Cutting Power Costs for Wastewater Treatment
by Scott A. Williams
You will do it today – more than once. So will essentially all the people you see at home, at work and out in public. What is it? Flush a toilet. It’s something most people take for granted, but of course wastewater sent down a toilet doesn’t simply go “away.” If the toilet you flushed leads to a public sewer system, you owe a nod of recognition to people like Richard Lyons who work to protect public health and the environment.
Lyons is Executive Director for the Albany County Sewer District (ACSD) in New York State. The ACSD operates two wastewater treatment plants (North and South) that together treat 44.4 million gallons of wastewater on an average day. On an average day in 2005, the North and South plants removed more than 22 tons of bio-chemical oxygen demand (BOD5) and more than 31 tons of total suspended solids (TSS). Both plants had zero State Pollution Discharge Elimination System (SPDES) permit violations in 2005. What’s more, ACSD removed greater than 95% of bio-chemical oxygen demand (BOD5) and total suspended solids (TSS), which is 10% greater than SPDES permit requirements.
To achieve these excellent results, Lyons and his team must keep the plants running smoothly, continuously. “The treatment process takes place in two stages,” Lyons says. “The primary process is sedimentary, as solids drop out to remove half the strength of the waste. The secondary process is accomplished with a biological reactor.
“In the secondary treatment, via the activated sludge process, we apply microorganisms in an aerobic environment to the waste stream,” Lyons explains. “The reactors oxidize the waste. By-products of the process are more of the organisms, CO2, and water. Basically we have live bugs in the basins and we keep them happy with lots of oxygen.”
Wastewater plants introduce the oxygen needed to treat the wastewater through aeration. While a variety of specific methods are used across the industry, they typically fall into either of two general categories:
- Mechanical systems agitate wastewater, generally from the surface level, using propellers, blades or brushes to introduce air from the atmosphere.
- Fine Bubble systems introduce air in the form of very small bubbles from diffusers submerged in the wastewater.
While both methods are effective in terms of introducing the needed oxygen to the wastewater stream, there is one substantial difference: fine bubble diffused aeration consumes much less power.
“In 1994, our mechanical aerators were nearing the end of their lifecycles,” Lyons says. “The repair frequency was increasing and we wanted a new system that was more energy efficient. We went from 2-speed mechanical aeration to fine bubble diffused aeration in order to take advantage of the more efficient oxygen transfer with the fine bubble system. Our old mechanical system was effective at treatment, but the fine bubble system promised to be just as effective and much less expensive to operate.”
Lyons details the improvement. “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, were down to 29,000 kW per day.”
The math is simple: annual savings = kW reduction per day x 365 days per year x rate per kW. At ACSD’s current electric power rate of 10.8 cents per kilowatt-hour, the cost reduction is 6,000 x 365 x 0.108 = $236,520 per year. That is for the North plant alone!
“The original project cost was $2.7 million including engineering, and we received a $900,000 grant for Energy Reduction for Non Profit Agencies from our local utility (Niagara Mohawk, which today is 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.”
As power costs have gone up, so have the savings, Lyons says. “We saw a 42% increase in the cost of electricity in 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’d we be right now if we never did this project.”
Fine bubble diffused aeration depends on a steady supply of air. The compressors that create it for ACSD are HA9 single stage oil free compressors from Atlas Copco. “We use high volumes of low pressure compressed air to keep the environment aerobic for the biological reactors,” 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 starts up automatically.
“The Atlas Copco single stage compressors provide the turn down 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. With our system, the operator simply enters the required output on the controller. We can go easily from 7,500 cfm at 250 hp up to 15,000 cfm at 450 hp, and any point in between.”
Lyons says he and his team have found the Atlas Copco compressors to be excellent performers all around. “In the last 10 years, other than lubrication we have done no major maintenance. Zero. 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.”
According to Lyons, the biggest impact of the fine bubble system has been the reduction of the amount of energy required to provide the same level of treatment. “The savings with the Atlas Copco compressors have been huge, and as energy costs go up the savings go up, too. “
The Oil Free Advantage: Atlas Copco’s HA series compressors create oil-free air, which is important in wastewater treatment. “Most wastewater plants are right next to a river,” explains Glenn Shultz, Product Manager for Atlas Copco Compressors Inc. “You simply can’t introduce oil into the basins that are eventually going to return treated water into a river. The HA9 at Albany County Sewer District is a high efficiency single stage oil free compressor that keeps power consumption low. Keeping power costs as low as possible it is a huge benefit for municipalities.”
Why Do Fine Bubbles Work Better? The biological reactors used in wastewater treatment are living aerobic organisms that require a steady supply of oxygen to thrive and do their job. The benefit of a fine bubble aeration system is revealed through geometry: as bubbles get smaller, they provide more surface area per unit volume. Greater surface area increases oxygen transfer efficiency, reducing the amount of power required to operate a wastewater treatment plant.
ACSD: The Albany County Sewer District is responsible for assuring compliance with New York State Pollution Discharge Elimination System Permits as well as the Federal Pretreatment Program. Under these guidelines, the Sewer District provides the essential services to member communities and county residents in the treatment of wastewater generated within the county to preserve community health and the environmental quality of waters. For more information, visit www.albanycounty.com/departments/sewerdistrict/