A California wastewater treatment plant uses hauled-in high-BOD wastes to maximize biogas production and generate more power than it uses.
By 2020, the East Bay Municipal Utility District wastewater treatment plant could be selling twice as much electricity as it uses.
With the addition of a new high-efficiency biogas-fueled turbine in early 2012, the plant in Oakland, Calif., is already selling excess electricity. The district is probably the first water or wastewater utility in the nation to sell excess electricity produced solely from waste material back to the grid.
Over the last 10 years, EBMUD has been updating the plant and expanding efforts to take full advantage of biogas resources. "It's exciting," says Dave Allen, power plant supervisor. "Every time I give a tour, people say 'You've got to be kidding me.' They think we just treat toilet water. They're shocked at what we're doing with electricity."
The initiative was the brainchild of wastewater director Dave Williams, whom Allen credits with driving the idea now called 2x20: two high-efficiency biogas-driven turbine generators by 2020. Biogas turbine fuel comes in large part from high-BOD food and process wastes trucked to the plant and added to the digesters.
Exceeding plant demand
The new high-efficiency turbine is a Mercury 50 model from Solar Turbines of San Diego, rated at 4.5 MW. While the typical turbine is 25 percent efficient, Solar rates the Mercury 50 turbine at 38 percent efficient. The company developed it as part of the U.S. Department of Energy's Advanced Turbine System Program.
Solar says it has the highest electrical efficiency for a gas turbine its size, along with ultra-low emissions. The plant's emission permit allows 20 ppm for NOx; the turbine actually produces about 7 to 8 ppm. The turbine is supplemented by three 2.1 MW, 3,000 hp Enterprise engines installed in 1985, giving the plant the capacity for about 10 MW of electrical generation fueled by biogas.
The engines used to generate about half the plant's electric power. The turbine quickly showed its potential: Over two days starting last Feb. 1, the plant's generation consistently exceeded its demand by about 2 MW, allowing EBMUD to sell excess power back to Pacific Gas and Electric, the local power company. "From March to October, our wastewater plant runs on 4.5 MW of electrical demand," says Allen. "Our power plant is producing anywhere from 4 to 7.5 MW." It normally produces about 6 MW or higher.
With the turbine, the plant is generating around 15 percent more power than it uses per year, and that is expected to grow to 25 percent or more in two years as the district's food waste program continues to grow.
Electricity sales are bringing in the equivalent of about $500,000 a year in revenue, and EBMUD is seeking some long-term contracts to sell its electricity to area businesses and industries that could double that amount. Income from waste hauled to the plant's digesters is about $8 million. Tipping fees range from 3 to 11 cents per gallon for liquids; food wastes, which require much more handling, have tipping fees from $30 to $65 per ton. In addition, the plant is saving about $2.5 million a year in electricity bills at today's rates.
Driven by change
When EBMUD began using biogas to generate electricity in 1985, the community had a much different industrial makeup — the plant served a dog food factory, several food processing companies, canneries and other food producers. That much BOD load coming into the headworks ultimately provided about 1,200 scfm of biogas from the digesters. When those industrial customers later closed down, gas production fell off to 800 to 900 scfm.
After a few years of studying ways to increase organic loading, EBMUD started a Resource Recovery (R2) program in 2002 to increase biogas production by adding fats, oil and grease (FOG) and high-strength waste to the digesters.
The program is managed by Sophia Skoda, a senior civil engineer. "We were put in a position where we had to either raise rates because of the departure of the large commercial customers, or we had to figure out some creative ways to use our capital and labor," Skoda says. "There were septage and FOG trucks looking for a place to go, and that's how it really started."
Allen adds, "Right now, we're averaging 1,900 to 2,000 scfm of biogas." That is expected to increase to around 2,700 scfm with some operational changes this year, including blending tanks, and passive overflow digesters to allow the addition of biosolids and more hauled waste to the digesters. "We found that pumping all of that product straight into a digester makes gas production go through the roof," says Allen.
A boon for treatment
The loss of BOD in the influent reduced biogas production but helped the plant's treatment process. "We have a very stable BOD load in the influent now," says shift supervisor John Cloak. "All the high-strength waste goes straight to the digesters, so we have a little more luxury on the secondary treatment side. We don't have to worry about too many large spikes of BOD coming in."
Skoda says operators were a bit hesitant as, 10 years ago, the plant began adding more and more hauled waste to the digesters and began operating outside the normal parameters. "We have mini-upsets," she adds. "It used to be a nerve-wracking thing. But the operators have gotten very good at spotting trends and figuring things out."
The 70 mgd (average) treatment plant uses a pure oxygen activated sludge process operating one of two cryogenic reactors at any given time. Cloak notes that pure oxygen plants like EBMUD's have a tendency to promote filament growth. "We combat that by running a low MCRT/SRT [mean cell resident time/solids retention time]," he says. "I'm able to do that because I don't have large BOD loads coming into the front end of the plant."
Change in digestion
The 11 digesters all had floating covers until recently, when they were replaced with fixed covers. "At the same time, we moved from the mesophilic to the thermophilic range on all the digesters — generating gas is all about organic loading," says Cloak.
"While thermophilic digesters may have a tendency to be a little harder to dewater, they seem to be able to take a pretty high organic loading. We bring in some wastes with high BOD values and put it straight into the digesters with the idea of producing more and more methane. They've handled it pretty well."
It is difficult to add large organic loads to mesophilic digesters because volatile acids will increase, making the digesters sour faster. "The thermophilics can take a pretty good hit," says Cloak. "I may run outside my alkalinity-to-volatile-acids ratios, but it's short-term, and it recovers fast. I still watch the chemistry on them, but they can take the loading, produce the gas, and not have the tendency to upset like mesophilics."
Despite the unusual chemistry, treatment levels are still well within normal parameters: quite often in single digits for effluent BOD and TSS discharged to the Pacific Ocean. Ammonia limits are less stringent than for a plant discharging to inland waters.
Cloak says the operations team also learned some lessons about the intricacies of high organic loading. "On a gas flowmeter, you might see a huge increase in immediate gas production," he says. "But sometimes, large volumes don't necessarily mean high-Btu fuel. That first stage of digestion with some wastes produces huge amounts of carbon dioxide, and you can't burn that."
Over the years, the plant has added 190,000 cubic feet of Dystor gas holding systems (Siemens Water Technologies). The first one went in 15 years ago because there were times when there wasn't enough gas to run all the engines, and some gas was lost through flaring. The first Dystor system helped increase electrical output at the time by 40 percent.
Seeking more waste
When the high-efficiency turbine was added this year, the infrastructure was built so that a second 4.5 MW turbine could easily be added. "All the breakers were installed, the pad was poured, and everything is ready to receive the second turbine," says Allen. That would increase total generating capacity to about 15 MW and give the plant the potential to sell twice as much electricity as it uses.
After the second turbine, the total required biogas flows would be about 4,000 scfm, according to Allen. EBMUD is taking steps to ensure that gas supply by expanding its R2 program to attract more FOG and high-strength waste to feed the digesters. "We have trucks coming from as far away as Bakersfield, about a 300 mile round trip," notes Allen.
The R2 program materials include chicken blood from a poultry processor, waste from wineries and dairies, septage from septic tanks, pumpings from portable restrooms, animal processing and rendering waste, water and wastewater sludge, and other commercial and industrial process wastes.
The number of customers varies greatly — for example, winery waste is highly seasonal, and dairy waste varies in character based on the market for byproducts. "We have received up to 20 trucks per day of FOG and have had hundreds of other customers with dairy, winery and other wastes that need to be disposed of in an environmentally friendly manner," says Skoda.
She says each type of waste has its own challenges. FOG has high levels of contaminants, as does food waste, which tends to include plastics and silverware. "You have to watch things, but it's been working out for us," says Skoda. "With chicken blood, we can get a lot of feathers, and wastewater plants are not used to dealing with feathers. Winery waste has grape stems. It's not easy, but we think it's the right thing and worth doing."
The plant also plans to add what Allen calls a Lettuce Digester, designated just for food waste. That would encourage people to buy the biosolids for fertilizer or compost.
Some people are reluctant to use biosolids that come from sewage. "But they like the idea if it comes from food alone and was segregated from other biosolids," says Skoda.
EBMUD continues to work on its policies and standards for accepting hauled wastes and continues to work with regulators, because sometimes there is a lack of clarity over what is allowed and which agency has authority over different waste streams.
Skoda likens it to doing research and development in real time. "That's what we do in the San Francisco Bay Area, that's our culture," she says. "Some people work on computer chips; we work at a wastewater treatment plant. Our ratepayers, through our elected board of directors, are looking for us to be innovative, do the right things for the environment, and be effective and efficient."
Cloak isn't certain yet how much loading the digesters will take, so he continues to be careful. "I've seen the ammonia run up higher than the textbook numbers, but the pH keeps us from getting into trouble with ammonia toxicity," he says. "We tend to have a little higher alkalinity also. We've pushed the limit quite a bit, sometimes to the point of nervousness, but so far it's been pretty good. Our sampling and lab work is the key to keeping things under control."