By Beth Brindle, Water Online
Best known as the home of outdoor retailer L.L. Bean, the community of Freeport is a popular summer destination on the shore of Casco Bay in southern coastal Maine. In 2010, the Freeport Sewer District brought in engineering firm Woodard & Curran (also of Freeport), to conduct an energy analysis of the Freeport Wastewater Treatment Facility. The assessment revealed that the Freeport plant consumed more than 8.9 kBtu per gallon of wastewater treated, more than double the national average of 4.1 kBtu per gallon. While an outdated oil heating system contributed to the overall inefficiency, the plant’s 30-year-old multistage centrifugal aeration blowers accounted for as much as 80 percent of the electricity consumption.
Limited Turndown Leads To Inefficiency
The Freeport Wastewater Treatment Facility has a design capacity of .44 million gallons per day (MGD) and an annualized average daily flow of .25 MGD. But the plant’s existing blowers were delivering a much larger volume of air than the plant required, particularly during the slower winter season, when the tourist population drops and average flow rates take a sharp decline.
“There’s a huge variation between Fourth of July conditions and middle-of-January conditions in Freeport,” said Michael Loncoski of Aqua Solutions, Inc., who aided Woodard & Curran with new equipment selection for the Freeport plant. “And for much of the period when demand is at its lowest, the air is also very cold and very dense.”
Cooler temperatures and decreased demand should have resulted in winter energy savings for the Freeport plant, but limited turndown in the multistage blowers meant that the blowers continued to run at or near full power, resulting in chronic over-aeration and low overall efficiency.
Multistage Blowers Replaced With Hybrid Blowers
Following the 2010 energy assessment, Woodard & Curran carried out extensive upgrades to the mechanical equipment in the Freeport plant, including the installation of new blowers, diffusers, and controls. The aging multistage blowers were replaced with two Aerzen Delta Hybrid blowers, making Freeport one of the first U.S. installations for the hybrid technology. But while the Delta Hybrid blower was new to the U.S. market, its combination of familiar operation and improved efficiency made it a clear choice for the project.
Two Delta Hybrid blowers replaced the original multistage blowers in the Freeport plant.
“We had already determined that a positive displacement blower would be the right fit for the Freeport Sewer District,” said Brian Cataldo, project engineer with Woodard & Curran. “When the Delta Hybrid blowers became available, they were a very similar product in terms of operation, but they delivered air more efficiently for this application.”
The Delta Hybrid achieves power savings comparable to turbo blowers while retaining all the advantages of a positive displacement blower, including the ability to operate under wide pressure variations and the turndown capability so important to the Freeport plant.
Unlike standard positive displacement blowers, which convey air with no internal compression, the hybrid blower uses screw compressor technology to compress the air within the machine. Aerzen describes the Delta Hybrid as a rotary lobe compressor or a low pressure screw compressor. The profile of the rotors is similar to that of higher pressure screw compressors, but the hybrid achieves higher efficiency by adapting the profile of the rotors for a lower compression ratio to meet the low pressure requirements of aeration systems.
The profile of the rotors is adapted for a lower compression ratio, resulting in higher efficiency in low pressure applications.
In addition to accommodating seasonal fluctuations in demand, the new blower had to meet the mixing energy requirements and physical limitations of the Freeport plant, which includes combined aeration basins and clarifiers in a package design. Woodard & Curran also converted the treatment units from an airlift pump system to an electric pump system, reducing the overall air requirement and providing better return-activated sludge flow control.
“To meet both peak airflow rate requirements and turndown requirements, we needed to select a blower that could provide enough air to meet the mixing requirement, resulting in a higher dissolved oxygen (DO) level,” Woodard & Curran’s Cataldo said. “Our solution included a cyclic on-off blower operation, where the blowers run at 80 percent output for a short duration to rapidly re-suspend, then trim back on dissolved oxygen.” With an operating range of 600 CFM to 1400 CFM, the Delta Hybrid blower easily handled the aeration requirements at both the high end and the low end of the operation.
Ease of maintenance was another factor in the Delta Hybrid’s selection. “It came down to the familiarity of the operators with the technology and the maintenance requirements for the new equipment,” Cataldo said. “With the hybrid blower, you do your oil changes and check your belts on a routine schedule, very much like a typical positive displacement maintenance schedule, and we felt very comfortable recommending it for the Freeport plant.”
Improved Efficiency Results In Immediate Energy Savings
The hybrid blowers were placed online in December 2011, with one blower handling the plant’s aeration requirements and a second identical blower installed for redundancy. Since the installation, the improved turndown capability of the hybrid blower has been a significant factor in the plant’s energy savings.
The turndown range of the Delta Hybrid blower contributed to the energy savings at the Freeport Wastewater Treatment Plant.
Before the plant upgrade, the Freeport facility’s electricity use was between 110,000 and 120,000 kWh per month. With the upgrades installed, electricity consumption was reduced to less than 40,000 kWh per month.
“As part of the plant redesign, we replaced the oil heating system with an electric heating and cooling system,” said Brian Cataldo. “So even with the increased electricity use for heating and cooling, the plant consumes only one third of the electricity that it was originally using.”
At full load, the old centrifugal blower consumed about 62,240 kWh per month, as compared with 50,560 kWh per month for the new hybrid blower. And while this top-end efficiency marks a clear improvement over the old blower technology, the efficiency at the low end of the operating range is what truly set the hybrid blower apart.
“Turndown capability is three times what it was before the installation,” Aqua Solutions’ Loncoski said. “The Freeport plant didn’t know what to expect from running at the low end; they had never been able to do that before.”
Because the mechanical upgrades represented such a significant improvement in efficiency over the plant’s original equipment, funding from an Efficiency Maine incentive program offset a portion of the capital expense, resulting in an immediate cash savings for the Freeport Sewer District.