Environmental Science & Engineering - www.esemag.com - June 2003
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Dissolved Oxygen technology is critical component in wastewater treatment
By Dr. Russell Young, Dr. John Woodward and Michael J. Rousey,
Hach Company
Dissolved Oxygen (DO) is one
of the most critical measurements
in a wastewater treatment
plant. Aerobic bacteria
require dissolved oxygen to live.
Historically, plant operators in
North America have used hand-held
DO analyzers to periodically check
aeration tank dissolved oxygen levels.
Hand-held DO instruments will only
provide a snapshot of the DO in an aeration
tank. They cannot detect when
more air is required to handle a heavier
biochemical oxygen demand (BOD)
that has the potential to kill the aerobic
bacteria, possibly causing the plant to
violate National Pollutant Discharge
Elimination System (NPDES) permit.
Hand-held DO instruments cannot
be used to control oxygen supply. For
instance, at low BOD levels typically
experienced during night-time hours,
the amount of dissolved oxygen
required to maintain proper bacterial
activity is less than that required during
peak daytime hours. If a hand-held
instrument is used, the reduction of air
supply must be initiated by an operator
on site if power savings are to be
attained. Similarly, during reduced
loads caused by storm water inflows or
colder temperatures, in the absence of
automatic DO measurement, too much
air can be used, wasting valuable energy
and causing sludge bulking problems.
With rising power costs and the
requirement that wastewater treatment
plants meet more stringent NPDES
permit requirements, hand-held DO
instruments coupled with human intervention,
have proven to be an ineffective control mechanism for the operation
of aeration basins. Proper control
of aeration tank DO requires the use of
continuous DO analyzers. Properly
maintained on-line DO analyzers, used
in conjunction with blowers using variable
frequency drives, and plant integrated
control systems (SCADA or
PLC), can reduce plant power costs by
up to 25%.
Two dissolved oxygen technologies
have been used in wastewater treatment
facilities for over 40 years; unfortunately
both have had many problems.
Galvanic technology consists of an
anode and cathode made from different
metals; both of these are immersed
in an electrolyte solution and covered
with an oxygen permeable membrane.
When these dissimilar metals are
immersed in the electrolyte a spontaneous
voltage occurs between the
anode and cathode. As oxygen permeates
the membrane, the cathode
reduces the oxygen. This then creates
a potential across the electrodes that is
equal to the amount of dissolved oxygen
in the system. The voltage is converted
from a milliamp current and
then converted in the instrument to an
mg/l or ppm reading. Some galvanic
systems do not use any membranes.
These systems have had problems
with low DO applications as well as
with wastewater chemicals such as ferrous
sulfate, ferric chloride and aluminum
sulfate. Any chemical that produces
an electrical charge can interfere
with effective readings.
The second dissolved oxygen system
that has been used is a
Poloragraphic DO sensor. This system
also uses an anode and cathode; however,
it does not create a spontaneous
voltage. Poloragraphic systems supply
constant electrical voltage across the
anode and cathode, and oxygen is
reduced as the current supplied to the
cathode is increased. The amount of
electricity required to reduce the oxygen
at the cathode is equivalent to the
DO present in the system.
These systems have had problems
due to time-intensive maintenance
problems caused by the cleaning of the
electrodes and the degree of stretch
manually applied to membranes used
with the older systems.
Today a new technology has been
developed that does not require an
anode, cathode or electrolyte. This
system greatly reduces maintenance
time and costs. The system is based on
luminescent technology. A sensor is
coated with a luminescent material;
blue light from an LED strikes the sensor
causing the luminescent material to
become exited. As the exited material
relaxes, a red light is emitted. This
light is measured by a photo detector.
As oxygen molecules come into contact
with the luminescent dye, its luminescence
is quenched, and the amount
of light given off by the sensor is
reduced. The higher the oxygen content
the less luminescence is observed.
What does this technological breakthrough
do for plant operations personnel?
Elimination of the anode,
cathode and electrolyte means that
there can be no poisoning of the sensors.
This now provides a means of
consistently measuring oxygen content
in an aeration basin so that it will not
be affected by the interferences that
cause traditional DO sensor technologies
to give erroneous readings.
The life of the sensors is considerably
longer than the life of traditional
sensor technologies. There are no
membranes or electrolyte solutions
that need replacement. In fact, the
only replacement part of the system is
an inexpensive sensor cap. The cap is
easily cleaned and is made of a plastic
material that will not break and is not
easily scratched. As this system is
self-calibrating, no time-consuming
calibrations are required. Frequent
cleanings are not required as the system
will produce accurate DO readings
even with inorganic build-up on the
sensor.
Unlike other luminescent DO technologies,
the HachLDO™ does not
require that the system be sent back to
the factory for calibration and sensor
replacement. The system will provide
consistent DO readings, and it is easy
to use and maintain. In conjunction
with the Hach SC100™ controller, this
technology represents a very efficient
method of operating and controlling
plant blowers and aeration systems.
Russell M. Young, Ph.D., Director of
Advanced Technology (Research),
Dr. John R. Woodward, Sensor
Technologist, and Mike Rousey,
Product Manager, Wastewater
Products, are with Hach Company.
For more information contact Hach
Company at 800-227-4224.
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