By: John Volbeda
Wastewater treatment professionals understand the need to
continuously measure dissolved oxygen in aeration basins for improved process
control. There are two primary technology options available for continuous
dissolved oxygen measurement in aeration basins--bare- or open-electrode
sensors, and membrane sensors.
Both options are viable and offer plants specific benefits.
The key is for plants to select which sensor will work best for their
application and production workflow needs.
Maintenance challenges
Time-consuming and expensive maintenance requirements represent
the primary challenge plants face in dissolved oxygen measurement in aeration
basins. Aeration basins in wastewater treatment plants are dirty, high-coating
applications. As a result, they have proven to be one of the toughest and most
challenging measurement environments requiring extensive maintenance.
When a sensor in an aeration basin becomes coated, it is
rendered inaccurate and ineffective, so it's important that plants conduct
regular maintenance on dissolved oxygen sensors to avoid this problem. However,
this maintenance can become time-consuming and costly if the dissolved oxygen
instrumentation being used is not designed specifically to address the
application's maintenance needs to reduce the time and cost involved in regular
sensor upkeep.
Both of today's open-electrode and membrane dissolved oxygen
sensor technologies are designed specifically to meet the needs of this harsh
application environment and incorporate several features to reduce maintenance
requirements.
The primary benefits open-electrode dissolved oxygen systems
offer are a very long life, significantly reduced maintenance and low overall
cost of ownership. These sensors are ideal for low-flow or zero-flow
environments and those with substantial greases, fats and oils where membrane
sensors will be challenged. They also are the best choice for plants that find
maintenance schedules difficult to meet consistently due to, for example,
staffing challenges or because instruments in aeration basins are difficult to
physically access. These kinds of parameters can offset the relatively high
initial investment required by the open-electrode sensor over the membrane
sensor.
The electrodes in the open-electrode design are two
independently spring-loaded concentric rings that are insulated from each
other. Fresh sample is pumped to the electrode through an oscillating sample
chamber to protect the bare electrodes from exposure to air bubbles and
suspended solids in the liquid being treated. Even in low-flow or zero-flow
wastewater, this chamber ensures that sufficient sampling occurs.
The electrodes are subject to polluting substances in the
process liquid being treated, so a critical element in the design is a
self-cleaning feature. Open-electrode sensors incorporate a rotating diamond
grindstone that continuously polishes the electrode surfaces, cutting through
and cleaning off the material that would otherwise coat the sensor and render
it ineffective. This automatic self-cleaning capability can significantly
reduce maintenance time over membrane systems. Also, unlike membrane sensors,
open-electrode systems do not require plants to clean and replace membranes and
replenish the electrolyte solution, which can be time-consuming. In fact,
virtually the only maintenance requirement plant personnel must plan for is the
replacement of the rotating diamond grindstone every eight to 18 months, and
the replacement of the electrodes every three to five years.
Bare-electrode sensors provide the longest life of any
dissolved oxygen measurement system available due to the self-cleaning housing.
These sensor housings typically last 15-20 years, while membrane sensor
housings last three to five years, on average. The initial cost of the
open-electrode is higher than a membrane instrument, but over time the very
long life and reduced maintenance requirements of the open-electrode options
can work best because the probe is built so mechanically rugged that it can
typically withstand many applications better than membrane sensors.
Membrane sensors
The primary benefits membrane sensors with air-blast
cleaning systems offer include initial cost-effectiveness, ease of service,
lightweight structure, and a resistance to heavy metal electrode poisoning.
Membrane sensors are an ideal choice for applications where low initial cost is
a primary factor, maintenance schedules can be followed consistently,
instruments in basins are relatively accessible for maintenance purposes and
heavy grease and oil are not present.
The membrane sensor uses a semi-permeable membrane to
isolate the measuring electrodes and the electrolyte solution from the process
liquid solution. By limiting the flow to gases alone, and especially to oxygen,
this membrane also protects the electrode from contamination. A polarizing
voltage applied externally drives the electrodes in the most commonly used
technique for membrane measurement. Using this methodology, dissolved oxygen is
measured by correlating the current flow between the electrodes to the amount
of oxygen present in the process.
Traditionally, membrane sensors in aeration basins have been
vulnerable to coating and as a result, have required plant personnel to
regularly clean the sensor, usually on a weekly basis, by removing the sensor
from the process. This is time-consuming and labor-intensive for plant
personnel and potentially slows down the process treatment.
New technologies overcome this issue to make membrane
dissolved oxygen sensors essentially self-cleaning. Plants can purchase
air-blast sensor cleaning systems that can be integrated with a membrane sensor
to automatically clean the sensor, thereby reducing plant maintenance
requirements.
These air-blast sensor-cleaning systems blow a jet stream of
air across the sensor membrane, usually for a duration of one to three minutes,
to clear away any material coating the sensor membrane. The plant sets the
system to shoot the air blast at certain times, such as every eight hours, and
the cleaning frequency is controlled by a programmable timer in the analyzer.
The air supply for the instrument is provided by a small, high-efficiency
compressor that's situated near the sensor. Membrane sensors with an air-blast
cleaning system can last three months or longer before any sensor cleaning
maintenance is required--a dramatic improvement over the weekly cleaning needed
by most traditional membrane sensors.
Some wastewater treatment plants have found flotation balls
to be another valuable component of dissolved oxygen membrane sensor systems.
In this measurement technique, the sensor and air-blast cleaning device is
attached to a floating ball. This can be beneficial because the sensor comes in
contact with the process solution on motion, achieving a higher flow rate, and
in some cases, rendering a more accurate measurement.
Additionally, some manufacturers contend that this
implementation can reduce maintenance requirements by keeping the sensor
cleaner longer as it floats in the aeration basin. Unfortunately, in long-term
testing conducted at a major municipal wastewater treatment plant, the
flotation-based method has not proven to be an effective cleaning technique.
Overall, the dissolved oxygen membrane sensor with an
air-blast cleaning system is cost-effective, easy to calibrate and easy to
service because it does not incorporate any moving parts.
Both open-electrode and membrane sensors can effectively
monitor dissolved oxygen on a continuous basis in aeration basins. Each offers
valuable benefits in terms of cost savings and maintenance reduction.
Wastewater treatment professionals must simply evaluate
their specific plant needs to determine which solution will work best for
them.
About The Author: John Volbeda has over 20 years of industrial process and control experience. He currently serves as industrial manager for the water and wastewater and power industries for the Liquid Division of Emerson Process Management, Rosemount Analytical. Volbe
