Wastewater treatment is an energy-intensive operation. The U.S. Department of Energy (DOE) estimated that in 1994, the water and wastewater segment accounted for 36% of the total electricity consumed by American industrial pumps. At a more practical level, the energy required for supplying and treating local water and wastewater represents the largest municipal energy cost for many communities.
One of the most critical elements in reducing these expenses is to optimize pump system performance, which can represent upwards of 10% to 20% of the energy needed to operate a treatment facility.
System engineers, therefore, work to capture these potential savings by closely matching a project’s flow requirements to a pump’s ideal performance, or its best efficiency point (BEP)—the flow rate at which it operates with the lowest energy use (volume moved per kWh). Pumps that operate outside the BEP—even small variations in flow and head rate—tend to waste energy and wear out more quickly.
Small Efficiencies, Big Savings
Raising pump efficiency by a few percentage points can have dramatic energy and cost savings. Recently, project engineers working on a wastewater treatment plant expansion near Atlanta found that efficiently matching a facility’s flow rate with a pump’s BEP performance curve could shave hundreds of thousands of dollars in energy costs over a project’s life-cycle.
In 2005, Georgia’s second-largest county began a program to grow its wastewater processing capacity by 50%. Instead of renovating five smaller and much older wastewater treatment plants, the Gwinnett County Department of Water Resources (DWR) chose to shutter the aging facilities and consolidate treatment operations at its Yellow River Water Treatment Facility, located about 23 miles southwest of Atlanta, in Lilburn, Ga.
The goal of the six-year, $275 million modernization project was to upgrade and expand the 26-year-old Yellow River plant’s capacity from 14.5 to 22 million gal per day (mgd) and leverage membrane ultrafiltration (UF) treatment technology.
A key component of these plant improvements was the specification of Morris Pumps, responsible for feeding return activated sludge into the membrane bioreactor (MBR) filtration process.
“Our design criteria required pumps capable of efficiently delivering 500% of the 22-mgd membrane capacity flow, or 110 mgd,” said Yellow River Plant Supervisor Ben Bagwell. “Because power consumption accounts for 85% of a pump’s life-cycle costs, project engineers required extremely high operational efficiency that was relatively constant at our targeted flow rates.”
Increasing operational efficiency by a mere three percentage points, as was the case between the Morris product and another brand, translated into significant energy and cost savings for the county’s 800,000 ratepayers.
The new pumps will cut the plant’s daily power consumption by 71.04 kWh, or roughly $7, per day, according to Grundfos Pumps, which acquired the Morris product line in 2007. Over a standard 20-year life-cycle, the small efficiency boost across each of the operation’s four primary pumps translates into more than $200,000 in cost savings for area residents.
The design/build team selected a pump model that provided them with the best operating efficiency point for the project. According to Morris Pumps, the engineers opted to use the same model at four other facilities as well, due to its BEP criteria. Pumps that operate within their BEP keep maintenance, repair, operation and energy costs low.
The Morris 7100 pumps, which handle raw sewage, grit and septage waste, as well as the return activated sludge process, consist of five 30-in.-diameter vertical ball-bearing mixed-flow pumps—four primary and one standby pump. Each pump is rated 27.5 mgd, or the capacity to process 19,097 gal per minute at the specified 20-ft total equivalent height, or total dynamic head. The pumps are driven by 150-hp variable-speed vertically mounted induction inverter-duty motors with integrated variable frequency drive controls conveniently located adjacent to the motor drive systems.
In addition to its monetary rewards, pump performance added to the project’s process efficiencies, as the MBR process requires a significant amount of recirculation to keep solids in suspension and limit membrane fouling (the most serious system performance issue).
The massive undertaking—led by Jacobs Eng. in collaboration with CH2M Hill, Precision Planning Inc. and contractor PC Construction Co.—is the second of three wastewater treatment plants that the county plans to convert to membrane technology.
The combination of biological wastewater treatment and membrane filtration is ideal for many municipal and industrial applications like the Yellow River facility, where biodegradable pollution is reduced using bacteria and microorganisms.
The MBR treatment processes consists of fine screening; multi-cell, plug-flow activated sludge tanks; and submerged membranes for liquid-solids separation. Following biological treatment, the biological sludge is separated from the treated water by a hollow-fiber UF membrane cartridge filter manufactured by Zenon Environmental, part of GE Water.
Lime is added just upstream of the biological reactor basins to supplement the alkalinity in the wastewater so that the pH is maintained near neutral to support the biological activity in the mixed liquor. Alum may be added upstream of primary sedimentation and upstream of membrane filtration to increase the removal of suspended solids, organics and phosphorus. Feeding a precipitant increases the amount of sludge produced and decreases its volatile solids fraction.
Using MBR, the Yellow River plant, originally built in 1979, will now have the capability to remove nutrients and filter pathogens as small as bacteria and some viruses. The Yellow River is part of the Ocmulgee River basin, which eventually flows into the Atlantic Ocean.
In addition to dramatically improving the cleanliness of the water discharged into the Yellow River basin, the use of better filtration technology drastically reduces the plant’s footprint while increasing its capacity. By combining aeration and solids separation (filtration) into a single step, the MBR footprint will occupy just 25% of the land area required by more conventional treatment processes.
“During the 1980s, as our region was experiencing significant population growth, the county commissioned a patchwork of several smaller treatment operations,” Bagwell said. “Around 1995, with population levels still rising, water officials knew the best way to strengthen the network was to close the smaller, older plants, and to focus on upgrading and expanding the larger operations to accommodate the added capacity.”
According to Bagwell, the closing of the five smaller plants will yield roughly $2.4 million in annual operational savings, plus a staff reduction of 29. But Bagwell noted that the savings are actually much higher, because all of the shuttered plants required millions of dollars in treatment upgrades to meet required purification standards.
Pump technology helps Atlanta WWTF lower energy use