Performance and Mechanism of Fe₃O₄ Improving Biotransformation of Waste Activated Sludge into Liquid High-Value Products

Abstract

This study demonstrated that Fe3O4 simultaneously improves the total production and formation rate of medium-chain fatty acids (MCFAs) and long-chain alcohols (LCAs) from waste activated sludge (WAS) in anaerobic fermentation. Results revealed that when Fe3O4 increased from 0 to 5 g/L, the maximal MCFA and LCA production increased significantly, and the optimal fermentation time was also remarkably shortened from 24 to 9 days. Moreover, Fe3O4 also enhanced WAS degradation, and the corresponding degradation rate in the fermentation system increased from 43.86 to 72.38% with an increase in Fe3O4 from 0 to 5 g/L. Further analysis showed that Fe3O4 promoted the microbe activities of all the bioprocesses (including hydrolysis, acidogenesis, and chain elongation processes) involved in the MCFA and LCA production from WAS. Microbial community analysis indicated that Fe3O4 increased the abundances of key microbes involved in abovementioned bioprocesses correspondingly. Mechanistic investigations showed that Fe3O4 increased the conductivity of the fermented sludge, providing a better conductive environment for the anaerobic microbes. The redox cycle of Fe­(II) and Fe­(III) existed in the fermentation system with Fe3O4, which was likely to act as electron shuttles to conduct electron transfer (ET) from the electron donor to the acceptor, thus increasing ET efficiency. This study provides an effective method for enhancing the biotransformation of WAS into high-value products, potentially bringing economic benefits to WAS treatment.

Insights into Chain Elongation Mechanisms of Weak Electric-Field-Stimulated Continuous Caproate Biosynthesis: Key Enzymes, Specific Species Functions, and Microbial Collaboration

“…The electron transfer efficiency indicated the extent to which the electrons from lactate and acetate were transferred and was calculated using eq electron 0.25em transfer 0.25em efficiency = products 0.25em ( mmol e − ) lactate 0.25em ( mmol e − ) + acetate 0.25em ( mmol e − ) × 100 % where the number of electrons contained in products and substrates is (mol e – /mol) as follows: 12/lactate, 8/acetate, 14/propionate 20/butyrate, 26/valerate 32/caproate, 38/heptanoate, and 44/caprylate.…”

“…With the development of CE technology, more and more approaches − to facilitate caproate production were meditated, including optimizing operation parameters, applying online product extraction, , adding conductive materials, , electro-fermentation, and so on. However, most of the studies were conducted in batch experiments.…”

“…7,8 Nevertheless, the low caproate yield is a bottleneck for the CE technology application, calling for more research to achieve efficient caproate production. With the development of CE technology, more and more approaches 9−11 to facilitate caproate production were meditated, including optimizing operation parameters, 12 applying online product extraction, 5,13 adding conductive materials, 14,15 electro-fermentation, 16 and so on. However, most of the studies were conducted in batch experiments.…”

Electricity-Driven Microbial Metabolism of Carbon and Nitrogen: A Waste-to-Resource Solution

“…Without inserted electrodes, conductive biochar can serve as the electron shuttle between ethanol-oxidizing microbes and MCFA-forming microbes . In addition to acting as an electron shuttle, zerovalent iron (ZVI) can create a more favorable environment with a declined oxidation–reduction potential (ORP) during the valorization of waste activated sludge, whereas the addition of Fe 3 O 4 promotes the microbial activity of multiple stages, including hydrolysis and acidogenesis, as well as chain elongation . Apart from these conductive materials, a recent study suggested that with phenol addition Petrimonas mucosa FDU058 and Methanofollis sp.…”

“…81 In addition to acting as an electron shuttle, zerovalent iron (ZVI) can create a more favorable environment with a declined oxidation−reduction potential (ORP) during the valorization of waste activated sludge, 82 whereas the addition of Fe 3 O 4 promotes the microbial activity of multiple stages, including hydrolysis and acidogenesis, as well as chain elongation. 83 Apart from these conductive materials, a recent study suggested that with phenol addition Petrimonas mucosa FDU058 and Methanofollis sp. FDU007 might be involved in increased caproate production through direct interspecies electron transfer (DIET).…”