Fine bubble vs. agitation: Which proves more efficient?

An Albany County Sewer District compressor system proves to be more efficient running a fine bubble system than agitation. Find out the details, including the costs, in this case study.

The Albany County Sewer District (ACSD) in New York operates two wastewater treatment plants (North and South) that together treat 44.4 million gal. of wastewater on an average day, removing more than 22 tons of biochemical oxygen demand (BOD) and 31 tons of total suspended solids (TSS).

The maintenance team must keep the plants running smoothly and continuously. The primary process is sedimentation, as solids drop out to remove half the strength of the waste. In a secondary process using aerobic microorganisms, oxygen is introduced through aeration. While a variety of methods are used, they typically fall into one of two categories:

  • Mechanical systems agitate wastewater, generally from the surface, using propellers, blades or brushes to introduce air from the atmosphere.
  • Fine bubble systems introduce air in small bubbles from spargers submerged in the wastewater.

Both methods are effective, but fine bubble diffused aeration consumes much less power. “In 1994, our mechanical aerators were nearing the end of their lifecycles,” says Richard Lyons, ACSD executive director. “Repair frequency was increasing and we wanted a new system that was more energy-efficient.

As power costs have gone up, so have the savings.

– Richard Lyons, ACSD executive director

We went from two-speed mechanical aeration to fine bubble diffused aeration to take advantage of the more efficient oxygen transfer with the fine bubble system.”

The process relies on a steady supply of air. ACSD uses HA9 single-stage oil-free compressors from Atlas Copco (www.atlascopco.com). “We use high volumes of low-pressure compressed air,” Lyons explains. “Our Atlas Copco compressors provide it reliably. On average, in the North plant our compressor puts out 10,000 cfm at 6 psi, although we increase or reduce output depending on the demand of the waste.” Both North and South plants have redundant compressors, configured in sequence so if one shuts down the other automatically starts up.

“The Atlas Copco compressors provide the turndown efficiency we need,” Lyons says, “which is critical because the waste stream varies. We’re looking for dissolved oxygen residual to be about 0.5 ppm. Say we’re at 1.5 ppm — that would mean we’re adding more air than we need so we’re using more power than is necessary.” The operator enters the required output on the controller, and the system can supply from 7,500 cfm at 250 hp to 15,000 cfm at 450 hp.

“In 1993, the last full year we operated the mechanical aeration system, we used an average of 35,000 kW per day at the North plant. In 1995, the first full year of the fine bubble diffused aeration system, we were down to 29,000 kW per day.” At ACSD’s current electric power rate of 10.8 cents per kWh, the cost reduction is $236,520 per year for the North plant alone.

The original project cost was $2.7 million, including engineering, and the facility received a $900,000 grant for Energy Reduction for Non-Profit Agencies from its local utility, Niagara Mohawk, which is now part of National Grid. The out-of-pocket cost for ACSD was $1.8 million for the two plants.

“Payback took less than five years, and our Atlas Copco compressors keep paying us back every single day,” Lyons says. “As power costs have gone up, so have the savings. We saw a 42% increase in the cost of electricity from 2003 to the present. Minimizing the amount of electricity required to run the aeration system is crucial to the bottom line. I don’t know where we’d be right now if we never did this project.”

Lyons says he and his team have found the Atlas Copco compressors to be excellent performers. “In the past 10 years, other than lubrication, we have done no major maintenance. Zero,” he says. “In 2003, Atlas Copco performed a Schedule C maintenance. Everything inside — bearings, guide vanes, gears — all looked brand new. It was unbelievable. The vibration is still baseline after 10 years of constant operation. They’re as smooth as silk. The company’s support is terrific, too. They provided a technician and we conducted the Schedule C maintenance as a training seminar for our maintenance people.”

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