Methane and carbon dioxide content of produced biogas
The cumulative biogas production volumes in the anaerobic co-digestion for R1, R2, R3 and R4 were 399.21, 1380.22, 1740.43 and 892.99 mL, respectively (Fig. 3). The specific biogas production (mL/g·VS) ranges were 0.01–1.78 for R1, 0.01–3.27 for R2, 0.01–3.59 for R3 and 0.01–0.93 for R4, and the average biogas vs. VS in all samples was observed in a range of 0.59–2.04 (mL/g·VS) (Fig. 4). The production of biogas was low until 3 days after the reaction, and it started to increase after 5 days of the reaction in R1 and R4 samples. In particular, the biogas production increased significantly after 5 days for R2 and R3 samples, and the maximum cumulative biogas yield was observed for R3. The yield and composition of biogas is greatly affected by the carbohydrate, fat and protein content of the co-feedstock (Chanda et al. 2012). The critical toxicity level of inorganic substances in anaerobic co-digestion depends on whether the co-feedstocks are acting alone or in combination (Strazzera et al. 2018). That is, certain combinations may have synergistic effects, while other combinations may have antagonistic effects. It was confirmed that co-digestion using FSOC as a co-feedstock has a favorable effect on biogas production.
The average cumulative methane volumes produced over 30 days of the anaerobic co-digestion process were 207.59, 924.75, 1253.11 and 509.00 mL for R1, R2, R3 and R4, respectively, and the methane content of the produced biogas was determined to be in a range of 51.91–72.06% for all samples. The maximum average methane content reached 72.06% for R3, and the minimum average methane content was 51.91% for R1. In R2 and R3, both the process stabilization time was fast, and the biogas production was high, and the content of methane gas was also higher than that of R1 and R4 samples. The carbon dioxide content of the produced biogas was measured in the range of 127.75–348.09 mL for all samples, which was in the range of 20–32% of produced biogas. Sample R1 showed the highest ratio of 32%, and R3 was the lowest with 20%. According to the report of previous studies, the methane content was 60.7–68.0% and carbon dioxide content was 31.7–38.3% in biogas from the anaerobic co-digestion of jatropha oil cake as a co-feedstock (Chanda et al. 2012). After process stabilization, the average proportion of methane produced from biogas measured by Deshpande et al. (2012) ranged from 68 to 72%, and the amount of carbon dioxide was 28 to 31%. Similar results for the range of methane concentration of biogas in anaerobic co-digestion using various co-feedstocks were also observed in refs. (Chanda et al. 2012; Deshpande et al. 2012; Jhr et al. 2020; Pohl et al. 2019; Wang et al. 2018).
Figure 3 and Fig. 4.
Anaerobic co-digestion can produce synergistic interactions through nutrient balance, trace element supplementation, dilution of toxic and inhibitory compounds, and promotion of microbial diversity (Pan et al. 2019). FSOC also has a high content of nutrients and trace elements, so it increases the diversity of anaerobic microorganisms in anaerobic co-digestion, prevents partial accumulation of VFAs in the digester, and activates methanogenic bacteria at a balanced C/N ratio for biogas production (Esteban-Gutiérrez et al. 2018; Hassan et al. 2017; Pan et al. 2019). A combination of substrates through anaerobic co-digestion can lead to synergistic effects such as dilution of methanogenesis inhibitors, addition of nutrients to increase biodegradability and/or changes in the microbiome resulting in improved metabolism (Chen and Brewer 2021; Pan et al. 2019). Ebner et al. (2016) proposed comparing the biomethane potentials of co-digested substrates with a weighted sum of single-substrate biomethane potentials as a measure of synergistic or adversarial interactions. The co-digestion performance index (CPI), or synergy index, is defined as the specific methane yield (SMY) due to co-digestion divided by the weighted average of the SMY obtained from a single digestion of each co-feedstock (Pan et al. 2019). A high CPI value does not guarantee the maximum SMY if it is used as a performance indicator. Therefore, both SMY and CPI should be considered to determine the optimal mixing ratio of the common co-feedstock. CPI is used to evaluate antagonistic (CPI < 1), additive (CPI = 1) and synergistic (CPI > 1) interactions in co-digestion (Karki et al. 2021). The calculated CPI was 1.29 for R2, 1.39 for R3, and 1.10 for R4, indicating that R3 had the highest synergistic effect, and R1 was used as a control (Fig. 5).
According to a previous study, when pig manure: corn stover: cucumber residues were used as co-feedstocks, the CPIs were determined to be 1.9 and 1.8, respectively, indicating a high synergistic effect (Wang et al. 2018). On the other hand, when food waste: toilet paper and oat straw: cow manure were used as co-feedstocks, CPIs were − 1.0 and − 1.5, respectively, and no synergistic effect was found in anaerobic co-digestion (Kim et al. 2019; Zhao et al. 2018). As such, the synergistic effect for biogas production is determined depending on which co-feedstock is used for anaerobic co-digestion.
Figure 5.
Table 4 summarizes the amount of biogas produced using various co-feedstocks. Sugarcane straw, wheat straw, rice straw and fallen leaves showed low methane production of 0.067–0.174 (mL/g·VS), whereas Jatropha and sunflower oil cakes reached high methane production in the range of 0.422–0.460 (mL/g·VS). De-oiled seed cakes have a high VS content, including hard lignocellulose, proteins, digestible carbohydrates and lipids (Jhr et al. 2020), with the lipid content contributing the most to CH4 formation. Higher operating temperatures, lower substrate concentrations and lower lignin content provide higher biogas yields. Although the oil seed cake contains some lignin, most of the lignocellulosic matrix degrades during oil extraction, resulting in a low C/H ratio (9–12) and a high CH4 yield (Choi 2020a; Paritosh et al. 2021). In this study, 0.56 (mL/g·VS) of methane production was measured for R3, which produced about twice as much methane gas than sugarcane straw or fallen leaves.
Table 4
Biogas yield by various co-feedstock
Feedstock | Operating conditions | CH4 Yield (mL/gVS) | Reference |
C/N | Time (day) |
Wheat straw | 53.5 | 40 | 0.1779 | Hassan et al. 2017 |
Rice straw | 59.5 | 30 | 0.1743 | Kaintholaet al. 2019 |
Fallen leaves | 58.9 | 30 | 0.0673 | Elsayed et al. 2021 |
de-oiled rice bran | 16.5 | 40 | 0.270 | Jhr et al. 20208 |
Pig manure | 13.0 | 85 | 0.3587 | Wang et al. 2018 |
Suldge and fishwaste (7:3) | | 30 | 0.38 | Choi 2020a |
Food waste | 15.1 | 30 | 0.447 | Tang et al. 2019 |
Jatropha oil cakes | 12.7 | 30 | 0.422 | Chanda et al. 2012 |
Pongamia oil cakes | 8.7 | 30 | 0.448 | Chanda et al. 2012 |
Sunflower oil cake | 11.8 | 7 | 0.460 | Raposo et al. 2009 |
sewage sludge and fermented sesame seed oil cake (7:3) | 11.7 | 30 | 0.42 | This study |
sewage sludge and fermented sesame seed oil cake (5:5) | 13.4 | 30 | 0.56 | This study |
Lignocellulosic biomass feedstock has great potential in anaerobic co-digestion due to its abundance, low cost and high availability throughout the year. However, the slow hydrolysis rate of lignocellulosic-based biomass in general limits the single digestion of these highly recalcitrant feedstocks, which is often addressed by utilizing expensive pretreatment (Chen and Brewer 2021; Paritosh et al. 2021). Pretreatment to improve the stability of the anaerobic process has been used to loosen the structure of the biomass, release more readily degradable components, increase the accessible surface area, and remove toxic saponins from the oil seed cake (Chen and Brewer 2021; Grübel et al. 2019). In particular, some methanogenesis-inhibiting compounds, such as furfural and hydroxymethyl-furfural, are inevitably released from pretreatment of lignocellulosic biomass (Jhr et al. 2020; Karli et al. 2021; Monlau et al. 2013), meaning that not all pretreatments have a positive effect on CH4 production. It was recently reported that CH4 production was improved by more than 17% using an efficient two-stage anaerobic co-digestion (Chen and Brewer 2021; Karki et al. 2021; Pohl et al. 2019). According to a previous study, methanogenesis inhibiting compounds were not significantly produced in the two-step anaerobic digestion (Choi 2020a; Paritosh et al. 2021; Pohl et al. 2019). In addition, two-stage anaerobic co-digestion can reduce the use of chemicals (Choi 2020a,b; Siddique and Wahid 2018).
Table 4.
OLR (Organic loading rate)
OLR refers to the amount of organic matter supplied to anaerobic co-digestion, and excessively high OLR organic or inorganic substances inhibit bacterial growth (Choi 2020a; Pohl et al. 2019). For various samples, the cumulative biogas production increased from 399.21 to 1740.43 (mL/m3) when the OLR increased from 1.43 to 3.82 (g·VSin/L·d) and the COD loading rate increased from 5.67 to 14.72 (gCODin/L·d) (Table 5). The COD loading rate was higher than the VS loading rate in all samples. However, cumulative biogas production decreased in R4, which showed the highest organic content among samples with an OLR of 4.65 (gVSin/L·d) and a COD loading rate of 16.18 (gCODin/L·d). Since OLR is a major factor influencing the removal efficiency of VS or the productivity of biogas, many researchers have conducted various studies to find the optimal OLR (Cheng and Brewer 2021; Choi 2020a). According to previous studies, the optimal OLR of the anaerobic co-digestion is about 3–4 (kg·VS/m3·day), and the biogas production decreases when it exceeds 4.2 (kg·VS/m3·day) (Cheng et al. 2020; Paritosh et al. 2021). Jeong et al. (2019) reported that the maximum operable OLR in a single plug flow anaerobic digester was (4.2 kg·VS/m3·day), and Hassan et al. (2017) suggested 3 (kg·VS/m3·day) as the optimal OLR condition for the integrated digestion of goose manure and straw in a single CSTR reactor. The optimal OLR of anaerobic co-digestion depends on a number of parameters, including the nature of the feed organism, operating temperature and reaction time (HRT) (Choi 2021; Choi 2020a). Previous studies have shown that a balanced C/N ratio achieved through simultaneous anaerobic co-digestion of different feedstocks prevents the accumulation of VFAs due to the improved buffering capacity despite the high OLR (Choi 2020a; Hassan et al. 2017; Strazzera et al. 2018). For example, anaerobic co-digestion of food waste and pig wastewater, which are rich in trace elements or nutrients, can prevent VFA accumulation, resulting in stability and improved methane production rates (Esteban-Gutiérrez et al. 2018; Wang et al. 2018). Although food waste is deficient in trace elements, it plays an important role in activating the enzymes necessary for the growth of synthetic trophic bacterial communities and methanogens (e.g., carbon monoxide dehydrogenase, coenzyme M-methyltransferase complex, and coenzyme F430) (Latha et al. 2019). On the other hand, nutritional balance and supplementation of trace elements (e.g., Fe, Ni, Co, etc.) by pig wastewater can enhance the microbial diversity and enzymatic activity and support symbiotic and neotrophic associations (Aboudi et al. 2015; Wang et al. 2018). In general, a low OLR can cause nutritional deficiencies in the fermenting microorganisms, which can lead to low anaerobic digestion efficiency (Latha et al. 2019; Zhang et al. 2019). Conversely, a OLR that is too high can cause accumulation of VFA in the digester, and consequently inhibit the activation of bacteria, thereby causing a decrease in process efficiency (Aboudi et al. 2015; Choi 2020a). As a result of this study, it can be seen that co-digestion using FSOC is suitable for biogas production using wastes with high organic matter content because stable operation is possible even under high OLR conditions (R3 is 3.82) compared to a general single digester. In conclusion, the treated substrate in the anaerobic co-digestion using FSOC both produces a larger amount of biogas and significantly reduces the contents of TS and VS in the digester.
Table 5
Organic loading rate and biogas production by various samples
Samples | OLR (gVSin/L·d) | COD loading rate (gCODin/L·d) | Biogas production (L/gVSin·d) | Cumulative biogas production (mL) |
R1 | 1.43 | 5.67 | 0.85 | 399.21 |
R2 | 2.26 | 11.36 | 1.02 | 1380.22 |
R3 | 3.82 | 14.72 | 1.27 | 1740.43 |
R4 | 4.65 | 16.18 | 0.38 | 892.99 |
Table 5.