VFDs Speed Up Wastewater Processing
The operators of the Bellingham, WA, wastewater treatment plant are constantly seeking new ways to improve production to serve a growing population base, without dramatically increasing operating costs. In late 2006, they undertook an upgrade that helped them make a major advance.
Constructed in 1974 with a secondary plant upgrade in 1992, Bellingham’s current wastewater treatment plant provides primary treatment for a peak flow of 55 million gallons per day (MGD) for the area’s population served by sewers, including over 250 miles of collection mains located in Bellingham and the surrounding community. Septic tank waste is also collected from throughout the county and brought to the plant by truck for treatment.
Over the years, the plant has undergone several updates to provide the latest innovations in water treatment technology. The most recent, in 2006, included an improvement to its high-purity oxygen basin, in which oxygen dissolves into a treated solution as part of the purification process.
Before wastewater gets to the oxygen basin, it first undergoes preliminary and primary treatments, where it is cleaned of major debris, heavy particles, suspended solids, and grease. During secondary treatment, microorganisms (activated sludge) remove pollutants from the wastewater in large, enclosed aeration basins. Here, activated sludge and pure oxygen is mixed with the wastewater to create a perfect environment for microorganisms to trap and remove contaminants.
The objective of the oxygen basin upgrade is three-fold: To increase the rate at which oxygen can dissolve into the treated solution, provide greater control over the agitators, and reduce the amount of energy used overall. Denver-based consulting engineering firm, Carollo Engineering, which specializes in water and wastewater treatment plants, designed the new oxygen basins.
Resized motors, improved control
Bellingham’s high-purity oxygen basins have two identical channels through which the treated solution can pass to absorb oxygen. Wastewater traveling through either of the channels stops in each of three agitation chambers where oxygen mixes with it.
Prior to the upgrade, mixer blades within the first agitation chamber were powered by a 50 hp motor, followed by a 30 hp motor in the second chamber, and another 30 hp motor in the third. All three chambers in both channels ran at a constant speed, with only on-or-off control.
One of the main features of the oxygen basin upgrade was a change in the motor size in each of the six chambers and the addition of variable frequency drives (VFDs) to control the motors. This new feature allows greater control over motor speed, reducing the amount of power used by the agitators.
The system upgrade changed the initial chamber to a 40 hp motor, followed by a 75 hp and then a 30 hp. The new design also changed the shape of the mixer blades and moved them closer to the top of the basin. Previously, the blades were located very near the bottom of the 30-ft basins. The new basin design, coupled with the ability to speed up or slow down the blade rotation, keeps the bacteria in the solution more active, promoting faster oxygen absorption.
Even more noteworthy than the change in motor horsepower was the significantly increased control operators enjoyed because of the Telemecanique Altivar 71 VFDs connected to each of the motors. Prior to installing the VFDs, operators had no speed control, meaning that the motors generally ran 24/7 at full speed.
Since the Bellingham plant was already equipped with a SCADA system to monitor the treatment process, under the new design, the agitator speed can be controlled via the plant’s existing process control and communication network. Using a closed loop, the drives can increase or decrease agitation using the VFDs within any of the chambers depending on oxygen dissolution levels reported through the SCADA system.
“The new system is like installing a cruise control on the agitators,” says Mike Sowers, Bellingham maintenance supervisor. “In a car, you just set the speed, and you don’t need to worry about it. Similarly, we can set up the agitators to run at a certain dissolved oxygen percentage, and the operator doesn’t need to stop or start it at any point. The system will know if it should reduce or increase the speed. It really reduces the amount of time we spend monitoring the motors and considering if they should be turned on or off.”
Operators also have the option to control the drives manually via remote control points on the SCADA system or the drive enclosure, eliminating the need to return to the control room to shut down the motors.
In addition to replacing the old motors with high efficiency models and adding the six VFDs, the $750,000 upgrade also included new blades in the agitators, new gearboxes and the addition of several remote control units to control the motors from multiple points within the plant.
Power conservation benefits
It became much easier to reduce power consumption for the overall plant through the combination of motor speed and chamber activation controlled centrally through the SCADA network. According to John Rowe, manager of custom automation solutions for Schneider Electric’s Industrial Solutions Center, which designed and manufactured the custom drives for the plant, says the operators now have data that facilitates major gains in efficiency.
“The operators at Bellingham can now figure out the most efficient way to run their entire system because they’re monitoring six drives that can do totalizing calculations,” Rowe says. “For example, maybe they only need to run three motors to achieve the same application as opposed to six. Or maybe it’s more efficient to have all six motors running at lower speeds than it is to have three running at a higher speed. It gives them all the tools they need to experiment to find the most efficient way to run their application.”
Sowers concurred that the ability to trend data and experiment has been a useful tool. “We’ll use the trend data to help us improve the efficiency of the process,” says Sowers. “We might be programmed to run one of the motors at 40%, but we can do a manual override and run it at 35% to see how it goes. That little adjustment, if successful, can be significant over the course of the year.”
Avoiding hard line starts, during which the motor is initiated at 100%, can also be a major power saver for the plant. According to Rowe, frequent starts without a VFD can create costly energy bills for the plant and problems for the utility. “The current draw during the acceleration of a motor can be three times what the normal operating level is,” says Rowe. “That cost can be significantly more than running the motor constantly but at only a percentage of its maximum speed. Additionally, line starting the motor without a VFD hits the power company really hard, because they’re not expecting to see big surges in current like that.”
In fact, the reduction in power consumption was so great that the local utility, Puget Sound Energy (PSE), helped finance the upgrade project with a $300,000 grant. Sowers says the $750,000 investment is expected to be recaptured through energy cost savings within 10 years.
“This was the kind of project that PSE likes to support,” Sowers says. “They know the demand for electricity is increasing every year, and rather than build new power plants, they would rather have people reduce their consumption. That’s exactly what this upgrade does. It also fits into our long-range plan, which is to eventually increase our production with the same amount of energy.”
Rowe said motor applications that already have VFDs installed can conserve energy by upgrading to the latest technology. New drives are more efficient than older designs because there is less heat loss when passing current to the motor.
“Technology on semiconductor devices for variable frequency drives changes roughly every five years,” says Rowe. “Hence, manufacturers of VFDs, like Schneider Electric, release new families of drives around the same time frame.”
Because each of the Altivar drives have the built-in capability to monitor and trend power consumption, Sowers says the upgrade has also proven useful in identifying potential maintenance issues and resolving them before a system failure occurs. “A big part of this system is monitoring power that’s consumed by the motors,” he says. “That monitoring can also tell me if there’s a problem with one of the motors or drives.”
Rowe added that monitoring for maintenance and repair issues is a common practice for his customers. “A plant like Bellingham will look at how much current it’s normally drawing over an extended period of time,” he says. “If that current increases, it’s telling him that something is changing in the system. He may have a motor that’s slowly starting to fail or maybe it’s a drive that’s no longer running at peak efficiency.
“The components of this system are passing current and producing heat. Over time, failure is going to occur. It’s inevitable. By having this monitoring capability, operators can schedule when they want to review the system and repair a situation before an outright failure occurs.”
Sowers says conserving power through the new system wasn’t enough. Drives can commonly produce harmonics on the system, which can contribute to poor power factor and potentially damage other sensitive electrical equipment on the power line. Therefore, the new system design called for a solution to mitigate those harmonics. Rowe explains, “Harmonics are energy at frequencies other than 60 hertz that show up on the line power. They’re a reflection coming back from the drive. Those reflections reduce the available energy on the power line, so you’re actually drawing more current to compensate for the harmonics.”
The answer to the power quality question was to design each of the 75 hp drives as an 18-pulse unit, a common solution for harmonics abatement. With an 18-pulse drive, power is fed to a large transformer rather than directly to the drive. The transformer takes three power lines in and produces nine power lines out, all phase-shifted apart from each other. Those nine leads are fed into a rectifier bridge and the current is converted to dc.
As part of the installation, Schneider Electric conducted an analysis prior to the upgrade as well as after to ensure that the 18-pulse solution allowed them to meet the IEEE 519 power quality standard. Additionally, all the drives, including the six-pulse models, have a DC choke as an additional harmonic abatement feature.
“We’ll look at the incoming power and all the devices being driven by the incoming power,” Rowe says. “Then we’ll calculate what the harmonics would be. Oftentimes, with an 18-pulse drive we’ll have made the power even cleaner than it was before the installation, because even the conventional motors running off the line produce harmonics.”
Meeting tight schedules
Even though the project wasn’t initiated with Schneider Electric until October 2006, the Bellingham plant needed the system designed, manufactured, and delivered by mid-December of that year, due to heavy rains expected in late December and early January. Schneider Electric’s Seattle field office used the company’s industrial solutions center (ISC) to fill the customized solution.
“The process that Schneider Electric is using to keep communications flowing between everyone on their team is truly paying off in my opinion,” says Sowers. “In contrast, I’ve ordered other drives where I would be talking with the company sales rep and explaining what I need. Two weeks later I get a call from a systems integrator, who’s supposed to be working with that manufacturer, and he’s asking what it is I need. In this case, there was never any question; everyone at Schneider and the people they were working with were on the same team. That made a huge difference in getting it done quickly and done right the first time.”
According to Rowe, the intent of the ISC team members is to educate the customer, the engineering firms, and the installation contractors as to what the latest technology is. He says, “We look for what else can be done to create improved efficiency, more savings, more capability of control, and new features that a customer’s particular industry may not even be considering for monitoring and controlling their processes.”
Even the post-installation product training received high marks from Sowers. “The Schneider Electric people brought in miniature drives for the classroom training discussions so everyone would be able to get around it and actually play with the controls,” he says. “Training on new equipment isn’t always a favorite activity, but you could tell this session went well when my team came back actually excited about the new equipment and what it does.”
They’re not the only ones who are excited, as the new installations continue to make the plant simpler to run and cheaper to operate for the community. ce
|John O’Reilly is a product application engineer, drives & soft starters, Schneider Electric North American Division. Reach him at firstname.lastname@example.org .|
|John Rowe is a product application engineer, drives & soft starters, Schneider Electric North American Division. Reach him at email@example.com .|