Agricultural Engineering

Evaluation of anaerobic digestion of municipal waste and biogas production efficiency in combination with animal waste

Document Type : Research Paper

Authors

Davood Mohammadzamani 1
Mahdi Jafari 2
Mohammad Rasooli 3

1 Associate Professor, Department of Biosystems Engineering, Takestan Branch, Islamic Azad University, Takestan, Iran

2 Assistant Professor, Department of Power, Takestan Branch, Islamic Azad University, Takestan, Iran

3 Assistant Professor, Department of Breeding, Takestan Branch, Islamic Azad University, Takestan, Iran

https://doi.org/10.22055/agen.2024.46033.1712
Abstract
Introduction: The yield of methane production in the anaerobic digestion processes of municipal organic solid waste alone is low. Adding animal waste or other additives to municipal solid waste as feed for anaerobic digestion system not only increases the relative composition of methane, but also increases the rate of biogas production (Rivas-García, 2020). Carbon and nitrogen are essential elements for the growth and reproduction of aerobic microorganisms. The balanced ratio for C/N in the process is between 20-30. Simultaneous digestion is used to balance the C/N ratio (Yousefi & Bahri. 2021). This process has many advantages, including the synergistic effect of microorganisms, increasing the stability of the process, increasing the efficiency of biogas, increasing the recycling of nutrients and reducing odor.

Materials and Methods: This research was carried out with the aim of increasing the rate of biogas production, reducing the feed retention time in the digester and increasing the amount of biogas production, by investigating the effect of co-digestion of urban solid organic waste with cow excrement using anaerobic digestion method. For this purpose, 52 samples of mixed urban waste (during the year 1400, once a week and one sample each time) were prepared from the waste transfer station of Qazvin city, and in order to investigate the effect of animal manure on the studied variables, from a cattle farm located in 50 kg of fresh manure was collected in the region. After preparing the samples, a laboratory bioreactor was used to perform the experiments. The biogas production process was carried out in two stages. In the first stage, urban waste materials were used, and in the second stage, a combination of urban waste materials and animal manure was used.

Results and Discussion: The ratio of carbon to nitrogen (C/N) in the primary feed and residual materials was obtained in the first and second stages. In this way, this ratio was estimated as 19.39 and 27.64 for the primary feed and the remaining materials in the first stage and 18.60 and 28.23 respectively for the second stage. In this study, the amount of ash decreased during the process, which indicated the participation of this substance in improving the activity of microorganisms. In both stages of the experiments, the organic matter of the primary feed decreased during the digestion process, which indicates the decomposition of these materials during the process. Also, the conversion percentage of dry material from primary feed to secondary material in stage 1 and 2 was 8.2% and 10.5%, respectively, which shows that in the second stage, in which the combination of animal manure was used, the percentage of conversion The dry matter is more and the process has progressed towards the production of biogas. The changes in the pressure of biogas inside the tank in the experiment related to stage 1 reached its maximum value (0.19 bar/kg) within 23 days after the start of the process, and then stabilized at 0.14 bar/kg of solid material in the last seven days. Is. Since the criterion for the completion of the digestion process was pressure stabilization in seven consecutive days, therefore, after 38 days, the first stage process was completed and the biogas and residual (secondary) materials were discharged. The maximum biogas pressure in the second stage test was 0.28 bar/kg of solid material, which was achieved on the 15th day, and finally, after 26 days, the pressure reached 0.16 and stabilized at this pressure for seven days. Therefore, the digestion process in the second stage lasted for 32 days. Therefore, it can be seen that by using animal manure in the primary feed and keeping other variables constant, the retention time has decreased by 6 days compared to the first stage.  The maximum amount of biogas produced in stage 1 was equal to 6.27 liters/kg of solid matter and in stage 2 it was equal to 10.3 liters/kg of solid matter. As can be seen, by using animal manure in combination with urban organic waste, the volume of biogas production has increased under the same conditions. Taking into account the cumulative amount of biogas production, it was found that in stage 1 and 2, 140.89 and 230 liters/kg of solid biogas were produced during the digestion period, respectively. Therefore, the efficiency of biogas production has increased by 38%. Although the total amount of biogas produced in both stages of the experiments compared to the theoretical values obtained in this study (at the rate of 370 liters/kg of solid matter) and also reported by other researchers (Salehoun, et.al, 2020 and Kozminesky , 1995) has been less.

Conclusion: According to the results of this study, it was found that in the second stage compared to the first stage, the role of the two elements carbon and nitrogen in the biogas production process became more effective and one should expect more biogas production in the process, because the increase in the conversion of organic matter and nitrogen is The more effective decomposition of these materials by microorganisms has been achieved by adding animal manure to the primary feed. According to the results obtained from this study, it can be concluded that in the process of biogas production, the combination of animal manure with urban organic waste, in addition to reducing the retention time, can help to increase the efficiency of biogas production, which in this study A 38% increase in biogas production was observed in the case of using a combination of animal manure with urban organic waste compared to using only urban organic waste. Although the role of other variables such as temperature, type and amount of stirring, type of initial preparation of materials in terms of size, humidity, pH, addition of yeast and bacteria, degree of impurity and toxicity of materials, ratio of carbon to nitrogen, type and size of reactor and other examined the variables.

Keywords

anaerobic decomposition
biogas
municipal waste
animal manure
retention time
Subjects
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Agricultural Engineering
Volume 47, Issue 2
Spring 2024
Pages 143-160

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