title: Thermodynamic and physiological study of caproate and 1,3-propanediol co-production through glycerol fermentation and fatty acids chain elongation.
abstract: An alternative process for anaerobic wastewater treatment with methane recovery is to elongate the carbon chain of volatile fatty acids (VFAs) with a formation of medium chain carboxylic acids (MCCAs), e.g. n-caproic acid with higher monetary value. A potential electron donor is glycerol as a surplus byproduct from the rapid growth of waste-derived biodiesel industry. In the current approach, an industrial chemical, 1,3-propanediol (1,3-PDO) is produced from crude glycerol along with a formation of other soluble byproducts including ethanol and volatile fatty acids (VFAs), which necessitates a significant amount of energy input for separation and purification. To circumvent the energy sink requirement and upcycle both the wastewater treatment process and the biodiesel industry, it is highly beneficial to produce a valuable secondary product from the byproducts. This pioneer study reports on thermodynamic and physiological insights gained into the co-production of 1,3-PDO and caproate from glycerol. Thermodynamics analysis demonstrated that a higher pH range is more favorable when either glycerol or ethanol acting as an electron donor, whereas a high partial pressure (27% at 1 atm) and a low pH (≤5.5) are advantageous for caproate formation with hydrogen. With the glycerol-to-acetate molar ratio of 4 and pH of 7, the physiological experiments achieved a co-production of 1,3-PDO and caproate. However, the caproate yield was low and found to be kinetic-limited. Caproate formation was significantly increased by the intermediate ethanol addition with the optimal mono-caproate formation obtained at the ethanol-to-acetate molar ratio of 3. A synergistic relationship was evinced through microbial characterization, resulting in Clostridium kluyveri and some bacteria with function of converting glycerol to VFAs. This study demonstrates that sufficient ethanol produced as an intermediate is capable of enhancing caproate formation in a synergistic pathway along with 1,3-PDO. The knowledge gleaned paves new avenues for the biodiesel industry by upcycling the byproduct crude glycerol into 1,3-PDO and caproate.
authors.affiliation: Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. Electronic address: ling.leng@connect.polyu.hk.
authors.forename: Ling
authors.initials: L
authors.lastname: Leng
authors.affiliation: Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. Electronic address: ypxsunypx@gmail.com.
authors.forename: Peixian
authors.initials: P
authors.lastname: Yang
authors.affiliation: Department of Civil Engineering, University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong, China; Department of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, China. Electronic address: maoy@szu.edu.cn.
authors.forename: Yanping
authors.initials: Y
authors.lastname: Mao
authors.affiliation: Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. Electronic address: zhuoyingwu@gmail.com.
authors.forename: Zhuoying
authors.initials: Z
authors.lastname: Wu
authors.affiliation: Department of Civil Engineering, University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong, China. Electronic address: zhangt@hku.hk.
authors.forename: Tong
authors.initials: T
authors.lastname: Zhang
authors.affiliation: Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. Electronic address: phlee@polyu.edu.hk.
authors.forename: Po-Heng
authors.initials: PH
authors.lastname: Lee
mesh_terms: D002208:Caproates; D005227:Fatty Acids; D005285:Fermentation; D005990:Glycerol / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D002208:Caproates; D005227:Fatty Acids; D005990:Glycerol
keywords: 1,3-propanediol; Caproate; Glycerol; Microbial characterization; Mixed culture fermentation; Thermodynamics