By Nikolai Pitchforth, Research Analyst, Environmental Health and Safety, U.S., Frost & Sullivan
Turbidity measurement science has come a long way since 1865 when astrophysicist and Jesuit priest Angelo Secchi dipped a white disk into the Mediterranean Sea, in an effort to determine its transparency. Secchi disks are still used widely today, but it was an event in 1900 that led to modern turbidimetry; the development by Whipple and Jackson of the first turbidity standard, from distilled water and diatomaceous earth. Described simply, turbidity is a measure of the relative clarity - or cloudiness - of a liquid, compared to a predetermined standard. It is measured in a variety of ways, but each method relies on the light scattering, light reflecting or light absorbing qualities of suspended matter in a sample.
Turbidity measurements provide no information about the nature of particulate in
a sample, they simply allow us to evaluate the effect it has on light passed
through the sample and compare this to a known standard. In water treatment
applications this comparison provides a useful indication of overall water
quality and is used for such purposes as monitoring filter effectiveness.
Early last century, Jackson applied the standard he developed with Whipple to
create the Jackson Candle Turbidimeter, a glass tube that was positioned above a
candle. The user would add a turbid sample to the tube until the image of the
flame below diffused to a soft glow and the depth of the sample in the tube
would correspond to a standardized turbidity scale. The measurement was referred
to in terms of Jackson Turbidity Units (JTU). Jackson's standards were difficult
to formulate with consistency though and turbidity measurement remained
imprecise until 1926 when Kingsbury and Clark developed formazin.
Formazin, formed by dissolving precise quantities of hydrazine sulfate and
hexamethylenetetramine in distilled water, provided a standard so reliable that
it is still used today as the universally accepted standard for turbidity
measurement. Even with the advent of formazin however, the Jackson candle
turbidimeter remained problematic because it relied firstly upon an imprecise
light source and secondly on human judgment. Improved light sources were
introduced but these did not eliminate the possibility of erroneous human
judgment.
In response to this problem, photoelectric detectors were introduced as a more
accurate substitute for the human eye. While these were more reliable, they
produced inaccurate measurements from samples with very high or very low
turbidity since either too much light was scattered before reaching the
detector, or the change in light intensity across the sample was imperceptible.
The solution to this problem represented the next big advance in turbidity
science.
Rather than measuring the attenuation of light transmitted through a sample, the
detector could be situated at an angle to the incident light beam, permitting
measurement of light scattered by the particulate in the sample instead. This
measurement could then be used to calculate the sample's actual turbidity. A
detection angle of 90 was determined to provide the most accurate measurements
in most solutions and is the angle currently used for measurement of turbidity
in most water quality applications. Instruments that use this scattered light
technique are referred to as nephelometers or nephelometric turbidimeters.
Since the advent of nephelometry, most advances in water turbidity measurement
technology have been modifications of this technique for specific applications.
These have included variations on the scatter angle, an assortment of different
light detectors, and a variety of light sources as well as features to increase
ease of operation such as self-cleaning sensors, memory functions and networking
capabilities.
Despite the lack of incentives for technological innovation caused by the need
for end-users to comply with EPA or ISO prescribed methods for many
applications, nephelometry continues to advance. In the U.S., the EPA has recently
tightened turbidity limits for drinking water and has come under pressure to
reduce the limit further due to evidence pointing to a correlation between high
turbidity levels and the occurrence of harmful protozoa. This, along with
increasing interest in turbidity measurement from ultra-pure water users such as
semiconductor manufacturers, has resulted in increasing demand for instruments
capable of measuring low levels of turbidity. The demand has been met with
astounding effectiveness by instruments that utilize more consistent light
sources such as near infrared LED's, bubble traps to reduce error and multiple
light detectors to ensure measurement precision.
The most extreme manifestation of the growing demand for low-level instruments
is the recent introduction of an instrument with a laser light source by Hach
Company. Hach claims that this instrument is 150 times more sensitive than a
traditional turbidity meter, with a resolution of one-millionth of one NTU.
While some argue that such precision is unnecessary, the ability of this
instrument to detect solids that even a particle counter can miss may open up
entirely new applications for turbidity measurement in fields such as
electronics manufacturing and biotechnology.
The other trend evident in turbidity measurement technology is growing demand
for instruments capable of measuring over a very wide range. Such capabilities
are useful in applications where turbidity levels can be unpredictable and vary
greatly, such as some wastewater monitoring applications. While suspended solids
instruments are available to provide a quantitative measurement of high
particulate levels, they are time consuming and difficult to operate in
comparison to turbidity meters and readings from the two types of instruments
are not easily comparable. Demand therefore exists for an instrument capable of
measuring turbidity accurately over a range traditionally covered by suspended
solids meters, but this is technically difficult to achieve since large
particles or high levels of particulate disrupt measurement of the scattered
beam by absorbing a large proportion of the light.
One solution is ratiometric measurement, which utilizes detectors at several
different angles to the incident beam simultaneously. This eliminates much of
the error caused by the different light-scattering properties of different
particles and accounts for the absorption of light by including a transmitted
light detector to calculate light attenuation across the sample. Like laser
nephelometry, ratiometric measurement may encourage new applications for
turbidity measurement.
A hundred years ago Jackson would have been amazed to know what his candle
turbidimeter would become and how it would be used by the turn of the century.
Recent developments in turbidity science indicate that the heyday of its
measurement may not yet have arrived: modern instruments have created new
possibilities and will continue to improve the utility of turbidity measurement
throughout the new century.
For more information visit www.water.frost.com, or contact Kimberly Howard at 210.247.2488.
