Image courtesy of Brown and Caldwell

The digester facilities at Monterey One Water's Regional Treatment Plant in Marina, California. Image Courtesy of Monterey One Water

‘Perfect fit’

Looking to decrease greenhouse gas (GHG) emissions and minimize landfill waste, Roseville recently added two new anaerobic digesters to its Pleasant Grove Wastewater Treatment Plant as part of an expansion of the facility’s treatment capacity from 9.5 mgd to 12 mgd. 

Previously, the Pleasant Grove WWTP did not have primary treatment or digestion facilities, says Bryan Buchanan, the city’s wastewater utility manager. As a result, the expansion of the WWTP offered the city a “blank slate” on which to evaluate different scenarios for maximizing resource-recovery efforts, he says. “We were fortunate because we were building this expansion from scratch.” 

With the pending requirements of SB 1383, the prospect of maximizing digester gas and converting it to compressed natural gas for vehicle fuel amounted to a “perfect fit” for Roseville, Buchanan says. “We viewed vehicle fuel as the best, highest use of that gas.”

Waste to energy

To facilitate its waste-to-energy program, Roseville added a receiving facility to accept and process commercial high-strength organic waste — essentially fats, oils, and grease — from restaurants. When tankers carrying the organic waste arrive at the receiving facility, the material is pumped into tanks, where mixers make it homogenous. “We slowly meter that into the digesters,” Buchanan says. As a result of the added waste, the digesters “produce quite a bit more gas,” he says.

Ultimately, the city expects to divert up to 12,000 tons of high-strength organic waste annually from landfills. In the next 3 to 5 years, Roseville plans to begin accepting food waste from restaurants that will then be processed into a slurry before it is added to the digesters, says Devin Whittington, the city’s assistant director of environmental utilities. “We're hoping that we'll have an additional gas-production benefit,” he says.

Before it can be compressed and used as vehicle fuel, the methane captured from the digesters must undergo a “fairly complex” conditioning process to remove pollutants, including siloxanes, hydrogen sulfide, and carbon dioxide, Buchanan says. 

A by-product of the conditioning process is a wastestream known as tail gas. But even this wastestream is put to use, Buchanan says. The tail gas is mixed with utility-provided natural gas and fed to four microturbine cogeneration units that “generate between 400 and 500 kilowatts [of electricity] all the time, 24 hours a day,” Buchanan notes. This electricity, which is used to help power the gas-conversion process and heat the digesters, somewhat offsets the WWTP’s energy requirements.

Fuel savings add up

Roseville completed construction of the energy-recovery features approximately 6 months ago and the city is bringing them online at full-scale now, Buchanan says. The environmental engineering firm Brown and Caldwell designed the biofuel production facilities.

Ultimately, the city aims to have a fleet of about 50 sold waste collection vehicles that run on the natural gas produced at the Pleasant Grove WWTP. By fueling the vehicles with compressed natural gas instead of diesel, the city will reduce its GHG emissions by approximately 7,000 tons of carbon dioxide equivalents per year and lower emissions of nitrogen oxides by 5 metric tons per year, enabling it to earn certain credits, including from the Low Carbon Fuel Standard program. Administered by the California Air Resources Board, the program encourages the production and use of cleaner, low-carbon transportation fuels to reduce GHG emissions.

All told, designing and constructing the receiving facility, the system for conditioning and compressing the gas, and the vehicle fueling facility cost approximately $23 million, Buchanan says. The payback period for the biofuel production facilities is expected to take around 20 years, though that timing could change depending on the duration and value of the credits received by the city.

Solid-waste collection vehicles that run on natural gas cost between $30,000 and $60,000 more than similar vehicles that run on diesel fuel, Whittington says. What makes the higher-priced vehicles an attractive option is the lower cost of natural gas. “In some cases, the fuel cost is a quarter to half the price of diesel,” he says. “That's where you get your return on investment — by purchasing a cheaper fuel for a longer period of time.”

Switching all of Roseville’s solid-waste collection trucks will take some time. To date, the city has purchased 15 trucks that run on natural gas, Whittington says. Because of supply-chain problems, the city is still waiting to receive another 10 trucks it ordered 2 years ago. “We're hoping in the next two to three years we'll have about 30 vehicles that will be fueling at this location,” he says.

Monterey One Water to boost digestion

Rather than converting methane from digesters into vehicle fuel, Monterey One Water is looking to use digester gas as a major source of renewable energy to power operations at its 29.6 mgd Regional Treatment Plant in Marina, California. To this end, the wastewater utility aims to add food waste to its digesters to increase the amount of renewable energy generated during the digestion process.

Under the terms of an $8.1-million turnkey contract that was announced June 5, Anaergia Inc. — a provider of waste separation, anaerobic digestion, and biogas upgrading technology — will furnish Monterey One Water with the necessary equipment for receiving and processing food waste before it enters the facility’s digesters. The company also will provide a total of 12 mixers for the four digesters.

“The new technology will simultaneously improve performance and save energy,” according to a June 5 news release from Anaergia. “Along with the ability to co-digest food waste with biosolids, Monterey One will benefit from a significant expansion of digester capacity to provide operational flexibility.”

150 percent increase in biogas

Organic matter diverted from a local landfill will comprise the materials to be added to the digester. However, the precise types of food waste to be accepted remains to be determined as part of an ongoing study of converting organic material to energy, says Rachel Gaudoin, the communication services administrator for Monterey One Water. Funded by the U.S. Environmental Protection Agency, this study is “one component” of a “larger renewable energy feasibility study” evaluating the possibility of establishing a “renewable energy microgrid to power the campus we are located within,” Gaudoin says. 

Biogas production from the four digesters is expected to increase by more than 150 percent, according to Anaergia’s release. Monterey One Water’s existing onsite combined-heat-and-power engines will use the biogas to generate up to 1.6 MW of renewable energy. 

“The added biogas production is anticipated to meet the needs of our wastewater treatment facility and possibly provide excess power,” Gaudoin says. The project also will enable Monterey One Water to avoid more than 9,000 metric tons of carbon dioxide emissions per year, according to Anaergia’s release.

‘Model solution’

California’s Department of Resources Recycling and Recovery, commonly known as CalRecycle, will provide grant funding to the project in the amount of $4.2 million.

“With support from CalRecycle, we have a unique opportunity to co-digest wastewater solids with food waste to produce more green energy, which will both reduce our operating costs and cut our carbon emissions, said Paul Sciuto, the general manager of Monterey One Water, in Anaergia’s release. “This is a model solution for other wastewater utilities,” Sciuto said.

Ultimately, Monterey One Water’s operating costs are expected to decrease between $175,000 and $366,000 per year, Gaudoin says. Depending on particular assumptions, the payback period for the project is estimated to last from 11 and 23 years, she says.

“By upgrading its infrastructure to enable co-digestion of food waste along with its wastewater, Monterey One Water will now not only recycle water, it will also recycle organic waste that would have otherwise created methane emissions in landfills,” said Andrew Benedek, the chairman of the board of directors for Anaergia, in the June 5 release. 

“This turns a big problem into a huge benefit and ultimately is what will make a net-zero future possible for planet earth,” Benedek said. “These proven technologies will eventually be implemented at hundreds of other wastewater treatment plants around the world, and Monterey One Water is leading the way.”

Work on the project is scheduled to begin in quarter two of 2023 and end by quarter three of 2024.

Uncertain future for SB 1383

Whether SB 1383 will motivate other California wastewater treatment to pursue co-digestion remains to be seen. On June 8, the Little Hoover Commission, a nonpartisan state oversight agency, issued a report calling on the California Legislature to “enact a temporary pause to the implementation” of the legislation’s requirements. 

In its report, the Little Hoover Commission noted the state’s failure to comply with the law’s mandate to reduce, by 2020, the amount of organic material deposited into landfills by 50 percent below 2014 levels. “Instead, the amount of organic waste going into landfills increased by a million tons from 2014 to 2020,” the report stated. What is more, California is “unlikely to meet” the law’s requirement to reduce the amount of organic material entering landfills by 75 percent below 2014 levels, according to the report.

“Successfully achieving the goals [of SB 1383] will require changes in law and regulation, additional funding, and creating a more holistic approach to reducing landfill methane emissions,” according to the report, which included 12 recommendations for improving the state’s organic waste reduction program.