Methane-arrested anaerobic fermentation of orange peel waste: impact of headspace gas composition and nitrogenous co-substrates on carboxylate profiles

Details

Supervisors

Gerin, Patrick A.

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en chimie et bioindustries, à finalité spécialisée

Abstract

enWithin the transition toward a circular bioeconomy, the carboxylate platform provides a "Waste-to-Chemicals" route to convert agro-industrial residues into high-value platform molecules, particularly medium-chain carboxylic acids (MCCAs) via anaerobic chain elongation (rBOX). However, the anaerobic valorisation of orange peel waste faces two major hurdles: a nitrogen deficiency and the antimicrobial toxicity of endogenous essential oils, predominantly D-limonene. This master’s thesis aims to evaluate and optimize the anaerobic fermentation of orange peel waste inoculated with sewage sludge, investigating how the combination of nitrogenous malting co-substrates and headspace gas control can overcome bioenergetic barriers, suppress methanogenesis naturally, and selectively steer metabolic fluxes toward elongated carboxylates. Batch anaerobic fermentations of orange peel pulp were carried out over 64 days using native municipal return activated sludge. Two industrial malting residues (rootlets vs green malt) were evaluated as co-substrates under two contrasting headspace atmospheres: a nitrogen control versus a reducing hydrogen/carbon dioxide mixture. Process performance was monitored through periodic pH regulation, headspace gas composition profiling, chromatographic quantification of soluble carboxylates, and cumulative COD mass balances. Endogenous D-limonene from orange peel waste naturally arrested methanogenesis, securing a productive, chemical-free window that enabled carboxylate accumulation following re-inoculation. While a N2 atmosphere remained thermodynamically restricted to short-chain acids, supplying an H2/CO2 headspace overcame this bioenergetic bottleneck, driving homoacetogenesis and rBOX to selectively boost valerate, butyrate, and caproate synthesis. Malting by-products effectively rebalanced the C/N ratio without synthetic additives, where green malt accelerated initial startup kinetics while rootlets ensured superior conversion completeness in the final mass balance. However, the late-stage recovery of hydrogenotrophic methanogens acted as an electron vacuum that depleted carboxylate yields. Integrating pending metagenomic sequencing data could validate the functional succession of key elongating guilds and archaea.