Currently, global energy consumption and environmental pollution have become global issues. Oil, as one of the most important economic energy developments, has been excessively exploited and consumed. Therefore, the development and utilization of primary energy and the exploration of alternative renewable clean energy have become a focus of global concern [1
]. As a large agricultural country, China produced large amounts of agricultural organic waste such as straw and livestock manure every year, which was an important source of biomass energy [3
]. Straw and livestock manure can be converted into clean energy such as methane, thereby realizing the reuse of agricultural organic waste [4
]. However, the technology for the treatment of straw and livestock manure is not sufficiently advanced. At present, the utilization rate of straw waste in China is insufficient and a large amount of straw is incinerated, resulting in large amounts of soot and bad weather. In addition, a large amount of livestock manure was discharged without treatment, causing adverse effects for humans and livestock, including the spread of the plague [5
]. Therefore, innovative and effective processes for the resourceful utilization of agricultural organic waste were urgently developed.
Pretreatment proved to be one simple but effective method for improving the biodegradability of crop residues. Our previous research showed that NaOH pretreatment could significantly improve biodegradability and enhance biogas production of corn straw [6
]. This could develop new feedstock without affecting biogas production efficiency. Generally, an anaerobic biological treatment method was also widely applied for agricultural organic waste treatment. It not only solved the excessive accumulation of agricultural organic waste but also produced clean energy, thereby realizing the recycling and harmless treatment of organic waste [7
]. According to the widely accepted theory of four stage anaerobic fermentation, the anaerobic digestion process was composed of hydrolysis, acidogenic fermentation, H2
-producing acetogenesis, and methanogenesis [10
]. However, the traditional single anaerobic digestion process could not meet the requirements for the simultaneous growth of acidogenic bacteria and methanogenic bacteria and also led to competition between the two groups of bacteria, resulting in low operating efficiency in the reaction unit. In order to solve this problem, the concept of two-phase anaerobic digestion was introduced in the 1970s [11
]. The two-phase anaerobic digestion process could achieve the separation of the acidogenic phase and the methanogenic phase via a regulation of operating parameters [12
]. Furthermore, the acidogenic bacteria and methanogenic bacteria in the two-phase anaerobic digestion process could maintain the optimal growth conditions to achieve the efficiency and stability of the anaerobic fermentation system.
In fact, the two-phase anaerobic digestion process is affected by various factors, such as pH, C/N ratio, inoculum, and temperature [14
], which directly determine the composition of fermentation products and the methane production efficiency. As one of the main abiotic factors in determining anaerobic digestion efficiency, temperature plays an important role in the performance of the two-phase anaerobic digestion system. In practice, sudden environmental changes, e.g., dramatic increases or drops in temperature, may cause severe disturbances in all parameters of the process, and the system requires a long period of time to adapt to a stable state. Furthermore, the temperature has a significant impact on the growth and metabolism of microorganisms and the interactions between the microbial groups [18
]. In the process of anaerobic digestion, the temperature can regulate microbial intracellular enzyme activity, thus affecting the metabolic activity of microorganisms and the anaerobic fermentation efficiency. In addition, changes in the microbial metabolism or the community dynamics affect the operation of the anaerobic digestion system [22
]. It was reported that mesophilic conditions (30–40 °C) have been generally adopted for the anaerobic digestion of agricultural organic waste and show good performance in biogas production [27
]. The low temperature condition has become the main reason for the limited application of anaerobic digestion technology for the treatment of agricultural organic waste in North China. It was found that low temperatures resulted in a low biogas production and unstable operational performance in the anaerobic digestion system [29
]. The effects of increasing or decreasing temperatures followed by the re-establishment of the initial temperature have been assessed in some previous studies. These studies show that a decrease in temperature typically causes a lower solluted chemical oxygen demand (SCOD) removal efficiency, a lower biogas production, and a lower volatile fatty acids (VFA) accumulation. However, the studies on the effect of daily temperature fluctuations on the biogas production and the microbial community of the two-phase anaerobic digestion process were rarely reported. Therefore, it is important to study the influence of temperature on the two-phase anaerobic digestion process.
In the present study, a modified two-phase anaerobic digestion reactor was studied to assess the effects of daily temperature variations on the semi-continuous anaerobic digestion by treating a cow manure and corn straw mixture. The daily temperature variations were simulated by a forced square-wave oscillation of the reactor temperature of an anaerobic digester. Meanwhile, the effect of temperature on the efficiency of volatile fatty acid and biogas production and the microbial community structure was also investigated. Examining the effects of daily temperature variations and microbial composition on the performance of anaerobic digestion will aid in providing the necessary regulation to derive the optimal fermentation liquor for efficient biogas production.