Xitong Wang, Hairong Yuan,*, Xiaocong Song, Xiujin Li, Xiaoyu Zuo

1 Beijing Center for Pollution Control and Resource Recovery, Beijing University of Chemical Technology, Beijing 100029, China

2 Chinese Research Academy of Environmental Sciences, Beijing 100029, China

Keywords:Hydrothermal pretreatment (HP)Spent cow bedding Anaerobic digestion (AD)Methane yield

A B S T R A C T The anaerobic digestion (AD) performance of spent cow bedding was investigated with different hydrothermal pretreatment (HP) conditions. Spent cow bedding was pretreated with low temperatures(50, 70, and 90 °C) and different pretreatment times (2-72 h) with ammonia and without ammonia.The results showed that spent cow bedding was a good raw material for AD.After pretreatment,the concentration of volatile fatty acids(VFAs)in the group of hydrothermal pretreatments with ammonia(HPA)was higher than that in the HP group at the same pretreatment temperature and time. The optimal pretreatment condition was achieved with an HPA of 50 °C holding for 72 h. At the optimal condition, the highest concentration of VFAs was 1.58-10.85 times higher than that of the other pretreated groups.The highest hemicellulose and lignin removal rates were 58.07% and 10.32%, respectively. The highest methane yield was 163.0 ml·(g VS)-1, which was 48.9% higher than that of the untreated group. The VFAs, pH, and reducing sugars showed positive relationships with the methane yield. Therefore, HP at low temperature can enhance the AD performance of spent cow bedding.

1. Introduction

China is one of the world’s largest agricultural countries and produces an enormous amount of spent bedding materials in deep-litter housing for horses or cows. Spent bedding consists of animal faeces with loose straws,rice husks or softwood[1].Historically, spent bedding has been used directly as a fertiliser or soil conditioner [2].

Recently, the anaerobic digestion (AD) of spent bedding has been considered an environmentally friendly means of generating renewable energy sources such as biogas,which contains methane[3]. For example, Wartellet al.[1] obtained methane of different volatile solids (VS) ratios, ranging from 0.01:1 to 4:1, from degraded horse manure and softwood bedding. However, the different types of bedding materials vary in degradability. For example, Taitet al.[2] batch-digested wheat straw, barley straw bedding, and rice husk bedding and discovered that the wheat straw bedding was the most degradable at 60%, barley straw bedding at 45%and rice husk bedding was the least degradable at less than 20%. Riggioet al.[3] compared the AD performance of six types of spent bedding from deep-litter, housing for sheep, goats,horses or cows in a batch Leach-bed reactor (LBR), and found that biochemical methane potential of the beddings ranged from 192 to 239 ml·(g VS)-1.Also,they discovered that thermophilic conditions increased the methane production of spent cow bedding in LBR,compared to mesophilic conditions, the AD of spent cow bedding was a slowly-degradable substrate[4].While there has been much research focusing on the methane potential of different beddings,there are few studies on the pretreatment of bedding. Since beddings have high lignocellulosic content, suitable pretreatment is necessary for enhancing their degradation and improving AD’s efficiency [5].

There are mechanical, chemical, thermal, biological, and combined modification methods of pretreatment. Hydrothermal pretreatment (HP) is environmentally friendly; it uses no chemical solvents or external catalysts[6].During HP process,lignocellulose materials are cooked in water at high temperatures for an extensive period to destroy the hemicellulose and modify the lignin[7]. What’s more, ammonia pretreatment can open the ester bond among hemicellulose, cellulose and lignin, also dissolve part of hemicellulose,cellulose and lignin[8].Thus,the pretreatment temperature (25-240 °C) and pretreatment time (2-4800 min) were considered the two essential parameters [9]. Low temperatures are considered optimal for AD since those lower than 180 °C can enhance biogas production [10]. Luoet al.[11] examined the AD performances of rice straw under different pretreatment temperature of 90-130 °C and achieved the highest methane yield at 127.6 ml·(g TS)-1was achieved at 100 °C. Also, the methane potential of dairy cow manure was optimized at 100 °C for 5 min with 10%NaOH [12]. Taherdanak and Zilouei [13] found that with 8% NaOH for 60 min at a pretreatment temperature ranging from 0 °C to 100 °C, the highest methane yield of wheat occurred at 75 °C;the yield was 54.5% higher than that of the untreated group. Relevant research reported that greater methane yield appeared at lower pretreatment temperature range from 55 °C -90 °C [14].Yuanet al.[15] examined the AD performance of corn stover pretreated with ammonia at 50 °C-90 °C for 24-72 h and identified the highest methane yield appeared at 50°C for 24 h.These studies show that the optimum pretreatment conditions may vary for different materials.

Therefore,to improve AD performance of the bedding materials,spent cow bedding was used as AD material.The objectives of this study were: (1) to investigate the AD performance of spent cow bedding under lower temperature HP conditions, (2) to compare the effects of with and without ammonia on methane production rate of cow bedding.

2. Materials and Methods

2.1. Raw material and inoculum

The spent cow bedding was collected from a dairy farm in Xiaodian Village of the Shunyi District in Beijing,China.Spent cow bedding consisted of cow manure and rice straw with a ratio of 3:2(dry matter). Before its basic characteristics were determined, the spent cow bedding was dried naturally, crushed with a grinder to pieces shorter than 425 μm(YSW-180,Zhengde,China)and heated in an oven at (105 ± 5) °C for 10 h. The total solids (TS), VS, total carbon (TC) and total nitrogen (TN) of spent cow bedding were 91.76%, 74.06%, 40.27%, and 1.98%, respectively. And the contents of cellulose, hemicellulose, and lignin in spent cow bedding were 18.21%, 13.35% and 2.70%, respectively. The inoculum was obtained from a mesophilic biogas plant that used swine manure in Donghuashan Village of Shunyi District,Beijing,China.The main characteristics of the inoculum were TS content of 14.74%,VS content of 8.92%, TC content of 29.83%, TN content of 3.14%, and pH value of 7.50.

2.2. Hydrothermal pretreatment

The spent cow bedding was put in a blue cap bottle of 500 ml,and was heated with a constant temperature water bath. The pretreatment temperatures were 50, 70, and 90 °C, and pretreatment time was maintained for 24,72,2,and 4 h.In addition,these experiments were carried out in two groups: with and without 2%ammonia (mass fraction). HP with and without ammonia is called HPA(where‘‘A”represents‘‘with ammonia”)and HP,respectively.The solid(spent cow bedding,20 g TS)and liquid(total water contents, 120 g) ratio of the pretreatment was 1:6. At the same time,the untreated spent cow bedding (without ammonia and water)was set up the control group. For the convenience of description,each of the experimental groups was expressed in the format of‘‘HP/HPA-Temperature- Time”, as shown in Table 1.

Table 1 The description of experimental groups

2.3. Batch AD experiment

The batch AD experimental setup consisted of a 500 ml blue cap bottle acting as the digester, another 500 ml bottle acting as the gas collector and a 1-litre beaker as the water tank. Each digester contained 50 g TS·L-1spent cow bedding and 15 g of inoculum(mixed liquid suspended solid,MLSS) per liter [16];then, the final working volume was brought to 400 ml with deionised water.Finally,the digester bottle was placed in the water tank with a constant temperature of (35 ± 1) °C. Meanwhile, untreated spent cow bedding and inoculum acted as two separate control groups; the inoculum control group was used to offset the effect of the inoculum during quantification. The batch AD experiment was conducted for 40 days. Each group was run in triplicates, and its result was the mean value of the triplicate experiments. For the convenience of description, each of the experimental groups was expressed in the format of ‘‘HP/HPA-Temperature-Time”.

2.4. Analytical methods

The TS and VS of the spent cow bedding and inoculum were measured according to the American Public Health Association(APHA) standard methods [17]. The TC and TN were determined by elemental analyzer (Vario EL/cube, Germany). The pH of this study was analyzed with a pH meter (CHN868, Thermo Electron,USA). The contents of cellulose, hemicellulose and lignin were measured on the basis of the procedures reported by Van Soest[18]viaa Fiber Analyzer(ANKOM,A2000i,USA).The concentration of volatile fatty acids(VFAs)was determined with the GC-2014 Gas Chromatograph (Shimadzu, Japan). The reducing sugar content in the samples was measured by using the 3,5-Dinitrosalicylic acid(DNS)method[19].Daily biogas production was achieved by water drainage method. The methane concentration was determined by using the GC (GC1120, Shunyuhengping, Shanghai, China).

Principal component analysis(PCA)was calculated out with the SPSS software(Version 17.0).The diagrams in here were drawn by using Excel 2019 and Origin 2020 software.

3. Results and Discussion

3.1. The characteristics of pretreated spent cow bedding

3.1.1. The degradation rate of VS

VS content of spent cow bedding was 74.06%, which similared to the spent bedding from deep litter piggery housing [2]. After HP, the VS degradation rate of spent cow bedding changed to different extent (Fig. 1). At different pretreatment temperatures of 50, 70, and 90 °C, the VS degradation rate of spent cow bedding were 1.50% -3.79%, 2.22% -3.96%, and 2.95% -3.46%, respectively.The addition of ammonia also affected VS degradation. For with ammonia group,the VS degradation of spent cow bedding reached its peak of 3.96% at 70 °C holding for 72 h, which was 4.5 %-164%higher than that of the HP group.Also, the level of VS degradation of spent cow bedding was 98% -296% higher than the rice straw(1% -2%) at 150 °C [20]. However, this result was lower than the VS degradation rate of corn stover (13.57%) [15]. The difference in VS degradation rates may be attributed to the spent cow bedding having undergone biophysical processes such as chewing and churning in the cow’s digestive system and composting and pre-fermentation in the cattle sheds [2].

Fig. 1. VS degradation rate of spent cow bedding in different pretreatment conditions.

3.1.2. The changes of pH, VFAs, and reducing sugar

During the HP process, pH value reduced to an acidic level due to water acted as an acid by ionizing to generate hydrogen ions[9].Therefore,pH value was considered an important parameter using as evaluating the pretreatment effect [21,22]. The changes of the pH value of spent cow bedding are shown in Fig. 2 at different HP conditions. After pretreatment, the pH value of the spent cow bedding increased with rising temperature. The pH value of HP spent cow bedding with ammonia was higher than that of HP without ammonia at the same temperature.The pH value after HP was in the range of 6.29-8.41, while the addition of ammonia further increased the pH value to 7.57-9.09.This was because the neutralization reaction of OH-in ammonia water and acid (VFAs) in the HP process occurred after the addition of ammonia [15].

At different pretreatment conditions, the VFAs concentrations of spent cow bedding is listed in Fig. 2. After pretreatment, the VFAs concentrations of spent cow bedding was 1601.64-10944.3 5 mg·L-1, which was 1.69-18.41 times than that of the control group (594.59 mg·L-1). At the same pretreatment temperature and duration, the VFAs concentration of HPA group was higher than that of HP group. The highest VFAs concentration was 10944.35 mg·L-1with HPA at 50 °C for 72 h, which was 1.58-10.85 times higher than other HP groups. Among VFAs components, acetic acids concentration was 673.50-9045.14 mg·L-1,which was 16.5-22.17 times higher than that of the control group.Lastly,during the measurement of VFA,valeric,isobutyric and isovaleric acids had been removed due to their negligible quantity.

Fig. 2. The changes of pH, VFAs, and reducing sugar at different pretreatment conditions.

The changes of reducing sugar concentration of spent cow bedding are listed in Fig. 2. After pretreatment, at the same temperature and time, the reducing sugar concentrations for with ammonia groups were higher than that of the HP without ammonia groups. The reducing sugar concentrations of spent cow bedding for with ammonia groups were in the range of 54.78-74.80 mg·L-1, which was 1.22-2.12 and 1.96-2.67 times than that of without ammonia groups and the control group, respectively.Furthermore,the changing trend of reducing sugar is roughly consistent with pH.

3.1.3. The changes of lignocellulose compositions

The lignocellulose compositions of spent cow bedding at different pretreatment conditions are shown in Fig.S1.With HP,the cellulose, hemicellulose, and lignin contents of spent cow bedding were 13.17% -17.18%, 10.71% -12.81%, and 3.34% -4.46%, respectively. After HP with ammonia, the cellulose, hemicellulose, and lignin contents decreased to 10.42% -16.89%, 6.85% -11.89%, and 2.96%-4.59%,respectively.The cellulose,hemicellulose,and lignin contents for HP were higher than that of HPA group. This changes of cellulose and hemicellulose was consistent with the results of Song’s use of ammonia during the HP of corn stover [16]. Compared to the untreated spent cow bedding, the lignin relative content of after pretreatment increased. The highest lignin content was 4.46% at HPA of 90 °C holding for 4 h, which was 70% higher than that of the untreated group.Duet al.[23]found that the lignin content of rice straw increased. Additionally, Yuanet al.[15] also reported that the lignin content of corn stover after HP increased compare to the untreated corn stover.

The removal rate of cellulose was in the range of 21.34% -53.23%(Fig.3).At the same temperature,the removal rate of cellulose and hemicellulose increased with the increasing maintain time. The highest hemicellulose removal, 58.07%, occurred with HPA at 70 °C for 72 h. And the removal rate of hemicellulose in the HPA group was higher than that of the HP group at the same temperature and time, due to ammonia can hydrolyze the polymers of hemicellulose [24].

The lignin removal rate of spent cow bedding was 10.32% with HPA at 70 °C for 72 h. Apart from that, the lignin removal rate of others pretreatment groups was all negative, indicating that the lignin content of spent cow bedding increased after different pretreatment conditions. A possible for this increased lignin content that condensation reactions of lignin with other degradation products occurred during the pretreatment process [25]. Nitsoset al.[26] also found that the lignin content of beach wood increased after HP. At the same time, the lignin content of the hardwood slices increased to 40.3% with increasing HP severity [27].

3.2. Methane production

The cumulative methane production (CMP) of spent cow bedding also varied with different pretreatment conditions (Fig. 4).The CMP of spent cow bedding at 50, 70, and 90 °C was 2081.1 1-2630.15 ml, 1992.16-2213.92 ml, and 2082.88-2617.22 ml(Fig. 4(a)-(c)), respectively. At 50 °C, the CMP of pretreated spent cow bedding increased with longer pretreatment time. At 70 °C,the CMP of spent cow bedding decreased with lengthening pretreatment time for HP. Conversely, for HPA, the CMP of spent cow bedding increased with increasing pretreatment time. For 90 °C pretreated spent cow bedding, the CMP increased with increasing pretreatment time, and the maximum CMP was 2617.22 ml at pretreatment time of 4 h, which was 48.14% higher than that of the untreated group. The highest CMP of spent cow bedding was 2630.15 ml at HPA of 50°C holding time 72 h,which was 48.87% higher than that of the untreated group. This result was similar to Yuanet al.[15] using corn stover as material at HPA of 50 °C holding time 24 h. Similarly, the maximum methane yield of rice straw was 23%higher than that of the untreated group by Heet al.[20].

The methane yield of spent cow bedding at different pretreatment conditions are listed in Fig. 4(d). At 50 °C and 70 °C HP, the methane yield of spent cow bedding decreased from 139.0 to 134.5 ml·(g VS)-1, when pretreatment time lengthened from 24 h to 72 h, the methane yield rose from 137.2 to 125.3 ml·(g VS)-1.In contrast,the methane yield of spent cow bedding increased with longer pretreatment time for HPA and HP of 90°C.Addition ammonia has greatly influenced on methane yield during the HP of spent cow bedding process.The methane yield HPA of spent cow bedding was 136.5-163.0 ml·(g VS)-1, which was 8.9% -30.1% correspondingly higher than that of HP (except HPA of 50 and 70 °C holding for 24 h). The highest methane yield was 157.1 ml·(g VS)-1and 163.0 ml·(g VS)-1for HP and HPA, which was 43.5% and 48.9%higher than that of the untreated group, respectively. The maximum methane yield of the spent cow bedding appeared to HPA of 90°C holding for 4 h.Lower temperature pretreatment can avoid the formation of complex, difficult-to-biodegradable substrates,and undesirable pyrolysis reactions [28]. This was also due to under a lower temperature condition, the addition of ammonia can result in weak alkaline solution in the water solution, which can swell the lignin structure of spent cow bedding, and result in the reduction and division of lignin-carbohydrate meshwork [9].A similar the highest methane yield of corn stover was 148.2 ml·(g VS)-1at an HPA of 50°C held for 24 h,which was 31.35%higher than that of the untreated group [15]. At the lower temperature pretreatment conditions, water can also penetrate the spent cow bedding, leading to the cellulose hydrolysis, solubilize hemicellulose solution and partially lignin removal [29]. Thus, the methane yield can increase at the lower temperature pretreatment conditions.

Fig. 3. The removal rate of lignocellulose at different pretreatment conditions.

Fig. 4. Methane production at different pretreatment conditions.

3.3. The relationship of methane yield and the pretreated characteristics parameters

Methane yield was correlated with different pretreatment conditions. The different pretreatment conditions lead to the changes of different parameters after different pretreatment conditions.The relationship between methane yield and parameters such as pH, VFAs, reducing sugars, and lignocellulose compositions after pretreatment was calculated by the Pearson correlation method(Table S1) and investigated by PCA (Fig. 5). The principal components 1 (PC1) and principal components 2 (PC2) made up 66.96%and 24.04%,respectively. These parameters were positive or negative correlations with methane yield. The VFAs, pH, and reducing sugars were shown to have a positive correlation with the methane yield. On the other hand, the content of cellulose, hemicellulose and lignin showed negative correlations with methane yield. This indicated that high lignocellulose contents disadvantaged of methane production, after HP, lignocellulose composition were converted into VFAs, they contributed to better methane production.The pH values negatively influenced the VFAs,suggesting that lower the pH was more beneficial to VFAs production. Similarly,Pedersen and Meyer also stated that a high pH positively influenced the enzymatic hydrolysis[30].The lignin content was negative associated with methane yield.At the same time,a negatively associated with lignin content and methane yield was found for lignocellulosic biomass [31]. In addition, a negative relationship cellulose and lignin content with anaerobic biodegradability was reported by Buffiereet al.[32]. In summary, HP could improve methane yield of spent cow bedding by enhancing the removal rate of cellulose and hemicellulose and increasing VFA production.

Fig. 5. Correlation of methane yield and intermediate parameters.

4. Conclusions

In this study, spent cow bedding has been confirmed to be a good raw material for AD.After pretreatment,the VFAs concentration of HPA group was higher than that of HP group at the same pretreatment temperature and time. The optimal pretreatment condition was obtained at HPA of 50 °C holding for 72 h. At the optimal condition, the highest VFAs concentration was 1.58-10.85 times higher than that of other groups.The highest hemicellulose and lignin removal rate were 58.07% and 10.32%, respectively. The highest methane yield was 48.9% higher than that of the untreated group. The VFAs, pH, and reducing sugars showed positive relationship with the methane yield. Therefore, lower temperature HP can improve AD performance of spent cow bedding.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (grant numbers JD2006).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.10.012.