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The Process
The most effective way to remove dissolved organics from wastewater
is to allow bacteria to consume them before the water leaves the
wastewater treatment plant.
Required for this process are food (the dissolved organics),
bacteria, and dissolved oxygen. This process is most frequently
carried out in a large "aeration" basin, where the
wastewater is mechanically aerated to provide the bacteria with
enough oxygen to support a high rate of consumption of the dissolved
organics. The bacteria are later removed from the water as suspended
solids. The simplified reaction can be represented as:
bacteria
Dissolved Organics + O2
--->
Insoluble Organics + CO2
The Problem
Water is aerated mechanically, either by churning it in air with
large impellers or by bubbling compressed air through it from a
level near the bottom of the basin. Controlling the amount of
aeration is important. If too little dissolved oxygen is available
to the bacteria, the process will slow; in an extreme case, the
bacteria will die from lack of oxygen, leading to "losing the
basin." Recovery of a lost basin is expensive and
time-consuming. If the aeration provides more oxygen than the
bacteria can use, the excess usually does not upset the process but
represents a significant unnecessary cost in the electrical energy
required to run the compressors or aerators. (In some wastewater
treatment plants, energy required for aeration is the largest line
item expense after salaries.)
Required is first an accurate measurement of the dissolved oxygen
concentration and then efficient control of the dissolved oxygen
concentration. The dissolved oxygen concentration that provides the
most efficient treatment depends on the specific process being used
and on the nature of the organics being consumed. Typical values are
in the range of 2 to 4 ppm—that is between one-fourth and
one-half of the air-saturation value.
Honeywell's Dissolved Oxygen Solution
Long-term accuracy depends on both the inherent stability of the
probe and analyzer and independence from variables that can
interfere with measurement. For instance, non-Honeywell dissolved
oxygen probes produce signals that depend on diffusion rate of
oxygen through the membrane. Variations in flow rate or cleanliness
produce variations in signal, which are erroneously interpreted as
variations in dissolved oxygen concentration. Furthermore, results
can be compromised as a probe ages because parts of the probe are
consumed during the measurement. Probes must be maintained by
changing internal electrolytes and etching internal electrodes. Or
the whole active portion of the probe must be replaced with some
regularity.
The patented Honeywell probe is unique in that the oxygen consumed
to make the measurement is balanced by oxygen produced within the
probe. The oxygen within the probe is in equilibrium with that in
the sample, so results are independent of flow rate and inert
fouling. Since no internal parts of the probe are consumed, the
probe is permanent. Eliminated is maintenance to replace electrodes,
electrolyte, or entire probes. The fact that no probe materials are
being consumed leads to great probe stability.
Automatic Cleaning and Calibration
Although the Honeywell probe accuracy is unaffected by inert
fouling, there are two conditions where probe cleaning may be
required. (These conditions affect all conventional dissolved oxygen
probes as well.) The first is where the fouling is so thick that the
response time of the probe becomes unacceptably long. The second is
where organic fouling is consuming oxygen before it reaches the
surface of the probe. A feature allowing automatic cleaning at
preconfigured times is included in the
7020 series analyzer.
Cleaning may be initiated with a frequency of every few minutes to
monthly. Functionally, relays within the analyzer are tripped,
allowing withdrawal of the probe from the sample, turning on a
cleaning spray, turning off the spray, and reinserting the probe
into the sample. Execution of automatic cleaning and calibration
requires the user to install a drive unit, a solenoid valve, and
mounting hardware.
Similarly, all probes drift with time. Although the Honeywell probe
is very stable, included in the analyzer is a feature that allows
withdrawal of the probe into air for automatic air calibration at
user-configured times. The sequence of calibrations and cleanings
are user-configurable.
One symptom of the need for cleaning is a low dissolved oxygen
reading. The 7020 series dissolved oxygen analyzer can be configured
to execute a cleaning cycle if the measured dissolved oxygen falls
below a user-selected value. An alarm can be configured to alert the
user if the cleaning fails to restore the dissolved oxygen to a
higher level. The alarm will indicate either a true decrease in
dissolved oxygen concentration or unsuccessful cleaning.
A dedicated calibration key may be configured to execute
calibration, cleaning, or both whenever it is pushed.
Variations in salinity affect the relationship between probe output
and dissolved oxygen concentration. The readings can be corrected
automatically by either entering a constant value or providing for
continuous variation in salinity using the signal from a
conductivity meter. While this condition is rarely observed within
the wastewater treatment plant itself, it can be present in
downstream monitoring of dissolved oxygen where the stream receiving
the effluent is brackish or tidal.
Control of Dissolved Oxygen
The 7020 Series analyzer can function as a controller with choices
of control, including On/Off or PID (proportional, integral,
derivative) control with CAT (current adjusting type) or DAT
(duration adjusting type or time-proportioned) outputs. For
instance, in simple On/Off control, one or more of the relays can be
dedicated to turn additional blowers or impellers on and off based on
dissolved oxygen concentration. If a continuously variable aerator
is being used, its output can be controlled with full PID function
based on dissolved oxygen concentration. A secondary input from a
flowmeter allows the control action to provide rapid response to
variations in process flow. Control based on remote setpoint is also
user-configurable. To facilitate control tuning, an active display
of a graph of the dissolved oxygen vs. time may be displayed.
Cascade Control
With a second analyzer, cascade control is available. In cascade
control, the output of a downstream controller is used to adjust the
setpoint of an upstream controller. To control the dissolved oxygen
of either the plant effluent or the receiving body of water, cascade
control may be useful. If the dissolved oxygen concentration in the
aeration basin is within the desired range, but the readings at
either the discharge point or farther downstream are approaching
some low limit, a cascade control action can automatically raise the
setpoint in the aeration basin to keep the downstream dissolved
oxygen concentration within compliance.
Probe Diagnostics
In addition to the low-dissolved-oxygen diagnostic associated with
cleaning, described above, additional tests identify such conditions
as AC noise on the probe signal and out-of-range dissolved oxygen
and temperature signals.
There is also a sophisticated diagnostic that allows operation under
unusual sample conditions. Under most conditions observed within a
wastewater treatment plant, the probe exhibits a current-voltage
characteristic compatible with factory settings. However, in some
industrial applications, particularly those in petroleum refineries,
active gases dissolved in the wastewater can cause probe
characteristics to change. The 7020 Series analyzer includes a
diagnostic feature that automatically varies the probe operating
voltage while displaying the probe output. At completion of the
test, an opportunity to change the operating voltage is provided.
Thus, even where significant gaseous contamination might otherwise
interfere with the response of the probe to dissolved oxygen, this
advanced feature allows the probe to operate successfully.
Analyzer Diagnostics
Extensive alarm summaries, alarm histories, and diagnostic
summaries are included within the software, greatly facilitating
documentation for operation and proving permit compliances.
Security
A fear of wastewater treatment plant supervisors is that operators
will change programmed operations. The 7020 Series analyzers’
security codes may be owner-configured to lock out many operations,
leaving the operator with the ability to execute routing operations
but not to interfere with configured control parameters.
Additional Applications
Dissolved oxygen is frequently monitored in at least two additional
locations in the plant, and each may benefit from 7020 series
features. The effluent from the plant is monitored to prove
compliance with regulatory agency requirements. While this water
should be very clean, its flow can vary, and the flow-independence
of the Honeywell probe proves particularly useful. Additional checks
may be required at one or more points downstream from the treatment
plant discharge.
Applications to Avoid
Not all wastewater treatment plants use energy-intensive aeration
such as impellers or blowers. Where biological treatment of
dissolved organics is by trickling filters or biological contactors
(bio discs), the control of aeration is not an important requirement.
However, the effluent and downstream monitoring requirements should
still be addressed.
There are rare applications where water can contain dissolved
sulfides. Continuous exposure to sulfides can interfere with the
Honeywell probe (as well as most other probes), and these
applications should be avoided.
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