WaterWorld - March 2014

system was manufactured by AirSep Corporation of Buffalo, N.Y., who worked closely with plant management and staff on oxygen generated versus oxygen needed. As a result, the system includes a 22-second idle feature. It also provides for fully automatic and unattended operation, including turndown capability from 100% to 0% fow capacity, and remote monitoring by AirSep engineers.

“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 oxygen, 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 D.O. was back to normal within a day.”

REPLACEMENT OF PREVIOUS OXYGEN PLANT

Oxygen is needed for the 21 MGD, Grade 4 WWTP’s pure oxygen, biological nutrient removal (BNR)-type activated sludge system, which is followed by tertiary fltration, chlorine disinfection, dechlorination, and post-aeration.

Removed solids are processed by dissolved air fotation and anaerobic digestion before pastureland application.

While the population served has increased since the plant’s 1983 startup, fow 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 fows would approach the
permitted 21 MGD,” Bass recalled.
“We pumped as much oxygen as pos-
sible into the basins. Pressure trans-
ducers showed 1 in. of water with the
vents wide open. Now the vents are
pretty much closed off and the pres-
sure is 0.1 in. or less.”
“We had been successful lowering
our oxygen costs by 15% by switching
to liquid oxygen for awhile but when
energy costs started going up, our di-
rector of water resources allowed us
to engage a local consulting engineer-
ing frm, The Wooten Company, to
work with myself and our wastewater
engineer to evaluate alternatives, in-
cluding VPSA, and follow through to
implementation.” Bass said they origi-
nally thought the payback on the VPSA
would be 7 years, and said they’re on

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Replacement of a cryogenic plant with a custom-engineered vacuum pressure swing absorption (VPSA) system is saving over 65% of Rocky Mount’s electric power costs for oxygen production. Buffer tank (left) holds oxygen, produced via indoor and outdoor equipment, prior to delivery into the plant’s activated sludge treatment process.