Advancing Chain Elongation Technology for Medium Chain Carboxylic Acids Production from Waste Streams

Abstract

Innovative approaches to convert organic waste streams into biofuels and biochemicals are gaining attention as they provide substitutes for fossil-fuel based products, address waste management problems, and provide economic return. Chain elongation for medium chain carboxylic acids (MCCAs) production from organic waste streams using anaerobic mixed-culture microbial communities is one such emerging biotechnology. MCCAs are platform chemicals used as building blocks for several industrial and agricultural commodities. This dissertation research focuses on the development and optimization of anaerobic bioreactor systems for efficient MCCA production from complex waste streams by integrating process engineering, microbial ecology, and modeling tools.

We demonstrated that pre-fermented food waste and brewery waste can be used for MCCA production by engineering anaerobic microbiomes. However, excessive ethanol oxidation to acetate, a competing reaction, led to inefficient usage of ethanol present in the brewery waste. Therefore, the competing reaction was suppressed by increasing hydrogen partial pressure through the addition of an inhibitor of hydrogen consuming methanogens, 2-bromoethanesulfonate. While the inhibition initially was successful, it was short-lived as a microbial community resistant to 2-bromoethanesulfonate developed over time. Thus, controlling competing processes is challenging with heterogeneous waste streams and the use of mixed cultures and other strategies need to be developed. Furthermore, the contribution of microbial immigration from the feed to the chain elongation bioreactor was characterized. A significant fraction of the microbial community in the chain elongation bioreactor originated from the influent. However, not all immigrant populations remained active in the bioreactor, while other populations that were present at relatively low relative abundance and activity in the influent contributed significantly towards the chain elongation function.

Given that MCCA recovery with an in-line membrane-based extraction system requires solids removal from the bioreactor effluent to avoid membrane fouling, a lab-scale anaerobic dynamic membrane bioreactor (AnDMBR) was developed. This system contained stainless steel meshes to support the formation of a biological cake layer termed a “dynamic membrane” that provided filtration. The dynamic membrane achieved efficient solid-liquid separation, resulting in higher than 95% suspended solids removal, despite high bioreactor solids concentration, enabling integration of the AnDMBR with the MCCA extraction system. Additionally, the development of the dynamic membrane biofilm led to the enrichment of highly active MCCA producing populations, thus promoting chain elongation activity.

Finally, the environmental life cycle impacts of the production of caproic acid, a six-carbon MCCA, from brewery waste using chain elongation were compared with the environmental impacts of a conventional palm kernel oil approach for caproic acid production using a life cycle assessment tool. The brewery waste based system provided environmental benefits compared to the conventional route on all impact categories assessed. The results also showed that the environmental footprint of the chain elongation system can be further improved by reducing sodium hydroxide addition and using renewable energy sources for heating the system.

As several cities, industries, and organizations are evaluating organic waste diversion through anaerobic bioprocesses, this dissertation research is highly relevant. It addressed knowledge gaps and technological barriers associated with MCCA production from waste streams and suggested strategies to guide future technology development.