Exploring Carbon Capture Opportunities at Water Resource Recovery Facilities

OBJECTIVE:
The increasing emphasis on circular economies has led wastewater treatment in recent years to become a renewable source for energy, heat, water, fertilizers, carbon products, and soil amendments. Despite these advancements, one significant and potential resource that remains unrealized is biogenic CO2, which can be used for geological sequestration or other manufacturing. Given the arising climate change policies and markets for biogenic, or green CO2, there will be greater financial incentives in the future for WRRFs to implement bioenergy programs with carbon capture, utilization, and sequestration (BECCUS) strategies. This paper explores a market assessment for biogenic CO2 end-uses, funding mechanisms and economic incentives, regulations and policies, technological solutions, and techno-economic analyses (TEA) for carbon capture at a WRRF (tail gas from RNG, aeration basins, post-combustion).

CURRENT CO2 MARKET
In 2022, commodity CO2 demand in the United States was estimated at 10.3 million MT per year with over 64% of that sourced from fossil fuel derivatives (Figure 1). About 25 percent of CO2 are mined, with the remaining demand supplied as a byproduct from chemical manufacturing processes (ammonia, ethanol, hydrogen, and natural gas processing). While most of the CO2 is used in enhanced oil recovery (EOR), the second most common use of CO2 is for the food/beverage industry (approximately 30% of all CO2 demand).

THE VALUE OF BIOGENIC CO2 AND THE FUTURE CO2 MARKET
CO2 can be categorized by its sourcing as traditional CO2 or green CO2. Traditional CO2 mainly comes from the burning of fossil fuels and results in an accumulation of CO2 in the atmosphere. As climate-driven initiatives and regulations become more pressing, the CO2 market will need to evolve to more sustainable alternatives such as green CO2.

The two main sources of green CO2 are direct air capture (DAC) CO2 and biogenic CO2. DAC separates CO2 from the atmosphere through chemical reactions and is not limited to point of emission locations. However, DAC is both an extremely expensive and energy intensive method of capturing CO2 (Figure 2).

Biogenic CO2 is a favorable alternative as it comes from the degradation (digestion/fermentation), combustion, or mineralization of organic matter, and tends to result in a higher concentrated CO2 stream. Currently, ethanol plants provide up to one quarter of the existing biogenic CO2 market's supply in the U.S., but its production is limited to the midwest region. WRRFs on the other hand, are situated in most urban centers, and generate biogenic CO2 as a natural byproduct, making them prime locations for CO2 capture.

OPPORTUNITIES AND CHALLENGES FOR BIOGENIC CO2
WRRFs can utilize two pathways for biogenic CO2: carbon capture and utilization (CCU) or carbon capture and storage (CCS). Figure 3 shows a general diagram of the two paths. A national survey will be performed to examine the potential end-uses of captured biogenic CO2 from WRRFs. Partial details of the survey are shown below, with full details included in the paper:
- E-Fuels — also known as 'electrofuels', are a class of synthetic fuels that are produced from combining the carbon in CO2 with green or blue hydrogen.
- E-Chemistry — Use electrosynthesis to produce chemicals through more sustainable avenues.

CCS is the process in which captured CO2 is compressed and injected in underground geologic formations for permanent storage or sequestration. WRRF would partner or contract with a regional or local CCS service provider, and the credits can be used to offset a utility's emissions or sold in the voluntary or compliance carbon market.

FUNDING MECHANISMS AND ECONOMIC INCENTIVES
The Inflation Reduction Act (IRA) was passed in 2022 and provided climate-related funding for domestic clean energy production, decarbonization technologies, climate change mitigation, and resiliency programs. Within Section 45Q of the IRA, provisions for CCU and CCS provide the financial incentives for utilities with bioenergy programs to invest in CO2 capture technologies (Table 1).

TECHNOLOGICAL SOLUTIONS
For CO2 capture off tail gas in RNG facilities, the cleaning process includes removing CO2, H2S, siloxanes, VOCs, and moisture. Each of these technologies have varying levels of compatibility with CO2 recovery technology in the tail gas as shown in Table 2.

TECHNO-ECONOMIC ANALYSIS
A TEA was developed to evaluate the financial feasibility of CO2 capture program at a 15 and 50MGD plant with and without IRA eligibility as an add-on to an RNG program (Table 3 and Figure 5). The future paper will explore TEA options for all potential carbon capture sites.

The TEA results that a 'no distributor model' with IRA funding reflected a positive NPV. Note that CO2 pricing and transportation costs may change which will impact the economics.

CONCLUSIONS
Capturing CO2 from municipal bioenergy programs is a significant opportunity for resource recovery and carbon management and stands out for its location, practicality, and effectiveness.

However, a limitation for WRRFs-sourced biogenic CO2 is its scale and size as many require a production of a minimum of 30,000 to 50,000 tons of CO2 per year for partnership. We anticipate that the competition for green CO2 sources will drive these end-users to seek WRRFs in the future.