Fractional factorial experimental design for optimizing volatile fatty acids from anaerobic fermentation of municipal sludge: Microbial community and activity investigation

Volatile fatty acids (VFAs) production from municipal sludge is a promising venture for resource recovery while ensuring wastewater treatment plants' ecological and economic sustainability. This study used a fractional factorial design (FFD) and response surface methodology (RSM) to optimize VFAs production from municipal sludge in semi-continuous flow acid fermenters (AFs) based on four critical parameters (i.e., sludge retention time (SRT), sludge composition, pH, and temperature) interactively and individually. To ascribe the mechanisms to VFAs production dynamics, non-methanogenic microbial activity assays and microbial community composition linked to the VFAs yields were explored. FFD and RSM successfully optimized VFAs production in the AFs, and a second-order polynomial model with an R-squared of 0.83 was derived. Optimal model conditions for VFAs production were 3-days SRT, 45 ?, pH 8.1, and 0.92 sludge composition (VS/TS ratio). Under these conditions, the model predicted a 3.47-fold increase in VFAs production, close to the experimental value of 3.48. AFs at pH of 8.1 and varying temperatures harbored the highest proportion of fermentative bacteria, mainly Clostridia, and lowest community diversity, indicating strong selective pressure for VFAs-producing populations. Furthermore, the microbial activities assays provided quantitative functional information linked to the microbial communities in each AF configuration consistent with VFAs production.