Study on the Effect of Two-Phase Anaerobic Co-Digestion of Rice Straw and Rural Sludge on Hydrogen and Methane Production

China is the largest rice producer in the world, and a large amount of rice straw is left behind after rice harvest every year. In addition, with the rapid development of urbanization, the use of activated sludge technology for wastewater treatment will also generate a large amount of surplus sludge [1]. Since the 18th National Congress of the Communist Party of China, with the continuous improvement of environmental standards in response to President Xi Jinping’s call for “green waters and green mountains”, efforts need to be made to develop renewable resources [2], purify and utilize rice straw and sludge through anaerobic co-digestion to produce hydrogen and methane, and provide an effective method for the treatment of agricultural solid waste [3]. Regarding resource utilization technologies for these two typical types of rural solid waste, Jinhe Jiang et al. [4] concluded that two-phase anaerobic digestion is better than single-phase anaerobic digestion for maximizing hydrogen and methane production from solid organic wastes, and has the advantage of better reaction stability. Two-phase anaerobic digestion can recover hydrogen and methane at the same time, and has the advantages of low energy consumption [5], sustainability, and high energy production. It is considered one of the most effective methods to cope with the energy crisis [6], which can realize the resourceful utilization of rural solid waste and generate clean energy, and has attracted the attention of scholars at home and abroad. Currently, research on two-phase anaerobic hydrogen and methane production using food waste is common [7,8], but there are few reports on hydrogen and methane production from agricultural (village) waste.
Rice straw and rural municipal sludge are two typical types of agricultural (village) solid waste, and their common treatment methods are open burning, direct return to farmland, and sanitary landfill, sludge incineration, sludge composting, and being sent to urban areas for centralized processing [9,10]. The disposal of municipal sludge has historically been a big issue. If not handled properly, it is easy to cause secondary environmental pollution and financial losses [11]. In recent years, with the continuous development of sludge treatment and disposal technologies, researchers have developed a number of new treatment technologies. For example, thermochemical transformations, hydrothermal liquefaction, etc. However, these new treatment technologies are difficult to apply on a large scale due to the constraints of technical means and treatment costs. Current sludge disposal methods are still dominated by stabilized landfills, aerobic fermentation, dry incineration, anaerobic digestion, and the use of building materials. On the other hand, China is a large country with a large population, and rice is one of the main staple foods in China, so China is rich in rice straw resources. Currently, the most common ways to reuse rice straw in China are directly returning it to the field and mixing it with manure for composting. Returning rice straw directly to the field is a simple, economical, and rapid method of reuse. However, as the quality of rice straw after returning to the field is low and it is difficult to spread evenly, it not only increases the difficulty of plowing, but also affects the seeds and emergence of crops to a certain extent. Although rice straw and sludge have good anaerobic resource utilization potential [12,13], their single digestion can lead to lower gas production rates due to factors such as an inappropriate C/N ratio. Conversely, co-digestion can significantly improve gas production rates due to its advantages of balancing nutrients, diluting toxic substances, and improving microbial activity [14]. Hairong Yuan et al. [15] explored the performance of the anaerobic co-digestion of sludge and wheat straw. The results of the study showed that the anaerobic digestion performance of sludge was affected by different C/N ratios, and the use of anaerobic co-digestion resulted in higher gas production than single sludge. Xuhui Wang [16] used mixed aerobic composting of rice straw and residual sludge to study the effect of C/N ratio and different particle sizes of straw on the treatment of composting system, and find the best C/N ratio. Jing Ning et al. [17] conducted a study on the gas production rate of anaerobic co-digestion using pig manure and corn stalk as substrates. The results showed that compared to single anaerobic digestion, when pig manure and corn stalk were co-digested at a C/N ratio of 25, the reaction system operated stably and had the highest specific gas production rate. Taotao Tang [18] found that sludge combined with corn stover (1:1.5) had a strong promotion of methane production using anaerobic digestion technology. Qiuying Huang [19] studied a co-fermentation method to improve the gas production performance of rice straw and obtained the same results. In order to construct an efficient and stable rice straw sludge with a combined anaerobic digestion system, Yiran Wang [20] conducted a batch experiment of combined anaerobic digestion using different types of rice straw and excess sludge. The research results showed that the addition of rice straw significantly increased the methane content compared to the single anaerobic digestion of sludge.

At present, anaerobic digestion using rice straw/sludge has limited methane production, but there is a lack of research reports on its co-digestion for hydrogen production and methane production. Therefore, this study uses a medium-temperature batch experiment to investigate the optimal mixing ratio of rice straw and rural municipal sludge for hydrogen and methane production in order to provide experimental support for its resourcefulness and provide reference for the low utilization rate of straw and sludge and the decline of land fertility in China.

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