Marta Carballa

Lignocellulosic agricultural residues represent a significant challenge for global waste management. However, their high organic content offers a unique opportunity for high-value resource recovery. This study investigates medium-chain carboxylate (MCC) production from a cellulose-xylan mixture, focusing on the influence of reactor configuration: a one-stage sequential batch reactor (SBR), a two-stage SBR, and a sequential fed-batch reactor (SFBR). The operational conditions, including hydraulic retention time (HRT), volume exchange ratio (VER) and pH, were systematically varied to enhance lactate production and its subsequent elongation into MCCs. Results demonstrated that shorter HRTs and pulsed feeding strategies improved substrate hydrolysis and lactate availability, steering the fermentation pathway towards MCC production. SFBR resulted in the optimal reactor configuration since the two reaction phases generated by the two pulse-feedings enhanced, firstly, lactate production (high load, short time), and secondly, caproate production (low load, long time). This study provides scalable process design guidelines to steer carbon from low-value streams toward high-demand biochemicals, enhancing the circularity of waste treatment plants.

Macroalgae waste accumulating in coastal regions has become an increasingly significant ecological issue worldwide. Anaerobic fermentation offers an economical and environmentally friendly alternative to landfilling or incineration. This study evaluates the impact of pH (6.0 and 7.5) and organic loading rate (OLR, 1.5 and 3.0 g volatile solids (VS)·L-1·d-1) on carboxylate production, mainly short-chain fatty acids (SCFA), and microbial communities during semi-continuous anaerobic fermentation of macroalgae. Macroalgae acidification reached 40% at low OLR under both pH condition. At pH 7.5, the dominant SFCA were acetic acid (51±3%) and propionic acid (35±2%), whereas at pH 6.0, the dominant SCFA were acetic acid (37±7%), propionic acid (17±4%), n-butyric acid (25±2%), and n-caproic acid (11±6%). Increasing the OLR decreased acidification at pH 6.0, but diversified the SCFA profile, leading to heptanoic acid production. In contrast, both acidification and the SCFA profile remained stable at pH 7.5. The microbial population was more influenced by pH than by OLR, exhibiting greater diversity at pH 7.5. Strong correlations between microbial populations and SCFA production were observed, particularly involving members of the phylum Firmicutes, Actinobacteriota and Proteobacteria. Overall, this study demonstrates that maintaining a neutral pH enhances process stability and microbial diversity, while acidic conditions promote chain elongation and the production of medium-chain carboxylates.

Among processes that control microbial community assembly, microbial invasion has received little attention until recently, especially in the field of anae