Synergistic of nanoparticles and pretreatment on biogas yield from anaerobic co-digestion of municipal waste and sewage sludge

doi:10.21203/rs.3.rs-3245008/v1

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Synergistic of nanoparticles and pretreatment on biogas yield from anaerobic co-digestion of municipal waste and sewage sludge

https://doi.org/10.21203/rs.3.rs-3245008/v1

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published 02 Apr, 2024

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Municipal solid waste (MSW) and sewage sludge (SS) have a great potential for producing greenhouse gases and environmental pollution, and as such it would be beneficial to treat them and change them into biogas. Yet, a serious challenge is the complex structure of lignocellulose and the difficulty of its bioconversion. A solution to degrade the lignocellulose structure is the use of pretreatments and nanoparticles with proper concentrations. For this purpose, the synergistic effect of 6%NaOH pretreatment with different magnetite nanoparticles (MNP) concentrations was assessed for maximizing the biogas production. In this study, the optimalmixing ratio of MSW and SS co-digestion was determined from our previous studies (MSW:SS: 60:40). Application of the 6%NaOH pretreatment alone increased biogas and methane production by 24.6 and 35.4% respectively, compared to the control. Also, the 6%NaOH pretreatment alone elevated cellulose by 85% while reducing lignin and hemicellulose by 64and 33% respectively. The greatest biogas and methane production was obtained in the reactor of 6%NaOH with MNP concentration 110 ppm, experiencing 86 and 162% increase compared to the control. The greatest reduction in total solids and volatile solids was also obtained in this digestor by 76.6 and 79% respectively compared to the control reactor.

Biogas

Pretreatment

Municipal Solid Waste

Magnetite Nanoparticles

Anaerobic Co-digestion

The modern society has a heavy dependence on fossil fuels in order to maintain its economic development and its standards. The physical constraints of earth resources including limited oil and natural gas reservoirs as well as the effect of greenhouse gas emissions resulting from these fuels in the Earth's atmosphere are among the major challenges of sustaining life in this planet. The inevitable persistence of global production faced with population growth and newly emerging economies requires sustainable supply of energy growth. Biomass energy is a sustainable renewable energy resource which stems from biological wastes resulting from solid wastes as well as urban SS. In the conventional landfilling and releasing solid wastes, when these materials become degraded, volatile organic compounds are emitted which have great potential to degrade the ozone layer. Thus, the estimates performed in Iran indicate that annually 15.6 million tons of solid wastes, 4.6 billion m³ urban wastewater, and 328.5 million tons of animal wastes alongside agricultural waste are produced. Approximately, methane production through anaerobic digestion (AD) technology in Iran in 1996 was around 9300 million m³ of methane per year, with 1000 million m³ being waste. Thus, collecting the resulting biogas in a suitable AD medium can first supply part of the energy demands (sustainable) and help reduce greenhouse gas and carbon emissions in the environment. Furthermore, the stabilized remnants of AD of wastes can function as sources of organic fertilizer suitable for soil remediation [1, 2].

AD process includes four stages, with each stage completed by various bacterial groups. Hydrolysis, acid production, acetatization, and methanation are the main stages of biogas production. All reactions occur concurrently and interdependently [3]. Nevertheless, acidification and sludge solution removal are among the limiting factors of biogas production. In order to overcome this problem as well as to improve the efficiency and accelerate production of short-chain volatile fatty acids (VFAs), heat pretreatment [4], chemical [5], and heat-chemical [6] pretreatment techniques are employed. In addition, enhanced microbial activity of the digester during chemical and biological pretreatments has been reported [7]. From among all these pretreatments, alkaline pretreatment yields the maximum biodegradability [8]. Common chemical pretreatments include strong alkaline materials such as sodium hydroxide (NaOH), calcium hydroxide, and potassium hydroxide. The mechanism of the effect of alkaline pretreatment is degrading ester bonds as well as glycoside bonds, reflected as changes in lignin structure as well as cellulose swelling [9].

In addition, another important aspect of AD process is degrading lignin and releasing cellulose, and hence enhancing the food demand of the microbial population in the digester. Iron is a principal metals in facilitating electron transfer between methanogenic and osteogenic bacteria, which intensifies conversion of VFAs to methane [10]. Methanogens obtain the required energy for their growth by degrading primary substances such as acetate, formate, and hydrogen. It has been proven that interfacial electron transfer occurs through H₂ and emission of formate between methanogens and proton reducing osteogens [11]. Also, It has been reported that addition of Fe₃O₄ causes stimulation and stability in AD, hence improved performance of biogas production [12]. MNP are unstable and are easily dissolved whereby Fe²⁺ and Fe³⁺ ions are produced. Indeed, iron ions are the main constituent elements of cofactors (biochemical) and enzymes, and their addition to AD causes enhanced activity of Archaea microorganisms (the most important methanogen microorganisms) [12, 13]. Although the addition of Fe²⁺ ions increases biogas production within the first 24–48 hours, its high concentration has toxic effects on bacteria, which would lead to diminished biogas production [14]. Accordingly, Fe₃O₄ nanoparticles because of lack of saturation and bioavailability are a suitable candidate to preserve the optimal concentration of iron during AD process.

Reportedly, upon adding magnetite nanoparticles (100 ppm) to AD containing waste, biogas and methane production increased by 180 and 234% respectively [15]. Kato et al. (2012) observed that adding conductive magnetite and hematite in AD can enhance within production biogas and reduce hydraulic retention time (HRT) because of the synthesis based on direct interspecies electron transfer (DIET) [16] (Kato et al., 2012) [16] [16] (Kato, Hashimoto et al. 2012) (Kato, Hashimoto, & Watanabe, 2012) (Kato et al., 2012) ¹⁶ (Kato, Hashimoto et al. 2012) (Kato et al., 2012) (Kato, Hashimoto, & Watanabe, 2012) (Kato, Hashimoto, & Watanabe, 2012) (Kato, Hashimoto et al. 2012) (Kato et al., 2012) [16] (Kato et al. 2012) [16] (Kato et al., 2012) (Kato et al., 2012) [16]. DIET is syntrophic metabolism in which free electrons flow from one cell to another without reaction to reductive molecules [17]. Conductive materials naturally foster DIET process in syntrophic methanogens [11]. Qiang et al. (2012) also reported that the growth of methanogenic bacteria is a function of effective presence of Fe, Co, and Ni in the synthesis of enzymes [18](Qiang et al., 2012)[18][18](Qiang, Lang et al. 2012)(Qiang, Lang, & Li, 2012)(Qiang et al., 2012)¹⁸(Qiang, Lang et al. 2012)(Qiang et al., 2012)(Qiang, Lang, & Li, 2012)(Qiang, Lang, & Li, 2012)(Qiang, Lang et al. 2012)(Qiang et al., 2012)[18](Qiang et al. 2012)[18](Qiang et al., 2012)(Qiang et al., 2012)[18]. Previously, researchers used iron salt in the alkaline pretreatment composition, while the synergism of nanoparticles of this metal in the alkaline pretreatment composition of AD substrate as well as its effect on improving biogas production from MSW organic materials has not been examined. Iron salt dissolves more rapidly compared to iron oxide nanoparticles, and thus the concentration of iron ions in the substrate grows sharply. This (high concentration of iron ions) has toxic effects on useful microorganisms in the primary stage of AD process [15]. Hence, maintaining the optimal concentration of iron ions in the digester is one of the principal measures to be taken for enhancing the activity of methanogen bacteria. The novelty of this study is the use of iron oxide as nanoparticles in a non-pretreated AD as well as in MSW and SS alkaline pretreatment.

Accordingly, the general aim of this study is to investigate the synergistic effect of alkaline pretreatment (NaOH) and different concentrations of Fe₃O₄ nanoparticles on biogas production from MSW and SS organic part under mesophilic thermal conditions in batch digesters. For this purpose, three series of experiments have been designed. In the first series, the effects of Fe₃O₄ nanoparticles on biogas and methane performance were examined on the control. The second series of experiments tested the effect of alkaline pretreatment in the composition and the amount of biogas produced. Finally, in the third series of experiments, the synergistic effect of Fe₃O₄ nanoparticles on biogas and methane function on alkaline pretreatment reactor was assessed.

2.1 Inoculum and substrate

The organic materials of the research were prepared from urban wastewater treatment plant (MSW and SS). After preparing MSW in several times and from several sites and mixing them, a 10 kg sample was prepared to sort and separate the impurities (such as bone, metals, paper, glass, and plastic). The raw materials for better digestion underwent crushing physical pretreatment with 3 mm mesh. After preparation, the materials were kept inside plastic at 4°C for further usages. The bovine fertilizer required for preparing the inoculum (F/I: Feedstock/Inoculum) was also prepared from industrial dairy farms of Samian Village in Ardabil province. After sieving (mesh size 2 mm), they underwent anaerobic conditions for 25 days. When loading the samples to enhance the microbial population, 20% of the digestor volume was allocated to the inoculum.

2.2 Preparation of nanoparticles and pretreatment

The MNP coated with L-lysine were prepared out of the chemicals including Iron chloride hexahydrate (FeCl₃.6H₂O) with 99% purity, iron chloride tetrahydrate (FeCl₂.4H₂O) with 99% purity, sodium hydroxide with 97%, hydrochloric acid (HCl) and L-lysine (as suspender and to prevent precipitation of the synthesized nanoparticles), according to the method presented by Patel et al. (2009) [19] (Patel et al., 2009) [19] [19] (Patel, Chang et al. 2009) (Patel, Chang, & Lee, 2009) (Patel et al., 2009) ¹⁹ (Patel, Chang et al. 2009) (Patel et al., 2009) (Patel, Chang, & Lee, 2009) (Patel, Chang, & Lee, 2009) (Patel, Chang et al. 2009) (Patel et al., 2009) [19] (Patel et al. 2009) [19] (Patel et al., 2009) (Patel et al., 2009) [19].

The alkaline pretreatment was performed using NaOH with 6% wt of the raw material used in incubator at room temperature for 3 h. The pretreatment conditions chosen for this study were based on the results of previous studies [20, 21]. After pretreatment, the mixture was washed with distilled water in order to achieve pH = 7, with the samples kept at 4°C [22].

2.3 Experimental digester

The laboratory digesters consisted of glass bottles (1000 mL), with 667 mL filled with substrate, and the rest sealed as vacuum. Nitrogen gas was used to make their conditions anaerobic inside the digesters at the beginning of experiments (for 1 min) [23]. The substrate contained 7% organic solid material. The digesters were loaded in warm water bath equipped with a digital system for temperature control under mesophilic conditions (37°C). Every day, before determining the volume of gas produced and the sampling, the digesters were shaken for around 30 s for better mixing [23].

2.4 Experiments performed

The experiments continued under mesophilic conditions at 37°C with organic material loading rate of 14 gVS/L for 3 days. The samples included (1) control sample (unpretreated MSW), (2) sample pretreated with 6%NaOH, and (3) sample pretreated with 6%NaOH plus different MNP concentrations. MNP concentrations were added at the beginning of the AD process. The volume of biogas produced by the inoculum was subtracted after inoculation in specific reactors from the volume of biogas produced in all other reactors, in order to obtain the pure biogas content.

2.5 Analytical methods

TS and VS of the experimental substance of the control and the pretreated one were measured according to the standard method presented by American Public health Association (APHA) with numbers 2540B and 2540C [24]. In order to determine the total organic carbon (TOC) as well as total nitrogen content using Kjeldahl method (TKN: Total Kjeldahl Nitrogen), APHA standard was employed [24]. Analysis of MSW for the lignin, cellulose, and hemicellulose was carried out according to Chesson (1978) method. The performance of AD process was examined based on daily production of biogas, cumulative production of biogas, methane performance, and solid material removal efficiency. The biogas volume was measured daily through water displacement method. The methane and carbon dioxide contents were measured in all digesters over 30 days of digestion using a laboratory biogas analyzer (CHEMIST600, seitron). The mean methane performance was calculated based on the percentage of methane produced. The biodegradability of TS and VS was calculated before and after digestion for analyzing the effects of pretreatment on reducing the solid substances. The acidity of each digester was determined on a daily basis by a pH meter (pH-201, Taiwan).

2.6 Nanoparticle characterization

The structural properties of MNP with L-lysine coating were characterized via X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) within the wavelength range of 400–4000 nm. In order to determine the crystalline structure of nanoparticles, XRD (PW1800, PHILIPS, the Netherlands) and to characterize the MNP structure, FTIR (TENSOR, BRUKER OPTICS, Germany) were used. Finally, in order to detect the type of the structure and size of nanoparticles, scanning electron microscopy (SEM, MIRA3, TESCAN, Czech Republic) was employed.

2.7 Statistical analysis

All of the experiments and statistical analyses were replicated three times, where their results with the mean value have been presented. For each of the parameters including TS, VS, TKN, TOC, C/N ratio, TS reduction, VS reduction, cellulose, hemicellulose, lignin, daily and cumulative biogas performance, standard deviation values alongside their values were embedded in the relevant diagrams and tables. One-way analysis of variance (ANOVA) was performed using MINITAB 17 software in order to assess the effect of different nanoparticle concentrations on the cumulative biogas performance.

3.1 The effect of alkaline pretreatment on the substrate chemical compounds

Presence of large amounts of organic materials in MSW has made this material suitable for biogas production [25]. One of the main effects of pretreatment is improving the biodegradability and increasing biogas production, with the high VS/TS ratio after the pretreatment confirming this point (Table 1). The MSW alkaline pretreatment increased the TS and VS/TS ratio, and reduced the C/N ratio. Elevation of TS and VS/TS ratio can be attributed to the diminished humidity after the pretreatment. The increase in the SS content of raw materials also leads to enhanced biogas performance [26]. The optimal mixing ratio of MSW and SS (MSW:SS: 60:40) was chosen based on the results of the studies by Ahmadi-Pirlou et al. (2017) [21] (Ahmadi-Pirlou et al., 2017) [21] [21] (Ahmadi-Pirlou, Ebrahimi-Nik et al. 2017) (Ahmadi-Pirlou et al., 2017) (Ahmadi-Pirlou et al., 2017) ²¹ (Ahmadi-Pirlou, Ebrahimi-Nik et al. 2017) (Ahmadi-Pirlou et al., 2017) (Ahmadi-Pirlou et al., 2017) (Ahmadi-Pirlou et al., 2017) (Ahmadi-Pirlou, Ebrahimi-Nik et al. 2017) (Ahmadi-Pirlou et al., 2017) [21] (Ahmadi-Pirlou et al. 2017) [21] (Ahmadi-Pirlou et al., 2017) (Ahmadi-Pirlou et al., 2017) [21]. The low C/N ratio of the SS after adding MSW lied within the proper AD range (15–30) [3, 27] (Table 1).

Table 1

The MSW and SS anaerobic co-digestion compounds before and after the pretreatment
Parameter	MSW	SS	Inoculum	6%NaOH MSW
TS (%)	22.80 ± 0.15	18.30 ± 0.15	9.80 ± 0.25	29.60 ± 0.17
VS (%)	77.50 ± 0.22	63.30 ± 0.20	59.70 ± 0.15	84.40 ± 0.12
VS/TS	3.40 ± 0.00	4.46 ± 0.00	6.09 ± 0.00	2.85 ± 0.00
MC (%)	77.20 ± 0.15	81.70 ± 0.15	90.20 ± 0.25	70.40 ± 0.17
C (%)	45.20 ± 0.20	32.50 ± 0.15	30.80 ± 0.18	69.80 ± 0.15
N (%)	1.79 ± 0.20	2.57 ± 0.20	2.85 ± 0.20	1.39 ± 0.20
C/N	25.20 ± 0.00	12.65 ± 0.00	10.80 ± 0.00	50.22 ± 0.00
P (%)	0.16 ± 0.15	0.72 ± 0.15	0.67 ± 0.22	-
K (%)	0.27 ± 0.25	0.96 ± 0.15	0.11 ± 0.15	-
pH	7.40 ± 0.30	6.90 ± 0.28	6.80 ± 0.22	8.10 ± 0.20
Cellulose (%)	33.20 ± 0.20	-	-	61.50 ± 0.15
Hemicellulose (%)	27.40 ± 0.15	-	-	18.40 ± 0.15
Lignin (%)	14.20 ± 0.12	-	-	5.10 ± 0.10

The biomass degradability can be determined using lignin to cellulose ratio [28]. Low lignin to cellulose ratio indicates high biomass degradability. The lignin to cellulose ratio of pretreated MSW and control (untreated) was 0.04 and 0.42 respectively, suggesting greater degradability because of the alkaline pretreatment. Moreover, the alkaline pretreatment led to 85% increase in cellulose, 64% reduction in lignin, and 33% decrease in hemicellulose. Increased cellulose results from dissolution of the lignin and hemicellulose solution, as well as the conversion of crystallized cellulose to amorphous cellulose [14]. Also, the increase in cellulose can be attributed to ash removal during the pretreatment [29].

3.2 Nanoparticles characterization

Availability of amino acid groups and Fe-O bonds of coated nanoparticles was assessed through FTIR. Emergence of strong IR band at 584.75 nm suggests presence of Fe-O bonds [19]. Presence of the weak bands at 1615.80 and 1380 indicates symmetric and asymmetric COO- stretching [20], while a strong bond at 3535.28 shows N-H stretching in the amino acid coating on MNP [30]. The crystalline structure and purity of synthesized nanoparticles were examined by XRD. The position and relative intensity of nanoparticles with amino coating matched JCPDS card: 19–0629 standard. Some other low intensity multiple diffracted peaks were also found, which might be due to the presence of an amino acid coating. The crystalline size of nanoparticles was estimated as 7.4 nm using Scherrer equation.

3.3 Synergistic effect of the alkaline pretreatment and nanoparticles on biogas production

The performance of the daily biogas production of control MSW (untreated) with different nanoparticle concentrations (50, 70, 90, and 110 ppm) was analyzed over 30 days within the HRT using one-way analysis of variance (p < 0.05) (Fig. 1). Biogas production began in the early days and without delay phase, resulting from adequate microbial population, suitable inoculation, and proper conditions of reactors. The maximum daily biogas performance for the control and 6%NaOH pretreated samples with different nanoparticle concentrations of 50, 70, 90, and 110 ppm was obtained 47, 58, 61, 64, 68, and 51 Nml/g VS, respectively; the maximum daily biogas performance was achieved by 6%NaOH pretreated with 110 ppm Fe₃O₄ (Fig. 1).

The cumulative performance of the biogas for all digesters of 6%NaOH pretreatment with MNP concentrations was obtained to be significantly greater than the biogas produced by the control (p < 0.05). The maximum cumulative biogas performance was achieved for the 6%NaOH pretreatment plus adding 110 ppm Fe₃O₄, as 988 NmL/g VS (Fig. 2). Biogas production in the sample 6%NaOH pretreatment MSW with 110 ppm Fe₃O₄, in comparison to the control and 6%NaOH MSW was greater by 46 and 28.5%, respectively. On the other hand, the biogas performance in the 6%NaOH pretreatment (no MNP) was 23.8% greater than that of the control. Figure 3 displays the daily changes of pH for all treatments throughout the experiment. In proportion with the biogas production trend, in the early days of experiment, pH had a descending trend in all treatments. The pH has been always slightly higher in the pretreated digester compared to the control digester. Furthermore, the minimum pH has been higher in the pretreated digester compared to the control digester. This shows faster stability of the digestion process by applying the alkaline pretreatment. As shown in Fig. 3, hydrolysis of compounds in the early days and subsequent acidification lead to pH decline and hence reduced biogas production. After several days, with further growth of methanogen bacteria and consumption of acids by them, pH was re-elevated, and biogas production also increased (which is a result of acid consumption [31, 32]].

The positive effect of MNP can be attributed to cellular absorption of MNP inside methanogens and integration with metabolic mediators as well as enzymatic activity involved in hydrolysis of sludge, acidification, and methanogenesis [33]. Verma and Stellacci (2010) investigated the interaction effects of nanoparticles and found that the shape and size of nanoparticles affect their cellular absorption [34](Verma & Stellacci, 2010)[34][34](Verma and Stellacci 2010)(Verma & Stellacci, 2010)(Verma and Stellacci, 2010)³⁴(Verma and Stellacci 2010)(Verma & Stellacci, 2010)(Verma & Stellacci, 2010)(Verma & Stellacci, 2010)(Verma and Stellacci 2010)(Verma and Stellacci, 2010)[34](Verma and Stellacci 2010)[34](Verma and Stellacci, 2010)(Verma & Stellacci, 2010)[34]. It was also observed that the size of nanoparticles heavily influences the attachment and activation of membrane receptors as well as the expression of the next protein. The results of the present research also confirmed this synergism of the size of nanoparticles in methane and biogas production. The results indicated that the cumulative biogas performance with the synergistic effect of the alkaline pretreatment and MNP was not only greater than that of the control, but it was also larger than that of 6%NaOH pretreatment or MNP alone. This feature can be attributed to the synergistic effect of the alkaline pretreatment and MNP [25]. The alkaline pretreatment helps in lignin removal and disrupts the hemicellulose structure; the cellulose released from that helps in increasing the microbial population of AD for holocellulose, thereby enhancing biogas production. The results of other studies also emphasize that Fe₃O₄ nanoparticles enhance electron transfer mechanism between methanogens and fermentation bacteria, leading to further augmentation in the biogas production [34, 35]. Ni et al. (2013) also noted that the interactions resulting from nanoparticles can promote the activated sludge activity and support formation of anaerobic medium, and also foster the activity of heterotrophic bacteria [36](Ni et al., 2013)[36][36](Ni, Ni et al. 2013)(Ni, Ni, Yang, & Wang, 2013)(Ni et al., 2013)³⁶(Ni, Ni et al. 2013)(Ni et al., 2013)(Ni, Ni, Yang, & Wang, 2013)(Ni, Ni, Yang, & Wang, 2013)(Ni, Ni et al. 2013)(Ni et al., 2013)[36](Ni et al. 2013)[36](Ni et al., 2013)(Ni et al., 2013)[36].

3.4 The effect of alkaline pretreatment and MNP on the methane content and methane performance

The trend of methane production in the control and 6%NaOH pretreatment MSW without the presence of nanoparticles and after its addition has been shown in Fig. 4 and Fig. 6. As can be seen, the alkaline pretreatment and adding nanoparticles have had a significant effect on the produced methane. The maximum methane content was observed for 6%NaOH pretreatment MSW with 110 ppm of Fe₃O₄, which was 62% higher than that of the control. The lowest methane content in the control might be due to lower performance of carbohydrates during hydrolysis as well as relatively high lignin content. The results suggest that 6%NaOH pretreatment MSW and MNP concentrations compared to the control enhance the methane performance by two times. Possibly, when the pretreatment and nanoparticles are applied to a biomaterial, a greater bioenergy would be obtained. Lu et al. (2017) employed alkaline pretreatment and 5% wt of Fe₃O₄ and added them to digester during AD. They observed 373% increase in methane performance compared to the untreated digester [37](Lu et al., 2017)[37][37](Lu, Wang et al. 2017)(Lu, Wang, Ma, Zhao, & Wang, 2017)(Lu et al., 2017)³⁷(Lu, Wang et al. 2017)(Lu et al., 2017)(Lu, Wang, Ma, Zhao, & Wang, 2017)(Lu, Wang, Ma, Zhao, & Wang, 2017)(Lu, Wang et al. 2017)(Lu et al., 2017)[37](Lu et al. 2017)[37](Lu et al., 2017)(Lu et al., 2017)[37]. Abdelsalam et al. (2017) observed 2.2-fold increase in methane performance for bovine fertilizer while using 20 mg/l MNP compared to the control [38](Abdelsalam et al., 2017)[38][38](Abdelsalam, Samer et al. 2017)(Abdelsalam et al., 2017)(Abdelsalam et al., 2017)³⁸(Abdelsalam, Samer et al. 2017)(Abdelsalam et al., 2017)(Abdelsalam et al., 2017)(Abdelsalam et al., 2017)(Abdelsalam, Samer et al. 2017)(Abdelsalam et al., 2017)[38](Abdelsalam et al. 2017)[38](Abdelsalam et al., 2017)(Abdelsalam et al., 2017)[38]. Also, Khatri et al. (2015) reported 22.4 and 62% increase in methane performance for 6%NaOH pretreatment maize straw with 1000 mg/l Fe as compared to NaOH 6% pretreatment and without maize straw pretreatment [25](Khatri et al., 2015)[25][25](Khatri, Wu et al. 2015)(Khatri et al., 2015)(Khatri et al., 2015)²⁵(Khatri, Wu et al. 2015)(Khatri et al., 2015)(Khatri et al., 2015)(Khatri et al., 2015)(Khatri, Wu et al. 2015)(Khatri et al., 2015)[25](Khatri et al. 2015)[25](Khatri et al., 2015)(Khatri et al., 2015)[25].

The elevation of methane performance over alkaline pretreatment with Fe₃O₄ nanoparticles might be due to the increase in interfacial electron transfer mechanism between osteogenic and methanogenic bacteria, causing increased conversion of VFAs to methane. In addition, higher availability of holocellulose for microbial degradation might also be involved in improving the methane performance of the control compared to the alkaline pretreatment. Furthermore, in order to control H₂S, by reducing the toxicity of sulfide to methanogens, iron is effective through accelerating it in AD process, thereby boosting the methane performance [39, 40].

3.5 The effect of alkaline pretreatment and magnetite nanoparticles on TS and VS reduction

TS and VS reduction can help assess the performance of AD process. TS and VS reduction for 6%NaOH pretreatment UCSW, 6%NaOH pretreatment MSW with different MNP concentrations of 50, 70, 90, and 110 ppm Fe₃O₄ increased by 31.5 and 15%; 40 and 25%; 42 and 28%; 46 and 31%; and 50 and 35% respectively, compared to the control (Fig. 5). In this study, VS reduction for the control digester was 57.5%, which is in line with similar studies performed by Abudi et al. (2016) and Ye et al. (2013) who reported respective percentages of 58 and 52 increase. 6%NaOH pretreatment MSW with 110 ppm Fe₃O₄ concentrations offered the maximum TS removal (78%) and VS removal (89%), whereby the highest biogas and methane was also observed from this treatment. If, in this study, a higher concentration of MNP were used (above 110 ppm), it would lead to the removal of volatile compound by 99% and even higher as well as a corresponding increase in the methane performance.

3.6 Volumetric methane production

Volumetric methane production in control and 6%NaOH pretreatment MSW with different MNP concentrations is shown in Fig. 5. The least volumetric methane production (9.6 L_methane/L_work) was observed in the control reactor and decreased 2.1 times compared to the alkaline pretreatment with 110ppm Fe₃O₄ concentrations. The volumetric of methane produced in reactors of 6%NaOH pretreatment MSW with different MNP concentrations of 50, 70, 90, and 110 ppm Fe₃O₄ compared to the control reactor increased by 32, 37, 46.4 and 53.3%, respectively. The highest volumetric yield was observed in 6%NaOH pretreatment with 110ppm MNP concentrations. The main cause of the better yield in 6%NaOH pretreatment with MNP concentrations can be explained by the synergistic effect of alkaline pretreatment and MNP due to the enhanced mechanism of DIET between osteogenic and metanogenic bacteria, which has led to an increase in the conversion of VFAs to methane [41].

3.7 Statistical analysis of MNP and alkaline pretreatment

A summary of the statistical analysis of the mean performance of methane and biogas production under alkaline pretreatment conditions with different MNP concentrations within the six-time period throughout the AD process has been shown in Table 2. Statistical analysis of cumulative performance of biogas produced by the pretreatment substrates with various MNP concentrations showed a significant difference with the control. All treatments had a significant difference at 5% probability level in terms of methane volume. The largest methane volume was obtained by adding 6%NaOH pretreatment with 110 ppm MNP concentrations to the substrate, as large as 988 Nml/g VS, which was significant compared to the control (Table 2). Figure 6 compares the mean cumulative production of biogas from control, 6%NaOH pretreatment UDSW, 6%NaOH pretreatment with 50, 70, 90, and 110 ppm MNP.

Table 2

Mean performance of the methane and biogas production under alkaline pretreatment conditions with different MNP concentrations at six time period throughout the AD process
Treatments	index	Time (day)
Treatments	index	(1–4)	(4–8)	(8–12)	(12–16)	(16–20)	(20–24)	Mean
6%NaOH	Biogas	5500	4125	5100	4140	1170	0	3339
6%NaOH	CH4	1882	1717	3111	3112	936	0	1793
6%NaOH with 50ppm Fe3O4	Biogas	6180	4310	5300	4350	1460	625	3704
6%NaOH with 50ppm Fe3O4	CH4	1947	1954	3801	3480	1168	454	2134
6%NaOH with 70ppm Fe3O4	Biogas	6135	4265	5775	4690	1925	700	3915
6%NaOH with 70ppm Fe3O4	CH4	2284	1962	3970	3722	1540	504	2330
6%NaOH with 90ppm Fe3O4	Biogas	6545	4900	6250	5190	2335	1045	4378
6%NaOH with 90ppm Fe3O4	CH4	2951	2587	4252	4090	1868	758	2751
6%NaOH with 110ppm Fe3O4	Biogas	6430	5150	6375	5610	3020	1350	4656
6%NaOH with 110ppm Fe3O4	CH4	2544	2797	4795	4488	2416	1051	3015

3.8 Environmental effects of using iron nanoparticles and alkaline pretreatment

NaOH recovery after pretreatment is environmentally important, since discharging a solution containing large concentrations of sodium ions (Na⁺) can lead to water pollution and soil salinity. Iron is a vital nutrient for plants and bacterial species in the soil, but high concentrations of iron in digestion can lead to toxicity. Iron concentration 500 ppm and above can cause toxicity in soil [42], which is far higher than the proposed concentration utilized in this study (110 ppm). In addition, iron remaining from digestion can be reclaimed through integrating the mechanisms of electrochemical discharge recovery such as the residual heat discharge of the digestive materials posttreatment [12]. This can recover reasonable amounts of iron used in the AD process. Recovery of NaOH and Fe can lead to reducing the costs related to pretreatment and application of nanoparticles. It can also eliminate the possible environmental contamination resulting from Na⁺ and iron accumulation in waste.

The aim of this study was to assess the synergistic effect of alkaline pretreatment and Fe₃O₄ nanoparticles on methane and biogas production from anaerobic co-digestion of MSW and SS organic parts. The maximum methane and biogas production occurred in the alkaline pretreatment reactor using 6%NaOH pretreatment with 110 ppm MNP concentration, which was 86% and 162% greater than the control respectively. The results indicated that the MNP and 6%NaOH pretreatment could effectively enhance the methane and biogas production, and could be a new approach to optimizing biogas production from MSW. Further research is required on AD process including the mechanism of NaOH recovery after pretreatment and recycling the iron remaining from the digestion.

Acknowledgment

We sincerely thank the esteemed officials of the Biogas Laboratory, Department of Biosystem Engineering, University of Mohaghegh Ardabili, who cooperated in this research.

Authors' contributions

Mansour Ahmadi-Pirlou: Conceptualization, Investigation, Methodology, Formal analysis, Data curation, Writing-original draft, Writing review & editing. Amirhamzeh farajollahi: Funding acquisition, Supervision, Writing review & editing. Ahmad Reza Rahmati: Investigation, Formal analysis, Data curation.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript

Competing Interests

The authors declare no conflict of interest.

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published 02 Apr, 2024

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Editorial decision: Reconsider pending major revisions
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Synergistic of nanoparticles and pretreatment on biogas yield from anaerobic co-digestion of municipal waste and sewage sludge

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Abstract

Figures

1 Introduction

2 Materials and methods

2.1 Inoculum and substrate

2.2 Preparation of nanoparticles and pretreatment

2.3 Experimental digester

2.4 Experiments performed

2.5 Analytical methods

2.6 Nanoparticle characterization

2.7 Statistical analysis

3 Results and discussion

3.1 The effect of alkaline pretreatment on the substrate chemical compounds

3.2 Nanoparticles characterization

3.3 Synergistic effect of the alkaline pretreatment and nanoparticles on biogas production

3.4 The effect of alkaline pretreatment and MNP on the methane content and methane performance

3.5 The effect of alkaline pretreatment and magnetite nanoparticles on TS and VS reduction

3.6 Volumetric methane production

3.7 Statistical analysis of MNP and alkaline pretreatment

3.8 Environmental effects of using iron nanoparticles and alkaline pretreatment

4 Conclusion

Declarations

References

Status:

Journal Publication

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