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Latest process control strategy includes the installation of multiple online dissolved oxygen instruments per treatment basin
For the past 20 years it has become more common to automatically control dissolved oxygen (DO) content in the aeration process. While some wastewater treatment plants still use a hand-held portable meter to measure the DO level once per day or maybe a few times per week, more plants are using online process instruments.
Years ago these instruments were a bit unreliable, but today wastewater treatment plants have access to accurate and reliable DO instruments to help with process control. Online monitoring can help improve the wastewater treatment process and save energy, often by as much as 25% to 50%.
Utilizing DO profile control, the latest process control strategy includes the installation of multiple online DO instruments per treatment basin. This new control technology allows cost-effective oxygen profiling throughout the aeration basin, thereby improving plant performance and leading to additional
Enhance capacity, savings
It has been well documented that the demand for oxygen changes throughout the aeration basin. At the inlet of the aeration basin the demand is typically high, whereas at the end of the basin there is little or no oxygen demand.
Furthermore, the demand for oxygen will change as the daily load cycles. In fact, a load that changes by a factor of five, over a 24-hour period, is not uncommon. All of this leads to a requirement for improved online measurement of DO.
The International Water Association developed a model of a wastewater treatment plant that indicates what DO level is present in several different positions within the basin, while the concentration of DO is controlled by only one system located in the middle of the basin.
The model shows the level of DO is too low in the first part of the basin and too high in the end of the basin. The objective of introducing oxygen profile control is to equalize the level of DO throughout the aeration basin.
This type of control strategy may require a few control changes, including modulating control valves and pressure controls, but these changes will provide two major advantages:
• More efficient use of the aeration basin; and
• Additional energy savings.
By being able to control several sections in the aeration basin individually, the overall treatment capacity of the basin can be increased by 40–50% and the energy savings increased an additional 20–25%. All these savings are on top of the savings already achieved by switching from fixed aeration control to online DO control.
Depending on the size of the aeration basin, the number of controlling sections needed to achieve the maximum energy savings varies—a typical number is four to six sections per basin. By increasing the number of sections in the aeration basin, the number of DO systems will increase proportionally.
Proven energy savings
Oxygen profile control is already being used at some wastewater treatment plants, both in the U.S. and Europe. Some actual data indicating the energy savings realized at a German wastewater treatment plant, the city of Lindau WWTP, reveal the energy consumption before and after implementation of oxygen profile control. Within one month of increasing the oxygen profile control strategy, energy consumption was reduced by 20% as compared to the control strategy using only one DO system per basin.
The city of Lindau is located on Lake Constance, at the foot of the Alps. The lake is one of Europe’s largest fresh water resources and, because it provides drinking water to several adjoining countries, it is subject to some of Europe’s most stringent wastewater regulations. As a result, not only is the efficiency of the plant critical, but the quality of the plant effluent is being very closely scrutinized.
Although performance of the plant using a single DO system to monitor each aeration basin was satisfactory, plant personnel questioned whether they could keep up with increasing plant loading conditions. They decided to investigate alternatives that would allow them to cost-effectively increase both plant efficiency and capacity.
They examined the process and found that oxygen was entering some zones that were meant to be oxygen-free. This unwanted disturbance in the denitrification process was caused by an excess DO level, based on the internal recirculation rate. They also discovered that the DO content in half of the aeration basins exceeded 2.5 ppm. This “surplus” of DO did not increase the effectiveness of the nitrification process, in fact, the energy used to reach this level of surplus DO was wasted.
In an effort to improve process efficiency and eliminate energy waste, plant personnel decided to install oxygen profile control. Three additional Evita OXY systems manufactured by Danfoss, which brought the total to four per basin, were installed in each aeration basin.
Each DO system controls a valve that supplies air to the bottom aerators in four dedicated zones within the aeration basin. The air is supplied by a blower that is controlled by a frequency converter. A pressure transmitter measures the air pressure. Consequently, the DO content is much more stable throughout the basin, saving energy and achieving a higher treatment capacity.
Friedrich Hutter, plant manager at the Lindau WWTP, stated that “with an annual energy savings of 13,000 Euros, oxygen profile control allowed the Lindau plant to achieve a payback of our investment in less than 18 months.”
On top of the operating cost savings, the new control strategy had a positive effect on the denitrification process, thereby improving the overall efficiency of the entire plant. The improved efficiency meant that increased plant loading conditions were no longer a concern for plant personnel.
New strategy, instrumentation
Successful implementation of oxygen profile control, as seen at the Lindau plant, puts a more stringent demand on the requirements necessary for the DO instrumentation.
It is not simply a question of installing more instruments, valves and pressure controls. Success will necessitate that the DO instrumentation fulfill new requirements such as:
• Quick and easy installation, at minimal cost;
• Simple calibration;
• Reduced maintenance; and
• Accurate, reliable measurements.
Accordingly, the Evita system has been specifically designed to meet these demands.
More accurate information
Embedded in all of the Evita sensors, the Danfoss USC 7000 signal converter utilizes built-in HART communication protocol. This system provides a power supply for the sensors and enables simultaneous communication of up to 15 sensors, using only two wires.
The HART platform also allows a parallel connection to a PLC system, enabling all 15 sensors to be simultaneously operated and controlled from both the USC 7000 signal converter and the plant control system.
Parameter settings can be changed and the user has access not only to the measurements of DO and temperature, but also to the remaining lifetime of the membrane cartridge for all 15 monitoring points.
A key factor when doing DO profiling is the time required to install and maintain the system. Many DO systems need a supply of clean water and/or compressed air to continuously clean of the sensor. Using a mechanical cleaning method can be a costly and time-consuming arrangement. In addition, electrical connections are usually complicated and, in many cases, there is a risk of destroying the instrument or sensor if improper connections are made during installation.
The Evita system overcomes these obstacles by providing a self-contained, self-cleaning membrane cartridge and a simple two-wire power and signal cable connection arrangement. This design makes the entire installation procedure simple to perform; it can typically be completed without a manual and in a matter of minutes.
Reduced sensor maintenance
Previously, time-consuming maintenance often has prohibited efficient use of DO profiling. The Evita system is specifically designed to be user-friendly. Some of its features are:
• Calibration required only twice
• Automatic calibration initialed by simply tilting the sensor;
• Patented, self-cleaning ball-float design; and
• Replacement of the membrane cartridge is only required every two or three years.
Today, many plants are ready to take the next step in process control—DO profiling. This new control strategy has set new standards for online DO instrumentation.
To achieve maximum energy savings and allow for tighter process control, it is very important that DO instrumentation is reliable and accurate, requires very little or no maintenance and is cost effective.