelometric Turbidity Units

• The calibrated output is measured in FNU’s – Formazine Nephelometric Units


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Reference:

• 1 NTU = 1 FNU = 1 FTU = 1 TU/F

Other standards are available for sludge/solids applications (e.g. Kaolin, Diatomite, SiO2, TiO2, and various sludges)


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Ongoing calibration/verification:



• Formazine: Unstable, difficult to prepare in the field, carcinogenic.

• Not suitable for site.

• Some form of verification/check to ensure sensor still working well

• Solid state block – various fixed outputs

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Interferences:


• Three main interferences:


• Gas bubbles/entrained air

• Incident light

• Build up

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Theory, technologies and practiceDanny O’Connor – United Utilities


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United Utilities

• During AMP6 United Utilities have deployed hundreds of new turbidity measurements.

• We have rolled out a large number across Water Treatment as part of a scheme to protect production.

• We have also rolled out a large number across Waste Water Treatment as part of a scheme to improve the companies Remote Monitoring & Control capabilities on de-manned sites.

• There have been a number of turbidity monitors installed across our Water Network.

• So if you can think of an application – we’ve done it!

• Whilst the vast majority have gone in without problem, any problems that have come up are around lack of application knowledge.



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Water Treatment – Example A

• Site: Edenhall WTW

• Project: DWI

• Vendor: E&H

• Un-Common Issues:

• Entrained air…

• Coating…

• Impact on Other Measurements…

• Changing behavior…



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Wastewater Treatment – Example B



• Site: Dukinfield WwTW

• Project: RMC

• Vendor: E&H


• Installation…

• Coating…

• Changing behavior…

• M Certs…

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• Site: WwTW

• Project: RMC

• Vendor: ATI / PPM

• Type: Pumped Solution


• Lack of water!

• Thanks to Mark Gibson at NWL and PPM!

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• Site: WwTW

• Project: RMC

• Type: Pumped Solution

• Thanks to Mark Gibson at NWL and PPM!

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Others….

• Whilst we have worked to standardize against the various applications, invariably when deploying at scale some applications slip through the net. We set a success criteria of 80% - we think we hit >95%.

• When looking at Whole Life Cost, the ability to standardize the same look/feel has delivered large scale cost reductions from cradle to grave.

• Ensuring the right process conditions is critical but not always possible within the constraints of a large Project.

• Be prepared for some failures – have three back up plans.

• Utilise your supply chain.

• Put your trust in the appropriate place.

• Whilst constrained by Kit Frameworks we have used other suppliers to try and resolve really challenging applications: Thanks to PPM, Swan and PI!

• Talk and learn from each other.



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Theory, technologies and practice – any questions?


05 David Claridge 2019 - SWIG

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