Wet lab

Batch

Fig. 2(a-b) shows that VFA concentration had a significant increase, reaching 8.99 g/L at 72 h. VFAs were mainly composed of acetic acid, propionic acid and butyric acid, accounting for 63.6%, 24.4%, and 8.45%, respectively, which were similar to the previous studies (Liang et al., 2021a; Liang et al., 2021b). The concentration of SCOD reached 13.7 g/L after 72 h fermentation. The degradation efficiency of dry matter, hemicellulose, cellulose, and lignin of corn straw was 57.7%, 54.3%, 40.2%, and 8.61% at 72 h, respectively (Fig. 2c). As shown in Fig. 2(a), the system pH decreased from 7.47 to 5.95, which basically kept in the normal pH range for rumen microorganism growth (Li et al., 2021a). The decrease in pH stood for an increase in VFA concentration in anaerobic fermentation. Therefore, rumen microorganisms were able to effectively and quickly hydrolyze and acidify the corn straw and produce VFAs within 72 h. Dry matter degradation efficiency is an important index to show the substrate conversion in anaerobic fermentation (Gharechahi et al., 2020). The dry matter degradation efficiency of corn straw in this study was similar to that of grass clipping (Wang et al., 2018), and slightly lower than that of rice straw in rumen (Gharechahi et al., 2020). The rumen microorganisms showed a shorter hydrolysis and acidogenesis time than that of traditional sludge fermentation (Tao et al., 2019). Furthermore, the VFA yield reached 0.42 g/g VS, significantly higher than that from lignocellulosic biomass fermented with anaerobic digested sludge and biogas slurry as inoculum (0.24-0.27 g/g VS) (Fang et al., 2018; Wang et al., 2019). Therefore, rumen microorganisms as inoculum have a good application prospect in anaerobic fermentation of biomass for producing VFAs.

Semi-continuous

Fig. 3 demonstrates the acidogenesis effect of the reactor at different corn straw loads for 180 d. As shown in Fig. 3(a), The VFA concentration in the reactor gradually increased as the corn straw load was raised. The average VFA concentration reached 8.5, 11.1, and 13.3 g/L at 2.5%, 5.0%, and 8.0% corn straw loads, respectively. Meanwhile, acetate, propionate, and butyrate as the main components of VFA demonstrated the same trend as VFA concentration at different corn straw loads, which was the same VFA composition in the sequential batch experiment (Fig. 3a). VFA concentration gradually increased with corn straw loads, while VFA yield gradually decreased. pH changes in the reactor corresponded to the changes in VFA concentration (Fig. 3c). In this study, rumen microorganisms achieved stable VFA production in a reactor at high corn straw loads for the first time. In other reports, although rumen microorganisms achieved a stable fermentation of straw biomass to produce sizable amounts of VFAs, the substrate loads were only at 1.0% or 2.5% (w/v) (Liang et al., 2023; Nguyen et al., 2020). The average VFA concentrations in this semi-continuous reactor at 2.5%, 5.0%, and 8.0% corn straw loads were higher than the rumen sequential batch fermentation of turfgrass, rice straw, and corn straw with the same substrate loads in literature reports. The average VFA concentration of this study reached 8.5 and 11.1 g/L at 2.5% and 5.0% corn straw loads, respectively, which were much higher than those of turfgrass at the same substrate loads (6.1 and 7.3 g/L) in batch fermentation (Wang et al., 2018). At a higher load of 8.0% (w/v), the VFA concentration of this study reached 13.3 g/L, which was significantly higher than that (10.8 g/L) of rumen fermentation of rice straw at 10% (w/v) load as reported by Liang et al. (2021). Moreover, the VFA concentration in this study was equivalent to the VFA production efficiency by biomass after pretreatment with activated sludge as inoculum (Zhang et al., 2023). In this study, rumen microorganisms demonstrated high efficiency and stability in VFA production, and have great potential for application in biomass anaerobic fermentation.

Scalability

In this study, the average VFA productivity reached 1.1, 1.3, and 1.7 g/(L·d) at 2.5%, 5.0%, and 8.0% corn straw loads, respectively. VFA productivity of this study at 2.5% corn straw load was significantly higher than that of rumen fermentation of wheat straw and corn silage at 3.0% load (0.6 and 0.4 g/(L·d)) in a sequential batch reactor as reported by Nguyen et al. (2019). At a higher load of 8.0%, VFA productivity of this study was significantly higher than that of rumen fermentation of food waste and corn straw co-substrate at 7.5% load (0.3 g/(L·d)) in a sequential batch reactor as reported by Xing et al. (2020a). Thus, VFA productivity further indicates that the 8.0% corn straw load is the optimal load of VFA production in this semi-continuous reactor. The efficient productivity of VFA may be because this semi-continuous reactor provides stable fermentation conditions for VFA production, and long residence time also provides sufficient conditions for efficient hydrolysis and acidogenesis (Liang et al., 2023). In addition, shortening the hydraulic residence time or increasing the corn straw load may further increase the VFA productivity of this rumen semi-continuous reactor. Except for the 5.0% corn straw load, VFA concentration showed a gradual increase, then a gradual decrease, and finally leveled off in this rumen semi-continuous reactor. Specially, the VFA concentration showed a significant decrease at 2.5% and 8.0% corn straw loads (Fig. 3a). The same decrease in VFA concentration was observed in a rumen membrane bioreactor with corn straw as the substrate (Nguyen et al., 2020). Also, a significant decrease in VFA was observed in long-term rumen fermentation of corn straw (Liang et al., 2023). Changes in the fermentation environment might cause significant changes in the rumen microbial community, further causing a decrease in the VFA concentration. Meanwhile, the accumulation of VFA might lead to a decrease in the hydrolytic and acidogenic activities of rumen microorganisms (Darwin and Blignaut; 2019).

efficiency of main components (c) during corn straw of rumen fermentation.

Pollutant Reduction
Microbial Analysis

Cost Analysis

Fig 1. Variation in pH, VFA accumulative production (a), VFA components (b), and degradation efficiency of main components (c) during corn straw of rumen fermentation.

Cost Analysis
Cost Analysis
Cost Analysis

Fig 2. Acidogenesis effect of different corn straw loads. The variation in (a) VFA concentration and yield, (b) VFA composition, and (c) pH.

Key Findings

Acetate, propionate, and butyrate were the main VFA components (Fig. 1b). VFA components in this study were similar to those of most studies in degrading various crop straws using rumen fluid (Bhujbal et al., 2022). The acetate percentage in the reactor at 8% corn straw load decreased, and the propionate percentage increased. The percentage of propionate in VFA produced from rumen fermentation of rice straw gradually increased with increasing substrate concentration as reported by Liang et al. (2021). The change in the percentage of VFA components might be attributed to low pH. The activity of acetogenic bacteria is strongly affected by low pH, whereas propionate-producing bacteria maintain high activity in an acidic environment (Li et al., 2023). This was also verified in the rumen environment of ruminants, where low pH led to a decrease in the acetate percentage and an increase in the propionate percentage (Bach et al., 2023). pH significantly dropped to 5.4 in the reactor at 8.0% load (Fig. 2c), the low pH affected the community of rumen microorganisms, further altering the VFA components.

Fig. 4 shows the variation in SCOD concentration, VFA/SCOD ratio, and VS removal in this reactor at different corn straw loads. Obviously, the higher the corn straw load, the higher the SCOD concentration, indicating that more soluble organic substances were converted from the solid phase to the liquid phase, referring to the stronger liquefaction capacity. The average SCOD concentration reached 14.2, 16.5, and 22.2 g/L at 2.5%, 5.0%, and 8.0% corn straw loads, respectively. The average liquefaction efficiency decreased, reaching 56.8%, 33.0%, and 27.8% at 2.5%, 5.0%, and 8.0% corn straw loads, respectively. VFA/SCOD ratio indicates how much soluble organic substances are transformed into VFAs, which refers to the acidogenesis efficiency. The average VFA/SCOD ratio reached 58.9%, 62.7%, and 59.8% at 2.5%, 5.0%, and 8.0% corn straw loads, respectively, indicating that this reactor maintained high acidogenesis efficiency with different corn straw loads. VFA concentration is equal to liquefaction efficiency multiplied by acidogenesis efficiency multiplied by corn straw load. Thus, as the corn straw load increases, the concentration of VFAs also increases.

Cost Analysis

The average VS removal reached 53.1%, 37.9%, and 29.2% at 2.5%, 5.0%, and 8.0% corn straw loads, respectively. The higher the corn straw load, the lower the VS removal, which corresponds to the lower VFA yield. This result is consistent with the results reported in other studies (Liang et al., 2021; Zhen et al., 2024). This is mainly because that as the corn straw load increased, the amount of corn straw being hydrolyzed also increased, but the amount of corn straw being hydrolyzed increased less than the amount of corn straw increasing. Meanwhile, the degradation of lignin in corn straw required high-potential oxidative enzymes, which were difficult to function in anaerobic environments (Zhen et al., 2024). Therefore, rumen microorganisms might only convert cellulose and hemicellulose in this study, leading to a high accumulation of lignin. In addition, an increase in corn straw load of the rumen semi-continuous reactor equated to a decrease in rumen microbial population, which further led to a decrease in the number of secreted hydrolytic enzymes by rumen microorganisms (Liang et al., 2021). Typically, the rumen digestive system of ruminants has high substrate fermentation, and its biomass degradation efficiency is much higher than that of this study (Weimer, 2022). Also, most studies used rumen fluid from filtered solids as an inoculum, and most rumen microorganisms attached to the fibers were removed, which unintentionally reduced the hydrolysis and acidogenesis capacity of rumen microorganisms (Basak et al., 2022). The reactor had the lowest VS removal at 8.0% corn straw load, but the VFA concentration was the highest. Thus, the 8.0% corn straw load is suitable for generating VFAs in this reactor.

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