Abstract
The cattle stomach was considered as the basis for simulating a proposed operation. Microenvironmental degradation mechanisms are understood to be key to the efficient utilization of straw and other resources. Through dynamic tracking of the change law of heat generated by microbial degradation of straw in the cattle stomach, this study used an orthogonal test to explore the optimal ratio of feeding feed, the degradation mechanism in the microenvironment, and the characteristics of cattle manure and straw anaerobic fermentation. The results showed that the number of days of fermentation and the ratio of straw and cattle manure had a significant impact on methane gas production, and the mixture ratio was 1:3, at 26 °C; within 20 days, the cumulative gas production was up to 78.9 L. The results also showed that rumen microorganisms, cattle manure, and mixed straw fermentation can be used at different ratios to obtain the change of methane production, and determine the best ratio to achieve the maximum gas production.
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Simulation of Cattle Stomach Processes Applied to the Fermentation of Mixed Manure and Straw
Enhai Liu,a Zhanghui Guo,a,* Yali Shi,a Biao Qi,a Yu Wang,b,* and Yine Jiang a
The cattle stomach was considered as the basis for simulating a proposed operation. Microenvironmental degradation mechanisms are understood to be key to the efficient utilization of straw and other resources. Through dynamic tracking of the change law of heat generated by microbial degradation of straw in the cattle stomach, this study used an orthogonal test to explore the optimal ratio of feeding feed, the degradation mechanism in the microenvironment, and the characteristics of cattle manure and straw anaerobic fermentation. The results showed that the number of days of fermentation and the ratio of straw and cattle manure had a significant impact on methane gas production, and the mixture ratio was 1:3, at 26 °C; within 20 days, the cumulative gas production was up to 78.9 L. The results also showed that rumen microorganisms, cattle manure, and mixed straw fermentation can be used at different ratios to obtain the change of methane production, and determine the best ratio to achieve the maximum gas production.
DOI: 10.15376/biores.18.1.1096-1114
Keywords: Cattle stomach; Straw degradation; Microenvironment; Cattle manure; Corn straw; Anaerobic fermentation
Connect information: a: School of Petroleum Engineering, Changzhou University, Changzhou, 213164, China; b: School of Politics and Public Administration, Guangxi Minzu University, Nanning, China;
*Corresponding authors: guozhanghui1999@126.com; XN2430490561@163.com
INTRODUCTION
A shortage of energy has become a main problem facing global economic development. Straw, as the main source of biomass energy, has the characteristics of rich resources, low price, and environmental favorability, since it can replace non-renewable energy. As a large agricultural and animal husbandry country, China produces many different types of straw annually, and the amount of recycled straw reached more than 1000 million tons, but the straw utilization rate has been relatively low (Dai et al. 2021). In addition, China has an increasingly prominent problem of soil pollution that is caused by livestock and poultry manure emissions (Li et al. 2020), The potential for these unused biomass energy to be converted into the equivalent of standard coal is one billion tons (Feng et al. 2021). Liu et al. (2022) established a gas production model of mixed anaerobic fermentation based on a Box-Behnken design. Zhang et al. (2016) studied the effect of steel slag micropowder on methane production through anaerobic fermentation of cattle manure. Li et al. (2020) studied the feed ratio and semi-continuous anaerobic fermentation of corn straw, and they found that when the ratio of corn straw to cow manure was 1:3, the gas production was at its maximum. Zhang (2011) studied the characteristics of efficient anaerobic fermentation of wheat straw and found that when the ratio of wheat straw to cattle manure was 1:2, the maximum gas production rate was at 40 ℃ and the minimum gas production rate was at 20 ℃. Huang (2019) studied the efficient anaerobic fermentation technology of rice straw and found that there was no significant synergistic effect between the pretreated rice straw and sheep or cattle manure. Xu et al. (2020) studied the mixing ratio of corn straw and cattle manure separation solution, and they concluded that the maximum yield was achieved under the fermentation test conditions. Jin et al. (2021) analyzed the effect of the addition of corn straw on the fermentation of cow manure compost. It was concluded that the ratio of corn straw to cow dung was 2:3 and the seed germination rate could reach 97.35 % after 23 days of fermentation. Cai et al. (2022) studied the enhancement of anaerobic fermentation of cattle manure, and the results showed that cellulose pretreatment and micro-voltage have significant influence on anaerobic fermentation of cattle manure. Bułkowska et al. (2022) studied the biogas production process with glycerol as the substrate of anaerobic fermentation of cattle manure. Van et al. (2022) found that the anaerobic decomposition of wetland grass using cattle manure could reduce carbon dioxide and methane in the biogas. Singh et al. (2022) synthesized IONPs (Iron Oxide Nanoparticles, IONPs) using neem leaf extract. Therefore, the effect of methane production by anaerobic fermentation of cattle manure and straw could be improved.
In recent years, many experts and scholars in China and abroad have conducted in-depth research on anaerobic fermentation of cattle manure, but there have been few reports on the application of the gas method. In this paper, bionic simulation and an orthogonal testing were performed for cattle feed matching, and a bionic cattle stomach model was established. The orthogonal test of three factors and three levels was run on the influencing factors of anaerobic fermentation of cattle manure. Further specific analysis showed that at 26 ℃, the corn straw:cattle manure ratio was 1:3. Within 20 days, the maximum cumulative gas production could reach 78.91 L. Thus, the days of fermentation and the ratio of cattle manure to straw had significant effects on methane gas production. The experiment of co-fermentation of cow dung with different types and different ratios of straw and studying the change of methane production can provide a theoretical basis for the construction of a bionic system for efficient and stable control of straw degradation.
Bionic Digestion System Simulation
The stomach is the place where all animals digest, decompose, and ferment food; thus, a lot of digestion reactions take place. The stomachs of some organisms not only can degrade food, but they produce methane and carbon dioxide. Such a stomach, as in cattle, is called the rumen. It contains a lot of methanogens that produce methane and carbon dioxide through biochemical reactions during food digestion.
The fermentation process of other rumen organisms was explored by simulating the anaerobic fermentation of the cattle stomach. The main parameters of cattle stomach microenvironment include acid-base buffering capacity, volume, etc. Basic data of cattle stomach environment is shown in Table 1. A cattle stomach model is shown in Fig. 1.
Table 1. Basic Data of Cattle Stomach Environment (Wang and Mao 2005)
1. Rumen; 2. Reticulum; 3. Omasum; 4. Rennet
Fig. 1. Cattle stomach model
The stomach (Fig. 1) of cattle is the place for digestion of cattle’s food intake, consisting of the rumen, reticulum, omasum, and abomasum, in which there are a large number of microorganisms (Pei 2012). These microorganisms can complete many biochemical reactions and maintain various life activities of cattle (Beauchemin et al. 2016).
Fig. 2. Dynamic tracking of feeding changes in cattle stomach
Dynamic Tracking of Microbial Degradation in the Stomach
Based on the principle of bionics, combining biology and biomass energy, it is expected that the application and analysis of rationally fermenting the mixed raw materials of cattle manure and straw to prepare gas production, the cattle were fed with different straw, and the variation of heat generated by microbial degradation of straw in the stomach was dynamically tracked. The differences and similarities of degradation parameters and microenvironment of straw degradation in the cattle stomach were compared and studied.
Mechanism of Anaerobic Fermentation
Cattle stomach microorganisms mainly consist of bacteria, methanogens, fungi, protozoa, and a small number of bacteriophages. The anaerobic fermentation process is shown in Fig. 3.
Fig. 3. Anaerobic fermentation process
Production of hydrogen and acetic acid
According to Fig. 3, the reaction equations can be written as follows:
Methanogenic stage
According to Fig. 3, the specific reaction equations were as follows (Yang and Chen 2021):
Predicted Methane Production
This study employed the findings of earlier experimental work as the basis for simulating and predicting the results of biodegradative processes. Methane production was predicted based on the cellulose, hemicellulose, crude protein, crude fat, and other components. The prediction formula was as follows (Liu and Wang 2022), assuming that the mass of a cattle was 300 kg, the daily intake of food was 15 kg, in which the dry matter was 10 kg, the hemicellulose was 0.5 kg, and the cellulose was 1 kg. The daily gas production forecast was as follows:
In these equations, φ is the dry matter intake (kg/d), and γ is a constant (-0.9363);
Through substituting the data into Eqs. 8 and Eq. 9, the daily gas production was predicted to be 385.3 g. Methane gas production is further given as follows:
where H and C are the hemicellulose and cellulose intake, respectively (kg/d). By substituting the data into Eq. 10, the daily gas production was predicted to be 394.7 g. By substituting the data into Eq. 11, the daily gas production was predicted to be 371.1 g.
According to the change of season and temperature, the internal environment of cattle stomach also changes to some extent. Equations 12 and 13 provide the prediction of changes in cattle stomach gas production in spring and winter:
According to Eq. 12, the daily gas production in Spring could reach 392.3 g.
According to Eq. 13, the daily gas production in Winter could reach 387.81 g.
Equations 14 through 17 were combined so that the daily gas production was 392.7 g. In the above equations, CH4 is the methane emission (g/d), θ is the soluble intake of neutral detergent (kg/d), F is the crude fiber intake (kg/d), N is the nitrogen free extract intake (kg/d), P is the crude protein intake (kg/d), E is the crude fat intake (kg/d), B is the weight of cattle (kg), and ρ is the digestible dry matter intake (kg/d). According to the calculation, the methane emission of 300 kg cattle was 370 to 395 g per day, indicating that the prediction was in line with the reality.
Model building
Rumen is the main site for rumen organisms to conduct anaerobic fermentation. The model of rumen biological anaerobic fermentation was established according to the shape of rumen biological stomach, as shown in Figs. 4 and 5.
The stomach model of biomimetic rumen animals can be adjusted according to the stomach conditions of different rumen organisms, and the relevant orthogonal test could be conducted.
Simulation and Analysis
In this work, the cattle stomach was simulated and analyzed by ANSYS (ANSYS Company, v.2020, Canonsburg, PA, USA) to study the status of its internal microorganisms. According to the actual shape of the cattle stomach, a 3D model of the front side was drawn. To better highlight the flow state of the cattle stomach, the front side model is shown in Figs. 6 and 7.
The simulation started from the feeding of cattle, as shown in Fig. 8. The distribution of temperature in the cattle stomach just after eating is shown in Fig. 9.
Fig. 8. Changing trend of flow state in the cattle stomach
Note: the black line represents the content flow rate, the pink line represents the feed flow rate, the light blue line represents the microbial flow rate, the green line represents the air flow rate, carbon dioxide flow rate is shown in red, and methane flow rate is shown in blue
Fig. 9. Distribution of temperature in the cattle stomach just after eating
Note: red means high temperature, blue means low temperature
According to the simulation results of Fig. 8, it can be seen that: at the beginning of feeding, the flow rate of some liquid in the cattle stomach suddenly changed from 0.1 m/s to 20 m/s, and then it dropped to the usual 0.1 m/s after 20 min of feeding. The flow rate of methane was 1 × 10-2 m/s, and then with the digestion of food, the flow rate of methane in the cattle stomach gradually increased. As shown in Fig. 9, just after eating the cattle stomach temperature was 41 ℃, and the temperature of the food only 25 ℃ (ambient temperature).
According to the simulation results: After 10 min of feeding, the gas in the stomach of the cattle had been flowing at a speed of 0.4 m/s. The undigested food was in a solid state with a slow flow rate or even not flowing at all. At the same time, biochemical reactions were also taking place in the digestive juices, which could also produce gases; thus, the situation shown in Fig. 10 occurs.
According to the simulation results: one hour after eating, the cattle stomach pressure changes are shown in Fig. 11, the food is present in the center of the cattle stomach, gas diffusion to the cattle stomach around the cattle stomach around the pressure is greater than the cattle stomach center.
SIMULATION EXPERIMENTAL
Microenvironmental Degradation Scheme
Firstly, by analyzing the simulation process, the differences of microbial degradation environment in cattle stomach under different feeding conditions were compared, and the reasons for the differences were analyzed. Then, the differences of microbial degradation in straw microenvironment at different time were analyzed. Finally, the optimal degradation state in the cattle stomach was determined by using the corresponding results of different simulated temperatures, so as to determine the optimal gas production rate.
Optimum Feed Ratio
Simulation methods: 1) The cattle of the same type, age, and weight were simulated, and different kinds of feed were simulated for feeding. According to the changes of gas content parameters in the simulated cattle stomach, the optimal feed ratio was determined. 2) The best feed ratio was put into the bionic stomach for simulated culture.
Table 2. Microenvironment of Cattle Stomach under Different Conditions
Simulation steps: The simulation used the same proportion of feed to feed cattle of the same type, age, and weight, and observed and recorded changes in parameters such as daily methane production, peristaltic frequency, pH, and microbial activity during the simulation (results are shown in Table 2 ) (Li 2018).
According to the comparison of simulation experiments and summary findings, the fermentation of corn straw in the cattle stomach was the most active microorganism at about 1.5 h. Because of the loose and porous structure inside corn straw, it could contact better with cattle stomach contents, and thus produce a better reaction effect. The decrease of gas production after 2 h was caused by the depletion of feed and the decrease of bacterial activity.
According to previous experts and scholars (Wang et al. 2016; Zhou et al. 2019), with the formula (feed formula as shown in the table below) for analysis and contrast.
Table 3. Formula 1 of Cattle Feed
Table 4. Formula 2 of Cattle Feed (Wang et al. 2016)
Table 5. Formula 3 of Cattle Feed (Zhou et al. 2019)
The simulation analysis showed that the Formula 2 in Table 3 contributed most to the growth and development of cattle, and Formula 3 in Table 3 promoted more CH4 in the stomach. In Table 3, Formula 5 fed the cattle in the stomach when the microbial was most active. Formula 4 in Table 4 could maintain the most stable pH value in the cattle stomach, which was 6.3. Formula 1 in Table 4 could keep peristalsis frequency of cattle stomach.
Orthogonal Test Verification
Simulation test method
The conditions in the bionic cow stomach were simulated by ANSYS, and the composition, pH and temperature of different straws were simulated. No other conditions were changed during the simulation process, so as to generate gas generation data during the simulation process.
Simulation test design
After simulating the conditions of the bionic cattle stomach, the method was applied to simulate 15 kg of different kinds of crushed straw, temperature, and pH conditions. The change of gas production during simulation was observed after 2.5 h to determine the best fermentation scheme.
Simulation test results
With straw type, temperature, and pH as the observation objects, three levels were set for each object, and nine experiments were conducted to observe the parameters changes in the cattle stomach by simulation the crushed straw, pH, and temperature of different kinds in the stomach at the same time. The influencing factors of the orthogonal test are shown in Table 6, and the test results are shown in Table 7.
Table 6. Influencing Factors and Levels in the Test
Table 7. Test Results
Ki(i =1,2,3) represents the actual gas production of a factor at the level of i, Ki represents the average value of a factor at the level of i, R = Ki(max) – ki (min). A larger R value means the influence of this factor on gas production was more obvious.
Table 8. Significance and Variance