Volume 9, Number 3 (April 2022):3465-3474, doi:10.15243/jdmlm.2022.093.3465 ISSN: 2339-076X (p); 2502-2458 (e), www.jdmlm.ub.ac.id
Open Access 3465 Research Article
Influence of waste type with co-digestion system on methane production of patch digester stirred with exhaust gases
Mostafa Ashmawy1, Osayed Abu-Elyazeed2, Youssef Ahmed Attai3, Mina Danial4*
1 Civil Engineering Department, Faculty of Engineering, Materia, Helwan University, Egypt
2 Mechanical Engineering Department, Faculty of Engineering, Materia, Helwan University, Egypt
3 Mechanical Engineering Department, Faculty of Engineering, Materia, Helwan University, Egypt
4 Civil Engineering Department, Faculty of Engineering, Materia, Helwan University, Egypt
*corresponding author: [email protected]
Abstract Article history:
Received 7 January 2022 Accepted 13 February 2022 Published 1 April 2022
Investing in biogas is a viable option for the production of renewable energy.
Production of renewable energy such as biogas has an impact on improving the environmental function and health impacts for all beneficiaries such as humans, lands, and ecosystems. As well recycling of solid waste could be considered waste management for economic development and protection of degraded and polluted lands. Anaerobic co-digestion has been practically applied in sewage sludge processing, agricultural and waste treatment and is recognized as an economical effective way for waste reuse, treatment, and disposal. This paper presents three co-digestion experimental batches for thickened sludge with food waste, rice straw and cow waste. The mixtures were digested for thirty days, with 15 minutes of daily stirring using laboratory generator exhaust. A steel fixed dome anaerobic digester was used for experimental batches. The produced methane was recorded to be 65 %, 45.9 %, and 55 % when using thickened sludge with food waste, rice straw and cow waste, respectively. Cumulative methane was investigated for 7, 15 and 30 days to show the effect of time in methane production. The results showed that approximately 80 % of the produced methane was produced between 15 and 21 days. Anaerobic co-digestion increases the pH value of the three mixtures. The pH value was increased during anaerobic co- digestion due to the mineralization of the organic matter. However, pH values stayed between 6.0 and 8.0, which is better for growing and activating the methanogenic microorganisms as a reason for methane formation.
Keywords:
anaerobic co-digestion batch digester biogas
methane production sewage sludge
To cite this article: Ashmawy, M., Abu-Elyazeed, O., Attai, Y.A. and Danial, M. 2022. Influence of waste type with co-digestion system on methane production of patch digester stirred with exhaust gases. Journal of Degraded and Mining Lands
Management 9(3):3465-3474, doi:10.15243/jdmlm.2022.093.3465.
Introduction
Renewable energy plays an important role in the process of sustainable living and environmental protection. In particular, biomass could contribute in a significant way because it is a carbon nature fuel and could be considered as a sustainable power source (Tas, 2010; Deublein and Steinhauser, 2011; Manea et al., 2012). Biogas is a gaseous biofuel derived from the anaerobic digestion/co-digestion of biodegradable
solid waste products such as sewage sludge, food waste, animal manure and rice straw (Bouallagui et al., 2009; Mussoline et al., 2012; Linke et al., 2013; Liu et al., 2020). The produced biogas from anaerobic co- digestion processing could be used in automobiles, residential heating, power generation, etc. (Bacenetti et al., 2016). It may be utilized for enthusiastic transformation through various processes, like biochemical or warm substance ones, contingent upon the biomass properties (Saracevic et al., 2019). The
Open Access 3466 production of biogas depends on many factors, but the
temperature affects it mostly and plays a very crucial role (Tumutegyereize et al., 2016; Babaei and Shayegan, 2019).
Anaerobic digestion is an ecological method for the sustainable disposal of biodegradable municipal and agricultural waste for environmental management to protect human health from pollution impacts (El Ibrahimi et al., 2021). Anaerobic digestion of sewage sludge from wastewater treatment plants or biodegradable materials is a promising technology for simultaneous treatment wastewater treatment plants production of renewable energy biogas and fertilizer (Luste and Luostarinen, 2010; Babaei and Shayegan, 2019). When organic biomaterials decompose, they undergo biological and chemical changes that produce new compounds (Ferdeș et al., 2020). The quality of these compounds varies according to the conditions of the reaction in which they were carried out. If the reaction takes place in an open place, aerobic decomposition takes place and produces ammonia gas and carbon dioxide (Tas, 2010). But if the reaction was conducted in an isolated reactor called an anaerobic digester, methane gas and carbon dioxide are produced mainly (Nagao et al., 2012), forming biogas which is a low-cost fuel, and organic fertilizer that could be used as an agricultural enhancer (Rabii et al., 2019). A technique called anaerobic co-digestion, based on bacterial interactions has been utilized for sludge digestion and/or biodegradable wet solid waste for environmental protection (Vincenzo and Trulli, 2015).
Absorption occurs in biomass mixtures for treating natural compost, accomplishing biogas to produce and deliver energy (Hamawand and Baillie, 2015). Co- digestion of different biomaterials and sludge could be beneficial due to the dilution of inhibitive substrates, improved nutrient content and synergistic effects between the treated materials resulting in better degradation of both (Liu et al., 2020). Moreover, the addition of different biomaterials with sludge digester increases the organic loading of the digester, thus resulting in higher methane production (Luostarinen et al., 2009; Rabii et al., 2019).
The anaerobic digestion process is defined by a sequence of biochemical transformations induced by various bacterial consortia: Extracellular enzymes must first liquefy the organic substrate materials such as cellulose, hemicellulose, and lignin (Bayané and Guiot, 2011) which are subsequently handled by acidogenic bacteria; the rate of hydrolysis is determined by pH, temperature, composition, and concentration of intermediate chemicals (Fantozzi and Buratti, 2009; Manea et al., 2012; El Monayeri et al., 2013; Ferdeș et al., 2020). Acidogens then transform soluble organic components, including hydrolysis products, into organic acids, alcohols, hydrogen, and carbon dioxide (Mechichi and Sayadi, 2005). The products of acidogenesis are converted to acetic acid, hydrogen, and carbon dioxide. Methane is produced by methanogenic bacteria from acetic acid, hydrogen, and
carbon dioxide, as well as other substrates such as formic acid and methanol (Chynoweth et al., 2001;
Gupta et al., 2019; Ferdeș et al., 2020). The process is catalyzed by a consortium of microorganisms (inoculum) that converts complex macromolecules into low molecular weight compounds (Magdalena et al., 2018; Liu et al., 2020).
Energy production research from anaerobic digestion in wastewater treatment plants have witnessed remarkable progress in the past years. This encouraged researchers to move forward in this field, as the production of biogas from sludge biomass generated from wastewater treatment plants is one of the most promising ways to maximize investments in sewage treatment for the production of high-quality water and consequent reduction in operating costs by recovering energy within wastewater treatment plants, as well as the optimal environmental management for lands of projects. The present work focuses on studying the anaerobic co-digestion process using a fixed dome laboratory digester. A mixture of thickened sewage sludge and three different wastes (food waste, rice straw, and cow waste) were used. Also, a new stirring technique by utilizing exhaust gas was applied.
The research scope was to examine the methane production from the mixtures and study the corresponding responses that occur behind the anaerobic reaction.
Materials and Methods Materials
Anaerobic co-digestion of the mixture of thickened sludge with food waste, rice straw and cow waste mixture sludge was investigated in a feed ratio of 1:5 by volume (one-unit thickened sludge and five units, co-digestion agent). The anaerobic co-digestion reaction was processed at natural lab temperature which was between 25-35 oC, to remain in mesophilic stage temperature. Three different biomass mixtures were digested in a single-stage, batch-scale anaerobic digester at the Biofuel Laboratory - El Mattria Faculty of Engineering - Helwan University. For a thirty-day batch period, the experimental work trials were conducted to evaluate the methane gas production percentage with mixing by generator exhaust for 15 minutes daily. The mixture biomasses were prepared to determine the best methane production percentage from thickened sludge with food waste, thickened sludge with rice straw and thickened sludge with cow waste.
The thickened sludge sample was collected from El-Berka WWTP at Madinet El Salam District in Cairo, Egypt. Table 1 presents the average characteristics of the collected thickened and secondary sludge samples. The exhaust gas was taken from a lab-scale generator employed as a source of exhaust for mixing in the digester in order to apply a new stirring approach. The mixing procedure was
Open Access 3467 performed every day for 15 minutes, and the
temperature operating range was still from 25 to 35 oC.
The hot exhaust gas increased the temperature of the mixtures by 1 or 2 degrees, but the temperature remained in the mesophilic range. The main exhaust gas component was carbon dioxide with small amounts of other hydrocarbons. Table 2 shows exhaust gas components.
Table 1. Sludge average characteristics data.
No. Parameter Thickened sludge
1 pH 6.17
2 TS, % 1.87
3 TVS, % 1.37
4 TS, mg/L 19183
5 Temperature, oC 20
Table 2. Exhaust gas components.
No. Component name Amount of molar (%)
1 CO2 86.08
2 Propane 1.78
3 i-Butane 6.52
4 n-Butane 4.88
5 i-Pentane 0.55
6 n-Pentane 0.19
Pilot setup
Steel sheets were used to construct a laboratory pilot scale digester, with a feeding system on the top cover and beneath the drainage system. The digester is a rectangular steel tank with an internal cavity for the substrate and a surrounding compartment that serves as a jacket and stiffeners. The digester's substrate capacity (internal zoon) is roughly 160 liters, with inner dimensions of 0.4 * 0.4 m and a clear height of
1 m. The exterior dimensions, including the stiffener rooms, are roughly 1.5 m3 volume, 1 x 1 m dimensions, and 1.5 m height, including the unloading hopper cone.
A 6 mm, fixed steel cover with three top holes was installed, the first for the pressure gauge, the second for filling, and the third for exhaust gas entrance. To ensure that the mixing gas is passed through all of the mixture volumes, the digester cover is attached with an internal perforated 0.5-inch pipe that begins at the bottom of the internal tank. The digester was built to hold the generated gases, and the final gas outlet is located at the top of the tank. Figure 1 illustrates a schematic diagram of the pilot setup and a real photo of the experimental pilot.
Experimental batch procedure
The anaerobic digester was filled with a known quantity of sludge (5 units) and a known quantity of selected co-digestion agent (1 unit). Three co- digestion agents were used in this experimental work, food waste from home use, rice straw and cow waste which were applied as co-digestion agents. All connections were checked to be sure the digester was isolated from the natural air entering. The pH value of the mixture was measured by a calibrated pH 340i (WTW), at the beginning of the batch and then after 30 days. The temperature of the mixture was measured daily by a calibrated thermocouple K-Type. The thermocouple was put in the mixture inside the digester to record the temperature readings continuously every day. Mixing and stirring continued daily for 15 minutes by generator exhaust, and the initial and final temperature readings were recorded.
Biogas samples were collected after thirty days of anaerobic co-digestion. The produced biogas was then analyzed by gas chromatography in Cairo Oil Refining Company (Chemical and Research Department). The previous steps were repeated with another co-digestion agent and a new thickened sludge sample.
(a) (b)
Figure 1. Digester pilot: (a) a schematic diagram of the experimental setup, (b) a real photo of the experimental pilot.
Open Access 3468 Results and Discussion
Biogas production rate and related cumulative curve were examined to compare the data related to different biomasses; samples of biogas were periodically analyzed and evaluated to clarify the cumulative methane production. Over a period of time (7, 15 and 30 days), the thickened sludge with co-digestion agents was able to create methane gas from a mixture
of volatile fatty acids and amino acids. Throughout the batch digestion period (approximately 30 days), the cumulative biogas production increased at a fast rate at the period from day 7 to day 15; however, the increasing rate at the first 7 days was slow due to the internal initial shock for the bacterial action. However, the first fifteen days produced approximately about half of the total methane released in batch digestion, as shown in Figure 2.
Figure 2. Cumulative methane CH4 production rate for various types of biomasses.
As a result of the high methane formation rate between day 7 and day 15, biogas production was increased significantly, probably as a result of the use of co- digestion agents. This indicates the effect of using rich substrates with biomass as an aiding factor in the digestion process for biogas production. From Figure 2, it could be observed that two experimental batches gave satisfying results in the first 15 days for the thickened sludge with food waste and thickened sludge with cow waste. They produced approximately 50% of the methane from the total gas content. These substances are considered biomass on their own with a rich number of biodegradable matters, and they could be used as the main biomass in the digestion process.
An opposite situation for the thickened sludge with rice straw was observed as the biodegradable matters were low. The moisture content doubtless probably plays an effective role in the decomposition process, as it is considered a good environment for anaerobic bacterial growth.
Influence of food waste on methane production For thickened sludge with food waste, the results were satisfactory. The methane gas production rate was increased from day to day, which means that the anaerobic reaction is in progress. It could be observed
that the co-digestion agent “food waste” enhances the anaerobic reaction as the production of methane gas reaches 65% at the end of this batch. After 7 days, 17.4% of methane was produced and increased to 53.4% after 15 days. Figure 3 demonstrates the different gases generated from the digestion process of the thickened sludge and food waste as a digested biomass. This batch achieved a high ratio due to the presence of a high amount of active anaerobic bacteria in the thickened sludge and a high amount of biodegradable organic matter in the food waste and the thickened sludge. As shown in Table 3, the produced biogas for thickened sludge with food waste consists of methane gas (65%), carbon dioxide (30.6%), i- Butane (0.8%), n-Butane (0.6%), i-Pentane (0.5%) and Propane (2.5%). Table 4 shows the results of methane gas production were 65 % after 30 days, 53.4% after 15 days and 17.6% after 7 days of anaerobic digestion.
During the experiments, it was noticed that about 80%
of methane gas was produced after day 15, which means that the batch time probably decreases in the future due to using of food waste as a co-digestion agent. The use of food waste as a co-digestion agent was useful for methane production as it enhances methane formation and decreases carbon dioxide creation as an environmental impact.
00
17.6
53.4
65
00 9.3
36.6
45.9
00
16.9
46.7
55
0 10 20 30 40 50 60 70
0 7 1 4 2 1 2 8
%CH4
Time (days)
%C H4
Thickened sludge with food waste Thickened sludge with rice straw Thickened sludge with cow waste
Open Access 3469 Figure 3. Biogas components ratios produced from thickened sludge with food waste co-digestion.
Table 3. Biogas components ratio for different mixtures.
No. Biomass type CH4 Co2 N2 i-Butane n-Butane i-Pentane n-Pentane Propane Ethane 1 Thickened
sludge with food waste
65 30.6 0 0.8 0.6 0.5 0 2.5 0
2 Thickened sludge with rice straw
45.9 33.2 0 8 6.8 0.2 0.1 4.5 1.3
3 Thickened sludge with cow waste
55 25.3 0.6 7.4 7.7 0.4 0.1 3.5 0
Table 4. Methane cumulative ratio.
No. Biomass type At 7 days CH4 At 15 days CH4 At 30 days CH4
1 Thickened sludge with food waste 17.6% 53.4% 65%
2 Thickened sludge with rice straw 9.3% 36.6% 45.9%
3 Thickened sludge with cow waste 16.9% 46.7% 55%
Influence of rice straw on methane production Table 3 shows the thickened sludge with rice straw as a digested biomass; the maximum ratio of the generated methane was 45.9%, while the carbon dioxide ratio was 33.2% of the produced biogas. The third level was for i-butane with a ratio of 8%. Some gases traces were formed due to the digestion action, such as Propane, n-Butane, Ethane, and i-Pentane.
Several local studies promote the use of rice straw because it is abundant in Egypt, and it is disposed of in an environmentally detrimental manner, such as by burning. Figure 4 shows the biogas components ratios
produced from thickened sludge with rice straw. Table 4 shows that the obtained methane production ratios were 45.9% after 30 days, 36.6% after 15 days and 9.3% after 7 days of anaerobic digestion. There is no doubt that the use of rice straw affects the digestion process. Two reasons were affecting the amount of the produced methane. The first one is the low moisture content of the rice straw, which decreases the amount of methane (Rabii et al., 2019). The second one is the lignin effect, lignin compounds act as glue between polysaccharide filaments and fibers, slowing their degradation, and it is estimated that 12% of cellulose 65
30.6
0 0.8 0.6 0.5 0 2.5
0 0
10 20 30 40 50 60 70
Thickened sludge with food waste
Biogas %
Component
Biogas components from thickened sludge with food waste
Methane Carbon dioxide Nitrogen i-Butane n-Butane i-Pentane n-Pentane Propane Ethane
Open Access 3470 remains in the flotation layer of a biogas reactor
prevents biogas from escaping to the upper layer ( Fantozzi and Buratti, 2009; Luste and Luostarinen, 2010). Despite several studies that have proven successful strategies in lab-scale experiments to break down the lignin and accelerate anaerobic decomposition (Bayané and Guiot, 2011).
Experimental results showed that using rice straw without treatment of the lignin effect is not useful for biogas production. However, the break of his effect may be costly due to the high-pressure requirements (Mussoline et al., 2012). Probably the hot exhaust for stirring enhanced the co-digestion processing with rice straw (Gupta et al., 2019).
Figure 4. Biogas components ratio produced from thickened sludge with rice straw co-digestion.
Influence of cow waste on methane production Table 3 also shows the thickened sludge with cow waste as a digested biomass. The maximum generated ratio was 55% for the methane gas, and the carbon dioxide ratio was 25.3%. Figure 5 shows the biogas components ratios produced from thickened sludge with cow waste in the co-digestion process. The high amount of active anaerobic bacteria and the high amount of organic matter in the thickened sludge and cow waste increase the generated methane. Hence, the present research promotes to use of cow waste as a co- digestion agent because it is present in richness in the environment and is disposed of without any benefit.
From table 4, the produced methane gas ratios were 55%, 46.7%, 16.9% after 30, 15, and 7 days, respectively. These results showed the effect of cow waste as a co-digestion agent in biogas production.
Cow waste, as a co-digestion agent, was effective for completing approximately 85% of methane production in 15 days and 31% in 7 days, which means that the co-digestion agent can rapid the digestion process. The addition of cow waste led to process support which is indicated by a high volatile fatty acids content as the animal manure has plenty of many types of acids that support the anaerobic digestion process, as proved previously by (Linke et al., 2013). Several studies depend on using animal manure waste as the main biomass because it is rich in organic matter, volatile
fatty acids, and bacteria also (Alsouleman et al., 2016).
The other produced gases during the co-digestion process are considered as fuels as their own and could be used as natural gas (Abd Elfattah et al., 2016).
Influence on pH value
To indicate the effect of bacterial action on the different biomasses, the influence of anaerobic co- digestion on the pH value of digested mixtures was analyzed. The pH values were measured before and after the digestion batch. The results revealed that the pH values were increased in the three experimental batches which are close to the previous work of (Bouallagui et al., 2009, Mussoline et al., 2012). To elucidate the effect of the digested sludge on pH values and to join the relation between pH increasing ratio and methane gas production, the percent of pH increasing ratio was calculated from Equation (1). Figure 6 shows the initial and final pH throughout the anaerobic co- digestion process. Figure 7 shows the pH increasing ratio percent and the production percent rate of methane. Table 5 shows the initial and final pH of the digested biomass. Also, it shows the calculated increasing ratio percent in pH values and methane gas percentage in the produced biogas. It could be observed that the highest methane production ratio was 65% which is compatible with the lowest increasing ratio in pH of 8.96%, for thickened sludge with food waste. For thickened sludge with rice straw and 45.9
33.2
0
8 6.8
0.2 0.1
4.5 1.3
0 5 10 15 20 25 30 35 40 45 50
Thickened sludge with rice straw
Biogas %
Component
Biogas components from thickened sludge with rice straw
Methane Carbon dioxide Nitrogen i-Butane n-Butane i-Pentane n-Pentane Propane Ethane
Open Access 3471 thickened sludge with cow waste, the pH increasing
ratios percent were 18.22%, 9.51% with methane production ratios of 45.9% and 55%, respectively. This
means that rice straw is not recommended in the anaerobic digestion process because of its negative effect on pH value.
%pH Increasing Ratio = ∗ 100 … … . . …Equation (1)
Figure 5. Biogas components ratios produced from thickened sludge with cow waste co-digestion.
Figure 6. Effect on pH values throughout the anaerobic co-digestion.
55
25.3
0.6
7.4 7.7
0.4 0.1 3.5
0 0 10 20 30 40 50 60
Thickened sludge with cow waste
Biogas %
Component
Biogas components from thickened sludge with cow waste
Methane Carbon dioxide Nitrogen i-Butane n-Butane i-Pentane n-Pentane Propane Ethane
6.2 6.24
5.9 6.81
7.63
6.52
0 1 2 3 4 5 6 7 8 9
Thickened sludge with food
waste Thickened sludge with rice straw Thickened sludge with cow waste
pH
Anaerobic co-digestion effect on pH
intial pH Final pH
Open Access 3472 Figure 7. The pH increasing ratio % and production ratio of methane CH4.
Table 5. Influence on pH, corresponding increasing ratios, and methane gas percentage.
No. Biomass Type Initial pH Final pH Increasing ratio % CH4 %
1 Thickened sludge with food waste 6.2 6.81 8.96 65
2 Thickened sludge with rice straw 6.24 7.63 18.22 45.9
3 Thickened sludge with cow waste 5.9 6.52 9.51 55
Several experimental studies have proven that pH decreases profiles for experiments with HRT and OLR were similar while pH decrease with temperature change was slight (Magdalena et al., 2018; Ren et al., 2018). The pH and alkalinity of the influent mixture on the digester are important to be monitored (Karwowska et al., 2020). Furthermore, Organic acids are normally found in the anaerobically digested substrate due to the presence and formation of volatile fatty acids and amino acids during digestion (Tumutegyereize et al., 2016). Mechichi and Sayadi (2005) suggest that pH is not a good parameter for the detection of the anaerobic digestion process, especially at the beginning of the process. During methanation, the organic matter is decomposed formed acids in acidogenesis and acetogenesis stages (Deublein and Steinhauser, 2011). They can be found in both the undissociated and dissociated forms. Aromatic amino acids probably lead to inhibition of the formation of methane gas (El Monayeri et al., 2013; Alsouleman et al., 2016). However, Bouallagui et al. (2009) stated that pH value increased during anaerobic co-digestion due to the mineralization of organic matter in the major cases of digester operations as the same in this study.
The pH values which stayed between 6.0 and 8.0 are better for the growth and activity of methanogenic microorganisms as a reason for methane formation.
Conclusion
In recent years, energy production research has made great progress. Finding out a renewable energy source such as biogas production from different biomass is an interesting area of research. Biomass recycling is considered a waste management technique to improve environmental impacts on humans, lands, and organisms. Anaerobic co-digestion of biomaterials was applied as an efficient and environmentally sustainable technology that enables energy production as heat, electricity, or fuel, as well as stabilization and volume reduction of waste for environmental protection. In this experimental study, a single-stage batch, exhaust gas stirred, mesophilic, steel, pilot-scale anaerobic digester was designed and built to evaluate the biogas production of methane from different substrates in the co-digestion process. Thickened sludge, food waste, rice straw and cow waste were evaluated as co-digestion biodegradable materials and proved successful continuous operation for biogas production. Cumulative methane production through thirty days was studied; thickened sludge with food waste and thickened sludge with cow waste show positive results; however, thickened sludge with rice straw produced a low methane ratio due to low moisture content and lignin effect. The effect on the 8.96
18.22
9.51 65
45.9
55
0 10 20 30 40 50 60 70 80 90 100
Thickened sludge with food
waste Thickened sludge with rice
straw Thickened sludge with cow waste
pH Increasing ratio %, Methane CH4%
pH Increasing ratio % , Methane CH4%
pH Increasing ratio % Methane %
Open Access 3473 pH value of the co-digested mixtures was observed to
be adjusted in future work.
Acknowledgements
The authors would like to acknowledge the very helpful support with Cairo Oil Refining Company - Chemical and Research Department - for the appreciated help in doing all the biogas analysis. Also, the authors wish to thank El-Berka WWTP, Cairo, Egypt for obtaining sludge samples. This study was conducted at the "Center of Excellence for Research and Development of Bio-fuel Technology" (CER
& DBT), which was supported by the Egyptian Ministry of Higher Education's, Project Management Unit and Helwan University.
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