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[Display omitted] • Fruit and vegetable waste has greater potential for LA production than kitchen waste. • Using LAB as inoculum improved the maximum LA concentration by 50.8%. • Optimal temperature of LA production was 37 °C for Fruit and vegetable waste and 50 °C for Kitchen waste. • Micro-oxygen limited Acetobacter growth then decreased VFAs formation. Lactic acid (LA) is an important chemical with broad market applications. To optimize LA production, food waste has been explored as feedstock. Due to the wide variety of food waste types, most current research studies have obtained different conclusions. This study focuses on carbohydrate-rich fruit and vegetable waste (FVW) and lipid-rich kitchen waste (KW), and the effect of inoculum, temperature, micro-oxygen, and initial pH were compared. FVW has a greater potential for LA production than KW. As an inoculum, lactic acid bacteria (LAB) significantly increased the maximum LA concentration (27.6 g/L) by 50.8 % compared with anaerobic sludge (AS). FVW exhibited optimal LA production at 37 °C with micro-oxygen. Adjustment of initial pH from 4 to 8 alleviated the inhibitory effect of accumulated LA, resulting in a 46.2 % increase in maximum LA production in FVW. The expression of functional genes associated with metabolism, genetic information processing, and environmental information processing was higher at 37 °C compared to 50 °C. [ABSTRACT FROM AUTHOR]

[Display omitted] • Air-micro-aeration had significant improvement with VFAs increased by 22.4%. • Air-micro-aeration at ORP +5.5 mV higher than reference ORP was favorable to VFAs. • Different gas micro-aeration did not achieve uniform mixing. • Air-micro-aeration promoted electron transfer activity and energy generation. • Consumption and conversion of VFAs by methanogens were slow by Air-micro-aeration. Micro-aeration is considered a new strategy for improving volatile fatty acids (VFAs) production of agricultural waste. This study investigated the effect and mechanism of micro-aeration of air and oxygen (O 2) on VFAs production from swine manure. The results showed that Air-micro-aeration had the most significant improvement effect, with the highest VFAs of 8.21 g/L, which was increased by 22.4%. Moreover, the mixing effects of different micro-aeration were limited, and the microbial communities significantly varied. Firmicutes and Bacteroidota were the dominant hydrolytic and acidogenic bacteria, and Air-micro-aeration preferentially promoted electron transfer activity and energy generation. Methanosarcina , Methanocorpusculum , and Methanobrevibacter can adapt to environmental changes according to their different oxygen tolerance, and the consumption and conversion of VFAs by methanogens were slow under Air-micro-aeration condition. This study revealed mechanism of micro-aeration for improving VFAs production from swine manure, providing a theoretical basis for micro-aeration regulation optimization. [ABSTRACT FROM AUTHOR]

[Display omitted] • Micro-aeration enhanced VFAs and LA by 20.3% and 19.1%, respectively. • The conversion of LA to VFAs was limited in micro-aeration condition. • Micro-aeration reduced the time to reach the maximum VFAs from 18 days to 10 days. • More obvious growth of Clostridium and Terrisporobacter accelerated VFAs production. • Micro-aeration increased Bifidobacterium to produce more LA by Bifidum pathway. Micro-aeration was proven to be an environmentally friendly strategy for efficiently enhancing volatile fatty acids (VFAs) and lactic acid (LA) production. The roles of micro-aeration on mono-digestion of swine manure (SM) for VFAs production and co-digestion of SM with corn silage (CS) for LA production were investigated, respectively. In this study, micro-aeration increased the maximum VFAs concentration by 20.3% to 35.71 g COD/L, and shortened the time to reach the maximum from 18 days to 10 days. Micro-aeration limited the conversion of LA into VFAs, leading to LA accumulation effectively to be 26.08 g COD/L. Microbial community analysis suggested that Clostridium and Terrisporobacter were always the dominant bacteria with or without micro-aeration for VFAs production, but the relative abundance increased notably during the same period. However, Bifidobacterium , which could use the higher productivity metabolism pathway, i.e., Bifidum pathway to produce LA, increased from lower than 1% to 22.9% by micro-aeration. [ABSTRACT FROM AUTHOR]

• 1. Co-AF of SM with AW for LA production was optimized using BBD-RSM. • 2. The optimum conditions were 60.4% AW in feedstock, 34.7 ℃, and 5.0% VS. • 3. LA concentration of 31.18 g/L was obtained in the optimum condition. • 4. Experimental result was in good agreement with the predicted value of the model. • 5. Both Lactobacillus and Clostridium were dominant in the co-AF. Co-anaerobic fermentation (co-AF) of swine manure (SM) and apple waste (AW) has been proved to be beneficial for lactic acid (LA) production. In order to further improve the LA production, three important parameters, namely AW in feedstock, temperature, volatile solids (VS) of feedstock, were evaluated using Box-Behnken design and response surface methodology. The quadratic regression model was developed and interactive effects was found between the three parameters. Results showed that the maximum concentration, 31.18 g LA/L (with LA yield of 0.62 g/g VS), was obtained under optimum conditions of 60.4% AW in feedstock, 34.7 ℃, and 5.0% VS. At the optimum conditions, the solubilization of organic matter was enhanced compared with mono-fermentation of SM. Microbial community structure of the reactor diverged greatly with fermentation time. Clostridium and Lactobacillus were dominant bacteria in the fermentation process, resulting in a remarkably LA accumulation. [ABSTRACT FROM AUTHOR]