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| Figure 1: Clarifier and Profile: Shows the Theory of the ILA. Ashbridges Bay uses rectangular moving bridges, not circular as drawn. |
By Dean Rudd and Jake Alaica, Cancoppas Ltd., and
Cordell
Samuels and Walter Geith, Ashbridges Bay Treatment Plant
The unique challenges of automating moving bridge primary clarifiers using ultrasonic detection and digital radio technologies, were confronted at Toronto's Ashbridges Bay (ABTP) wastewater plant. This is the first major wastewater facility in North America to attempt this level of complex online measurement of moving bridge primary clarifiers. As this is a very common style of clarifier in Canada, this project leads the way for other facilities to look at online automation of a crucial process parameter.
This project was started in early 1999 and took almost one year to complete the initial testing of the detection systems, connect the radio modem and complete the interface with the plant's SCADA (Supervisory Control and Data Acquisition) system. It involved close collaboration between the detection equipment supplier, the local technical representative and the plant instrumentation department.
The first aspect of the project was to test the Royce Interface Level Analyzer on the moving bridge primary clarifier. This system uses an ultrasonic sensor mounted on the moving bridge with a sensor submerged below the liquid surface. The ILA used the standard dual head sensor for transmitting and receiving of the ultrasonic signals. Primary clarifiers at ABTP are large rectangular systems inside a building and the inside location of the tanks inside added to the complexity of the interfacing of the ILA system and the plant's SCADA, referred to later.
The ILA produces a graphical, representative display of the sludge settling characteristics and sludge depth indication based on the system configuration. An ultrasonic pulse is produced four times per second; these pulses travel through the tank from top to bottom. After emitting a pulse, the system listens for return echoes from the sludge interfaces within the clarifier. It listens to sixty seconds of return echo information and produces an echo profile for the clarifier. This profile is essential information for the plant as it shows the settling characteristic of the sludge present in the tank. Based on the profile produced, the plant personnel can examine the sludge characteristics and make process decisions. The ability to produce and view the settling profile of the clarifier sludge was a key factor in the selection of the Royce ILA system.
The Interface Level Analyzer also interprets the graphical profile, based on a set of customer-configured parameters, and produces a digital value of sludge depth. This is displayed on a local LCD display. Plant personnel can customize the program parameters so that the instrument detects and tracks the desired interface or blanket.
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| Figure 1: Clarifier and Profile: Shows the Theory of the ILA. Ashbridges Bay uses rectangular moving bridges, not circular as drawn. |
There were some problems encountered during the initial testing of the instruments to measure primary clarifier interfaces. The clarifiers at the Ashbridges Bay plant can have clarifier off-gassing problems. Off-gassing occurs when biological activity in the sludge produces a gas by-product. The production of gas within the clarifier can be a significant problem for ultrasonic interface detection systems. Gas bubbles can accumulate on the sensor face and decouple the sensor face from the water and could cause the instrument to go blind and it is unable to detect reflections from the interfaces in the tank. Instrumentation will produce an alarm if this situation occurs.
Gas bubbles were a problem for the ILA until the sensor face was placed on an angle. The sensor face was put on about a two-degree angle off the perpendicular. This allowed the bubbles to roll off the sensor face due to the movement of the bridge along the tank. Angling of the sensor face eliminated the problem of the sensor face decoupling from the water.
Figure 1 depicts the operation of the ultrasonic interface analyzer. All interfaces are translated into a graphical profile which is transmitted to the plant SCADA system.
The second phase of the project was the installation of the radio modems which allowed the retrieval of the process information from the ILAs. As the ILA units were mounted on the moving bridge structures, hard wiring was not an option. Selection of the radio equipment was made by the local representative. Such selection is normally made by the customer or the local representative on these projects, as different jurisdictions have different regulations for radio equipment.
GINA (CRE) Radio Modems were chosen for several reasons:
Method of communications
The radio equipment was integrated into NEMA 4x enclosures to withstand the environment in the primary clarifier building. The radio modems are connected to the RS-485 output of the ILA systems. They use a 1200-baud rate for communications.
Interconnection between the modems and the plant SCADA is through a PLC5 Processor, and a 1771-DB Basic Module.
Radios were set up in a point multipoint configuration. The program in the basic module uses the Royce Communications Protocol to access each bridge ILA and the system is programmed to request two reports from the ILA, a data report and the profile report.
The basic module begins by sending out an ASCII string requesting the two reports. The ILA replies to the data report request with a 14 character pair with the tank height and the sludge blanket depth. The second request is for the profile report to which the ILA replies with a 249-character pair message. Each pair represents the position and relative strength of the return echo. In the system at Ashbridges Bay Treatment Plant, the largest return echo indicates the position of the interface or sludge blanket. Each response is spaced at 2.7 cm (0.1 feet) in depth; because of this, only the first 155 character pairs are used, as the tanks are 418.5 cm (15.5 feet) deep. The raw ASCII data from the ILAs is converted to floating point and then sent via shared memory to the PLC5 using BTR (Block Transfer Read) instructions.
There were some hurdles to clear when setting up the radio systems and the communications, as the communications protocol of the Royce instruments is limited. There is no command for end-to-end flow control, so in order to read in a long message like the profile without getting buffer overflows it is necessary to read in a 200-character pair block at an uncontrolled faster rate. The last 60 character pairs are read in at a slower rate in a controlled fashion. The buffer will check for data pairs before reading them in. If there is no data pair then the buffer reads this as the end of the message. This method normally works well.
Occasionally the data string will become corrupted or be truncated; if this happens, the port will wait indefinitely. This effectively hangs up the application. In order to deal with this problem, a ladder logic program in the PLC monitors the data from the interface level systems. When the system detects a flat line for one hour, it resets the Basic Module. A relay closes briefly, which shorts out two wires from the reset switch of the Basic Module. This relay closure restarts the Basic Module program.
Two main reasons for selecting a Basic Module for the communications were that reading/writing serial ports is much easier, and, if the program in the Basic Coprocessor hangs up it does not affect the main program in the PLC.
Testing of the system comprised two Royce ILA units and two radios. Once the system had been proven, a permanent installation of three Royce Model 2505 Interface Level Analyzers and radios was undertaken.
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| Figure 2: Sludge Height vs. Time. The saw tooth graph shows the bridge moving from one end of the tank to the other and returning. |
Benefits of the ILA/radio system
This extensive program was undertaken for several reasons and the least of the benefits was that it eliminates the need for manual sludge blanket measurements. Since the primary clarifiers in the primary building are enclosed, it creates a very inhospitable environment for plant personnel. The ability to report sludge blanket level without having to enter the building, frees up staff for more important tasks.
The first and most important benefit of the system is that the operations staff now have real-time online process values. This allows the plant to make proactive decisions as to the process and its condition. The radio modems allow the process people to not only see the sludge blanket level but also the complete settling characteristics of each clarifier. The plant personnel have setup view screens in the SCADA that allow for the sludge depth, in feet, the tank profile and a trend of sludge depth vs. time (Figure 2). This trend shows the movement of the bridge and the sludge at each position of the bridge. The operator can see the entire tank behaviour.
A second function of the online information is to allow the plant to monitor the operation of the collection mech-anism and to see that the raw sludge pumps are functioning. If either of the above functions were to malfunction, the trend graph would change drastically.
Thirdly, this information can be used to help with raw sludge distribution. The plant operation personnel can monitor the sludge levels in the different primary tanks and adjust raw sludge pumping schemes to ensure proper distribution of solids. This is important as the Ashbridges Bay Treatment Plant has 11 different aeration trains in the plant and several different dewatering points. The ability to get real-time online sludge levels allows for better solids handling and treatment.
Movement of sludges to digesters from the primaries is a key point of control within the plant. It is essential that the digesters receive the proper flow of solids to maintain proper temperatures and sludge treatment times. Raw sludge pumping can be controlled by either time or by density control. The limitation of time-based pumping is that it makes assumptions based on historical data. The better method of raw sludge pumping is density-based pumping.
A crucial component of the density-pumping scheme is the maintenance of controlled primary blanket levels. It is important to maintain at least a minimum sludge level in the tanks to ensure sufficient sludge density. When the blanket readings are provided by manual measurements it is quite possible to over pump the clarifier and send low level solids to the digesters. These low level solids create problems for the digestion process. The more constant the solids fed to digesters, the better the treatment process.
Finally, there is the problem of septic sludge and odour. As was discussed previously, there is the problem of off-gassing and septic sludge. If the primary sludge is allowed to remain too long in the primary clarifier, it will go septic and start to biodegrade. This biological activity produces odorous gases that give treatment plants their smell. One of the best ways of controlling primary sludge is to know the levels on real-time bases, which allow for proper distribution of the raw sludge before the odour occurs.
Conclusion
While the total scope of the project is still in its infancy, the plant is able to use the online interface level information to better control the overall plant process. A project to control sludge pumping based on interface level will be looked at in the future. This project proved the feasibility of the use of ultrasonic interface detectors and digital radios and interfacing with an existing SCADA system. The plant has other moving bridge primary clarifiers and there are plans to automate in the same manner.
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