3.1.1 Gas production
Daily biogas yields and cumulative gas yields of lignocellulosic materials are illustrated in Fig. 1. At the beginning stage, hydrolysis and acidification were dominant, and the daily biogas production was instable. The daily biogas yields of WS, extruded WS, WSD, and extruded WSD increased rapidly and reached peaks of 32.50, 36.06, 8.04, and 16.77 mL/g TSadded on day 2, 2, 1, and 4, respectively. After extrusion treatment, the peak daily biogas yields of extruded WS and extruded WSD were significantly increased by 10.95% and 108.58%, respectively, indicating the much greater effect of extrusion treatment on WSD compared to WS. The daily biogas yield of WS was much higher than that of WSD in the first five days due to the greater lignin content and less easily biodegradable organic matter in WSD (Fig. 1). A previous study (Wahid et al., 2015) showed that, after extrusion pretreatment, an increase in sugar availability (7 − 42%) accelerated the degradation of biomasses at the early digestion phases, leading to a higher peak in daily biogas yield, which is consistent with results reported by Victorin et al. (2020) and Hjorth et al. (2011). Victorin et al. (2020) also found that the extruded WS had a 28% increase in maximum daily methane production. After 15 days of reaction, the daily biogas yield of all runs decreased slowly, keeping at a low and stable level in the remaining days. Similarly, Perez-Rodríguez et al. (2017) found that the daily biogas yield was increased only at the beginning of the digestion period and appeared to diminish after 32 days, which is consistent with the results of this study.
The results of cumulative gas yield after 31 days of reaction are shown in Fig. 1b. As can be seen, the cumulative biogas yield of WS was 178.96 mL/g TSadded, while that of WSD was only 56.68 mL/g TSadded. WS and WSD contained similar amounts of cellulose and hemicellulose, but the lignin content in WSD was much higher compared to WS, which may explain the lower biogas production exhibited by WSD (Komilis & Ham, 2003). After extrusion pretreatment, the cumulative biogas yields of WS and WSD were enhanced significantly and were increased by 22.55% and 152.33%, respectively, compared to unextruded biomass. The enhanced biogas production of WSD resulting from extrusion was much higher than that of WS, which may be related to the higher lignin content in WSD. The effect of extrusion pretreatment on promoting methane production was similar to that of biogas, with the cumulative methane yields of extruded WS and extruded WSD being 27.01% and 191.58% higher than that of unextruded biomass, respectively, indicating that extrusion on lignocellulosic materials with higher lignin content resulted in a greater increase in biogas and methane production. It also showed that extrusion pretreatment is better at improving the production of methane from lignocellulosic materials than biogas production.
As a physical pretreatment, it will not produce new chemical materials that are toxic to anaerobic microorganisms and will only destroy the lignocellulosic structure. After extrusion treatment, the particle size was reduced while the SSA was increased, and the accessibility to bacteria and enzymes was simultaneously increased (Hjorth et al., 2011), leading to an increase in gas production. Much research has been conducted on extrusion pretreatment to promote biogas production from lignocellulosic materials, and the relevant research results have been summarized in Table 2. As can be seen, most of the research results show that extrusion treatment is beneficial to the biogas production of lignocellulosic materials. The improvement in the biogas production of WS in this study was 22.55%, which is similar to the results reported in Table 2. However, the biogas yield of extruded WSD in this study was increased by 152.33%, which is much higher than the results reported in Table 2. This may be attributed to the following reasons: (1) after 15 days of AD, the surface layer of starch, crude protein, and other easily biodegradable organic matter was consumed and utilized by anaerobic microorganisms, while the more challenging to degrade lignin and crystalline cellulose were exposed. In addition, hemicellulose is degraded faster than cellulose during AD (Sambusiti et al., 2015), leading to the accumulation of cellulose and lignin in WSD (See Table 1). The extrusion process is more targeted and effective on WSD to break the resistant layer of residual lignin and reduce the crystallinity of cellulose. (2) After AD, the lignocellulosic skeleton softened and engaged better with the extruder gears; thus, the extrusion treatment was more efficient and effective. (3) The moisture content of the substrate may affect the extrusion process. Kupryaniuk et al. (2020) found that the extrusion of straw with an initial moisture content of 25% was more effective at producing biogas than with an initial moisture content of 40%. In this study, the moisture content of WS was 8.32%, while the moisture content of WSD was 84.83%. Hjorth et al. (2011) also noted an increase of 70% in methane yield after 28 days of AD of extruded barley straw. Chen et al. (2014) found that extrusion treatment increased the SSA and biogas production of rice straw by 49.35% and 72.2%, respectively, and that there was a relationship between SSA and biogas production.
Table 2
Effect of extrusion on biogas production of lignocellulosic materials
Feedstock | Extrusion conditions | Extrusion effect | References |
WS | twin-screw extruder | 22.52% increase in biogas yield | This study |
WSD | twin-screw extruder | 152.31% increase in biogas yield | This study |
WS | twin-screw extruder | No increase in methane production compared to the untreated group | Victorin et al. (2020) |
WS | co-rotating twin-screw feeder | a 14–28% and a 1–16% methane yield increase after 28 and 90 days of digestion | Wahid et al. (2015) |
10% rice straw silage + maize silage + triticale silage | two counter-rotating screw extruder | 15.70% increase in methane yield | Menardo et al. (2015) |
30% rice straw silage + maize silage + triticale silage | two counter-rotating screw extruder | 10.60% increase in methane yield | Menardo et al. (2015) |
Rice straw | twin-screw extruder | 72.20% increase in methane yield | Chen et al. (2014) |
Barley straw | two counter-rotating screws | 11.08% increase in methane yield | Hjorth et al. (2011) |
Corn Straw | single screw extruder | 9–11% increase in methane yield | Kupryaniuk et al. (2020) |
Corn cob | twin-screw extruder | 6.54–6.94% increase in methane yield | Perez-Rodriguez et al. (2017) |
Maize silage | S45-12 series extruder | 3.00–6.40% increase in methane yield | Witaszek et al. (2020) |
Maize straw silage | S45-12 series extruder | 9.00–12.40% increase in methane yield | Witaszek et al. (2020) |
Maize silage | single-screw extruder | 12.40% decrease in methane yield | Pilarski et al. (2016) |
Maize straw | single-screw extruder | 35% increase in methane yield | Pilarski et al. (2016) |
vine trimming shoots | twin-screw extruder | 15.70–21.40% increase in methane yield | Pérez-Rodríguez et al., 2018 |
maize straw | a self-made device | 7.50% and 8.51% increase in biogas and methane production | Kozłowski et al.,2019 |
Although in most studies the extrusion treatment has been effective at improving the biogas production of lignocellulosic materials, it does not always have a positive effect on biogas production. Pilarski et al. (2016) found that the methane yield of extruded maize silage was decreased by 12.4% compared to the control. Meanwhile, the extrusion itself required high energy consumption (Duque et al., 2017), which may break the energy balance. However, previous studies (Simona et al., 2013; Menardo et al., 2015) have shown the energy balance of extrusion to be positive and more effective. Panepinto & Genon (2016) found that an improvement in the electric energy can lead to values ranging from 0–6.5% (excluding the extruder self-consumption) after extrusion pretreatment. However, Menardo et al. (2015) found that when extrusion is employed (co-digestion), methane production is significantly increased by as much as 16%. For the feeds containing 10% rice straw, the energy balance is positive (excluding the extruder self-consumption), but the energy balance is close to zero when the percentage of rice straw in the feed is 30%, meaning that the effect of extrusion on biogas production is influenced by biomass composition.
3.1.2 Physicochemical properties of liquid digestate during the AD process
The pH value is one of the most important indicators, reflecting the stability and buffering capacity of the AD process (Chen et al., 2021). Anderson et al. (1992) reported that the proper pH range for anaerobic microbes is 6.40–7.60, beyond which a state of inhibition may occur by toxic effects induced by hydrogen ions, which is believed to be closely related to the accumulation of VFAs. Figure 2a shows the pH values of liquid digestate during the experiment. As can be seen, a similar pH trend was noted between all treatments, with the pH decreasing in the first 15 days and then increasing until the end of the test. At the beginning, the easily biodegradable organic matter in lignocellulosic materials was hydrolyzed and converted to volatile fatty acids, leading to the decrease in pH. The lowest pH values of 6.84, 6.84, 6.81, and 6.71 in runs with WS, extruded WS, WSD, and extruded WSD, respectively, were obtained on day 15, with a decrease of 0.60, 0.76, 0.9, and 0.99 compared to day 0, respectively. The decrease in the pH value of extruded lignocellulosic materials was lower than that of the unextruded control, indicating that extrusion destroyed the lignocellulose structure, leading to the release of easily biodegradable organic matter in the lignocellulosic materials (Zheng et al., 2014). At the end of the experiment, the pH value in all runs ranged between 7.03 and 7.17.
The EC value reflects the total concentration of soluble minerals in the AD system (Ding et al., 2018). Changes in the EC of the liquid digestate during the experiments are shown in Fig. 2b. As can be seen, a similar trend in changing EC values was noted between runs with WS, extruded WS, WSD, and extruded WSD, where the EC increased sharply and peaked on day 2 at 8.39, 7.77, 6.54, and 6.63 ms/cm, respectively, and then declined gradually until the end of the test. After day 2, the EC value in the run with WSD was always higher than that of extruded WSD, and the EC value in the run with WS was always higher than that of extruded WS. This indicates that digestion with extruded lignocellulosic materials can reduce the EC value in the liquid digestate, while higher EC has an adverse effect on plant growth and inhibits nitrate reduction and ammonium assimilation (Debouba et al., 2006). After 31 days of reaction, the EC values in the runs with WS, extruded WS, WSD, and extruded WSD were 7.95, 7.13, 6.36, and 5.80 ms/cm, respectively.
Volatile fatty acids (VFAs), the main products of biomass hydrolysis and acidification, are the main source of methane production by methanogens (Fu et al., 2016). Changes in VFA content can reflect the stability of the system during the AD process (Boni et al., 2016). Meanwhile, the VFA content is an important factor affecting the pH value and, in many cases, in the AD system, is inversely proportional to the pH (Chen et al., 2010b). The variation in VFA content is shown in Fig. 3. As can be seen, changes in the VFA content in runs with WS, extruded WS, WSD, and extruded WSD were similar, where the VFA increased quickly and peaked on day 5 at 1920, 2818, 1630, and 2422 mg/L, respectively. The peak values of VFA content in extruded WS and extruded WSD were increased by 46.77% and 48.59%, respectively, compared to the corresponding unextruded biomass, indicating obvious breakdown by extrusion of the lignocellulosic structure, which led to a large amount of dissolved easily biodegradable organic matter in the liquid digestate (Zheng et al., 2014). Although the WSD was already digested for biogas production before the experiment, the improvement in the peak value of VFA content in extruded WSD was slightly higher than that of extruded WS, which is consistent with the result of gas production (Fig. 1b). This indicates that extrusion pretreatment has a greater effect on lignocellulosic material with higher lignin content and lower easily biodegradable organic matter content in breaking down the lignocellulosic structure compared to lignocellulosic material with lower lignin content and higher easily biodegradable organic matter content. After day 5, the VFA content in all treatments decreased gradually until the end of the test. Throughout the experiment, the VFA content in WSD was always lower than that in WS, while the VFA content in extruded WSD was always lower than that in extruded WS. This indicates that the easily biodegradable and biodegradable organic matter content in WSD was much lower than that in WS, because the organic matter was decomposed by anaerobic microorganisms before the experiment.