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Article

Comparison Among Thermal Pre-Treatments’ Effectiveness in Increasing Anaerobic Digestibility of Organic Fraction in Municipal Solid Wastes

by
Marco De Sanctis
1,
Valerio Guido Altieri
1,
Emanuele Barca
1,
Luigi di Bitonto
1,
Francesco Tedeschi
2 and
Claudio Di Iaconi
1,*
1
Water Research Institute, National Research Council-C.N.R, Viale F. De Blasio 5, 70132 Bari, Italy
2
ICMEA S.r.l., Via Gravina 156, 70033 Corato, Italy
*
Author to whom correspondence should be addressed.
Energies 2024, 17(24), 6293; https://doi.org/10.3390/en17246293
Submission received: 12 November 2024 / Revised: 4 December 2024 / Accepted: 9 December 2024 / Published: 13 December 2024
(This article belongs to the Topic New Research on Waste Treatment, Disposal and Valorization)
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Abstract

The organic fraction of municipal solid waste (OFMSW) is widely recognized as a possible substrate for anaerobic digestion processes. However, the heterogeneity of this matrix and the presence of slowly biodegradable compounds can slow down anaerobic digestion and reduce its performance. This study compares the effectiveness of different thermal pre-treatments in increasing OFMSW anaerobic digestibility. Thermal pre-treatments were compared with OFMSW shredding, considered as the minimum pre-treatment required in order to reduce particles size of the OFMSW. The pre-treatments were performed in autoclave (121 °C and 1.4 bar for 20 min) or in an ad hoc hydrolysis reactor designed for the experimental trial (140 °C and 7 bar for 30 min) with air or nitrogen as gas phase. The thermal pre-treatments affected methane yield (NmLCH4/gVS), depending on the pre-treatment strategy, with autoclaving allowing for an 80% increase with respect to the control run, and leading to a methane yield of 476 ± 194 NmLCH4/gVS. The pre-treatments in the hydrolysis reactor caused a loss of organic matter (due to its volatilization) reducing the organic loading rate of the digester. Nevertheless, the digester performance in terms of COD (chemical oxygen demand) and VSS (volatile suspended solid) removal showed limited differences among the pre-treatments applied and ranged on average 79–94%.

1. Introduction

The constant growth in world population (~1%/year) together with urbanization and economic development are leading to an increase in municipal solid waste (MSW) production. Global MSW production has been estimated as high as 2 billion tons/year [1], and it is predicted to exceed 9 billion by 2050 [2], generating serious issues for its proper treatment and disposal and representing a constant risk for the environment. Furthermore, the recent energy shortage experienced by several developed countries and the negative effects of fossil fuels use on climate change are leading to increasing interest in alternative and renewable energy sources [3]. The latter are also supported at European level, with the European Commission asking for a 27% increase in renewable energy use by the 2030 [4], as well as a 40% reduction in greenhouse gas (GHG) emissions with respect to 1990 levels by the same year [5].
A significant fraction of MSW is actually represented by organic material (46%) [6], suggesting that the organic fraction of MSW (OFMSW) might become an important source of alternative energy if utilized as feedstock for the anaerobic digestion process. The latter is an attractive strategy for OFMSW treatment because it allows for the production of energy, in the form of biogas or hydrogen, and compost for agricultural use, after proper stabilization of the digestate [6].
However, OFMSW is characterized by high heterogeneity and includes a variable share of poorly or slowly biodegradable materials such as lignocellulosic biomasses or greases. The anaerobic digestion process is carried out by different microbial populations each dedicated to a specific phase of this process, namely, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The hydrolysis phase is widely recognized as the limiting step in presence of a high share of particulate organic matter. Therefore, several studies focus on pre-treatments of OFMSW aimed at solubilizing particulate organic matter and improving its anaerobic digestibility, leading to an increase in biogas yield and/or a reduction in the hydraulic retention time (HRT) required by the anaerobic digestion process [3,7]. The investigated OFMSW pre-treatments are based on physical (i.e., shredding, heating), chemical (i.e., acid or alkaline hydrolysis), biological (i.e., fungi, enzymes), or combined approaches [3]. Among them, thermal pre-treatments are considered more sustainable, as they do not require the addition of chemicals, and the thermal energy required can be obtained by the biogas produced, as well as recovered and reused for heating an anaerobic digester. However, conflicting results are available in the literature, reporting positive but also negative effects on the AD process [7], therefore suggesting further investigation. Deepanraj et al. [8] observed only a limited increase in organic matter removal and cumulative biogas production, 5% and 6%, respectively, by autoclaving food waste (120 °C and 30 min). Another study [9] investigating the effect of pre-treatments in a wide temperature range (from 55 °C to 160 °C) reported methane production increasing along with pre-treatment temperature increase until 120 °C, resulting in 48% higher methane production than the control. Conversely, the test conducted at 140 °C and 160 °C resulted in a reduction in methane production. Indeed, high temperature pre-treatments might enable release or generation of inhibiting compounds [10,11]. For instance, the generation of phenolic and heterocyclic compounds has been reported when a relevant share of lignocellulosic wastes is present, with negative effects on anaerobic microorganisms conducting the anaerobic digestion [12].
In the present study, the authors conducted an experimental campaign comparing the effectiveness of different thermal pre-treatments in increasing OFMSW anaerobic digestibility. Thermal pre-treatments were also compared with OFMSW shredding, considered as the minimum pre-treatment required in order to reduce particles size of the OFMSW. Indeed, particle size is widely reported to influence anaerobic digestion efficiency, with large particles slowing the process speed [13]. The thermal pre-treatments were conducted under different operating conditions in terms of temperature, pressure and gas phase (air or nitrogen). They were performed in autoclave (121 °C and 1.4 bar for 20 min) or in an ad hoc hydrolysis reactor designed for the experimental trial (140 °C and 7 bar for 30 min).

2. Material and Methods

2.1. Experimental Setup and Operation

The experimental trial was conducted at a pilot-scale plant, including an anaerobic digester, a reactor for thermal hydrolysis, a home garbage disposal unit for OFMSW shredding, centrifugal pumps for feeding and drawing, peristaltic pumps for buffer addition, an online biogas analyzer, and a flow meter for biogas production estimation (Figure 1). The digester comprised a 60 L steel reactor (liquid working volume: 35 L) equipped with pH and temperature control and operating in the range 38 ± 2 °C. Plant heating and temperature maintenance were ensured by an electric trace and a polystyrene coat. The liquid phase was maintained under mixing by a mechanical stirrer. A pipe on one side of the digester provided additional mixing of digestate from the top to the bottom of the plant, by means of a centrifugal pump, and allowed for real-time monitoring of pH and temperature. The digestate purge occurred with the same centrifugal pump carrying out digestate recirculation by opening a dedicated valve. Biogas production and quality were on-line monitored with a flow meter (AWM3100V, Honeywell, Charlotte, NC, USA) and an infrared biogas analyzer (ADEV 4400, ADEV, Cesano Maderno, Italy), respectively.
The thermal hydrolysis of OFMSW occurred in a 20 L steel chamber, provided with temperature and pressure probes. After thermal treatment was performed, OFSMW was transferred in a second chamber (20 L) for cooling by opening a motorized valve. Then, a dedicated pump supplied the OFMSW to the digester. Thermal hydrolysis occurred at 140 °C and 7 bar, and lasted 30 min. The hydrolysis was conducted in presence of air or nitrogen. The second condition included OFMSW flushing with nitrogen gas in order to remove oxygen from the liquid phase and the headspace of the hydrolysis reactor. As alternative and milder strategy, thermal pre-treatment of OFMSW was also performed in autoclave at 121 °C and 1.4 bar for 20 min. Autoclaving occurred in sealed vessels (pyrex bottles) with air as headspace.
OFMSW mainly composed of food waste from households was collected from municipal waste collection plant located in Corato (Bari, Italy). To obtain a homogenized mixture, substrate was subjected to trituration using the home garbage disposal and a mixer–grinder. The OFMSW characterization (Table 1) highlighted a shortage of nitrogen and phosphorous with respect to organic matter; thus, an external source of these nutrients was supplied in the form of NH4Cl and KH2PO4, respectively.
During shredding operation, 1.75 kg of raw OFMSW was added to 1.5 L of tap water. Successively, the shredded OFMSW was diluted to a final volume of 5 L with the effluent of the anaerobic digester (digestate), or thermally treated and then diluted and supplied to the anaerobic digestion plant. This approach allowed for reducing the energy required for thermal pre-treatments (less OFMSW volume to be heated), the freshwater demand, and the addition of nutrients (nitrogen and phosphorous) to the feedstock. Furthermore, it allowed for extending the anaerobic digestion sludge retention time without affecting the organic loading rate (OLR) applied to the digester, 2.1 kgCOD/m3·d on average. Nevertheless, during the experimental campaign, variations in the ORL occurred due to the heterogeneity of the OFMSW collected and on pre-treatment applied effects. The digester operated in batch mode, with feeding/drawing operations occurring twice a week. The digester was inoculated with anaerobic sludge coming from the wastewater treatment plant of Monopoli (Bari, Italy) and operated at an average total solids (TS) concentration of 34 ± 6 kg/m3.

2.2. Analytical Procedures

The OFMSW suspension after shredding and thermal pre-treatments, and the digestate were analyzed for chemical oxygen demand (COD), soluble COD (sCOD), total nitrogen (TN), ammonia, total phosphorous (TP), total and volatile suspended solids (TSS and VSS), total and volatile solids (TS and VS), electrical conductivity (EC) and pH. EC and pH were measured by selective probes. COD, TN, ammonia, and phosphorous were monitored by Hach–Lange tests for water analysis, namely: LCK314 and LCK514 for COD, LCK138 and LCK238 for TN, LCK303 for ammonia, and LCK348 for TP. sCOD was measured after sample filtration through a 0.45 µm nylon filter. TS, VS, TSS, and VSS were determined according to the standard methods [14]. The bromatological characterization of OFMSW included free sugars, proteins, greases, starch, hemicellulose, cellulose, and lignin. The analyses were conducted according to the procedures reported in di Bitonto et al. [15,16].
In order to provide a more robust and accurate characterization of biogas composition, the latter was collected in gas sampling bags and analyzed by an Agilent 490 Micro-GC (Agilent, Santa Clara, CA, USA) twice a week. The Micro-GC was equipped with two columns, the first one (Molsieve 5A) for measuring O2, N2, H2, and CH4, and the second one (PoraPlot U) for detecting CO2 e H2S.

2.3. Statistical Analysis

An analysis of variance (ANOVA) was applied to evaluate the relevance of the differences observed among the thermal pre-treatments on organic matter solubilization, and the digester removal efficiency for COD, sCOD, and VSS. This procedure compares multiple sample means, for example, the outcomes of different treatments. Before applying ANOVA, it is necessary to check the assumptions of normality of data and homogeneity of variance. If the p value is less than 0.05 (significant), it implies that the means of the treatments differ significantly. In such cases, a post-hoc test (least significant difference test—LSD) is required to identify the specific differences between the sample means.

3. Results and Discussion

3.1. OFMSW Characterization

Table 1 reports the chemical and bromatological characterization of the OFMSW treated during the current experimental campaign and that one reported in the review by Campuzano and González-Martinez [6]. The latter compared OFMSW from twenty-two countries located in Europe, Asia, America, and Oceania. The study revealed a non-homogeneous OFMSW composition due to different habits but also countries definition of OFMSW. Notwithstanding this intrinsic variability, the characteristics of the OFMSW selected for the current study are in line with the data reported in the cited study [6]. The most relevant outcome is the 98% VS/TS ratio, indicating very low presence of inorganic substances in accordance with the waste type (i.e., food waste). The high share of organic matter supports the suitability of this OFMSW as substrate for anaerobic digestion process, suggesting high potential for biogas production and TS reduction.
Another positive aspect is the moderate presence of a lipid fraction that is reported to have a methane potential double than starch [17]. However, inhibition of the anaerobic digestion process has been observed in presence of high concentration of greases [18,19]. On the contrary, the limited presence of nitrogen and phosphorous suggests the need for an external source of these nutrients, in order to support a balanced growth of the microorganisms [20]. Therefore, additional sources of these nutrients were supplied to the digester by adding nitrogen and phosphorous salts, whereas in a full-scale plant the co-digestion with other substrates rich in nutrients might be considered. The non-negligible presence of lignin, cellulose, and hemicellulose, which are reported as slowly biodegradable or refractory to biodegradation during anaerobic digestion [21], is a relevant aspect also. In particular, lignin, which is recognized as the most challenging polymer for hydrolyzation in anaerobic digesters [3], acts as a net entrapping biodegradable organic matter and preventing its hydrolysis by microbial enzymes [22]. The presence of lignin, cellulose, and hemicellulose was expected and can be attributed to vegetable and fruit residues, as well as to paper towels or tissue paper.

3.2. Effect of Pre-Treatments on OFMSW

In order to enhance the substrate biodegradability, many pre-treatments—mechanical, chemical, thermal, or thermochemical—can be involved before the anaerobic digestion of OFMSW [23,24,25,26]. Among these, thermal treatments are one of the most investigated pre-treatment categories [23]. The prevalent consequence of their application is the breakage of cell membranes, which in turn determines an increase in the solubilization of the organic fraction [27], in order to overcome the rate-limiting step of anaerobic digestion process, namely, substrate hydrolysis by extra-cellular microbial enzymes [7]. As frequently reported [27,28], sCOD can serve as a viable representation of the content of soluble organic matter. Clearly, this parameter may change significantly with the heating temperature and the treatment duration. The effect of the investigated pre-treatments on organic matter solubilization in terms of sCOD/COD ratio is shown in Figure 2. Shredding turned out to solubilize on average 43% of the total COD. These data are in line with what was observed by Izumi et al. [29], reporting a 40% solubilization after food waste grinding by bead mill. Differently, almost no additional benefit was observed when the thermal pre-treatments were performed on the shredded OFMSW.
This result is close to the conclusions drawn by Ma et al. [30], which focused on pre-treatments of kitchen wastes. The authors observed a sCOD/COD ratio of 32 ± 8% after waste shredding. When autoclave treatment was added, this value increased to 39 ± 9%, and a similar but lower increase was observed after pressure–depressure treatment (at 10 bar) with CO2, indicating that most of the organic matter solubilization occurred due to shredding. In our study, autoclaving seems to have a negative or no effect on the sCOD/COD ratio, whereas the high-pressure treatment (145 °C and 7 bar) caused only a limited increase in this ratio (i.e., from 43% to 44%).
Better results were obtained by Tampio et al. [31], focusing on food waste collected from the South Shropshire Biowaste digestion plant in Ludlow (UK) and comparing sCOD after grinding and autoclaving at high temperature and pressure (160 °C and 6.2 bar). The authors observed a 20% increase in sCOD after autoclaving.
However, these authors [29,30,31] did not report the bromatological composition of the food waste; therefore, it is hard to point out if the differences in COD solubilization were only due to the pre-treatment conditions or food waste characteristics.
To better evaluate the significance of the differences among the sCOD/COD obtained in the current study, the ANOVA test was applied to check if the results from each treatment could be considered statistically equal on average. The data were Gaussian and homoscedastic; therefore, a parametric ANOVA was performed. The test confirmed there were no statistical differences among the sCOD/COD obtained after the four pre-treatments (Figure 3).
Studying the anaerobic digestion of food waste carried out on a batch system, Ding et al. [32] tested different temperatures (ranging from 100 to 200 °C) applied for 20 min on feedstock. After this treatment, sCOD resulted in always being higher than that obtained for the untreated food waste: this highlighted how thermal pre-treatments aided the solubilization of the organic components of OFMSW. The unique exception was represented by the 200 °C treatment, which resulted in a decrease in the sCOD. Jin et al. [27] studied anaerobic digestion performances on food waste after thermal pre-treatments, but taking into account even moderate temperatures (from 55 to 160 °C), with treatment lasting up to 120 min. Regarding the thermal pre-treatment effectiveness, the main conclusions drawn by this study focus on the nexus between temperature and time. For temperatures lower than 100 °C, the solubilization of organic matter was low and at least 30 min was required to reach a constant sCOD value, whereas for higher temperatures and prolonged treatment durations, the solubilization of organic fraction was significantly higher. Also in this case, excessively high temperatures and reaction times resulted in a decrease in the sCOD amount. Furthermore, substrates subjected to prolonged reactions at high temperature can even change their visual characteristics, appearing black.
Pecorini et al. [33] studied different thermal pre-treatments (namely, autoclaving and microwave heating) to evaluate the biodegradability enhancement of the anaerobic digestion of two synthetic OFMSWs, characterized by different lignocellulosic contents. The thermal pre-treatments were carried out at 134 °C and 2 bar for 30 min and 96 °C for 4 min, respectively. Among the two approaches, microwaving appeared more efficient in increasing the solubilization of organic components for both the studied OFMSWs. An interesting outcome from this study is that the significant increase in sCOD does not translate into an increase in methane production of the same magnitude, because most of the soluble COD produced during the thermal treatments was constituted of non-biodegradable compounds. This finding is in agreement with what is reported in the literature regarding treatments performed at excessive temperatures [25,34]. Notably, the study by Razavi et al. [34] focused on the assessment of the effects of hydrothermal pre-treatments on the solubilization of source separated organics (i.e., the mixture of several organic components in municipal solid waste generated from different sources, together with other organic matter such as pruning plant and wood wastes and paper fibers). In this study, 15 different pre-treatment conditions and five severity index values (i.e., a calculated index in which operative factors as temperature and time are combined into a single parameter) were taken into account. The best working conditions resulted in being 220 °C and 10 min, when a maximum sCOD content was reached, whereas the highest methane yield was reached at 190 °C. As widely reported in the literature, by increasing the intensity of the pre-treatment, a decrease in organic solubilization, following the formation of insoluble compounds, was observed. A mismatch between the optimum conditions linked to the highest methane yield and those related to the maximum solubilization of organic matter was reported also by Dasgupta and Chandel [23]. In their study, anaerobic digestion of OFMSW subjected to autoclaving occurring at different conditions (temperatures ranging from 80 to 160 °C, with duration time up to 120 min and pressure up to 6 bar) was investigated. Operating at the highest temperatures and duration times (160 °C, 120 min), it was possible to almost double the sCOD concentration, without any observed decrease in solubilization of organic components.
The downside of the thermal treatments is that, unless they are carried out in controlled conditions, they can generate undesirable effects. One of the most common outcomes is the formation of different classes of refractory compounds [24], which are produced during the Maillard reaction or the caramelization reaction. These are non-enzymatic browning reactions that occur when carbohydrates are exposed to high temperatures (or even to low temperatures, if enough reaction time is given), with or without proteins or amino acids, respectively [35,36,37,38]. These compounds are resultingly quite difficult to degrade, and some of them are even reported to have adverse effects on microbial activity [39,40]. The decrease in solubilization of organic matter and the appearance of a blackish color [27,32,34] are symptoms of the occurrence of the reactions described. In order to limit reactions that hinder the solubilization of organic matter, Liu et al. [35] used the response surface method to study the best combination among the main parameters involved in the anaerobic digestion process (temperature, reaction time, pH, carbon/nitrogen ratio, and moisture content). In this study, six different temperatures were evaluated (ranging from 110 to 160 °C), under high pressure, with a reaction time up to 30 min. According to this simulation, the best condition appeared to be 132 °C, with a reaction time of 27 min and a pH of 5.6.
A different drawback of thermal pre-treatments is reported by Ariunbaatar et al. [26], highlighting the possible loss of volatile organics and easily biodegradable substrates causing a decrease in biomethane production. Table 2 shows that this is the case in the current study. Indeed, the high-pressure thermal pre-treatments occurring in the hydrolysis reactor caused a net loss of organic matter (close to 40% of VS on average). This phenomenon was not observed during autoclaving and was attributed to the different pre-treatment conditions. During autoclaving, pressure and temperature were gradually decreased in order to avoid OFMSW suspension boiling. Moreover, this pre-treatment was performed in sealed vessels, preventing steam or organic compound loss in the atmosphere. Conversely, in the hydrolytic reactor, the higher operating temperature and pressure (140 °C and 7 bar), faster cooling, and larger steam discharge caused the OFMSW suspension to boil. Furthermore, hydrolytic reactor heating was performed by submerged resistance, which might have caused a temperature increase higher than expected in its proximity during the early phase of the thermal hydrolysis. Therefore, some organic matter was supposed to leave the suspension due to evaporation. The large scattering in the share of the organic matter loss reflects both the variability in OFMSW composition, and aging phenomena, which might occur between OFMSW household production and its collection by the authors. The latter affects the presence of readily biodegradable substrates due to waste fermentation and/or their partial biodegradation in the garbage bag itself. The review by Scherzinger and Kaltschmitt [41] dwells on several options of viable thermal pre-treatments in order to enhance anaerobic digestibility. Taking into account lignocellulosic feedstocks, the paper highlights how different categories of thermal processes adopted as pre-treatments can show the same loss of organic solids reported in the present study. However, the loss of organic solids does not occur every time, as it is related to the operating conditions and to the chemical composition of the substrate [25].

3.3. Digester Performances

Figure 4 shows a comparison among the mean removal efficiencies for COD, sCOD, and VSS, recorded during the four runs. Overall, all these parameters were effectively removed, with removal efficiencies close to or above 80%. Comparing the different periods, it can be observed that the high-pressure pre-treatments of OFMSW negatively affected COD and VSS removal, with the worst performances in VSS removal obtained when pre-treatment in presence of nitrogen was performed. It could be due to the fact that the organic matter lost during these pre-treatments was mainly composed of biodegradable compounds, causing an enrichment of the recalcitrant ones in the digester feed. Indeed, the anaerobic digestion process was more effective in terms of COD and VSS removal after autoclave pre-treatment, with the second best performances obtained when OFMSW was subjected only to shredding. The autoclave pre-treatment allowed for an average COD and VSS removal as high as 93.0 ± 4.3% and 94.1 ± 3.2%, respectively, also showing less variability.
To confirm the statistical relevance of the difference observed, ANOVA and LSD tests were performed; VSS removal efficiency was the only parameter showing significant differences. In particular, the tests confirmed the VSS removal efficiencies obtained when the digester was fed with OFMSW subjected to high-pressure pre-treatment in the presence of nitrogen (i.e., absence of oxygen) and after autoclaving were different between them and from the others. Figure 5 shows the result of the LSD test for VSS removal efficiency. The picture highlights that the VSS removal efficiencies group together when OFMSW is subjected to shredding or high-pressure pre-treatment in presence of air (group “ab”).
The limited positive effect of autoclaving on VSS removal is in line with the negligible effect on organic matter solubilization shown in Figure 2 and discussed in the Section 3.2. However, it has been widely reported in the literature that solubilizing VSS does not always reflect in increasing biodegradability [25,33,34]. On the contrary, thermal pre-treatment might also enhance particulate organic matter hydrolyzability by microbial enzymes [4]. This effect could justify the slight increase in VSS removal efficiency observed during the anaerobic digestion process when autoclaving was performed (from 90.8 ± 4.2% to 94.1 ± 3.2%, Figure 4).
The high digester removal performances come from the high share of organic matter in the OFMSW and the digester feeding strategy applied in this study. Indeed, the thermal pre-treatments of OFSMW were performed on a concentrate suspension of OFSMW. Successively, this suspension was diluted to a final volume of 5 L with digester’s effluent, and then supplied to the digester itself. This approach allowed for sparing freshwater for OFSMW dilution and increasing the digester HRT up to about 60 d. The feeding strategy provided the anaerobic microorganisms enough time to hydrolyze particulate organic matter even in absence of the thermal pre-treatments. Conversely, the long HRT might have had detrimental effects on sCOD removal efficiency, due to a not negligible presence of recalcitrant soluble compounds, which are recycled back to the digester during feeding operations.
The positive effect of the long HRT on the anaerobic digestion process is supported by the high removal efficiency observed for VS, which ranged between 90.9 ± 2.9% and 93.9 ± 2.2% when OFSMW was subjected to the high-pressure treatment in presence of nitrogen and autoclaving, respectively. These removals are higher than those reported in the review by [7], comparing the performances of a wide number of mesophilic, thermophilic, and temperature-phased anaerobic digestion processes. The only exception is represented by Chu et al. [42], where the authors obtained a 95.7% VS removal efficiency in a two-stage plant including a thermophilic first stage (for hydrogen production) and a mesophilic second stage (for methane production).
However, it should be pointed out that the removal efficiencies reported in Figure 4 were calculated according to the concentrations in the digester feeding and effluent, thus neglecting the organic matter loss caused by the high-pressure pre-treatments. This means that, despite similar removal efficiencies, if an analogous amount of OFMSW enters the treatment train (i.e., shredding followed by thermal pre-treatments and anaerobic digestion), the actual ORL applied to the digester would be lower in the case of OFMSW subjected to the high-pressure treatments than to autoclaving or shredding.
Table 3 reports the average methane yield and methane percentage in the biogas recorded during the four operative conditions tested. The methane yield obtained when OFMSW was subjected only to shredding (264 ± 223 NmLCH4/gVS) is in line with the data reported by several authors. For instance, Jiang et al. [43], treating OFMSW at mesophilic conditions, found a methane yield ranging 250–278 mLCH4/gVS depending on process ORL and HRT. Similarly, Borowski [44] found 230 mLCH4/gVS in a digester treating OFMSW and sewage sludge.
The volatilization of organic matter during the high-pressure thermal pre-treatments was supposed to negatively affect biogas production. The data reported in Table 3 show this assumption was true only when the high-pressure pre-treatment was performed in presence of air. In this condition, the specific methane production resulted 37% lower than that in absence of any thermal pre-treatment. Furthermore, such reduction in methane production is consistent with the 37% loss of VS observed in this operative condition. The low methane yield recorded in this operating condition might also reflect an inhibition phenomenon caused by the presence of residual oxygen in the digester feed due to the use of compressed air during the thermal pre-treatment.
An opposite trend was observed when the high-pressure thermal pre-treatment was performed in absence of oxygen (i.e., with nitrogen). During this run, methane yield increased and the highest methane percentage in the biogas was obtained. This result was attributed to a different composition of the OFMSW treated by the digester. Indeed, despite the same amount of raw OFMSW (1.75 kg for each feed) being sent for pre-treatment, the resulting ORL applied to the digester was the lowest of the study and averaged 1.0 ± 0.4 kgCOD/m3·d. The increase in methane yield as the result of an ORL reduction has already been observed by other authors [43,45].
The best result in terms of methane yield was obtained when OFSMW was subjected to autoclaving, with the average methane yield increasing by 80% with respect to the control run (i.e., shredding). Biogas production rate increased also with the respect to the control, with average values of 2.2 ± 1.1 and 2.9 ± 0.4 and 2.9 NL/h, respectively. As a drawback, the faster biogas production caused a reduction of the methane content in the biogas, which averaged 50%. The higher methane production during this period is in agreement with the increase in VS removal by the digester and suggests the biomass did not suffer for the generation of inhibiting compounds during the thermal pre-treatment of OFMSW.

4. Conclusions

The current study focuses on the effectiveness of three hydrothermal pre-treatment strategies of OFSMW aimed at increasing its anaerobic digestibility. Before the application of the thermal pre-treatments, OFMSW was subjected to shredding, considering particle size reduction as the minimum treatment required to support a stable and effective anaerobic digestion process. Two pre-treatments were conducted in a dedicated hydrolysis reactor at high pressure and temperature (140 °C and 7 bar), and in presence or absence of oxygen, namely, with air or nitrogen as the gas phase, respectively. The other was conducted in milder conditions (121 °C, 1.4 bar) by autoclaving OFMSW in sealed vessels with air as headspace.
All the thermal pre-treatments were ineffective in increasing sCOD concentration, which already represented about 40% of the total COD after OFMSW shredding. Furthermore, the high-pressure thermal pre-treatments caused a loss of organic matter (close to 40% in term of VS), leading to a reduction in the ORL applied to the digester. This phenomenon did not occur after autoclaving. Despite autoclaving had no effect on organic matter solubilization, it improved OFMSW biodegradability, leading to an 80% increase in methane yield with respect to the control, 476 ± 194 and 264 ± 223 NmLCH4/gVS, respectively.

Author Contributions

Methodology, C.D.I.; software, E.B.; formal analysis, E.B.; investigation, M.D.S., V.G.A., L.d.B. and F.T.; resources, M.D.S. and F.T.; data curation, M.D.S.; writing—original draft preparation, M.D.S. and E.B.; supervision, C.D.I.; project administration, C.D.I.; funding acquisition, C.D.I. All authors have read and agreed to the published version of the manuscript.

Funding

This study was co-funded by “Programma POR Puglia FESR-FSE 2014-2020”, INNOLABS call.

Data Availability Statement

No new data available.

Acknowledgments

The authors wish to thank Domenico Bellifemine (from CNR-IRSA) for the technical support in collecting samples and conducting the experimental campaign.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Sketch of the pilot plant. S1: buffer solution; S2–S3: tanks for biogas accumulation; R1: anaerobic digester; R2: reactor for thermal hydrolysis; R3: reactor for OFMSW cooling before anaerobic digester feeding; A1: biogas analyzer; P1: pump for buffer solution; P2: pump for recirculation and digestate drawing; P3: pump for feeding; LT: level sensor; TT: temperature probe; pH1: pH probe; PT1: pressure meter; FT1: biogas flow meter; TI1: temperature indicator; SP: sampling port.
Figure 1. Sketch of the pilot plant. S1: buffer solution; S2–S3: tanks for biogas accumulation; R1: anaerobic digester; R2: reactor for thermal hydrolysis; R3: reactor for OFMSW cooling before anaerobic digester feeding; A1: biogas analyzer; P1: pump for buffer solution; P2: pump for recirculation and digestate drawing; P3: pump for feeding; LT: level sensor; TT: temperature probe; pH1: pH probe; PT1: pressure meter; FT1: biogas flow meter; TI1: temperature indicator; SP: sampling port.
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Figure 2. Effect of pre-treatments on organic matter solubilization in terms of COD.
Figure 2. Effect of pre-treatments on organic matter solubilization in terms of COD.
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Figure 3. Distribution of the sCOD/COD ratio of the OFMSW subjected to the pre-treatments. The black lines represent the median, the boxes represent the I-III quartile, and the vertical bars represent the range within 1.5 IQR.
Figure 3. Distribution of the sCOD/COD ratio of the OFMSW subjected to the pre-treatments. The black lines represent the median, the boxes represent the I-III quartile, and the vertical bars represent the range within 1.5 IQR.
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Figure 4. Average COD, sCOD, and VSS removal efficiency for OFMSW subjected to different pre-treatments.
Figure 4. Average COD, sCOD, and VSS removal efficiency for OFMSW subjected to different pre-treatments.
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Figure 5. Result of the statistical analysis (LSD test) on VSS removal efficiency by the digester during the four runs.
Figure 5. Result of the statistical analysis (LSD test) on VSS removal efficiency by the digester during the four runs.
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Table 1. Main chemical and bromatological characteristics of OFMSW in this study and in the review by Campuzano and González-Martinez [6].
Table 1. Main chemical and bromatological characteristics of OFMSW in this study and in the review by Campuzano and González-Martinez [6].
This StudyCampuzano and González-Martinez
ParameterAverage ± st. dev.ParameterAverage ± st. dev.
pH6.1 ± 1.5pH5.2 ± 0.95
TS (%)26.4 ± 8.8TS (%)27.2 ± 7.6
VS (%)25.9 ± 8.7VS (%)22.9 ± 6.3
COD (g/kg)308.7 ± 116.7COD (g/kg)332 ± 121
TN (g/kg)3.7 ± 1.9TN (g/kg)7.9 ± 5.4
P (g/kg)0.5 ± 0.2P (g/kg)1.7 ± 2.5
Bromatological analysis of OFMSW fraction (% of VS)
Grease7.8 ± 1.9Fat, oil and grease17.5 ± 6.6
Lignin7.3 ± 6.3Lignin9.7 ± 5.3
Cellulose12.7 ± 4.5Cellulose18.6 ± 15.0
Starch/hemicellulose25.3 ± 2.5Hemicellulose8.6 ± 4.6
Starch17.1 ± 2.5
Protein9.0 ± 2.3Protein17.7 ± 5.5
Free sugars6.4 ± 3.0Free sugars10.5 ± 6.0
Table 2. Percentage of recovery of OFMSW (in terms of COD and VS) after thermal pre-treatment with respect to shredding. The data are reported as mean value ± standard deviation.
Table 2. Percentage of recovery of OFMSW (in terms of COD and VS) after thermal pre-treatment with respect to shredding. The data are reported as mean value ± standard deviation.
Percentage of Recovery (%)
High-Pressure AirHigh-Pressure N2Autoclave
COD65.1 ± 23.965.5 ± 13.3105.2 ± 13.9
VS63.2 ± 12.664.0 ± 15.7100.6 ± 13.7
Table 3. Methane yield and methane concentration in the biogas during anaerobic digestion of OFMSW subjected to different pre-treatments.
Table 3. Methane yield and methane concentration in the biogas during anaerobic digestion of OFMSW subjected to different pre-treatments.
ShreddingHigh-Pressure AirHigh-Pressure N2Autoclave
Methane yield (NmLCH4/gVS a)264 ± 223167 ± 99352 ± 62476 ± 194
Methane content (%)65 ± 764 ± 769 ± 750 ± 5
a gVS entering the treatment train (i.e., OFMSW after shredding but before the thermal pre-treatments).
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De Sanctis, M.; Altieri, V.G.; Barca, E.; di Bitonto, L.; Tedeschi, F.; Di Iaconi, C. Comparison Among Thermal Pre-Treatments’ Effectiveness in Increasing Anaerobic Digestibility of Organic Fraction in Municipal Solid Wastes. Energies 2024, 17, 6293. https://doi.org/10.3390/en17246293

AMA Style

De Sanctis M, Altieri VG, Barca E, di Bitonto L, Tedeschi F, Di Iaconi C. Comparison Among Thermal Pre-Treatments’ Effectiveness in Increasing Anaerobic Digestibility of Organic Fraction in Municipal Solid Wastes. Energies. 2024; 17(24):6293. https://doi.org/10.3390/en17246293

Chicago/Turabian Style

De Sanctis, Marco, Valerio Guido Altieri, Emanuele Barca, Luigi di Bitonto, Francesco Tedeschi, and Claudio Di Iaconi. 2024. "Comparison Among Thermal Pre-Treatments’ Effectiveness in Increasing Anaerobic Digestibility of Organic Fraction in Municipal Solid Wastes" Energies 17, no. 24: 6293. https://doi.org/10.3390/en17246293

APA Style

De Sanctis, M., Altieri, V. G., Barca, E., di Bitonto, L., Tedeschi, F., & Di Iaconi, C. (2024). Comparison Among Thermal Pre-Treatments’ Effectiveness in Increasing Anaerobic Digestibility of Organic Fraction in Municipal Solid Wastes. Energies, 17(24), 6293. https://doi.org/10.3390/en17246293

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