By replacing inefficient pumps and installing variable-frequency drives (VFDs), the Iowa city of Cedar Rapids has reduced power usage over the past few years. In addition, the city’s Water Division has cut electrical costs by optimizing operations at off-peak hours and earning special rates in an interruptible power program. The combined savings have allowed the city to add UV disinfection to the water treatment process without increasing the total electricity bill.

The 56 Water Division staff members serve a population of more than 127,000 in a 70-square-mile area. Total capacity at the city’s two treatment plants is 60 mgd (average demand 36 mgd). Water is sourced from an alluvial aquifer and collected in 45 vertical wells and four collector wells along the Cedar River.

Both treatment plants use aeration, softening, recarbonation, chlorination, filtration (sand and gravel beds), UV disinfection, fluoridation and phosphate addition before pumping to 11 storage tanks. The staff maintains nine pumping stations and 660 miles of mains.

More efficient

The city added UV disinfection to comply with U.S. EPA requirements for virus removal from surface water. In the evaluation process, Water Division staff saw a need for other plant updates, especially at the J Avenue facility, built in 1929. To offset the power required for UV, the city looked for efficiencies, most notably from the high-service pumps.

“The UV process uses 9 kW per million gallons, so by updating our pumping structure and adding VFDs, we maintain flat energy consumption,” says Tariq Baloch, water plant manager. The $41 million project was funded mainly by a state revolving fund loan and an $80,000 grant from the local utility, Alliant Energy. The J Avenue plant received upgraded high-service pumps, new VFDs and power factor correction capacitors to optimize power quality.

Steady improvement

Meanwhile, the city saw a need to monitor its electrical usage, a major contributor to operating costs. Over time, an energy management program has evolved. “We’ve been looking at electrical usage, peak demand and power factor for every million gallons of water pumped per kW used,” says Roy Hesemann, plant manager. “We want to see how much electricity it takes to push water through the plant and find ways to minimize it. We serve an industrial area, and raising rates may discourage industries from keeping operations here. We look at everything we can to keep tabs on our cost.”

In 2011, a dozen high-efficiency pumps (Fairbanks Nijhuis) and motors (Nidec Motor Corporation) replaced the older units. Six Allen-Bradley VFDs (Rockwell Automation) were added to well pumps and finished water high-service pumps at J Avenue to reduce energy use and control of flow, which operators previously adjusted manually.

“Operators have full control of both ends of the facility so they can regulate how much comes in and goes out,” Baloch says. “It makes their jobs so much easier. Previously, an operator had to run downstairs and adjust a 450 hp pump to get the needed pressure for the system. It was labor-intensive and took a lot of art instead of science. Now the controller automatically detects the change in pressure and adjusts the pumps accordingly. There’s less chance for error and less chance of rupturing lines from pressure fluctuations.”

Operators closely monitor real-time power usage through the SCADA system — especially helpful when the division is required to stay below a required power limit.

Power interrupted

In summer the Water Division is enrolled in Alliant’s interruptible rate program. It authorizes the utility on any given day to direct the Water Division to shed up to 1,420 kW in demand with two hours’ notice. The reduction may last several hours and can happen several times each summer. In exchange, the division gets a $7.06 credit per kW during June, July and August and a $4.50 credit per kW during the rest of the year.

To shed power quickly, loads like general plant cooling and lighting are curtailed off first, and a 900 kW diesel-powered backup generator at each treatment plant is activated. The operating team must always plan ahead and be prepared for interruptions. “We try to anticipate when those calls may come in,” says Dustin Elin, operations supervisor. “Going into a very hot day, we will fill the tanks to the maximum the previous night so we are ready for an extended interruption, which could be nine hours long. We want to reduce usage of as many pumps as we can.”

The city also reduces its electricity rates by using energy off-peak when possible.  By filling tanks, running centrifuges and backwashing filters during the evening when rates are lower, the division saves some $12,000.

Through its energy-management programs, the water department has set an example for other city departments and citizens alike. “We are the leader in the city for demonstrating how we can measure and use electricity wisely,” says Steve Hershner, utilities director. “The drinking water and wastewater departments are responsible for 66 percent of the city’s total electrical usage, and we want to use it as effectively and wisely as possible.”