Enhancing lactic acid production by organic loading rate regulation during anaerobic co-fermentation: Insights into metabolic and bacterial responses

Anaerobic co-fermentation (AcoF) offers an efficient approach to convert swine manure (SM) and apple waste (AW) into valuable products such as lactic acid (LA), promoting resource recovery and sustainable utilization. This study investigated how stepwise increases in organic loading rate (OLR) influenced LA production at a temperature of 35 ℃, revealing the associated microbial and metabolic responses during semi-continuous AcoF of SM and AW. The results demonstrated that increasing the OLR to 7 g volatile solids (VS)/L/d significantly enhanced LA accumulation to a concentration of 35.70 ± 1.08 g/L, achieved with a hydraulic retention time (HRT) of 10 d and a substrate mixing ratio of SM:AW=40:60, which was 1.92 times higher than that observed at 5 g VS/L/d. The initial pH declined sharply and then stabilized around 3.80. Moreover, in-depth studies demonstrated that increasing OLR greatly improved SM and AW disintegration, with hydrolysis and acidification rate reaching 72.60 ± 5.62% and 83.22 ± 2.92%, respectively, under an OLR of 7 g VS/L/d, thereby enhancing substrate conversion efficiency. Microbial community analysis revealed that functional bacterial (i.e., Clostridium_sensu_stricto_1 and Terrisporobacter ) associated with hydrolysis were enriched at high OLR, providing sufficient substrates for Lactobacillus to metabolize and produce LA. Metagenomic analysis revealed that an elevated relative abundance of carbohydrate metabolism and membrane transport functions under an OLR of 7 g VS/L/d promotes LA accumulation. This research could offer a technological foundation for efficient large-scale LA production using agricultural waste. • Increased OLR resulted in an increase of the LA production. • An OLR of 7 g VS/L/d produced the highest LA concentration at 35.70 ± 1.08 g/L. • High OLR promotes the hydrolysis and acidification of SM and AW. • Hydrolytic bacteria enriched at high OLR provide ample substrate for Lactobacillus . • High OLR strengthened metabolic pathways related LA production.