1. Turbidity measurement
Introduction
Turbidity is the amount of cloudiness in the water.Turbidity is a principal physical
characteristic of water and is an expression of the optical property that causes light
to be scattered and absorbed by particles and molecules rather than transmitted in
straight lines through a water sample.
This can vary from a river full of mud and silt where it would be impossible to see
through the water (high turbidity), to a spring water which appears to be completely
clear (low turbidity).
Turbidity can be caused by following:
silt, sand and mud
bacteria and other germs
chemical precipitates
It is very important to measure the turbidity of domestic water supplies, as these
supplies often undergo some type of water treatment which can be affected by
turbidity. For example, during the rainy season when mud and silt are washed into
rivers and streams, high turbidity can quickly block filters and stop them from
workingeffectively. High turbidity will also fill tanks and pipes with mud and silt, and
can damage valves and taps.
Where chlorination of water is practised, even quite low turbidity will prevent the
chlorine killing the germs in the water efficiently.
Some treatment systems, such as sedimentors, coagulators and gravel prefilters are
designed to remove turbidity. It is important for operators of both large and small
treatment systems to know how well these systems are working.
Measuring the turbidity of the water before and after each part of the system can
tell the operator where maintenance or cleaning is needed.
Measuring turbidity
Turbidity can be measured using either
an electronic turbidity meter
or
a turbidity tube.
Turbidity is usually measured in
nephelometric turbidity units (NTU)
or
Jackson turbidity units (JTLJ),
depending on the method used for measurement. The two units are roughly equal.
2. Turbidimeters
An instrument for measuring the turbidity of water or other fluids is called turbidity
meter.
An instrument for measuring and comparing the turbidity of liquids by viewing light
through them and determining how much light is cut off.
Advantages
very accurate
useful for measuring very low turbidities (less than 5 TU)
Disadvantages
high cost
need power supply (mains or battery)
easily damaged
Types of Turbidimeters
There are three common types of turbidimeters employed today.
Bench top
Bench top units are typically
used as stationary laboratory
instruments and are not
intended to be portable.
Portable
Bench top and portable
turbidimeters are used to
analyze grab samples.
On-line instruments
On-line instruments are
typically installed in the field
and continuously analyze a
sample stream spilt off from a
unit process.
3. Nephelometric principle of turbidity measurement
Generally most of the Portable and Bench type Turbidimeter operates on the
nephelometric principle of turbidity measurement.
The optical system includes a tungsten-filament lamp, a 90° detector to monitor
scattered light and a transmitted light detector. The instrument’s microprocessor
calculates the ratio of the signals from the 90° and transmitted light detectors. This
ratio technique corrects for interferences from color and/or light absorbing materials
(such as activated carbon) and compensates for fluctuations in lamp intensity,
providing long-term calibration stability.
Measuring principles of turbidimeter
The turbidimeter uses following methods to measure turbidity.
Surface light scattering method
Light scattering/transmission method
Light transmission method
Integrating sphere method
1) Surface light scattering method
The sample is sent to the measurement chamber, and light is radiated to the
surface of the liquid being measured. The light scattered from that surface is
used to measure the turbidity.
4. The intensity of the scattered light is in proportion to the concentration of
suspended substances in the sample. This relation is used to obtain the
turbidity. Since this method requires no window exposed to the sample to be
measured, there is no error which would result from a contaminated window.
2) Light scattering/transmission method
Light is radiated to the sample cell. The scattered light resulting from the
suspended substances in the sample and the light which passes through the
sample are measured. The difference between the two values is in proportion
to the concentration of suspended substances in the sample. Using this
relation, the turbidity is obtained.
In this method, the ratio between the scattered light and the transmitted light
is obtained. Therefore, the effects of fluctuations of the power source,
deterioration of the lamp, and the color of the sample are minimized as they
offset each other. Thus, stable measurements of the sample can be obtained.
5. 3) Light scattering method
In the Light scattering method, light is radiated to the sample cell. The
radiated light is attenuated by the suspended substances in the sample. The
transmitted light attenuates according to the amount of the suspended
substances, and is used to measure the turbidity.
4) Integrating sphere method
In the integrating sphere method, parallel beams are radiated to the cell. The
light scattered with the suspended substances is focused with the integrating
sphere. The ratio between the scattered light and the transmitted light is used
to measure the turbidity.
Applications of turbidimeters as Analyzers and Monitors
Turbidity meters monitor and in some cases analyze the condition of the liquid
in the process. The advanced systems available today transmit this data in
real time for true real-time automated process control, and offer an immediate
capability to react to changing process conditions.
Reliable, repeatable turbidity analyzers can then be further utilized to baseline
process parameters and system performance across multiple process lines,
even across multiple processing facilities. Well-placed turbidity measurement
techniques will not only respond to changing process conditions, but can
provide useful information to assist in troubleshooting any turbidity related
process issues.
Turbidity meter applications include turbidity detection as in a solids break-
out, or a carry-over of an undesirable constituent, separation and filtration
monitoring and control, solids concentration, for dosing or biomass growth,
interface between product phases, water quality, wastewater BOD, blending
control of cloudifier agents, haze control, concentration, clarity and stability
assessments.
Precautionary measures
Measurement with these units requires strict adherence to the manufacturer’s
sampling procedure to reduce errors from dirty glassware, air bubbles in the
sample, and particle settling.
Power supply should be ensured.
Proper lightening should be provided.
Light source should neccessitate proper straight beam.
Lens should be stationary.
