The Source Civil Engineering Magazine California would benefit from increased anaerobic digestion of food waste, study says
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California would benefit from increased anaerobic digestion of food waste, study says

By Jay Landers

California is looking to municipal wastewater treatment facilities to help it meet two goals: reducing statewide emissions of greenhouse gases and diverting food waste from landfills. A recently released study finds that making the necessary investments to enable California’s wastewater treatment plants to accept food waste and add it to their anaerobic digesters would prove cost-effective while at the same time reducing overall emissions and generating renewable energy.

In 2016, the California Assembly adopted Senate Bill 1383, requiring a reduction in the emissions of pollutants having short life spans, including methane. In part, the legislation mandated a reduction of methane emissions by 2030 to a level 40 percent below levels emitted in 2013. Because California’s landfills generate approximately 20 percent of the state’s methane emissions, SB 1383 set ambitious targets for diverting organic food waste from landfills, where such materials degrade and form methane. Under SB 1383, California is supposed to reduce by 50 percent the amount of organic waste it disposes of in landfills in 2020, as compared with its 2014 total. The legislation calls for a 75 percent reduction by 2025. 

CO-DIGESTION SYSTEM COMPONENTS (Carollo Engineers Inc.)

Funded by a grant from the U.S. Environmental Protection Agency, the California State Water Resources Control Board hired Carollo Engineers Inc. in December 2017 to analyze the extent to which wastewater treatment facilities in the state could help achieve the goals of SB 1383 by accepting food waste diverted from landfills and adding it to their anaerobic digesters. Known as co-digestion, this process increases the production of biogas, which in turn can be captured and reused. Such uses include heating digesters, converting biogas to compressed natural gas —  either for use as vehicle fuel or to be injected into a natural gas pipeline — or converting biogas to electricity, either for use on-site or to be sold to power utilities.

Released publicly in August, the study by Carollo is titled Co-Digestion Capacity in California. The authors estimated that California generated 6.8 million short wet tons of food waste in 2017, an amount that they expect to remain more or less constant through 2030 due to a continued reduction in per capita disposal. By 2025, the authors estimate that 3.30 million to 4.37 million short wet tons could be diverted to municipal wastewater treatment facilities, while these figures could grow to as much as 3.41 million to 4.55 million short wet tons by 2030. “As food comprises approximately 30 percent of total organics disposal, this suggests that co-digestion at California (wastewater treatment plants) may play a major role in helping the state meet its food waste diversion goals,” the study notes.

Known as co-digestion, this process increases the production of biogas, which in turn can be captured and reused.

Before it can be co-digested, food waste must be removed from the mixed waste stream or collected separately at the source, processed to remove non­organic contaminants, and converted into a slurry that is easy to pump. These steps typically occur at a materials recovery facility rather than a WWTP. The resulting slurry is trucked to a treatment facility.

To accept food waste for co-digestion and biogas production, a WWTP must have a receiving station to accept food waste slurry and sufficient capacity in each of the following systems: anaerobic digestion, biosolids dewatering, biogas conditioning and utilization, and biogas flaring. Based on the results of a 2018 survey developed by Carollo and sent to 223 WWTPs by the California Association of Sanitation Agencies, the authors of the report determined that the individual systems having the greatest excess capacity statewide are anaerobic digestion, dewater­ing, and flares. In fact, under most of the combinations of operating conditions considered, the state’s WWTPs “have adequate excess anaerobic digestion capacity to accommodate all of the estimated divertible and digestible food waste in 2025 and 2030,” the report states.

Only seven facilities will have food waste receiving stations and all of the other processes needed for co-digestion by 2025.

That said, only seven facilities will have food waste receiving stations and all of the other processes needed for co-digestion by 2025, according to the results of the survey. Absent any modifications, these facilities could handle approximately 118,000 short wet tons/year, or 3.5 percent of the total projected food waste. “If just these seven facilities were modified such that the overall system capacity matched their excess digestion capacity, they would be able to annually handle between 850 thousand and 2.2 million additional short wet tons of food waste diverted from landfills,” according to the report. “This represents 25 to 64 percent of the recoverable and digestible food waste in 2030.”

The report included an analysis of the cost-effectiveness of increasing the capacity of the other processes at California WWTPs to match the available excess capacity of anaerobic digesters. The analysis looked at two scenarios: the costs associated with co-digesting 2.4 million and 3.4 million short wet tons of food waste. The statewide capital costs for the two respective scenarios were estimated to be $968 million and $1.4 billion.

In either case, the expenditures needed to maximize co-digestion in California would represent a “net positive investment,” the report concludes. Under the first scenario, the authors found “expected revenues would exceed capital and (operations and maintenance) costs for a normalized value of $98,000,000, or $41 per short wet ton of diverted food waste,” the report states. Under the second scenario, the authors determined “expected revenues would exceed capital and (operations and maintenance) costs for a normalized value of $132,000,000, or $39 per short wet ton of diverted food waste.”

The expenditures needed to maximize co-digestion in California would represent a “net positive investment,” the report concludes.

Along with its positive findings, the report offers some caveats. The statewide benefits associated with maximizing co-digestion “could be lower for numerous reasons,” the report notes. For example, tipping fees could decrease as could the value of renewable energy. Existing credits for renewable energy programs could be lowered, while regulatory changes might increase costs. The report also does not include potential costs associated with municipalities switching the vehicles that they own and operate to those that run on renewable natural gas in order to take advantage of the extra biogas.

However, investing in efforts to maximize co-digestion capacity at WWTPs would achieve significant reductions in greenhouse gas emissions. The two statewide scenarios evaluated in the report would reduce greenhouse gas emissions in California by an estimated 1.6 million to 2.4 million metric tons of carbon dioxide equivalents.

Ultimately, greater use of co-digestion could enable California to divert 50 percent of its food waste from landfills and achieve 40 to 60 percent of the state’s goal to reduce landfill emissions by 4 million metric tons of carbon dioxide equivalents in 2030, says Rashi Gupta, P.E., a vice president of Carollo and the project manager for the team that prepared the report. “Those numbers are big, and they are significant,” Gupta says.

Gupta adds that it may make more sense to divert food waste to larger wastewater treatment facilities rather than smaller ones. Most food waste is generated in the more populous areas of the state, where large wastewater treatment plants tend to have excess anaerobic digestion capacity. “There is an economy of scale in the facilities that serve large, dense populations,” Gupta notes. “I think the economic picture would likely be more positive for those facilities than ones where a greater amount of investment is needed right off the bat.”

Armed with the results of the study, California’s State Water Resources Control Board plans to begin conducting outreach this fall to discuss next steps with the EPA and other stakeholders, including industry and community groups, says Charlotte Ely, a supervisor in the climate and conservation unit of the board’s Office of Research, Planning, and Performance. The agency will also hold internal discussions “about how we can best realize the opportunities that were outlined in the report,” Ely says.

This article first appeared in the October 2020 issue of Civil Engineering.

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