Energy efficiency is always on the mind of the staff at the Waco (Texas) Metropolitan Area Regional Sewerage System. While improving compliance, increasing capacity, and reducing BOD load, management and staff at the wastewater treatment plant are continually looking to save energy.
“The decisions to pursue the green initiatives have been easy,” says Kristy Wolter, operations program manager for the City of Waco Water Utilities, which operates the plant for the regional group that also includes the communities of Woodway, Lacy-Lakeview, Lorena, Bellmead, Robinson and Hewitt. “It hasn’t just made environmental sense. It makes financial sense.” The initiatives have included:
• Digester upgrades.
• Major aeration basin improvements.
• Diversion of fats, oils and grease (FOG) directly to the digesters to expand methane production and reduce treatment process energy consumption.
• Use of lift station emergency generators in a utility load management program.
Driving down demand
The wastewater treatment plant has steadily increased efficiency over the last eight years, cutting its energy use by 35 percent through innovative practices for an annual savings of about $675,000. Its focus on renewable energy and efficiency earned an Environmental Excellence Award from the Texas Commission on Environmental Quality in 2010.
From 2002 to 2005, the plant reduced electricity demand from the grid an average of 12 percent per year by increasing methane production in its anaerobic digesters. It now produces one-third of its own power.
With a current average flow of 25 mgd, the 37.8 mgd (design) plant will soon be generating even more of its own power while increasing its total capacity to 45 mgd and its high- strength waste processing by 200,000 gpd.
That will be achieved when three retired digesters are brought back to life this year and updated with dome covers, new mixers, heat exchangers and waste gas burners to achieve full acetogenic and methanogenic anaerobic digestion.
“We’re not at full two-phase digestion yet,” explains Wolter. “We’ll repurpose the tanks for the faster acetogenic digestion.” The other four will be reserved for methanogenic digestion, making methane production more efficient.
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Operational quality was at the heart of the plan to improve aeration basin performance in 2003, but energy efficiency was also a goal. The nitrification process was deficient because the diffusers were overworked, causing coarse rather than fine bubbles in the plant’s five aeration basins.
The first step was to increase the oxygen supply by increasing the number of fine-bubble diffusers in each basin from 2,800 to 3,500. Dissolved oxygen probes were added to the three zones in each basin to feed the plant’s PLC system and enable proper control of the blower inlet and throttling valves.
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The $400,000 project took just 2.4 years to achieve payback. In the first year alone, energy consumption dropped 17 percent. In the years since, energy use has been 22 to 44 percent below the baseline. Better treatment has also reduced chlorine use from a daily average peak of 6,000 pounds per day to a daily average peak of 800 to 1,200 pounds.
High-strength organics and fats, oils and grease (FOG) used to challenge the Waco plant, which received 33 percent of its daily BOD from industrial customers.
“Wastewater plants aren’t designed to treat such greasy waste, and it can cause blockages in the collection system,” says Wolter. “We converted an unused 40,000 gallon tank into an industrial receiving station in 2006 so customers could truck that waste to us. It bypasses the traditional treatment train, where the most energy is used, and goes directly to the digesters, where it generates more methane than traditional solids.”
Solving the BOD problem held off a costly expansion and in the process increased biogas production. The nine Green Partners involved in the FOG program include local industries, a convenience store, a fast food restaurant, and a convention center. With about six to nine truckloads of FOG and high-strength waste being delivered to the plant daily, the amount of grease going through the treatment process has been cut by 90 percent and solids by 50 percent.
Every month, about 65,000 gallons of FOG and 775,000 gallons of high-strength waste is placed directly into the digesters, generating 50 percent of the gas needed to run the biosolids dryer, while reducing energy use for the digesting treatment process by 30 percent —
a savings of about $120,000 a year. It also reduces loading in the aeration basins, in turn requiring less oxygen and energy to keep the process healthy.
Producing heat and power
“When we started this work in 2002, we were using 14 million kWh a year,” says Wolter. “We’re down to 9 million kWh. We’re saving $675,000 a year. As we continue to grow the program and take in more industrial waste, we hope we’ll be able to continue to use all of the methane we produce. ”
Methane not used for the dryer supplements natural gas in three 500 kW Caterpillar combined heat and power units. The plant’s digesters, in turn, receive heat recovered from those units. A planned dryer replacement will include gas cleaning to displace even more, perhaps all, of the dryer’s natural gas demand.
And the environmental benefits don’t end there. “Instead of sending the solids to a landfill, we make Class A biosolids used as fertilizer by agricultural customers and the public,” Wolter says. But 5,000 metric tons per year hasn’t been enough: “We have back orders because it’s a pretty popular product.”
Waco’s unique industrial base includes a meatpacking plant, a candy manufacturer, and a poultry processing facility that all provide energy-rich material. “We’re going to study feather byproducts —whether ground up feathers will give us any useful methane,” says Wolter. Another possibility is adding algae from the drinking water plant’s dissolved air flotation unit to the digesters to further increase methane production.
The agency replaced its dissolved air flotation unit in 2009 with rotary drum thickeners that use one-third the energy but produce more solids for the digesters — saving about 500,000 kWh per year.
Use of the lift station emergency generators during times of peak electricity demand has turned them from a source of backup power into a new revenue source. During times of high demand, the generators power the lift stations and reduce load on the area’s electrical grid. In exchange, the local utility pays the plant $70,000 a year.
Recycling of yellow oil (FOG not contaminated by other wastes) has proven beneficial, as well; keeping it out of the collection system to prevent clogs, bypassing the treatment process to save energy, and creating ten times the methane of other wastewater solids. “We started the Green Turkey Initiative for Thanksgiving in 2009 by setting up collection sites for used fryer oil,” says Wolter.
Now called Clean Up the Grease, the program has five collection sites centrally located around Waco. “It keeps people from pouring it outside where it can affect stormwater, and keeps them from pouring it down the drain and clogging the collection system,” says Wolter. “We’ve since started picking up fryer oil from restaurants and places like convenience stores and little league parks.”
A dedication to high-quality performance and a keen understanding of the treatment process has allowed Waco to produce clean water while capturing renewable resources to further limit the plant’s impact on the environment.
Details of the Waco aeration basin improvements can be found in a case study in the U.S. EPA “Evaluation of Energy Conservation Measures for Wastewater Treatment Facilities.” Visit www.epa.gov and search for Document No. EPA 832-R-10-005.
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