Future of Water - Fall Issue | 11
T
urbidity is the measure of relative
clarity of a liquid. It is an optical
characteristic of water and is an
expression of the amount of light that is
scattered by material in the water when a
light is shined through the water sample.
The higher the intensity of scattered light,
the higher the turbidity. Material that causes
water to be turbid includes clay, silt, finely
divided inorganic and organic matter, algae, soluble colored organic
compounds, plankton and other microscopic organisms.
Turbidity makes water cloudy
or opaque. The picture shows
a hydrologist sampling highly
turbid water in Arizona. The
water collected in the bottle
will be used to find out the
turbidity, which is measured
by shining a light through
the water and is reported in nephelometric turbidity units
(
NTU). During periods of low flow (base flow), many rivers are
a clear green color, and turbidities are low, usually less than 10
NTU. During a rainstorm, particles from the surrounding land
are washed into the river making the water a muddy brown color,
indicating water that has higher turbidity values. Also, during
high flows, water velocities are faster and water volumes are
higher, which can more easily stir up and suspend material from
the systems lines, causing higher turbidities.
Turbidity and Water Quality
High concentrations of particulate matter affect light penetration
and productivity, also with increased sedimentation siltation
can occur, Particles also provide attachment places for other
pollutants, notably metals and bacteria. For this reason, turbidity
readings can be used as an indicator of potential pollution in a
water system.
Turbidity and Human Health
Excessive turbidity, or cloudiness, in drinking water is aesthetically
unappealing, and may also represent a health concern. Turbidity
can provide food and shelter for pathogens. If not removed,
turbidity can promote regrowth of pathogens in the distribution
system, leading to waterborne disease outbreaks, which have
caused significant cases of gastroenteritis throughout the United
States and the world. Although turbidity is not a direct indicator
of health risk, numerous studies show a strong relationship
between removal of turbidity and removal of protozoa. The
particles of turbidity provide “shelter” for microbes by reducing
their exposure to attack from disinfectants. Microbial attachment
to particulate material has been considered to aid in microbe
survival. Fortunately, traditional water treatment processes have
the ability to effectively remove turbidity when operated properly.
Measuring Turbidity
State-of-the-art turbidity meters
(
left)
are beginning to be installed in systems
to provide an instantaneous turbidity
reading. The picture below shows a close
up of the meter. The large tube is the
turbidity sensor; it reads turbidity in the
system by shining a light into the water
and reading how much light is reflected
back to the sensor. The smaller tube
contains a conductivity sensor to measure
electrical conductance of the water, which
is strongly influenced by dissolved solids
(
the two holes)
and a temperature gauge
(
the metal rod)
.
Turbidity
By Vernon Camp, Executive Director
feature