• High-strength wastewater can be processed into volatile fatty acids by utilizing an anerobic submerged membrane digester in combination with a Resin Water Electrodeionizer. 
• The volatile fatty acids are further purified through a combination of filtration and distillation and then converted into sustainable aviation fuels (SAFs) through a sequence of catalytic reactions. 
• A life cycle assessment determined that production of SAFs through the use of volatile fatty acids leads to a 71% reduction in greenhouse gas emissions compared to the conventional pathway used to produce petroleum-based jet fuel.

The aviation industry is undergoing a transition to using sustainable aviation fuels (SAFs) derived from renewable sources to reduce greenhouse gas (GHG) emissions. While the ultimate goal for aviation is to find a strategy to power airplanes using electrification, this objective is probably not achievable in the short-term.

In a previous TLT article,¹ researchers utilized lignin, a renewable raw material that consists of a series of complex polymers and is used as support materials in most plants, to produce SAFs. The appeal of lignin is availability, and its chemical structure contains a very cyclic structure that is similar to the aromatics used in conventional aviation fuel. Processing of converted biomass containing lignin produced C12-C18 cyclic hydrocarbons that are suitable for use as jet fuels. Analysis of a blend of 10% of the lignin derived SAF candidate in conventional jet fuel demonstrated the feasibility of using this renewable source as all specification limits were met. 

Another potential raw material to be used to produce SAFs is volatile fatty acids derived from wet-waste feedstocks. Dr. Meltem Urgun-Demirtas, department manager – sustainable materials and processes at Argonne National Laboratory in Lemont, Ill., says, “Volatile fatty acids are short chain carboxylic acids with one to four carbon atoms.”

Past work has shown that volatile fatty acids can be produced from wet-waste feedstocks through a process known as arrested methanogenesis. Urgun-Demirtas explains, “A consortium of anaerobic microorganisms can be used to convert wet-waste feedstocks such as cheese, whey and brewery wastewater into volatile fatty acids. Care must be taken in selecting a suitable microbial community to prevent the formation of methane (the main product of methanogenesis) and for minimizing build-up of lactic acid.”

Another factor that is of concern is that those microorganisms able to produce volatile fatty acids cannot survive in the presence of high concentrations of these acids. Urgun-Demirtas says, “One approach for circumventing this problem is to literally expedite the removal of volatile fatty acids through a process known as in situ product removal.”

Wastewater is a potential feedstock as long as sufficient organic content is present to facilitate efficient conversion into volatile fatty acids. According to Urgun-Demirtas, the key is to ensure the wastewater displays a high enough carbon oxygen demand (COD). She says, “COD is a measure of the carbon content in water. Typically, COD values in effluent from residential areas will not exceed 500 milligrams per liter. But that figure can rise when the effluent originates from industrial operators. COD values in excess of 5,000 milligrams per liter are classified as high-strength wastewater. Highly enriched effluent streams are produced from the production of cheese and beer. By utilizing these sources, COD values for high-strength wastewater increase to a range between 30,000-100,000 milligrams per liter.”

Urgun-Demirtas and her colleagues have now used high-strength wastewater as the raw material to produce volatile fatty acids that are then ultimately converted into hydrocarbons which are usable as SAFs. 

Anaerobic submerged membrane digester
The researchers devised an approach for producing volatile fatty acids by utilizing an anaerobic submerged membrane digester in combination with a Resin Wafer Electrodeionizer (RW-EDI). Urgun-Demirtas says, “The digester operated under a semi-continuous feeding mode that consisted of a modified 14-liter digester, gas-volume measuring device, harvesting and membrane clean-in-place capabilities. Introduction of the RW-EDI is needed to remove the volatile fatty acids in order to ensure their continuing production by the microbial community. When present in high concentrations, volatile fatty acids are toxic to the microbes required to produce them.”

Once volatile fatty acids are produced, they are purified through filtration and distillation to increase their concentration so it will be suitable for direct conversion to SAF. Controlling the digester pH is important for the process. Urgun-Demirtas says, “Sodium hydroxide is utilized to keep the pH of the process at 6.0, which is the optimum environment for the microbial community to produce volatile fatty acids.”

To complete the production of SAFs, volatile fatty acids are sent to a biorefinery and undergo a sequence of catalytic reactions (ketonization, aldol condensation and hydrodeoxygenation) to produce crude material that can be separated and purified by distillation (see Figure 4 for a schematic of the complete process). 


Figure 4. A new approach has been developed to produce sustainable aviation fuels (SAFs) through arrested methanogenesis and SAF catalytic upgrading. Figure courtesy of Argonne National Laboratory.

A techno-economic analysis was conducted on three proposed scenarios for volatile fatty acid production to find the most cost-effective approach. One scenario was identified as being the most cost competitive and sustainable. This involved maximizing wastewater flow rate to 1,100 metric tons per day and by using a weak base anion exchange resin process that separates volatile fatty acids from the digestion broth, which contains the microbial community cell mass. As part of this process, sodium hydroxide is used and then regenerated in a recovery loop at the RW-EDI, which increases the purity of volatile acids, hence reduces cost. 

A life cycle assessment (LCA) was then conducted on the best-case scenario using Argonne’s Greenhouse, Gases, Regulated Emissions, and Energy Use in Technologies (GREET) model. GHG emissions and water consumption were evaluated for the three scenarios. The most cost competitive scenario also is the most sustainable scenario. A 71% reduction in GHG emissions compared to the pathway used to produce petroleum-based jet fuel was realized. This includes lower-water and fossil-fuel consumption.

Urgun-Demirtas says, “With our successful demonstration that SAF can be produced from wastewater, our next step is to move to the pilot scale with the eventual hope of commercialization in the future.”

Additional information can be found in a recent article² or by contacting Ben Schiltz, media relations lead for Argonne, at bschiltz@argonne.com. 

REFERENCES
1. Canter, N. (2022), “Sustainable jet fuel based on lignin,” TLT, 78 (8), pp. 14-15. Available at www.stle.org/files/TLTArchives/2022/08_August/Tech_Beat_II.aspx.
2. Wu, H., Kim, T., Ferdous, S., Scheve, T., Lin, Y., Valentino, L., Holtzapple M., Hawkins, T., Benavides, P. and Demirtas, M. (2024), “Sustainable aviation fuel from high-strength wastewater via membrane-assisted volatile fatty acid production: Experimental evaluation, techno-economic, and life-cycle analyses,” ACS Sustainable Chemistry Engineering, 12 (18), pp. 6990-7000.