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College of Agronomy, Yanbian University, Yanji 133000, China

Abstract In order to investigate the effects of slurry recirculation technology on anaerobic digestion performance of maize straw silage, maize straw silage was fermented with recirculated biogas slurry, and the gas production, pH value, methane content, volatile organic acids (VFAs) contents, chemical oxygen demand (COD) removal rate and other indicators were studied. The results showed that the fermentation time was positively correlated with daily gas production, methane content, cumulative gas production, VFAs and COD removal rate. Although the pH value fluctuated, it was still in the normal reaction range. The daily gas production was about 1.26 L. The acetic acid content increased first, then decreased, then increased, and finally stabilized. The biogas slurry recirculation technology saves water resources by 40 mL/d without affecting the normal gas production of anaerobic fermentation, and reduces the consumption of environmental resources. It has important development significance for the sustainable use of biomass resources.

Key words Biogas slurry recirculation, Maize straw silage, Anaerobic digestion performance, Biomass resources

1 Introduction

China is a big agricultural country, and the base of straw production is huge. Crop straw has always been one of the main sources of raw materials in biomass energy[1-2]. China is the world’s largest straw producer, with an estimated annual production of about 900 million t[3]. The use of anaerobic fermentation technology can not only reduce the harm to the environment, but also alleviate the shortage of energy. However, a large amount of biogas slurry is produced during the fermentation process. If not handled properly, it is easy to cause secondary damage to the environment. Biogas slurry treatment mainly includes microfiltration[4], reverse osmosis[5-7], ion exchange[8]and other technologies. However, due to high cost and complicated operation, they cannot be used in a wide range. In contrast, biogas slurry recirculation technology[9-13]can effectively reduce water consumption, improve fermentation efficiency, and reduce the pose-fermentation processing cost of biogas slurry, and it is a cheap and efficient method for treating biogas slurry. In this experiment, the biogas slurry produced by anaerobic fermentation was mixed with maize straw silage for fermentation experiment, which improved the organic matter content of the fermentation substrate, reduced the secondary pollution caused by the massive discharge of biogas slurry to the environment, and increased biogas production. It is of great significance to the research and technical application of biogas anaerobic fermentation technology.

2 Materials and methods

2.1MaterialsThe maize straw silage used in the experiment was taken from the experimental base of Yanbian University. The discharged supernatant was added to 500 mL of water and re-introduced into the reactor. The fermentation time was 197 d.

2.2Measuredindicatorsandmethods

2.2.1Gas production and methane content. The gas production was measured by the drainage method. The methane content was measured using a gas analyzer (Biogas-5000, Geotech, UK).

2.2.2pH. The fermentation broth was taken out of the fermenter at the same time every day, and its pH was measured using a pH meter (PHS-3C, Shanghai Leici, PRC).

2.2.3Volatile organic acids (VFAs). The contents of volatile organic acids (VFAs) were measured referring to the method of Zhengetal[12].

2.2.4Chemical oxygen demand (COD). The COD was analyzed using the potassium dichromate method[8].

2.2.5Data analysis. The data were processed and analyzed using SPSS 17.0 and Orgin 8.0.

3 Results and analysis

3.1ChangesindailygasproductionandcumulativegasproductionThe changes in daily gas production and cumulative gas production during the biogas slurry recirculation phase are shown in Fig.1 A and Fig.1 B. The gas production of the reactor was 0.96 L in the first two days, and it increased in the first six days. As the reaction proceeded, the daily gas production during the recirculation process was in a state of fluctuation. The maximum daily gas production (1.66 L) appeared on the 6th day, and the minimum daily gas production (0.9 L) appeared on the 177th day. At the end of the reaction, the daily gas production was maintained at around 1.1 L, reaching the gas production standard in the biogas project.

The total cumulative gas production during the entire recirculation process was 248.71 L. The average daily gas production was 1.26 L. This was mainly because that the recirculated biogas slurry contained organic matter that was not decomposed and used, such as nitrogen and phosphorus, which increased the organic matter content in the reactor. In the early reaction process, a certain amount of organic matter added in the reactor was beneficial to the increase in gas production. With the proceeding of the recirculation, the ammonia nitrogen and free ammonia contained in the biogas slurry accumulated to a certain extent, producing ammonia poisoning and inhibiting the growth and metabolism of anaerobic microorganisms. To a certain extent, this affected the progress of anaerobic fermentation, so the gas production decreased[14-15].

Note: A. Changes in daily gas production with biogas slurry recirculation; B. Changes in cumulative gas production with biogas slurry recirculation.

3.2ChangesinpHThe pH is an important parameter to measure the anaerobic digestion process. As shown in Fig.2, the pH on the first day of recirculation was 6.30. As the reaction proceeded, the pH fluctuated, and the overall trend was on the rise. As the fermentation time was prolonged, the amplitude of the fluctuation of the pH decreased, and the pH gradually stabilized. This was mainly due to the alkaline substances contained in the recirculated biogas slurry. In the 197-d recirculation, the pH gradually stabilized at around 6.58, within the appropriate pH range for anaerobic fermentation.

Fig.2 Changes in pH with biogas slurry recirculation

3.3ChangesinmethanecontentMethane content is a basic indicator for measuring the efficiency of anaerobic digestion systems. As shown in Fig.3, the methane content of the entire recirculation process was less than 50%. In the initial stage, the methane content was on the rise. This was mainly because that the methanogens carried in the biogas slurry restored their activity after returning to the anaerobic environment, and they continued to decompose the organic matter. As the amount of biogas slurry recirculated increased, the methane content decreased, but it remained above 30%. This was mainly because that the biogas slurry recirculation gradually broken the original balance of various substances in the fermentation system, as well as accumulating organic matter, and finally, a new stable state was obtained[16]. As the biogas slurry recirculated, the organic matter in the fermentation system accumulated, inhibiting the activity of methanogens, thereby reducing the methane content.

Fig.3 Changes in methane content with biogas slurry recirculation

3.4ChangesincontentsofVFAsThe changes in the contents of VFAs in the biogas slurry recirculation process are shown in Fig.4. In the initial stage, the formic acid was decomposed and converted to acetic acid in the first 10 d, and the acetic acid content reached a maximum value of 1.12 g/L on the 25th day. As the reaction proceeded, formic acid appeared again on the 30th day and was completed converted and decomposed until the 100th day. In the first 10 d, propionic acid and butyric acid appeared. However, the contents were low, and they were completely decomposed on the 10th day. During the entire recirculation process, the acetic acid content increased first, then decreased, then increased, and finally stabilized. In the initial stage, the reactor was unstable and the anaerobic digestion process was not complete, and the recirculated biogas slurry contained organic matter that was not completely degraded, resulting in the accumulation of organic matter. Therefore, the acid content was high. As the reaction proceeded, the anaerobic digestion flora gradually adapted to the habitat, and their digestion capacity gradually enhanced, so the organic acids were decomposed and utilized continuously. As the organic acid content in the biogas slurry was high, exceeding the decomposition ability of anaerobic bacteria, acetic acid had a surplus. From the 105th day on, the acetic acid content was relatively stable.

Fig.4 Changes in contents of VFAs with biogas slurry recirculation

3.5ChangesintotalCODremovalrateThe COD removal rate mainly reflects the utilization efficiency of methanogens for the substrate in the anaerobic digestion system. The COD concentration of the product after fermentation reflects the amount of substances that can be oxidized in the product after fermentation, and also indirectly reflects the degradation of organic matter in the anaerobic process. The initial COD removal rate in the recirculation phase was 80.33%. As shown in Fig.5, as the anaerobic digestion reaction progressed, the COD removal rate gradually decreased, and finally stabilized at around 25%. The continuous recirculation of the biogas slurry increased the organic load of the COD entering the anaerobic digestion system. When a certain value was exceeded, the substances that were not completely degraded accumulated in the whole anaerobic digestion system and could not be eliminated from the fermentation system in time, resulting in the microbial activity in the system being affected, and the accumulation of organic acids, further resulting in a decrease in COD removal rate.

Fig.5 Changes of the removal rate of COD with biogas slurry recirculation

3.6CorrelationanalysisbetweenvariousfactorsandgasproductionpotentialofslurryrecirculationAs shown in Fig.6, the fermentation time was positively correlated with daily gas production, methane content, cumulative gas production, VFAs contents and COD removal rate. The correlation coefficients between fermentation time and COD removal rate, cumulative gas production were smaller, almost zero, indicating that the fermentation time had little effect on the COD removal rate during the recirculation process. The daily gas production was positively correlated with VFAs content, and the correlation coefficient was 0.32. The methane content was positively correlated with fermentation time, and cumulative gas production. The correlation coefficients were 0.42 and 0.38, respectively.

Note: CODRR, COD removal rate; CB, cumulative gas production; DB, daily gas production

3.7SavingenergyThe biogas slurry recirculated in this experiment was mainly the supernatant of the discharge. The supernatant of the discharge was generally above 400 mL every day, so a minimum of 400 mL of water was saved per day. In the 197-d recirculation process, a total of 78 800 mL of water was saved. In addition to saving water, it also reduced the environmental pollution of biogas slurry.

4 Conclusions

As the reaction ran throughout the recirculation process, the pH was always dynamic, but within the normal range of biogas projects. The average daily gas production was about 1.1 L, in line with the requirements of biogas engineering. The methane content first rose and then decreased. The acetic acid content increased first, then decreased, then increased, and finally stabilized. In addition to reducing the pollution of the biogas slurry to the environment, biogas slurry recirculation reduced energy consumption (saving at least 400 mL of water per day compared to general anaerobic digestion).