A custom-engineered vacuum-pressure swing-absorption (VPSA) system, used to replace a cryogenic plant, cut electric power costs for oxygen production 65 percent, report managers at a 21 MGD wastewater treatment plant in Rocky Mount, N.C.
The VPSA-system investment was spurred by the waste and increasing power costs associated with the plant’s pure oxygen, biological nutrient removal (BNR)-type activated sludge system.
"The new VPSA system costs us only $345 per day for electricity, compared to over $1,000 per day with the ‘cryo’ plant," Kirk Bass, wastewater treatment superintendent says. "When you go with a new process, you always hope for the best. This one has not only paid off, but has done so as quickly as was promised."
Bass says key to power savings with the 27-ton per day VPSA unit, installed last February, are idle-time and turndown options not available with the cryogenic plant.
In addition, the unit is customized to the tonnage needed, rather than having been chosen from available capacity increments.
"The cryo not only had no idle, it didn’t offer a turndown," he recalls. "You had to run its motor wide open, with no way to cut back. One-half to two-thirds of the oxygen discharged as excess to the atmosphere. We had the benefit of power at a very reasonable price, but eventually we had to look at shutting it down."
The custom-engineered VPSA oxygen system was manufactured by AirSep Corp., Buffalo, N.Y., working closely with plant management and staff on "oxygen-generated versus oxygen-needed." As a result, the system includes a 22-second idle feature. It is equipped for fully automatic and unattended operation, including turndown capability from 100 percent to zero flow capacity, and remote monitoring by AirSep engineers.
Idle time and turndown is automatic for the new system’s motor, and can be changed as needed, Bass says. "The system also stores data for a three-day period. If we have any problem, we can download it for evaluation ourselves, or jointly with the system manufacturer. They helped us through construction and startup, and sent a trainer to spend two days with us. He did a very good job teaching us troubleshooting."
"It’s a real user-friendly system, and easy to go in and make changes," Bass said. "When we had an episode where our dissolved oxygen dropped and we needed a little more, we called on them for help, and they were back to us in an hour. They told us to simply lower the idle time to 20 seconds, and our DO was back to normal within a day."
A need to breathe
Oxygen is needed for the 21 MGD, Grade 4 WWTP’s pure oxygen, BNR-type activated sludge system, which is followed by tertiary filtration, chlorine disinfection, dechlorination and post-aeration. Removed solids are processed by dissolved air flotation and anaerobic digestion before pasture-land application.
While the population served has increased since the plant’s 1983 startup, flow has decreased due to the closing of most of the local textile plants, and it now averages only 10-12 MGD.
"We used to average over 15 MGD, and high flows would approach the permitted 21 MGD. We pumped as much oxygen as possible into the basins. Pressure transducers showed one inch of water, with the vents wide open. Now the vents are pretty much closed off and the pressure is 0.1 inch or less," Bass recalls.
"We were successful lowering oxygen costs by 15 percent by switching to LOX for awhile, but when energy costs started going up, our director of water resources, Wayne Hollowell, had us engage a local consulting engineering firm, The Wooten Company, to work with myself and our wastewater engineer, Jim Connolly, to evaluate alternatives, including VPSA, and follow through to implementation. We originally thought the payback on the VPSA would be seven years, and we are on track to see it at least that fast."
Charles Davis, project manager at Wooten, based in Raleigh, N.C., notes that the projected energy savings via the VPSA option helped to fund the project.
"We were able to get a $500,000 grant from our North Carolina Energy Office that was tied to the American Recovery and Reinvestment Act (ARRA)," he recalls. "The remaining funding was from our state‘s Infrastructure Finance Section, a revolving loan at zero percent interest.
Savings and support
Davis says that — following an RFP vendor selection process — the choice of AirSep was based on more than just cost. "Proposals had to show benchmarks for system turndowns of 40 to 50 percent, and they were able to produce that. As it turned out, they were able to perform even greater than expected in that regard."
Equipment and fabrication drawings were provided up front, with assistance later in training plant operators in maintenance and troubleshooting, as well as with plant startup, Davis says. "And we worked together closely to achieve a seamless transition from the former oxygen system to the new one."
Shankar Mistry, Wooten’s senior process engineer, provided valuable insight in preparation of detail design plans and specifications around AirSep’s proposal. Ralph Mobley was the construction contract administrator. H.G. Reynolds Company of Henderson, N.C., was the construction contractor for the installation. "We also knew Dan Gay from previous projects as an oxygen generation expert, and were very glad to have him available to our team as a consultant, including input on drawings and on site," Davis says.
"The former system had large motors and considerable instrumentation and controls, and operators had to attend to it daily, babysitting it during multiple shifts each day," he says. "The new system has freed up operators for other tasks, and I’m looking forward to seeing further savings result from that."
Dan Gay’s DWG Associates of Cumming, Georgia, was the high-purity oxygen consultant to the City of Rocky Mount. His company assisted Wooten with the preparation of specifications for replacement of the old cryo plant with the state-of-the-art VPSA oxygen generator.
"The treatment plant was running off of LOX for quite some time since the cryo plant had been down, but that didn’t present any special challenge," notes AirSep’s project manager, Jim Hitro. "With our own plant design philosophy calling for minimizing complexity, and our extensive and diversified VPSA installation experience available to us, including replacing cryo plants, we were readily able to provide what their consulting engineer referred to as ‘a seamless transition’."
What once took 20 foot-long control panels, with hundreds of wires and multiple control stations, has been replaced by a shoe box-size PLC. Simple low-speed blowers replace the cryo plant’s high speed turbines and compressors.
AirSep’s VPSA engineering manager, Terry Daniels, says the company’s design philosophy lends itself to an easy understanding of the entire VPSA operation by the plant personnel now responsible for the new system. "The entire VPSA plant is turned on or off with the touch of a button, and the operator interface graphically displays real-time plant operation information in a user-friendly display format," he says. "The interface also collects and stores all operating data for trending, as well as for ease of trouble-shooting if required."
Operators easily identify potential issues before they become a problem. If a plant operator needs additional assistance, AirSep is available to assist. "Our data collection systems operate at speeds measured in milliseconds, which allows for very precise analyses. We support VPSA plants around the world, and are able to identify the source of any operational issues in the time that it takes to receive and send an email."
"After our equipment is installed and turned over to our customer," AirSep Vice President Lawrence Hughes says, "we continue to provide engineering support on an ongoing basis, at no charge, working to increase the customer’s expertise and comfort level with our equipment."
AirSep Corp., a division of Chart Industries, offers custom-engineered VPSA oxygen systems, with capacities ranging from 3,000 SCFH (79 Nm3/hr) to 115,000 SCFH (3,023 Nm3/hr), and has installations around the world representing a variety of applications. For further information, contact Lawrence Hughes, AirSep Corp., Commercial Products Division, 260 Creekside Drive, Buffalo, N.Y. 14228, Tel. 716-691-0202, ext. 6253, Fax 716-691-1255, www.airsep.com, cpd@AirSep.com.