A challenge issued by the Snohomish County (Wash.) Public Utility District (PUD) No. 1 to reduce energy consumption by 10 percent in three years motivated the Edmonds Wastewater Treatment Plant team to process solids more efficiently and offset escalating electric bills.

To evaluate energy savings for the challenge, plants must select a baseline representing typical energy use and develop dashboard indicators against which to measure changes. “The main energy indicator for most wastewater facilities is kilowatt-hours per million gallons treated,” says Pamela Randolph, plant manager. “However, our main indicator is kilowatt-hours per ton of dry solids processed, because we process solids from outside facilities.”

Edmonds’ energy team, led by Curt Zuvela, plant superintendent, chose 2009 as the plant’s baseline year, when processing only the city’s solids required on average 1,964 kWh per ton. Three years later, when solids processing almost doubled, usage dropped to 1,652 kWh per ton.

By the first quarter of 2013, the plant’s energy use decreased by 21.1 percent, enough to power 25 homes and reduce carbon dioxide emissions by 87,182 pounds. That year, the PUD recognized the plant and staff as a 10 Percent Energy Challenge Achiever.

How It Works

Upgraded to secondary treatment in 1990, the 11.8 mgd (design) activated sludge plant serves 75,000 people; average flow is 5 mgd. Wastewater enters through 1/4-inch bar screens, then flows through three primary sedimentation tanks, three aeration tanks and three clarifiers. Effluent is disinfected with sodium hypochlorite before discharge to Puget Sound.

Two belt filter presses (Alfa Laval Ashbrook Simon-Hartley) dewater biosolids to 23 percent solids. The material is then conveyed at 6 to 9 gpm to a fluidized bed incinerator (Dorr-Oliver). After the incinerator, the flow goes through a heat exchanger. A VenturiPak scrubber (EnviroCare International) treats the air, while the ash is cooled, separated and landfilled.

To establish energy policy and goals, the plant’s energy team joined the Wastewater/Water Sustainability Energy Cohort, a group of northwest wastewater and water agencies. Members shared best practices, helping the Edmonds team members clarify objectives, which they shared with other plant personnel.
“With an average annual O&M budget of $3.5 million and capital improvement fund of $650,000, it made sense to accept the PUD challenge,” says Randolph. “Since we joined the program in 2010, we’ve completed three PUD projects that reduced costs to the city and helped us estimate ongoing savings.”

Low-Hanging Fruits

The first project reduced pressure on the internal process water system. The staff replaced the 50 hp pump with a 30 hp pump, reducing overall pressure from 100 psi to 40 psi. In the one area needing 70 psi, they installed a 7.5 hp booster pump (all pumps from Goulds). The modifications saved 166,858 kWh, lowered annual electric costs by $11,513 and produced an incentive check for $31,451 from the PUD.

The next project replaced 40-watt T8 fluorescent tubes and ballasts with 25-watt units. “The contractor also replaced some light fixtures with LEDs, installed motion sensors in offices and restrooms, and added timers in work areas,” says Randolph. The annual savings of 20,033 kWh lowered the electric bill by $1,402 and drew a PUD check for $2,915.

The staff identified other projects through the state Department of Enterprise Services (DES) Energy Program, then used its list of local Energy Service Companies (ESCOs) to conduct an audit, evaluate projects and help design, install and finance them. ESCO contractors guaranteed the maximum project cost and projected energy savings, while DES energy engineers and the PUD provided long-term monitoring of project savings. The PUD also split projected energy savings between the plant and the contractor.

One such effort replaced a 300 hp centrifugal blower in an aeration basin with a 300 hp blower (Aerzen USA). The $325,000 modification saved 491,248 kWh annually. “Puget Sound Energy mounted meters on both blowers and measured the difference in power consumption,” says Randolph. “Based on the calculated energy savings for the first year of operation, PUD sent an incentive check for $178,849.”

Consultants And PUD

Two projects involved working with consultants. The first involved a complete mix aeration basin that had Nocardia asteroides bacteria problems. “The engineering firm suggested we switch to a plug flow mode to alleviate the problem and improve the activated sludge process,” says Randolph.

The $292,000 project replaced ceramic diffusers with 299 Sanitaire Platinum Series flat-panel neoprene diffusers. It also included new baffles to change the flow pattern, a new hatch in the tank, additional modulating airflow valves and one more dissolved oxygen probe. Plant operators installed the hatch cover, valves and probe. Saving 90,635 kWh lowered annual electric costs by $11,800 and earned an incentive check for $51,338.

“Our process control improved during the first year of operating the new configuration,” says Randolph. “Because the improvement is difficult to measure, an engineering consultant is evaluating the retrofit to see if we’re receiving the promised benefits. The study will help us make better decisions about retrofitting another aeration basin.”

The most difficult project upsized 5-inch diffuser ports to 8-inch openings on two 36-inch outfalls to reduce head on the effluent pump. “A marine engineering company evaluated dispersion and mixing models, and then the state Department of Ecology approved the selection and modified our permit,” says Randolph. The project, which enabled the plant to sustain gravity flow longer, saved 98,000 kWh annually, lowered the electric bill by $6,762 and earned a $29,400 check from the PUD.

More Improvements

A second energy audit identified additional projects, including installation of a variable-frequency drive and programmable timer on the plant’s decorative water fountain. The fountain now cycles off from 1 a.m. to 5 a.m.

Last fall, contractors replaced the main electrical switchgear feeding the plant, enclosed the switchboard in a sealed cabinet, and increased safety by adding a remote operator panel. While not specifically an energy-saving project, the staff also installed submetering devices to measure localized usage throughout the plant.

“Our operators don’t see electric bills or DES reports of reduced kilowatt-hours,” says Randolph. “They just know the electricity is on when they need it. To demonstrate that what they do does have an effect, we plan to develop dashboard indicators showing real-time energy usage. If we can see it and measure it, we can improve it.”