Automation of a plant aeration system in Cape Coral, Fla., brings substantial energy savings and a more consistent treatment process
With about 1,200 new residents a month, Cape Coral, Fla., is one of the nation’s fastest-growing cities. Only 30 years old, it grew by 8 percent (11,400 people) in 2005-06. Its current 165,000 population is expected to double in the next 30 years.
Both the city’s wastewater treatment plants have been expanding to keep pace. When planning began for another expansion at the older Everest plant in 2006, acting plant superintendent Brian Fenske knew it was the last shot to get everything right. “There is no room to make the facility larger,” he says.
That work included improvements that dramatically increase energy efficiency. In particular, automated control of the aeration system drives down power consumption, while making the treatment process more consistent and helping to reduce chemical consumption.
On the grow
Everest was first expanded in the late 1980s when the 4-mgd conventional activated sludge plant saw flows of about 6 mgd. The expansion to 8.5 mgd included conversion to a four-stage biological nutrient removal system. That worked well, but by 2006, the plant was running out of capacity again.
After $10 million in various improvements, the final $62 million upgrade, engineered by MWH Global, converted Everest from four-stage nutrient removal to a five-stage system (adding an anaerobic zone) and upgraded its capacity to 13.4 mgd. The real prize, to Fenske, was getting more out of the plant while doing less.
“We were looking for automation to optimize the plant and reduce electrical consumption and chemical use,” he says. That’s exactly what he got when two new aeration basins came online in June 2008 with a new control system. The following June, while two existing basins were being refurbished to operate with the new control system, Fenske had a chance to see the old working alongside the new.
“The new system is much more efficient,” he says. A single-stage 300-hp blower (Turblex Inc., a Siemens company) replaced the old multi-stage blower and immediately met expectations in controlling dissolved oxygen levels. “The single-stage blower’s efficiency comes from its design and a Turblex proprietary algorithm,” says Fenske. “It keeps the motor running more efficiently at lower amps throughout its electrical turndown.”
According to Turblex, the blower uses variable diffuser vanes on the discharge side of the impeller to vary air volume from 100 percent to 45 percent of capacity. The PLC controls variable inlet guide vanes to optimize inlet airflow by reacting to three variables that affect efficiency: ambient temperature, differential pressure, and machine capacity.
The old blower had an annual electricity cost of about $96,500, versus an expected $69,700 for the single-stage blower — a savings of $26,800 a year, or 27.8 percent.
Actual savings could be much greater if real-life to date experience holds true. “We ran the two systems side by side and found the Turblex blower ran with about 40 percent lower electrical consumption than the multi-stage,” says Fenske. The old blower consumed 5,089.3 kWh in 24 hours, while the new blower used just 2,928.6 kWh. That means annual savings of about $100,000 (though based on only a single day of operation).
Control is the key
To operate the new automated aeration system, operators simply enter the desired dissolved oxygen levels into the SCADA. A proprietary programmable logic controller from Turblex serves as the brain, providing an automated connection between the blower and the field instrumentation, mostly from Hach and Endress+Hauser, and motor-operated valves from Rotork Controls Inc. that maintain the airflow with input from the PLC.
“With the previous system, we had to go out and manually inspect the dissolved oxygen levels, and our only adjustment was with a manual valve,” says Fenske. “Every two hours, operators adjusted the valve by hand to increase or decrease the airflow to each zone of the aeration basin. Or they adjusted a butterfly valve by loosening or tightening a wing nut to increase or decrease the output at the blower directly.”
With little information about dissolved oxygen levels, operators had to run the airflow higher to maintain the proper margins. “We had to shoot for 3 ppm because we could have a significant drop in the two hours between adjustments that would affect the treatment,” Fenske says. “Now, we can run right at 2 ppm. That saves a lot of energy.”
Luminescent dissolved oxygen probes from Hach give a continuous reading in each zone. They replaced membrane probes that had to be constantly cleaned and replaced. “The process is only as reliable as the field instruments, and these are rock solid,” says Fenske. “You take them out every few weeks and wipe them down.”
Redundant probes and automatic diagnostics ensure constant and accurate readings. In addition, operators check the probe accuracy with a portable dissolved oxygen meter six times a day.
What is now automatic used to depend on the experience of operators. Essentially, they used their eyes and ears to keep dissolved oxygen at the proper levels. “There was some instrumentation, but it was done mainly by looking at the tank to see how the flow increased or decreased and listening to hear the hiss through the valves,” says Fenske. “That was the best fine-tuning we could do.”
With reliable probes and the PLC, operators now have trend information on dissolved oxygen. They have found very consistent results. “Dissolved oxygen will stay within half a part per million for hours and sometimes days, so we have very little change,” he says. “That optimizes the plant and makes it run more efficiently.”
The PLC also monitors the temperature on different mechanical parts of the blower, helping to detect abnormal conditions so the staff can make repairs before failure occurs. The Turblex PLC was also designed as a hybrid system: it can also operate the old blower, which remains as an emergency backup.
Because of the more consistent treatment performance (and the addition of the fifth-stage anaerobic zone), the plant has been able to reduce chemical usage substantially. Consumption of alum, used to keep phosphorous below the permit limit of 0.5 mg/l, has been reduced from 400 to 450 gallons a day to about 200 gallons.
A free chlorine analyzer ( Emerson Process Management, Rosemount Analytical) has helped cut the use of a 12.5 percent bleach solution for final disinfection. “The analyzer in the chlorination chamber tells if we need to increase or decrease the output of the bleach pump,” Fenske says. “We just put that online recently, and we have already significantly reduced our bleach usage. In the long run, I think we’re going to save tens of thousands of dollars per year.”
The plant expansion was a large investment, but well worth it considering how precious freshwater is in Florida. “We’re extremely happy with it,” says Fenske. “We’re more than glad to show people how well it works.”