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).
