Comparison of the Effects of Pulsed Electric Field Disintegration and Ultrasound Treatment on the Efficiency of Biogas Production from Chicken Manure

Abstract

This study used chicken manure classified as lignocellulosic biomass due to its high straw content. This paper compares the possibility of using pulsed electric field (PEF) pretreatment of lignocellulosic substrates with ultrasonic disintegration (UP) to increase methane production. As for ultrasonic treatment, the BMP increased from 210.42 ± 7.92 mL/g VS to 250.06 ± 8.68 mL/g VS, whereas with PEF disintegration, the BMP ratio increased from 210.42 ± 7.92 mL/g VS to 248.90 ± 9.29 mL/g VS. The use of PEF and UP pretreatment increased methane production from 307.29 ± 13.65 mL/g VS to 366.99 ± 14.18 mL/g VS and from 307.29 ± 13.65 mL/g VS to 365.07 ± 11.71 mL/g VS, respectively. This study showed that both ultrasonic treatment and PEF contribute to the biochemical potential of methane (BMP) from chicken manure.

1. Introduction

The beginning of the development of biogas technologies, which continues to this day, dates back to the 1930s when the basic principles of the process were elaborated and the first digestion tanks appeared. After the Second World War, the biogas achievements of Germany that pioneered the “biogas evolution” spread throughout the world, and biogas became an object of global interest, particularly due to the need to search for energy from renewable sources [1]. Recent years have seen a steadily growing interest in innovative and environmentally-friendly technologies aimed at a reduction in the use of fossil fuels and the prevention of environmental pollution, and therefore further development of biogas technologies [2,3].

Lignocellulosic biomass is the most abundant renewable feedstock in nature [4]. Lignocellulosic material, due to the origin of the raw material, can be divided into four groups: forest resources, biodegradable municipal waste, paper waste, and residue of energy crops [5]. Chicken manure, which is often one of the largest organic waste streams, is treated as lignocellulosic biomass due to its high straw content of over 60%. The production of chicken manure worldwide is estimated at 20,708 Mt per annum, which often raises environmental concerns while potentially providing a readily available feedstock for biogas production. In theory, using all of the chicken manure resources can generate 40 × 10¹⁴ kJ/year of energy [6].

The lignocellulosic complex comprises three polymers, i.e., crystalline cellulose, amorphous hemicellulose, and highly branched lignin. These three components form a strongly intertwined, stabilised compound which makes lignocellulosic biomass difficult to dissolve in water and resistant to direct hydrolysis with the formation of monosaccharides [7,8,9,10]. Therefore, the use of lignocellulosic biomass for biofuel production requires a minimum of three stages: the first stage is the pretreatment aimed at increasing the release of cellulosic polymers from the cross-linked biomass structure, the second stage involves the hydrolysis of pretreated biomass into monosaccharides, and the third stage involves digestion or rehydration of monosaccharides to biorefinery products [7,11]. The pretreatment mentioned above is of key importance to ensure a high yield of monosaccharides from polysaccharides of the feedstock under treatment. Hydrolysis omitting the pretreatment stage achieves an efficiency of less than 20%, whereas applying the pretreatment process increases its efficiency to 90% [5]. Effective pretreatment should satisfy several criteria, e.g., ensure the separation of lignin from cellulose, increase the proportion of amorphous cellulose, increase substrate porosity, eliminate loss of sugars, reduce inhibitor formation, and minimise energy costs. In order to increase the efficiency of energy production from biomass, several physical, physicochemical, chemical, and biological treatments are carried out. An important issue is establishing technological conditions that enable an efficient, economically viable, and safe substrate preparation process [9,12,13,14].

This study compared two innovative biomass pretreatment methods preceding the anaerobic digestion process, i.e., ultrasonic pretreatment (UP) and pulsed electric field (PEF). The research described in the article was aimed at testing whether the analysed methods improve the efficiency of methane fermentation of lignocellulosic substrates. Although the application of UP for biomass conditioning is a relatively new solution, the literature reports indicate the impact of the compression and cavitation effects on delignification, the reduction in biomass particle size, and the reduction in the degree of biomass polymerisation [7,15,16]. UP has been successfully applied for the pretreatment of agricultural waste [7,15,17]. PEF is a non-thermal method which uses electrical pulses to destroy plant structures by creating pores in the cell membrane. In addition to increasing the cell membrane permeability, which considerably accelerates the hydrolysis of the polysaccharides contained in the lignocellulosic complex, other advantages of PEF application include short exposure time and low energy consumption [18,19,20,21]. The electroporation effect is widely used in food sterilisation and is an effective and environmentally friendly method. As the literature data show, PEF can improve the efficiency of biogas production when applied. Compared to traditional methods, it is characterised by lower energy consumption. However, it is a new technology, and more data are needed to fully understand the impact of the method on the lignocellulosic biomass decomposition process. One of the effects of using PEF is to increase the release of intracellular components, which can lead to an increase in methane production efficiency, the effect of PEF on increasing biogas production from waste-activated sludge and wastewater has already been proven [22]. There are few literature reports treating the effectiveness of the use of PEF as a method of pretreatment of lignocellulosic substrates; in the described studies the authors see the finding of an alternative, more effective method to those commonly used.

2. Materials and Methods

2.1. Substrate

The research used chicken manure as the substrate; the substrate was brought from a poultry farm in Baboszewo (Poland, Mazowieckie province). Before pretreatment using PEF, the material was shredded with a milling shredder, and the particle size ranged from approximately 0.5 to 1 cm. The substrate hydration was increased to 95%. The raw substrate was characterised by a dry matter (TS) content at a level of 28.91 ± 1.09%, whereas the dry organic matter (VS) content was 73.03 ± 1.04% of TS (

Table 1. Parameters of chicken manure.

Parameters	Value
Hydration [%]	71.1 ± 1.09
Dry weight [%]	28.91 ± 1.09
Dry organic weight [% TS]	73.03 ± 1.04
Total carbon (TC) [mg C/g TS]	380.75 ± 11.22
Total organic carbon (TOC) [mg C/g TS]	326.9 ± 18.78
Total nitrogen (TN) [mg N/g TS]	44.28 ± 4.56
C/N	8.60
Cellulose	23.68 ± 2.23
Hemicellulose	17.81 ± 1.55
Lignin	4.27 ± 0.42

).

2.2. Equipment

This study used a PEF proprietary prototype disintegration facility (Figure 1) which included a cutting shredder that pre-cut the material. The material was cut with a MultiDrive control BT Package laboratory shredder (IKA, Warsaw, Poland) to a particle level of 2–4 mm. The shredded material was placed into a tank in which the substrate hydration was increased. The disintegration facility is equipped with a hopper that enables its operation under static conditions and additionally includes a peristaltic pump which enables operation under flow conditions. The substrate was disintegrated in a coaxial disintegration chamber with an active volume of approximately 0.5 L. The disintegration chamber has two stainless steel electrodes spaced 2 cm apart.

The UP was carried out using a UP400S laboratory disintegrator (Hielscher Ultrasonics, Teltow, Germany).

2.3. Pretreatment

Rectangular electrical pulses were used in the pretreatment process using PEF. A voltage of 40 kV amplitude was applied to the electrodes. Based on the preliminary studies, the process parameters were selected [23,24,25]. The generated electrical pulses were characterised by a width of 50 µs and a frequency of 5 kHz. Considering the chamber’s coaxial characteristics, the generated electric field intensity was non-uniform. The highest electric field intensity was at the inner electrode surface (38.66 kV/cm), and the lowest was at the outer electrode surface (11.66 kV/cm). Previously crushed and hydrated samples with a volume of 0.5 L were introduced into the disintegration chamber using a hopper. In the filled chamber, the substrate was sandwiched between two electrodes to which high-voltage electrical pulses were applied. As a result, during processing, the substrate was in the zone of interaction of a high-intensity electric field. The UP process was carried out in a beehive disintegrator with a HIUS probe with 400 W power, 24 kHz frequency, and 10 mm probe diameter. According to the manufacturer’s equipment specifications, the acoustic power density for the electrode tip is 300 W/cm². Samples with a volume of 0.25 L were placed in a flat-bottomed beaker, and then a titanium probe with a diameter of 10 mm was immersed to a depth of 4 cm in the liquid sample and subjected to ultrasound. Ultrasonic radiation was produced directly from the probe’s tip in continuous mode. Using a rotary controller, the mode was set for continuous acoustic irradiation without pulse interruption. The amplitude of the oscillations was adjusted, and the ultrasound output power was set at 100% of the nominal power. This study was divided into nine series. The criterion for the division was based on the energy consumed for the disintegration process: UP0, PEF0—0 Wh/ kg TS—control sample; UP1, PEF1—0.05 Wh/g TS; UP2, PEF2—0.1 Wh/g TS; UP3, PEF3—0.15 Wh/g TS; UP4, PEF4—0.2 Wh/g TS; UP5, PEF5—0.26 Wh/g TS; UP6, PEF6—0.3 Wh/g TS; UP7, PEF7—0.36 Wh/g TS; UP8, PEF8—0.41 Wh/g TS.

2.4. Biochemical Methane Potential (BMP)

Biochemical methane potential (BMP), often defined as the maximum volume of methane, provides and indicates the biodegradability of the substrate and its potential to produce methane in the anaerobic digestion process [26]. The BMP of the test substrate was determined using an AMPTS II analyser (BPC Instruments AB, Lund, Sweden). The anaerobic digestion process was carried out at 37 °C for 24 days. The analyser was equipped with glass reaction chambers with an active volume of 0.5 L. The chamber contents were mixed for 30 s at 100 RPM every ten minutes. 0.2 L inoculum (I) was placed along with the pre-prepared substrate (S) in the chambers to provide the I/S ratio of 5. Before the digestion process, the reactors were flushed with pure nitrogen to remove oxygen. Anaerobic sludge was characterised by a TS content of 2.89 ± 0.11%, whereas the VS content was 73.02 ± 0.07% of TS.

In order to reduce the impact of inoculum on methane production, a negative blind test with a non-disintegrated substrate was conducted. The experiment was carried out in three replicates.

2.5. Analytical Methods

The liquid fraction of the substrate was filtered before and after disintegration, and the above-mentioned determinations were carried out to determine the effect of PEF disintegration on the total organic carbon (TOC) concentration and the chemical oxygen demand (COD). Moreover, the effect of disintegration on the cellulose, hemicellulose, and lignin contents was investigated. To this end, the substrate was subjected, before and after pretreatment, to chemical fractioning using neutral and acidic detergents. The cellulose, hemicellulose, and lignin contents were determined using the method proposed by van Soest et al. [27]. The method involves analysing the contents of the following fibre fractions: neutral detergent fibre (NDF), acid detergent fibre (ADF), and acid detergent lignin (ADL). The calculations were made using the following equations:

Cellulose content:

Cellulose = ADF − ADL

(1)

Hemicellulose content:

Hemicellulose = NDF − ADF

(2)

Lignin content:

Lignin = ADL

(3)

The TOC concentration was determined using a TOC-L L analyser (Shimadzu, Kyoto, Japan), whereas the nitrogen and carbon contents in the substrate were determined using a Flash 2000 2000 analyser (Thermo Fisher Scientific, Waltham, MA, USA). The chemical fractioning using detergents was carried out using a semi-automatic fibre analyser ANKOM220 (ANKOM Technology, Macedon, NY, USA). The analyses of TS and VS contents were conducted by the gravimetric method (PN–75/C–04616.01).

2.6. Kinetic Evaluations

As methane fermentation most closely fits first-order kinetics, methane production can be described by the following equation:

C = C_e × (1 − e^k·t)

(4)

where C (NmL/g VS) is the cumulative biogas yield in the fermentation time t, C_e (NmL/g VS) is the maximum biogas yield, and k (1/d) is the kinetic biogas production factor. The values C, C_e, and k were estimated by non-linear regression using Statistica 13 (TIBCO) software.

2.7. Statistical Analyses

Levene’s test was used to check the homogeneity of variance in the groups. The obtained results showed the uniformity and homogeneity of experimental data for individual disintegration methods. On this basis, it was possible to check the significance of differences between the analysed variables. For example, the energy dose during the pretreatment and the COD concentration in the solution. Tukey’s Reasonable Significant Difference (RIR) test was used to test for significance between the analysed variables. The tests adopted a significance level of α = 0.05. The statistical analyses were conducted using Statistica 13 software (TIBCO, Palo Alto, CA, USA).

3. Results

This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.

3.1. The Effect of UP and PEF on Organic Matter Solubilisation

The TOC content and COD value can represent the concentration of soluble organic substances, and an increase in these concentrations indicates that pretreatment increases the organic matter penetration into the liquid fraction of the substrate. Before the pretreatment, the TOC and COD concentrations were 1922 ± 76 mg/L and 5957 ± 71 mg O₂/L, respectively. Pretreatment of chicken manure using ultrasonic radiation resulted in the release of organic matter expressed in the COD and TOC from the substrate. The solubilisation degree increased with the amount of energy used for the treatment (Figure 2a,c).

The highest TOC content of 2269 ± 60 mg/L was obtained in the UP8 series. The highest COD concentration of 6991 ± 86 mg O₂/L was also obtained in the UP8 series. Using a pulsed electric field to treat lignocellulosic substrate also increased the degree of solubilisation (Figure 2b,d). The highest TOC and COD concentrations (2252 ± 48 mg/L and 6973 ± 85 mg O₂/L, respectively) were obtained in the PEF5 series. However, it was observed that in the disintegration samples in which more energy than 0.26 Wh/g TS was used, the solubilisation rate began to decrease. In their study, Kuşçu et al. [28] described the effect of PEF disintegration on the COD concentration in the sewage sludge liquid fraction. They demonstrated that the application of PEF increased solubilisation by 65%. A similar study was conducted by Deng et al. [29], in which the COD content increased by approximately 29% following the application of the PEF treatment of sewage sludge. The effects of ultrasonic radiation treatment of lignocellulosic substrate on TOC and COD solubilisation were also investigated by Kisielewska et al. [25]. The ultrasonic disintegration of Sida hermaphrodita increased the TOC and COD contents in the liquid fraction of the substrate by 24.7 and 21.9%, respectively.

The cellulose, hemicellulose, and lignin contents in the substrate were determined to calculate the depolymerisation efficiency following the UP and PEF pretreatment. The initial contents of these polymers were 14.62 ± 0.86%, 11.49 ± 0.77%, and 3.34 ± 0.25% TS, respectively. As regards the substrate subjected to ultrasonic radiation pretreatment, the greatest reduction in the polymer content was obtained in the UP8 series, with the resulting values of 12.18 ± 1.10%, 9.98 ± 0.64%, and 2.87 ± 0.17% TS, respectively (Figure 3a). For the substrate disintegrated using a pulsed electric field, the lowest cellulose, hemicellulose, and lignin contents were obtained in the PEF6 series and amounted to 12.33 ± 1.36%, 10.13 ± 0.51%, and 2.94 ± 0.17% TS, respectively (Figure 3b). Pansripong et al. [30] investigated the effect of UP on biogas production from rice straw and found that the application of frequencies of 37 and 102 kHz reduced the hemicellulose content by approximately 25.78% and 20.82%, respectively. An increase in the power level and exposure time reduced the hemicellulose content. Kisielewska et al. [31] noted, in a study into the UP of Sida hermaphrodita, that the cellulose, hemicellulose, and lignin contents decreased with an increase in the amount of energy used for disintegration. Their study showed a reduction in the cellulose, hemicellulose, and lignin contents in the best variant by 33.6%, 27.2%, and 14.0%, respectively. Similar results were obtained by El Achkar et al. when PEF was used as a pretreatment method for grape pomace. The use of PEF did not affect the chemical composition of the substrate, the fibre fraction did not decrease [32]. The results provide evidence that the electroporation phenomenon induced by PEF leads to a drastic increase in permeability, due to the appearance of pores in cell membranes releasing some components into the extracellular environment.

3.2. The Effect of UP and PEF on BMP

Figure 4 presents the experimental results of the chicken manure methane potential. The BMP from the raw substrate with no pretreatment was 210.42 ± 7.92 mL/g VS. The biogas yield was 307.29 ± 13.65 mL/g VS. The application of ultrasonic radiation pretreatment increased BMP. The highest BMP was obtained in the UP8 series, in which 250.06 ± 8.68 mL/g VS was obtained, with biogas production amounting to 365.07 ± 11.71 mL/g VS.

Regarding the BMP rate (r), the highest value of 55.06 mL/g VS daily was obtained in the UP7 series. The highest reaction rate constant (k) value of 0.14 1/d was also observed in this series (Figure 3). The effect of UP on the productivity of biogas from rice straw was investigated by Pansripong et al. [30]. When rice straw was treated at a frequency of 37 and 102 kHz, the biogas production values during the 45-day anaerobic digestion amounted to 250.36 and 243.79 mL CH4/g VS, respectively, which accounted for an increase of approximately 21.95% and 18.75% compared to the straw subjected to no treatment. Pretreatment at 37 kHz ensured a greater methane yield than at 102 kHz. The difference between the results of Pansirpong et al. and those obtained for chicken manure was probably due to the twice as high content of hemicellulose (41.7%) and cellulose (37.5%) in the substrate compared to chicken manure. Zieliński et al. [33] compared physical pretreatment methods using ultrasounds and hydrothermal cavitation in mesophilic anaerobic digestion of cattle manure mixed with wheat straw. Their study demonstrated that the application of UP allowed biogas production to increase by 64.2%.

Applying a pulsed electric field for the pretreatment of the lignocellulosic substrate also increased methane production. The highest biogas and BMP coefficients in the samples disintegrated using PEF were noted in the PEF6 series and amounted to 366.99 ± 14.18 and 248.90 ± 9.29 mL/g VS, respectively (Figure 5). It was also observed that the BMP and biogas productivity dropped slightly in subsequent series. The application of PEF disintegration did not affect the methane content in biogas.

The highest methane production rate (r) value of 104.17 mL/g VS d for the samples subjected to PFE pretreatment was obtained in the PEF8 series. Regarding the reaction rate constant (k) value, the highest value of 0.43 1/d was also obtained in the PEF8 series (

Table 2. Kinetic parameters.

Series	UP		PEF
	k	r	k	r
0	0.13	28.57	0.20	41.26
1	0.18	38.20	0.24	49.64
2	0.21	47.35	0.24	51.62
3	0.20	47.32	0.23	53.99
4	0.15	36.15	0.24	57.58
5	0.14	34.10	0.21	51.65
6	0.11	28.46	0.29	69.24
7	0.23	55.85	0.29	69.85
8	0.14	35.84	0.43	103.34

). Gali et al., while studying the methane fermentation of agricultural waste, obtained constant k values of 0.15 1/d for apple waste, 0.23 1/d for sunflower, and 0.29 1/d for orange waste [34]. Bohutskyi et al. estimated parameters for different kinetic models describing the production of u and methane from bacterial–algal biomass co-fermented at different ratios with cellulose. For first-order kinetic, they obtained results for a k value ranging from 0.055 1/d to 0.091 1/d [35].

The methane content of biogas was unaffected by PEF disintegration or UP. The percentage of methane in biogas ranged from 67.0% to 69.3% (Figure 6).

Safavi and Unnthorsson [22] conducted a study involving pretreatment using a pulsed electric field to determine the effect of PEF on methane production. The test substrates included leachate from waste landfills and fruit and vegetables. In order to investigate the effect of pretreatment on BMP using the biochemical methane potential test, the following three treatment intensities were applied: 15, 30, and 50 kW h/m³. The BMP from landfill leachate increased significantly by 44% at the highest intensity. The pretreatment results using a pulsed electric field of fruit/vegetables demonstrated that 15 kWh/m³ is an intensity at which the highest BMP was achieved. Above this intensity, BMP decreased. Wang et al. [36] subjected grass from the Pennisetum family to pretreatment under different conditions of a high-voltage pulsed electric field (HPEF) in order to improve the material utilisation and BMP coefficients in the anaerobic digestion process. In the study, they demonstrated that, compared to the control group, nine Pennisetum groups subjected to HPEF pretreatment exhibited higher biogas production and a higher methane concentration than that of control samples. The best results were obtained under the HPEF conditions of 15 kV/120 Hz/60 min, as in this variant, the cumulative gas production over the period was 9587 mL, with the obtained amount being greater by 26.95% than that in the control group. Lindmark et al. [37] analysed pretreatment in the PEF technology to improve biogas production from ley crop silage. The BMP increased by 16% with the digestion time shorter by 30% compared to the substrate subjected to no pretreatment, using an electrical voltage of 96 kV/cm. According to Kovacic et al. [38], the PEF pretreatment of the crop remains increases the biogas production and BMP efficiency by 18% and 16%, respectively, in maize stalks, and by 18% and 17%, respectively, in soybean straw. El Achkar et al. studied the effect of various types of pretreatment, including US I PEF treatment, on methane production from grape pomace. The PEF system consisted of a high-voltage power supply and a pulse generator; pulse duration (10-5-10-4 s) and frequency (24–240 Hz) were adjustable. The highest recorded increase in methane production after PEF was 4%, compared to methane obtained from untreated substrate. Ultrasonic treatment involved the use of an ultrasonic processor with an operating frequency of 50 kHz and an effective output power of 60 W; the use of Us treatment resulted in a 10% [32].

The authors in an earlier study compared the effect of PEF on the increase in BMP efficiency from corn silage. They noted then an increase in the amount of carbohydrates in the dissolved fraction which resulted in an increase in BMP. The highest amount of methane was obtained after using PEF for 180 s; there was then a 14% increase in production. The BMP for the control sample was 401.83 mL CH₄/g VS, and for the sample after PEF was 465.62 mL CH₄/g VS [23].

The determination of BMP allowed the energy balance (Table 3). To determine the energy input (E_in (Wh/gTS)) determining the energy expenditure for the disintegration process, the actual energy consumption of the PEF voltage generator and ultrasonic disintegrator was measured. The amount of energy input determines the energy intensity of the substrate pretreatment process. E_out is the energy gained due to the increase in BMP caused by pretreatment, considering methane’s energy value. E_out was determined using the equation:

E_out (Wh/g TS) = ∆P × ξ

(5)

∆P denotes the increase in BMP after disintegration compared to the control sample, whereas ξ denotes the methane energy value of 9.17 kWh/m³ CH₄. The increase in energy E_T was calculated as the difference in energy input (E_in) and energy output (E_out), resulting in determining the energy gain or loss due to the pretreatment of the sub-loss. E_T was determined using the equation:

E_T (Wh/g TS)= E_out − E_in

(6)

The balance showed that the greatest increase of 0.04 Wh/g TS was obtained in the PEF3 series. The further increase in the amount of energy used for PEF disintegration did not increase the energy gain. Even though the highest methane yield was recorded in the PEF6 series, it turned out that the incremental methane production could not cover the disintegration process’s increased energy input. As regards the samples subjected to pretreatment using ultrasound, the energy balance was negative in all series.

Table 3. Energy balance.

Series	Ein (Wh/g TS)	Eout (Wh/g TS)		ET (Wh/g TS)
		PEF	UP	PEF	UP
0	-	-	-	-	-
1	0.05	0.03	0.00	−0.02	−0.05
2	0.10	0.08	0.08	−0.02	−0.02
3	0.15	0.19	0.11	0.04	−0.04
4	0.20	0.20	0.15	0.00	−0.05
5	0.26	0.22	0.17	−0.04	−0.09
6	0.30	0.24	0.20	−0.06	−0.10
7	0.36	0.24	0.22	−0.13	−0.14
8	0.41	0.24	0.25	−0.17	−0.16

Table 3. Energy balance.

Series	Ein (Wh/g TS)	Eout (Wh/g TS)		ET (Wh/g TS)
		PEF	UP	PEF	UP
0	-	-	-	-	-
1	0.05	0.03	0.00	−0.02	−0.05
2	0.10	0.08	0.08	−0.02	−0.02
3	0.15	0.19	0.11	0.04	−0.04
4	0.20	0.20	0.15	0.00	−0.05
5	0.26	0.22	0.17	−0.04	−0.09
6	0.30	0.24	0.20	−0.06	−0.10
7	0.36	0.24	0.22	−0.13	−0.14
8	0.41	0.24	0.25	−0.17	−0.16

4. Conclusions

This study showed that both pretreatment of the lignocellulosic material with ultrasonic radiation and disintegration with a pulsed electric field increased methane production.

The application of ultrasonic treatment in the UP8 series increased the COD value from 5957 ± 71 mg O₂/L to 6991 ± 86 mg O₂/L. In the same series, the TOC concentration increased from 1922 ± 76 mg/L to 2269 ± 60 mg/L. Regarding the pulsed electric field treatment, the highest increase in the COD and TOC was obtained in the PEF5 series. The TOC concentration increased from 1922 ± 48 mg/L to 2252, whereas the COD concentration increased from 5957 ± 71 mg O₂/L to 6973 ± 85 mg/L. Regarding the UP, BMP increased from 210.42 ± 7.92 mL/g VS to 250.06 ± 8.68 mL/g VS. Biogas production coefficient increased from 307.29 ± 13.65 mL/g VS to 365.07 ± 11.71 mL/g VS. The application of pulsed electric field disintegration increased the BMP from 210.42 ± 7.92 mL/g VS to 248.90 ± 9.29 mL/g VS. Biogas productivity increased from 307.29 ± 13.65 mL/g VS to 366.99 ± 14.18 mL/g VS. Despite the slightly greater methane gain in the sample treated with ultrasounds, both the reaction rate and the methane production rate were more advantageous in the PEF-disintegrated sample.

The data show that under the influence of PEF and UP, organic compounds were released into the dissolved phase, which caused an increase in BMP.

This study demonstrated that disintegration using a pulsed electric field could find application in anaerobic digestion to improve BMP. Considering the energy balance carried out, it turned out that only in one series with PEF application was there an energy gain. Pretreatment with ultra-sound in all series showed energy losses.