Energy-efficient costs help Alabama city save costs
The sewer utility for the city of Andalusia, Ala., is among scores of municipal and regional operations across the nation that benefited from the American Recovery and Reinvestment Act of 2009. Alabama alone received $55.57 million worth of stimulus funds for energy-related conservation projects. Andalusia qualified for a $250,000 U.S. Department of Energy Block Grant administered through the Alabama Department of Economic & Community Affairs for a relatively modest commitment of $25,000 in matching city funds.
However, the proposed use for replacing aging pumps at two lift stations along the municipal sewage collection system had to be fully demonstrated.
The local office of engineering firm Goodwyn, Mills and Cawood Inc. broke ground for the grant opportunity. The engineering firm communicates routinely with state and federal agencies to identify potential sources for grants to help underwrite their cash-stretched local government clients needing infrastructure projects, explained Bob Carter, P.E., general manager of the Andalusia office of Goodwyn, Mills and Cawood Inc.
Carter called the grant to the city’s attention and provided input to prepare the application and proposal before leading the eventual project. The final report on the project scope and month-long monitoring results required the scrutiny—and seal—of a licensed professional engineer to verify the facts for grant eligibility.
The prerequisite for eligibility called for at least 30 days of documented monitoring to substantiate a guaranteed 25% reduction in electrical consumption by the replacement pumps versus the existing pumps.
Beyond the Specs
Flygt N-pumps were specified as replacements. Jim House & Associates Inc., a Xylem factory representative, supplied performance input for the grant, installed the units, modified the controls and coordinated the monitoring; and results proved impressive. The features of the pumps included energy use reductions of 48.1% at one location and 56.1% at the other station. Although not addressed in the grant requirements, the pumps have a record of reliability in reducing the clogging experienced at lift stations elsewhere.
“I wish we could replace our entire system with this type of Flygt pump,” said Earl Johnson, multi-term mayor of Andalusia who also serves as chairman of the Utilities Board.
“I can’t help but like what the grant will accomplish in our case and wish we had more of them available. We not only replaced aging pumps that were well beyond their expected life, but the new pumps will help us save thousands of dollars worth of electricity every year.”
A city’s sanitary sewer collection system remains generally out of sight, out of mind—until the monthly electric bills arrive or a failed pump demands an immediate response to prevent a sanitary sewer overflow. The fact is that ￼energy consumption and reliability are often interrelated to a pump’s resistance to entanglement or clogging with the often fibrous debris carried in the wastewater.
Replacing existing wastewater pumps with more energy-efficient units may not bring to mind buzzwords like “green technologies,” such as rooftop solar panels, wind turbines or geothermal heating and cooling systems might do. However, the savings Andalusia will accrue in electricity costs should have a favorable—and compounding—impact on the bottom line of the Sewage Department’s Operations & Maintenance budget.
The semi-open, self-cleaning impeller on the pump makes the companion wastewater-handling pumps fitted with them more energy efficient. The design keeps the leading edges of the impeller vanes unobstructed where fouling often sets the stage for clogging. The leading edges of the impeller vanes pass across a stationary relief groove that clears any snared fibrous solids, grease or sludge, creating a self-cleaning f low path through the pump. The self- cleaning feature inherently reduces an impeller’s vulnerability to entangling material that results in an ensuing drag on pump speed. This occurs although the energy consumption remains constant. The patented volute and impeller are therefore the underlying features for improved energy efficiency.
“Like everyone else, our community has seen significant cost increases in electric power,” Johnson said. “In fact, we are in the midst of enacting a phased four-year rate increase so the cost of operating the pumps is going up proportionately to the electric rate increases. Furthermore, as the existing pumps aged, their efficiency declined to the point where they had to operate for longer periods to achieve the same results.”
That became the criteria used for selecting the two sites receiving the new pumps, he added. The existing units had been in place since the mid-1980s, so a proactive upgrade was easily justified. The contract enabled Andalusia to gain an enviable jump start in the already renewed focus by public works departments on ways to reduce the incremental electricity charges associated with the collection and treatment of water and wastewater utilities.
The contract replaced three of the five pumps at the wastewater treatment plant’s influent pump station at one of the two sites. The three replacement units involved model NP-3202, 45-hp, 1,425-gal-per-minute (gpm) pumps that historically incurred 90% of the duty hours. A $75,000 short- fall prevented replacing the two larger, 77-hp pumps, but they have experienced less operational wear. The 56.1% reduction in energy use equated to 18,527 kilowatt-hours for the measurement period.
The reduction was 31.1% more than the guaranteed minimum and a total comparative savings of $1,859.43 for the new pumps during May 2011, including demand charge.
The five solids-handling pumps at the utility’s Central Lift Station consisted of two, 47-hp and three 85- to 88-hp pumps. The two smaller units were replaced with model NP-3202, 45-hp pumps and the three larger pumps with model NP-3301, 85-hp pumps equipped with N-impeller technology. The comparative savings for the new versus old pumps ran $1,082.40, including a demand charge for the same month, representing a 48.1% energy use reduction.
“Replacing the largest power consuming pumps was the most logical,” Carter said. He added that the 25% energy reduction was a grant requirement, but the project team and city actually wanted replacement equipment to exceed that minimum. The pumps were preferred in part because the manufacturer would guarantee the grant program’s specific 25% requirements.
“We also made changes to the controls at the Central Station, including adding alternators to cycle the small pumps,” Carter said. “In the new configuration, the large pumps assume the work of the smaller units whenever peak flows exceed 1,750 gpm. The large pumps can handle a combined flow of up to 3,000 gpm. Like many other mature collection systems, some Andalusia lines experience infiltration and inflow.”
The engineer foresees future applications when energy conservation is a partial goal of pump station retrofits. “As engineers, we are always trying to get the most efficient equipment for our clients,” Carter said.