Reviewing Digestate Thermal Valorization: Focusing on the Energy Demand and the Treatment of Process Water

Abstract

Anaerobic digestion is a feasible solution for the treatment of organic wastes. The process can reduce the amount of biowaste by stabilizing the organic material and producing biogas susceptible to energetic valorization. However, the digestate needs further valorization when land application is considered unfeasible. Thermal treatments, such as gasification, pyrolysis, and hydrothermal carbonization, are alternatives capable of transforming this material into valuable syngas, obtaining, in many cases, a carbonized stream known as biochar. The feasibility of the process depends on the energy demand for the drying stage and the treatments available for removing contaminants from the syngas, attaining high-quality products, and treating the process-derived water. In the present manuscript, these critical aspects were reviewed considering the characteristics of digestates based on their origin, the modifications of this material during anaerobic digestion, and the way digestate structure affects the final thermal valorization outcome. Emphasis was placed on the energy demand of the global approach and byproduct treatments.

1. Introduction

Anaerobic digestion is a well-known technology that provides multiple benefits in converting organic wastes into biogas and allows for nutrient recovery when land application of digestate is the selected disposal option. Anaerobic treatment has a proven and excellent ability to reduce waste volume and stabilize putrescible material, thus resulting in a highly mineralized product. Biogas can be commonly valorized as a fuel in boilers, combined heat and power (CHP) units, microturbines, or upgraded to a quality similar to natural gas for injection into the gas grid [1]. However, the digestion process is still fraught with operational complexities and high installation costs [2]. A critical balance must be attained between the amount of biogas produced, the cost of transporting raw materials, and the treatment option selected for the final disposal of the digestate. Although the environmental benefits of the process are undeniable, like any other commercial process, it must also be economically feasible.

Anaerobic digestion has traditionally been used as a treatment stage in wastewater treatment plants (WWTPs) to stabilize sewage sludge and treat livestock wastes and agricultural residues. Current studies focus on increasing the variety of cellulosic materials as a way to increase the share of renewable energy, such as via the use of grasses and invasive plants [3,4,5]. The biogas yield is highly dependent on the elemental composition of the material, the nutrient balance, and the accessibility of the microflora to the readily degradable compounds. Carbohydrate-rich materials degrade rapidly with the risk of volatile fatty acid (VFA) accumulation and undesirable process inhibition. Therefore, codigestion with other substrates, such as manure, improves process performance by increasing alkalinity and allowing proper pH regulation [6,7]. On the contrary, the presence of high lignocellulosic material prevents degradation due to the encapsulation effect caused by lignin, resulting in poor biogas yield unless specific pretreatments are applied to achieve full conversion of organics into biogas [8]. Failure to do so may result in the accumulation of recalcitrant components in the reactor, increasing the solids content, creating dead volumes in the digester, and causing rheological problems [9]. Figure 1 shows a schematization of the digestion process with the primary raw materials and process parameters.

Pretreatments can enhance biogas production by hydrolyzing complex molecules and releasing encapsulated material. However, the additional energy demand associated with the selected pretreatment option and the characteristics of the ancillary equipment added to the process should be carefully evaluated to avoid excessive operational complexity and to avoid increasing energy requirements to a point that may offset the extra benefit initially intended. Thermal hydrolysis processes are commonly applied in wastewater treatment plants to increase digestion capacity, improve the quality of biosolids, achieve hygienization, and reduce their final amount. Several commercial processes are currently available and although they have a high thermal demand (CAMBI™, BioThelys™, Exelys™, TurboTec™), the different heat recovery stages integrated into the process and the extra amount of biogas produced can offset this negative feature [10,11]. However, there is still residual material that needs final disposal. If the costs associated with sludge handling and transportation are too high, or if constraints regarding the presence of pollutants prevent land application, then considering alternatives such as thermal conversion may be a feasible option.

Thermal valorization of biosolids or digestates has been studied by several authors [12,13,14]. Given the widespread application of biogas plants, recent work has focused on the thermal treatment of digestates [15,16,17,18] as a means of finding a practical solution to the great amount of digestate produced. Pyrolysis, gasification, and hydrothermal treatment have been frequently studied at the laboratory and pilot plant scale [19,20]. However, extrapolating the technology to a larger scale has proven challenging. Figure 2 shows a schematic representation of the different thermal treatments commonly studied.

In the present manuscript, the characteristics of digestates were reviewed, along with the different thermal processes currently available for the valorization of this material. Focus was placed on the complexities associated with treating byproducts and the energy required to attain full process integration. The thermochemical treatment of digestate has been reviewed by several authors, who have reported on char applications and characteristics as well as on critical aspects associated with byproduct recycling and different types of process integration [21,22]. The work of Sikarwar et al. [23] describes different research projects and industrial companies working on this topic, focusing on the aqueous byproduct of thermal treatment. Digestate is a complex matrix and, although several valuable compounds can be obtained from this material, as described by Selvaraj et al. [24], the different intermediate stages needed—cleaning steps, removal of inhibitory compounds, the excessive energy demand—make any thermal valorization approach a difficult task.

The innovative aspect of the present review derives from its analysis of the process parameters involved when attempting to integrate anaerobic digestion and thermal valorization of digestates. This review comprises a section dedicated to the characteristics of wastes commonly used as substrates in anaerobic digestion and the changes expected once the material is submitted to the digestion process. The subsequent sections are dedicated to reviewing experimental results on the thermal treatment of digestates by the most common techniques, namely gasification, pyrolysis and hydrothermal carbonization (HTC).

2. Materials and Methods

The present review was performed using the following main keywords for selecting relevant manuscripts: “anaerobic digestion” AND “thermal treatment”. Among the different thermal treatments considered, gasification, pyrolysis and hydrothermal carbonization were the main technologies reviewed. Priority was given to manuscripts published between 2010 and 2024. Manuscripts containing basic knowledge of the subject were also considered, even in cases where the publication date was before 2015–2024. In addition to the aforementioned keywords, the search was conducted using terms associated with digestate characterization and the analytical techniques commonly applied for organic measuring. “Agricultural waste” OR “animal manure” OR “sewage sludge” were also used as keywords. The databases consulted were Google Scholar, PubMed, and Scopus.

3. Characteristics of Wastes and Digestates

Sewage sludge from WWTPs, food waste, livestock waste, and agricultural wastes are the most frequent materials used as substrates in anaerobic digestion systems [2]. Sewage sludge is composed of primary sludge from the primary settling of wastewater and waste-activated sludge from the secondary settling of the water treatment system. Therefore, primary sludge has better digestibility characteristics compared with secondary sludge. This is because the latter also contains polymeric cellular and extracellular material as a result of the growth of microbial biomass during the waste activated process, thus requiring a longer time to attain anaerobic degradation [25,26].

Table 1. Characteristics of fresh materials commonly used as substrates in the anaerobic digestion process and digestates. (SS: sewage sludge, FW: food wastes, CM: cattle manure, SM: swine manure). See Supplementary Material for full content of this table.

Material	Methane Yield (mL CH4/g VS)	Ref.	Proximate and Ultimate Analysis (%)						HHV 2 (MJ/kg)	Ref.
			Vol. 1	Ash	C	H	N	S
Fresh SS	118–214 3 143–249	[27,28,29]	49.7–72.0	26.7–38.9	32.9–33.6	4.5–4.7	3.9–4.5	0.9–1.4	15.8–16.2	[13,30,31]
Digested SS	-	-	29.6–53.9	38.0–59.5	19.3–30.1	2.9–5.0	3.2–4.7	0.6–3.4	12.2–14.0	[13,32,33,34,35]
Fresh FW	212.0–675.2	[28,29]	73.4–82.8	3.3–17.0	41.4–46.1	3.9–5.9	1.7–4.4	0.2–0.5	14.4–16.1	[31,36,37,38,39]
Digested FW	-	-	53.5–61.8	25.6–42.1	23.1–42.1	3.6–6.0	3.0–5.8	0.6–0.9	13.3–29.1	[38,39,40,41]
Fresh CM	110–230	[42,43,44]	59.1–75.5	14.7–27.4	33.0–50.0	3.1–7.1	2.1–6.6	0.3–1.0	16.0–17.8	[45,46,47,48,49]
Fresh pig/SM	323–568	[42,43,50]	51.9–77.7	7.1–34.7	33.5–57.0	5.3–7.7	2.8–4.8	0.2–1.8	13.0–17.6	[51,52,53,54]
Chicken/poultry manure	140–259	[43,55,56]	43.6–71.6	16.6–61.6	25.6–39.7	3.3–4.8	2.2–6.0	0.4–0.8	12.0–14.2	[56,57,58,59]
Goat/sheep manure	159–309	[43,60,61,62]	48.1–84.7	11.1–40.7	22.5–43.9	1.5–6.1	2.3–3.1	0.1–0.6	10.4–17.5	[51,61,62,63]
Digested manure	-	-	55.0–72.6	12.4–40.8	34.1–42.5	4.4–5.9	1.8–4.3	0.3–1.9	10.4–19.7	[16,44,64,65,66]

¹ Vol.: volatiles, values are reported on a dry basis. ² HHV: higher heating value. ³ Value expressed in mL CH₄/g COD. COD: chemical oxygen demand. All other values are expressed in terms of volatile solids (VS).

lists the chemical characteristics of different wastes and their digestates.

The material’s chemical structure dictates the biogas yield obtained and the characteristics of the digestate. However, factors related to experimental conditions greatly influence the outcome of the process. The expected characteristics of the digestates depend on the type of fresh material and the reactor dynamics. The hydraulic retention time and the organic loading rate applied to the reactor influence the mineralization attained. Longer digestion times or lower feeding rates will result in a more stable organic material with a higher degree of mineralization [67,68]. However, reaching a highly stabilized product requires the installation of oversized digesters, which hurts plant capital investment.

3.1. Analytical Techniques for Characterizing Digestates

Spectroscopic techniques and thermal analysis have been employed to characterize organic wastes and the corresponding digested material. The main peaks reported in the Fourier transform infrared (FTIR) spectra of sewage sludge were those associated with OH stretching, aliphatic region (2930, 2850 cm⁻¹), the C=C and C=O bonds at 1640 and 1560 cm⁻¹, respectively, and protein structures [30,69]. Because the biogas produced is closely related to the organic structure of the feeding material, spectroscopic analysis has been used to predict methane yield, obtaining good correlations. This is the case of Kandel et al. [70], Yang et al. [71], and Mortreuil et al. [72], who used near-infrared reflectance spectroscopy (NIRS) to predict specific methane yield. Other similar spectroscopic techniques have also been tested for the same purpose and have been reviewed by Jingura and Kamusoko [73]. The advantage of the infrared (IR) spectrum is that it offers a global picture of the sample without the need for complex sample preparation and extraction procedures.

Thermal analysis is a widely used technique to assess the thermal behavior of samples. Thermogravimetric analysis and differential scanning calorimetry record a sample’s mass loss and energy release when a heating ramp is applied under specific conditions. These experimental conditions can be set under inert or oxidizing atmospheres, thus evaluating the degradation at different temperatures. Mass loss curves and their derivatives offer valuable information regarding the pyrolytic/combustion process’s starting point, the intensity, and the temperature range in which this process occurs. In the case of sewage sludge, the reactive pyrolytic region is located between 125 and 680 °C, just after the end of the drying stage, initiating the process with a devolatilization of light fractions and biodegradable organic material. The subsequent thermal degradation stage involves the transformation of stable and complex compounds, many of bacterial origin, and ends with the decomposition of char and inorganic salts [32]. When an oxidizing atmosphere is applied, the combustion of the sample is usually characterized by two primary, well-differentiated regions in the temperature range between 200 and 450 °C, finalizing at around 520–550 °C [74].

When comparing fresh sewage sludge to its digested form, there is not only an evident increase in mineral content, but also a great reduction in lighter compounds, which is easily discernible from the lower intensity in the mass loss profiles derived from the thermal analysis [75]. However, when considering manures, their different characteristics lead to distinct thermal profiles based on the protein and lignocellulosic content of the fresh material. Therefore, the reduced capacity of the anaerobic microflora for degrading lignocellulosic material will lead to the accumulation of these compounds in the remaining solids, which will result in fewer modifications in the thermal profiles between the feed and the digested sample [48,76]. Subsequent thermal valorization of this material will then take advantage of the higher energy content of the remaining solids. On the other hand, in the case of readily degradable material such as food wastes or sewage sludge, the mineralization reached during digestion will be higher, increasing the ash content of digestates.

Table 2. Techniques used for evaluating the performance and characteristics of substrates and digestates. (SS: sewage sludge, FW: food wastes).

Material	Techniques	Main Findings	References
SS	FTIR 1, solid-phase fluorescence excitation–emission matrix (SPF EEM), Py-GC–MS 2, X-ray photoelectron spectroscopy (XPS).	Raw sewage sludge and stabilized material derived from AD and aerobic digestion were analyzed. The degradation of proteins was higher than that of compounds containing phenolic groups, carboxylic acids, or cellulose. Digestates had a higher percentage of aromatics.	[78]
Pig manure	FTIR, fluorescence spectroscopy as excitation–emission matrix (EEM).	Pig slurry. Raw sample and digested and composted material. Spectra of digested material was characterized by complex structures derived from lignocellulosic material recalcitrant to anaerobic degradation.	[69]
Cow manure	FTIR, X-ray diffraction (XRD).	Co-digestion of cow dung with Ipomoea carnea and rice straw: FTIR indicated the presence in the digestate of lignin/cellulose materials (C-H stretching, 1381 cm−1). Quartz was the main mineral component of digestate ash.	[79]
Swine manure	FTIR, Py-GC–MS, SEM 3.	Swine manure digestion: Reduction in aliphatic and protein content after digestion with an increase in aromatic signals.	[80]
Lignocellulosic biomass	Thermal analysis.	Fresh and digested Sargassum horneri: Digestate showed a high content in cellulose and lignin.	[81]
Group of different digestates	Fractionation of the sample. Fluorescence spectroscopy.	Prediction of the organic quality of digestates. The operational conditions of the digester could not be well correlated.	[82]
Digested FW	Thermal analysis, TG–MS 4, TG-FTIR.	Digestate ash mainly contained CaCO3. A large amount of water was produced during digestate pyrolysis. Pyrolysis products contain ketones, aldehydes, and carboxylic acids.	[40]

¹ FTIR: Fourier transform infrared. ² Py-GC–MS: pyrolysis gas chromatography–mass spectrometry. ³ SEM: scanning electron microscopy. ⁴ TG–MS: thermogravimetry–mass spectrometry.

shows the different analytical techniques used for assessing anaerobic digestates and process performance. Jimenez et al. [77] evaluated several digestate samples and organic residues using an extraction methodology followed by 3D fluorescence spectroscopy to predict the methane production potential and fertilizing value of digestates. However, the methodology is cumbersome due to several extraction steps, and the interpretation of the results is more complex than with other simpler techniques. Several works in the scientific literature provide valuable information on the performance of digestion and thermal treatment of different materials. This information may be useful in obtaining a first approximation of the expected outcome when integrating biological and thermal processes is intended.

The recalcitrance of some manure components and agricultural wastes to the anaerobic degradation process leads to a lower biogas yield and, therefore, to an accumulation in the digested slurry, unless a proper pretreatment is applied to increase the accessibility to the anaerobic microflora. Hemicellulose shows a higher degradation rate than cellulose, while lignin shows insignificant structural changes [83,84]. Cuetos et al. [9] have reported an accumulation of solid material during the anaerobic co-digestion of swine manure and crop residues due to the complex structure of the latter. In contrast, Fierro et al. [85] have reported a preferential degradation of readily degradable compounds when glycerin was added as a co-substrate during the digestion of swine manure.

It seems reasonable to assume that the fate of degradation can be predicted by the digestion process’s operating conditions and the feed’s composition. Spectroscopic techniques and thermal analysis allow the user to understand the process globally. Most experimental work focuses on qualitative information, making it difficult to link spectroscopic and thermal results to the biological outcome. V. et al. [86] used these techniques to study the digestion performance of tannery fleshing, reporting profiles for the fresh and digested material. Similar work has also been carried out by Cuetos et al. [87], Rodríguez-Abalde et al. [88], and De Oliveira Silva et al. [89], among others. Data from these scientific studies can serve to build a useful database for correlating different parameters. Fernández-Domínguez et al. [90] reviewed different techniques available for assessing digestate stability, indicating that mid-infrared spectroscopy (MIR) and ¹³C nuclear magnetic resonance (NMR) are the most promising for understanding changes in the solid phase, but highlighting the need to obtain quantitative information from statistical analysis for these techniques to be practical. However, given the cost of the equipment needed, the first seems to be the most promising for industrial applications.

3.2. Thermal Analysis

Dziedzic et al. [91] studied the thermal behavior of digestates derived from apple pomace and corn silage. They reported a low ash content for the different mixtures of these digested materials (between 8.2 and 11.6%) and, therefore, high energy density (higher heating values (HHV) between 16.4 and 19.6 MJ/kg). On the contrary, food wastes were highly degradable under anaerobic conditions because of their higher carbohydrate content. The digestate obtained from this material was characterized by a thermal profile with a much lower intensity in the differential thermogravimetry (DTG) curve and observed a higher amount of minerals at the end of the thermal profile compared with the feeding substrate [39,92]. A more general trend can be derived from the work published in the scientific literature, which may allow the discovery of a link between material and digestate characteristics, along with the expected biogas yield. Manures such as swine, poultry, and cattle have been studied in mono-digestion cases or as co-substrates with different materials. Provenzano et al. [69] have reported profiles derived from fresh, digested, and composted swine slurry, describing a significant decrease in polysaccharides and aliphatic structures with an accumulation of aromatic signals. The results reported by González et al. [93] for the digestion of swine manure and residual glycerin, in this case using thermal analysis only, indicate the presence of excessive labile compounds as a signal of digestion instabilities. The advantage of using thermal analysis is that this technique can provide insight into the fate of the digestion process and gives some clues about the expected performance associated with subsequent thermal treatment as a valorization alternative.

Determining the activation energy (Ea) to characterize biomass material is a frequent approach to the study of the samples’ thermal behavior. The analysis can be carried out under an inert or oxidizing atmosphere, the latter usually reporting lower values due to the earlier onset of thermal degradation and the oxidative character of the atmosphere, which releases a higher amount of material at lower temperatures [94,95]. The Ea values reported for sewage sludge, animal manure, and food wastes were, on average, 141.2 kJ/mol under combustion [96]. In contrast, under an inert atmosphere, this value rises to 202.8 KJ/mol (average value obtained from different authors evaluating similar types of wastes under a nitrogen atmosphere [44,97,98]). The composition of the raw material has an impact on the Ea values, with a higher lignocellulosic content resulting in elevated Ea values [99]. There are a number of methods for estimating Ea values, including the distributed activation energy model (DAEM) [94] and isoconversional model-free methods, such as the Friedman differential method and those of Flynn–Wall–Ozawa (FWO), Vyazovkin and Kissinger–Akahira–Sunose (KAS) [94,95,97,99,100]. The majority of these methods demonstrate good agreement among the different studies published in the scientific literature.

When comparing the Ea values obtained from fresh samples with those obtained from digestate, the results indicate a significant decrease in Ea values after digestion, consistent with the loss of readily degradable material, the accumulation of minerals and recalcitrant components that undergo thermal degradation at higher temperatures [44,68]. Therefore, when considering a subsequent thermal stage for digestate treatment, higher energy benefits may be obtained by treating digestate derived from lignocellulosic biomass.

4. Thermal Valorization of Digestates

Despite the undoubted advantages of anaerobic digestion, it is evident that the final disposal of the digested slurry may be problematic, mainly if there is insufficient arable land available near the treatment plant or if land spreading may cause excessive nutrient runoff in sensible regions. The presence of a high lignocellulosic content in the feeding material may exacerbate this issue, as it may limit the access of anaerobic microflora to the full range of components. This could result in accumulating a recalcitrant fraction and increasing the mass of material requiring final disposal. In light of these considerations, many researchers have proposed valorizing digestate through thermal processes, including pyrolysis, gasification, and hydrothermal carbonization (for further details, please refer to

Table 3. Different configurations by which to integrate anaerobic digestion (AD) and thermal treatments such as gasification, pyrolysis and hydrothermal carbonization (HTC).

Configuration	Characteristics	Reference
AD + gasification.	Gasification of digestate. Main products are syngas (for energy production or to be used as fuel) and char. Gasification can use air, steam or CO2 as gasification agent.	[102,103]
AD + pyrolysis.	Multiple fuel products obtained (biogas, syngas, pyro-oil and char) increases energy gain.	[104,105]
AD + HTC.	Hydorchar is obtained as the main product. Drying is not required before thermal processing. The high thermal demand of the process makes the full valorization of digestate by HTC unfeasible. HTC has also been proposed as an intermediary stage between a two-phase digestion system, with the second stage treating HTC slurry.	[106,107]

). Combustion is another way of extracting energy from the residual material, but the high ash content may result in the formation of molten ash, which could present a challenge. In addition, emissions regulation is currently highly rigorous, rendering sludge incineration feasible only in large-scale systems where other materials, such as municipal solid wastes, are also treated. This approach allows for the effective abatement of sulfur, NOx, and organic volatile contaminants, as well as the management of ashes enriched in heavy metals [101].

Guilayn et al. [108] reviewed other valorization alternatives, such as the production of biopesticides, biosurfactants, and composite materials. However, barriers regarding public acceptance and health risks could complicate the implementation of these approaches. In addition, many digestates are treated mechanically by removing the water through the use of water dehydration systems (filter press or centrifugal decanter), leading to a material that still has a moisture content close to 70%, leading to high transportation costs and calling into question whether this is really a valuable material or just a nuisance that is inevitably derived from the digestion process itself [109].

The review carried out by Pecchi and Baratieri [21] dealt with the thermal valorization of digestates and reported on the advantages of producing syngas from digestate gasification and char from digestate pyrolysis and on the application of hydrothermal carbonization to reduce slurry volume. However, the authors point out that the different technologies must overcome the challenges associated with ash melting in the case of the gasification process, with the treatment of the aqueous phase either in pyrolysis or HTC and with complying with the physical characteristics required for char so that it can be considered to have an agronomic value. The European Biochar Certification gives guidance on the characteristics of chars that can be safely used as an agronomic supplement, indicates limits for heavy metals and organic contaminants (such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), etc.), and provides a list of biomass materials suitable for biochar production, including digestates and sewage sludge [110]. Therefore, it is recommended that any process intended for biochar production obtain certification. Thermal treatment can transform a problematic material into char, which may in turn find a wider application spectrum such as soil conditioning, adsorbent, aiding soil carbon sequestration and enhancing soil nutrient retention [111,112]. However, not all thermal processes can produce a char material low enough in metals and polycyclic aromatics as to allow them to obtain this certification.

Another interesting study on integrating AD and thermochemical processes is that of Sikarwar et al. [23]. Their review analyzed different configurations, showing the efficiency gain achieved when considering the integrated (biological + thermochemical) process. However, several factors regarding the economic feasibility of the approach and the adequate treatment of byproducts are not frequently analyzed. The integration of both processes must achieve energy sufficiency, and the benefits derived from such an approach should be evaluated in terms of the high reduction attained in the mass of material requiring final disposal and the inert character of the char produced.

4.1. Gasification of Digestates

Gasification is a well-known technology that has been around for more than 200 years and continues to be of interest to technical developers and scientists due to the advantage it offers in transforming solid fuels and organic materials into gaseous fuels (synthesis gas or syngas) [113]. Although widely studied and with some large-scale plants installed, the technology still suffers from several drawbacks related to the need for syngas cleaning and treatment of undesirable byproducts, such as tar and toxic compounds. These drawbacks mean that it still requires financial effort to ensure that demo plants can reach commercialization and become economically feasible [114,115]. The gasification process requires the presence of a fluidizing/reacting agent, which is generally air or an oxygen stream (in large-scale plants where air distillation is feasible). Other gasification agents commonly used are steam or CO₂. Steam is usually preferred as it increases syngas H₂ content [116], whereas the presence of oxygen is usually associated with dioxin and furan formation when chlorine is present in the organic composition, although this topic requires further research [117,118]. The different types of gasifiers can be classified as fixed bed, fluidized bed and entrained flow reactors, plasma reactors, and rotary kilns, with the first two being the most widely used (See Figure 3).

Fixed bed reactors are often used in small-scale applications and fluidized systems in large-scale ones [119]. The efficiency of organic waste gasification has been investigated with different waste types, including sewage sludge, manure, food waste, and high lignocellulosic content biomass, among others. The gasification of digestate has received great interest due to the vast amount of material produced globally. Although digestion is widely known as a viable and sustainable treatment option for sewage sludge, the process suffers from several drawbacks and challenges that hamper achieving the optimal benefits of its utilization at wastewater plants. Fluidized bed gasification is a common gasification technology method proposed for sewage treatment; this technology is of interest to researchers seeking to understand the operating conditions for better gasification performance [120,121,122]. Results show that temperature is the most influential parameter in increasing efficiency, as well as H₂ and CO yields [123].

The high ash content of sewage sludge and, of course, of its corresponding digestate imposes constraints on the gasification operating conditions (limiting the temperature operating range) and performance due to slagging, which increases tar formation and causes lower energy yields associated with the lower energy content of syngas [120]. For a detailed description of the gasification technology, see Santos et al. [115] and Mallick et al. [124]. The type of gasification agent employed has a significant impact on the composition of the resulting syngas, as well as on the quantity of tar produced. The comparative results from sewage sludge and digestate gasification using air and air/steam at 750 °C show that adding steam enhanced cold gas efficiency (CGE), gas yield (GY), carbon conversion efficiency (CCE), and the lower heating value (LHV) of syngas. It also improved the values of the tar dew point, which is crucial for avoiding undesired condensation and decreased tar content [125]. The intrinsic reactivity of char with a H₂O–N₂ mixture increases with the proportion of steam present, although this reactivity depends mainly on char particle size and steam temperature [126].

Other less common gasification technologies tested with sewage sludge as a raw material include the bubbling fluidized bed, circulating fluidized bed, and continuous bench rotary kiln reactor. The experiment of sewage sludge gasification using a bubbling fluidized bed (BFB) and carried out by Manyà et al. [127] showed that an increase in bed height enhanced the process efficiency, thanks to the higher gas residence time, which also mitigates the influence exerted by the high ash content. The findings of Petersen and Werther [122] and Zhu et al. [128] also dealt with sewage sludge gasification, albeit in a circulating fluidized bed reactor. In contrast with the aforementioned studies, these researchers demonstrated that a lower feeding height in conjunction with a higher fluid velocity facilitated the mixing of fuel particles, thereby enhancing the quality of the produced gas. The experiment performed by Freda et al. [129] on a continuous bench rotary kiln reactor showed that operation under a prolonged residence time was attained, allowing optimal bed mixing and low tar production. However, the sticky nature of the wet sludge led to the formation of plugs along the feeding screw system, making necessary a drying stage previous to gasification.

Gasification has been studied as a process by which to directly transform biomass into syngas or as a complementary technology to valorize digestate. Li et al. [130] compared the energy efficiency of both processes and found similar values (62–64% for anaerobic digestion and 65% for gasification). Given this similar performance, the coupling of these two processes may be justified in terms of the energy needed to dry the input. Anaerobic digestion requires a wet material with a water content between 80 and 95%. On the contrary, gasification requires a material with a solid content of more than 70%, which would be detrimental to the energy balance if proposed as a post-digestion stage. The coupling of digestion and gasification is justified whenever the initial solids content of the input is low, and the feed contains a readily degradable fraction. Therefore, the remaining digestate may be dewatered and dried using some of the energy extracted from the integrated approach.

Additionally, the increasing number of anaerobic digestion plants being installed means that municipalities are dealing with excessive digestate, with few landfill sites available, making thermal treatment a suitable alternative [131]. The drying stage is the most critical parameter affecting operating costs and energy demand. However, in many cases, drying is necessary to facilitate transport and handling and to reduce greenhouse gas (GHG) emissions, which are much higher for wet digestate than for dried material [132]. The research carried out by Guilayn et al. [108] indicates that, in biogas plants, the heat recovered from co-generators is insufficient to dry the whole digestate flow, thus requiring auxiliary fuels to attain digestate drying. The energy demand estimated for convective drying is between 2.52 and 5.04 GJ/t water evaporated [133], translating into an energy demand in the range of 176.4 to 403.2 MJ to dry 100 kg of dewatered digestate with a solid content between 20 and 30%. However, the energy contained in the material may account for 200–510 MJ if an LHV between 10 and 17 MJ/kg is considered, thus showing the difficulties encountered when attempting a process integration.

The simulation study published by Sanaye et al. [134] assumed a drying of biosolids from 75% to 5.6% and concluded that this high level of water removal severely penalizes the global balance. Mabalane et al. [135] also evaluated the integrated approach when treating municipal solid waste. They indicated that its technical feasibility was higher than that of either digestion or gasification as a single-treatment technology. However, the technical complexity of the integrated approach and the extra capital investment associated with tar removal and syngas cleaning may tip the balance against gasification.

There are several studies proposing the use of sewage sludge in gasification (see

Table 4. Gasification results from fresh and digested materials. (SS: sewage sludge).

Material	Type of Gasifier and Conditions	Main Results	Reference
SS	Rotary kiln. Mass rate input: 170–260 g/h. Temperature: 800–850 °C. ER: 0.15–0.24.	HHV of Syngas (dry): 6–9 MJ/m3. Gas yield of 1 m3/kg SS. Low tar production (4–6 g/m3 dry gas). Char with HHV of 14.9–15.3 MJ/kg dry. Ash content of 67.6–74%.	[129]
Digested SS	Circulating fluidized bed gasifier. Gasification agent: air, CO2-N2 mixture and N2. Temperature: 750–850 °C. ER: 0.3–0.6.	Better results were obtained at an ER of 0.3 and with a syngas with an LHV of 4.7 MJ/m3. Increasing the air proportion caused a detriment in performance. The carbon conversion was 85% at an ER of 0.3. Operating temperature was restricted to 750 °C due to ash agglomeration problems when ER was set at 0.3.	[122]
Digested SS	Low-temperature circulating fluidized bed (LT-CFB).	Sludge co-gasification with cereal straw was considered for avoiding sludge drying. The mixture with dehydrated sludge showed good performance, avoiding accumulation of inorganics in the bed. No bed agglomeration or ash sintering was observed. High tar production was obtained when testing the sludge–straw mixture due to the high water content affecting process temperature.	[34]
Digested SS	Fixed bed gasifier. Evaluating ER from 0.12–0.27.	Low CH4 content in syngas. The energy content of syngas was low (below 5 MJ/m3). Phosphorus recovery from char was proposed.	[136]
Digested SS	Down-draft fixed bed gasifier. Temperature: 1100–1150 °C.	Solid char residue and a glassy material was obtained. Syngas was used to feed a CHP engine. There was an estimated electricity production of 1 kWh per 1.2 kg of dried sludge.	[137]
Digested SS	Fixed by gasifiers (up-draft and down-draft) using air or oxygen as gasifying agent. Temperature: 700, 800 and 900 °C.	High content of hydrogen, 40–46% and even higher. When using pure oxygen, the HHV of syngas was 12.7–14 MJ/m3.	[138]
Fresh manure	Fluidized bed laboratory scale gasifier. Parameters studied were temperature, ER and O2 concentration using oxygen enriched air.	The effect of temperature was the most significant. Higher temperatures favor hydrogen formation. The maximum energy content of syngas was 8.0 MJ/m3 at 800 °C with an ER of 0.25 using air enriched up to 40% in oxygen.	[139]
Fresh manure	Fixed bed gasification. ER: 0.23–0.47 1. Steam addition as gasification agent.	Syngas with low HHV (1.7–4.3 MJ/kg).	[47]
Digested manure	Downdraft fixed bed gasifier. Temperature: 750–850 °C. ER: 0.14–0.34.	Digestate was a mixture derived from the digestion of pig and cow manure with maize–triticale silage and cereal bran. The mixture was characterized by a very low ash content (ash: 9.5%, volatiles: 89.5%). Gas yield at 850 °C was 65.5% (wt%) with an LHV of 2.88 MJ/m3.	[102]
Digested manure	Downdraft fixed bed gasifier. Temperature: 600–800 °C. ER: 0.25–0.3.	Digestate was obtained from a mixture of manure and straw. Increased temperature resulted in syngas with higher energy content (from 3.4 to 4.78 MJ/m3). Tar production decreased with temperature increase.	[140]
Digested manure	Laboratory scale gasifier. Temperature: 700, 900 and 1000 °C.	Digestate was derived from high solid digestion of chicken manure and grass. Co-gasification with wood chips was performed. Gasification temperature had a positive effect on syngas yield, with higher temperatures improving syngas production. A higher proportion of digestate in the feeding mixture led to a syngas with lower values of LHV.	[141]
Digestate	Fluidized bed reactor. Maximum operating temperature was 750 °C to avoid slagging and fouling.	Digestate and SS were used as raw material. No information was given regarding the type of digestate. Sludge had a high ash content (44.5%), whereas digestate had an ash content of 11.7%. Syngas had similar LHV (about 4.0 MJ/m3 for both materials). Steam addition favored digestate gasification but not SS gasification.	[125]
Digestate	Downdraft fixed bed gasifier. CO2 was used as gasification agent. Temperature: 650–950 °C.	Digestate was derived from a digester treating a mixture of corn straw, sludge and cattle manure. Ash content was 41.5%. Gasification was carried out in 1:1 mass ratio with lignite. Maximum LHV of syngas was 6.52. MJ/m3 at 950 °C.	[142]

¹ Authors used the definition of air number instead of ER. ER values were recalculated and reported in table.

). However, this material has a high ash content and requires special consideration regarding bed temperature limits. Manure digestate, especially when treated with straw, has the advantage of much lower ash content, but it is still high compared with typical lignocellulosic materials. Limiting the gasification agent ratio aids in attaining syngas with higher energy content. However, this is not always possible, and the operating equivalence ratio (ER) is highly dependent on the reactor configuration, so typical LHVs of syngas are close to 5.0 MJ/m³ and often lower, with common ER values found in the range of 0.15–0.6 (see Table 4).

The use of catalysts during the gasification process can contribute to reducing tar production and increasing gas LHV, which ultimately improves efficiency and carbon conversion [143]. Food waste gasification experiments carried out by Raizada et al. [144] have shown that the presence of a catalyst (Nickel (Ni)) increased syngas and hydrogen production significantly. Their experiments were carried out using a steam gasification reactor with the catalyst being supported on the ash derived from the combustion of the same raw material. The highest performance reported by these authors was obtained when using a 50% catalyst content in the feeding mixture with a 5% Ni-catalyst load.

Supercritical Water Gasification

Recent research has focused on thermal treatment technologies that avoid the need for a drying stage, thus carrying out the conversion of organics at high temperatures. When the temperature and pressure are increased above 300 °C and 20 MPa, water reaches supercritical conditions and behaves as a non-polar solvent, becoming miscible with gases and hydrocarbons and greatly reducing the solubility of minerals [145]. The supercritical water gasification (SCWG) of biomass is an example of a thermal process recently studied to transform biomass and waste. The process resembles gasification in that the thermal reactions (water–gas shift and methanation reactions) produce a gaseous stream containing H₂ and methane as valuable fuels, but with the added benefit of avoiding nitrogen dilution [146].

The extreme conditions allow the residence time inside the reactor to be on the order of a few minutes, thus benefiting from small reactor dimensions. However, the development of the process is still in its infancy due to the complex technical requirements to operate at an industrial scale. Boukis et al. [147] summarized their experience in a pilot plant after several years of operation treating lignocellulosic biomass, focusing on the challenges encountered. The extreme operating conditions require specific materials capable of withstanding the erosion and corrosion associated with the presence of salts and high fluid velocities. Although this technology may offer future opportunities for biomass conversion into hydrogen and methane, the separation of minerals from the feeding slurry at high temperatures and the resistance of materials to such high temperature and pressure conditions, along with a hydrogen-rich stream that may pose an additional risk of metal embrittlement, are issues that require further research.

The use of catalysts in the SCWG process aids in increasing efficiency, though the process is not free from tar formation problems, catalyst poisoning, or activity reduction due to the blocking of active sites where the surface is covered with char and mineral salts [148]. The advantage of operating at high moisture content comes with the drawback of releasing a large amount of process water requiring further treatment. Adar et al. [149] analyzed process water from SCWG sewage sludge and reported chemical oxygen demand (COD) values ranging from 323 to 939 mg/L, with a high content of proteins, carbohydrates, and phenol. Recycling process water is an alternative to reduce the mass of this stream. Zhang et al. [150] tested this feature and reported increased H₂ production due to the catalytic effect created by K and Na accumulation.

4.2. Pyrolysis of Digestates

Pyrolysis involves the thermal degradation of an organic material in the absence of oxygen, resulting in thermal cracking that generates three main byproducts in different phases: a syngas containing light fuel gases and small amounts of light hydrocarbons, a liquid phase rich in oil components but also containing an aqueous fraction, and a solid phase fraction known as char, where ash and carbon are concentrated [151]. The temperature and heating rate applied to the process serves as a mode of classification.

Table 5. Classification of pyrolysis processes.

Classification	Main Characteristics	Products	Reference
Torrefaction	Energy densification process. Reduces hemicellulose content. Main reactions are dehydration, deacetylation and cleavage of ether linkages. Torrefaction temperatures (200–300 °C).	High energy content biomass. Weight loss is achieved by releasing hydrogen and oxygen atoms.	[155,156]
Slow pyrolysis	Low temperature. Low heating rates favor char formation, whereas higher heating rates favor liquid products.	Higher oil and char yields, favoring char formation.	[157]
Fast pyrolysis	Temperature between 700–900 K. High heating rates.	Higher gas and oil yield. Increasing temperature favors H2 yield. Temperature control (heating rate) and residence time allows controlling product distribution.	[158,159,160]
Flash pyrolysis	Temperature > 1000 K. High heating rates.	Higher liquid fuel yield, low residence time (bio-oil yield between 60 and 75%). The low residence time and high temperature reduce secondary reactions.	[158,161,162]
Plasma pyrolysis	High (106–108 K) and low (2000–40,000 K) plasma temperature. Non-thermal plasma.	Under plasma conditions molecules change into atomic, ionic or excited states.	[163,164]

shows a classification based on process temperature and heating rate. The absence of a gasifying agent offers the main advantage of obtaining syngas with a higher energy content as the presence of diluting air is avoided. The different fractions obtained are associated with the stages occurring during thermal decomposition, where cracking of polymers is produced, giving rise to light fractions. However, recombination also takes place, where polycyclic structures suffer cross-linking and an aggregation cycle releasing hydrogen and oxygen in the form of water, which is derived mainly from intermolecular dehydration and which gives rise to char particles at temperatures starting from 200 to 240 °C [152,153]. At temperatures above 265–275 °C, the process becomes exothermic, and at temperatures above 400 °C, the remaining organic matter is aromatized, favoring the formation of graphite-like layers [154].

Torrefaction is usually considered a biomass densification process that can improve waste transport efficiency and facilitate further energy recovery when other thermal processes are applied. In contrast, plasma pyrolysis has excessive energy requirements and high investment and maintenance costs, which limit its use [165,166]. The pyrolysis temperature has a significant impact on the product distribution. Consequently, an increase in temperature from 300 to 750 °C can result in a notable reduction in biochar yield, from approximately 70% to values approaching 40% [167]. Furthermore, the residence time and heating rate exert an influence on product distribution. Chen et al. [168] reported char yields in the 50–55% range when sewage sludge was subjected to pyrolysis, with lower values within this range being attained when the pyrolysis temperature was increased. In considering the energetic valorization of biomass, it would appear that the application of elevated temperatures and heating rates represents the optimal strategy for increasing the yields of syngas and bio-oil. This is due to the fact that char has a relatively low energy content, estimated at 10 MJ/kg [169]. Shahraki et al. [170] have reported a significant decrease in char production when the temperature was increased from 400 to 700 °C and the heating rate was raised from 20 to 60 °C/s. The resulting oil yield was between 45 and 47%, while the syngas yield ranged between 36 and 52% (HHV of syngas being approximately 10–13 MJ/kg) [169,171].

The energy density of bio-oils is lower than that of conventional fuels due to the presence of oxygen and nitrogen in their composition (30–32 MJ/kg) [169,172]. The presence of oxygenated compounds in these oils results in a high degree of reactivity, which in turn affects their stability during storage. Conversely, the presence of nitrogen gives rise to complications associated with elevated NOx emissions during combustion [173]. Accordingly, one of the pending research objectives is to identify effective methodologies for deoxygenating and removing nitrogen.

Moreover, an energy input is necessary to increase the temperature of the raw material to a level commensurate with that of the pyrolysis reaction. In their study, Chavando et al. [174] estimated this value to be about 4.6 MJ/kg of raw material, whereas Salman et al. [175] estimated a value of 1.8 MJ/kg. Assuming an average value of 3.2 MJ/kg and an HHV of 16 MJ/kg for the waste material submitted to pyrolysis, the energy recovery of the process could reach a value of 72% when considering a char, bio-oil, and syngas yield of 50, 25, and 10%, respectively (the remaining assumed as pyrolysis water). This represents an energy loss of 28%, which is consistent with the findings of Cong et al. [176] in their study of energy losses in the slow pyrolysis process using different reactor configurations. In the event that bio-oil production is the preferred outcome, the potential for energy recovery can reach as high as 90%. In this scenario, the biochar yield is estimated to be 34%, while the bio-oil yield is projected to be 41%.

Residence time also has a significant effect on char characteristics. Increasing the residence time changes char pH and adsorption capacity. Therefore, an important parameter for evaluating thermal processes is to consider the severity of the treatment (determined by the combined effect of temperature and residence time in a single factor). The severity factor can be thus correlated with char yield and surface area [177,178]. A description of the pyrolysis technology and the type of commercial reactors can be found in Raza et al. [179] and Gholizadeh et al. [180].

One of the major drawbacks of pyrolysis is the production of an aqueous byproduct, in addition to the oxygen and nitrogen content of pyro-oils previously commented on. The amount of water produced can be estimated to be between 12 and 28% [169,181,182]. The proposed integration of AD and pyrolysis usually requires the treatment of the aqueous phase by the digestion unit and the addition of char as a supplement to the digestion process to enhance biogas production and aid in the degradation of complex organics contained in the same aqueous byproduct [17,183,184]. Fabbri and Torri [185] reviewed the integration of these two technologies and reported on the need to increase the knowledge of the possible inhibitory effects of some phenols and toxic compounds (including cresols, furfural and hydroxymethylfurfural (HMF) present in the liquid phase if treated back in the digestion reactor, which could potentially disrupt the biological process.

Wen et al. [186] showed that the anaerobic microflora can easily acclimate to the presence of these inhibitors, allowing organics contained in the liquid phase to be transformed into biogas. These authors reported a COD content of 500 g/L, which was high when compared with results reported by others. Their aqueous phase was derived from a condensation procedure that had the advantage of obtaining a concentrated stream that removed acidic and oxygenated compounds from the oil phase. On the contrary, when the aqueous phase was derived after phase separation of the bio-oil using a water trap, as in the study of Hübner and Mumme [187], the organic content of the liquid phase was much lower. Using this stream in a hypothetical operating digestion system will decrease the organic loading rate, which may result in lower digester productivity. In addition, the biogas yield reported by Hübner and Mumme [187] were in the range of 37.5 and 220.3 mL CH₄/g COD, whereas, for comparison, the values for cheese whey are in the range of 314–338.9 mL CH₄/g COD [188,189], demonstrating the lower biogas production capacity of this aqueous phase. This phase, produced in a pyrolysis process, can reach values close to 30% when expressed as product yield [169] thus giving an idea of the magnitude of the problem and the importance of finding a technical solution for treating this liquid.

Pyrolysis, like gasification, requires energy to dry the dewatered digestate. Although sewage sludge is one of the main suitable inputs for pyrolysis and other thermal processes [190], many studies do not clearly indicate whether digested or fresh material is used when describing the experimental work. This feature is relevant as AD changes the chemical characteristics of organic compounds. The study performed by Wang et al. [191] showed that pyrolysis of digested sewage sludge had lower gas release than that of fresh sludge. Opatokun et al. [192] also reported similar results, indicating not only differences in gas yields but also in bio-oil production and composition due to the effect of digestion. Petrovič et al. [14] compared the kinetic parameters of digestates with their fresh material and concluded that the thermal degradation of digestates showed higher activation energy than that of their fresh homolog.

The unique organic composition of digestates gives distinct properties to pyrolysis products. The higher ash content of digestates, and particularly that of digested sludge, (and therefore their lower volatile content) means that the integrated approach has a net energy demand that renders it unable to reach energy neutrality, requiring additional lignocellulosic material as input in the thermal stage [133]. González-Arias et al. [105] have reported the use of pig slurry digestate in a subsequent pyrolysis process, obtaining a negative energy balance unless lignocellulosic biomass was added as a raw material in the subsequent pyrolysis stage.

Table 6. Studies reporting on the pyrolysis of digested material. (SS: sewage sludge).

Material	Temperature (°C)	Main Results	Reference
SS	300–500	Increasing the pyrolysis temperature decreased the C, N, and H contents and the H/C atomic ratio while increasing the C/N ratio.	[200]
SS	400–700	Temperature increase reduces biochar yield while increasing syngas production, which is also promoted by the increase in the heating rate.	[170]
SS	250–700	Increased pyrolysis temperature increases metal stability in biochar.	[201]
SS	550–850	Temperature increase reduces bio-oil yield.	[168]
Freshly digested manure	350, 450, 550	The effect of pyrolysis temperature on manure (chicken and dairy)-derived char and its digestates was studied. Zn and Cu must be removed before biochar can be considered for land application. High pyrolysis temperature reduces phosphorus bioavailability.	[202]
Digested hog manure	300–800	Production of char. The resulting biochar was suitable for use as a soil amendment. Surface area and porosity were analyzed. Char was rich in Ca, P, Mg, Si, Fe and K. High temperature increased porosity and surface area.	[66]
Mixture of digested SS and digested cattle manure	525	Manure was treated in a co-digestion plant that also treats food and agro-industry wastes. Experiments were carried out in a laboratory reactor. Between 30–50% of the material can be transformed into char. Manure showed a lower energy recovery when considering the oil phase.	[169]
Mixture of digested SS and cattle manure	550	Demo pyrolysis plant where only two fractions were obtained. Char and gas containing condensable gases were sent to a combustion unit for energy production. Char fraction was 34%. The gas had an LHV of 21.7 MJ/m3, thanks to the low content of N2 and the presence of condensable gases.	[49]

shows different studies using digested material as input to pyrolysis. Char is obtained as one of the main products, considering this carbonaceous material as a valuable one. The number of scientific publications on biochar application, especially on soils and agronomic lands, has increased exponentially since 2000 [193,194]. The correct use of biochar application requires obtaining a European Biochar Certificate (EBC), following the guidelines provided by the International Biochar Initiative. However, sewage sludge and material originating from mixed municipal wastes should be excluded for biochar production [195] based on the amendment of Regulation (EU) 2021/2088 of 7 July 2021 [196]. However, one of the main solutions proposed for treating sewage sludge is the treatment by thermal processes leading to char production. Given the previous restriction, the resulting char must then be used as a low-rank fuel. Another technical proposal is to carry out the pyrolysis of sludge under a CO₂ atmosphere, which not only allows a decrease in tar formation along with an increase in CO evolution, but also reduces the amount of char produced when the pyrolysis temperature reaches values over 800 °C [197,198,199], greatly reducing the problem of char disposal.

4.3. HTC for Treating Digestates

HTC is a process whereby high-water organics are heated to temperatures between 180 and 280 °C, producing mainly a solid material known as hydrochar and a liquid phase containing solubilized organics [203]. Hydrochar obtained at high severity exhibits analogous characteristics to pyrochar produced via slow pyrolysis methods but with a hydrophobic character due to the presence of amorphous aryl and alkyl C [204,205]. However, pyrolysis has the added advantage of recovering some of the energy from the gaseous phase and, in some cases, from the liquid fraction, depending on the process conditions.

During the HTC process, water is at subcritical conditions, which increases reaction rates due to a higher concentration of ionic products (acids and bases) that catalyze reactions [206]. The types of reactions that take place during HTC have been reviewed by Nicolae et al. [207], focusing also on the influence of the feedstock on the process outcome and the characteristics of the char produced. Another recommended review for insight into this process is that performed by Ischia et al. [208], which also mentions the main challenges of the technology regarding the co-processing of different wastes. However, a description of commercial plants is not found in the scientific literature, unlike reviews dealing with pyrolysis and gasification technology. It is worthy of note that the following commercial plants, which warrant particular attention, include those belonging to the companies Biatex GMBH (Karlsburg, Germany) [209], Terranova^®Ultra (Düsseldorf, Germany) [210], and the Ingelia HTC plant (Valencia, Spain) [211].

The advantage of HTC technology is that it eliminates the drying stage required by pyrolysis and gasification. Nevertheless, reactors capable of withstanding high pressures and the energy associated with heating the whole material, mainly water (organic inputs usually have a moisture content of 70–85%), are factors that work against this technology. In any case, the process needs a drying stage to remove hydrochar moisture —although, due to its hydrophobic character, the thermal demand is expected to be much lower compared with that of drying the digestate— in order to prepare the hydrochar for subsequent handling, storage and transport operations. Therefore, the initial benefit claimed is only partially eliminated. In addition, a considerably higher quantity of process water is generated, necessitating additional treatment before final disposal. There is no consensus on the optimal methodology for treating this complex aqueous phase. While anaerobic digestion is usually touted as a promising candidate, inhibitory complex compounds in the process water may pose additional difficulties [212,213].

The HTC process was compared with conventional digestion by Metyouy et al. [36] using food wastes as raw material and while considering the energy inputs needed in these two different processes to attain the treatment of the selected waste material. In their study, the authors reported a significantly better energy balance for digestion than for the HTC process. The rationale for employing this technology may be based on two key factors: firstly, the treatment of recalcitrant biomass in biological systems and, secondly, the processing of digestates that present challenges in terms of disposal.

The HTC process resembles the conventional hydrothermal pretreatment, which has been widely studied for its potential to enhance the digestion of sewage sludge. In the latter case, the temperature range typically falls between 140–180 °C. The temperature threshold is associated with the adverse effect experienced by digestion when treatment temperatures are beyond this range [214]. This effect is usually explained by the presence of Maillard reactions, which produce melanoidins derived from the reaction between sugars and amino acids at high temperatures [215].

Table 7. Results reported by different authors regarding the HTC of digestates. (SS: sewage sludge, MSW: municipal solid waste).

Material	Temperature (°C)	Main Results	Reference
Digested SS	250	Process time: 30 min. Process water was tested in BMP 1 tests. Net energy balance was carried out based on experimental results. A positive balance was obtained when digested sludge was treated in the HTC system with a solid content greater than 10% and by considering the use of hydrochar as fuel.	[216]
Digested SS	190	Process time was between 30 min and 3 h. HTC treatment for 1 h improved the dewaterability of the treated material, although the biogas production of the process water was lower than for the shorter duration treatment.	[217]
Digestate from a mixture of components	190, 220, 250	Digestate was obtained from a plant treating maize silage, liquid cattle manure and grass silage. Hydrochar was produced to recover phosphate and obtain activated carbon. Due to the composition of the feed, digestate resembles lignocellulosic biomass with a higher ash content (27.7%). Acid leaching of hydrochar allowed phosphate recovery and produced a product with high microporosity and better adsorption capacity after submitting it to an activation protocol.	[206]
Cow manure digestate	180, 220, 250	Hydrochar and water extracts from hydrochar were tested. Process temperature had more influence on hydrochar characteristics. The carbon content of hydrochar increased with increasing temperature. Phytotoxicity was detected in seed germination tests.	[218]
Digestate from a mixture of components	190, 220, 250	Digestate derived from a plant treating a mixture of corn and grass silage with cattle manure. Increasing the process temperature and duration increased hydrochar stability and reduced its carbon content. Process water was analyzed. At higher temperatures, its content in organics was higher. Phenol, lignin derivatives (guaiacol and syringol), cyclic ketones, cyclopentanones and N-containing compounds (pyrazines and pyrinidols) were measured in the process water.	[219]
Digested MSW	180, 200, 230	Process time was between 15 and 120 min. Liquid volume increased after treatment due to solid solubilization. The solid fraction was reduced from 29% (in the digestate) to 21.4% in the carbonized slurry. The energy content of this type of hydrochar is lower than that obtained from fresh equivalent material, as carbon extraction has already taken place during digestion.	[220]
Digested MSW	200, 250, 300	Process time: 30 min and 2 h. Increasing temperature produced a hydrochar with higher mineral content.	[221]

¹ BMP: biochemical methane potential.

lists the results of different authors who have performed HTC treatment of digestate.

The HTC treatment of digested sewage sludge was evaluated by Ahmed et al. [217], who indicated that the best operating condition was a temperature of 190 °C for one hour of treatment to favor solid dewatering and obtain a process water with good performance in batch digestion tests. The authors reported a 32% proportion of volatile solid (VS) destruction, a biogas yield of 157 ± 9 mL biogas/g COD (66.4% methane content) and a liquid with a COD concentration of 33.4 ± 0.6 g/L. Similar results are reported in a previous work by Ferrentino et al. [222], who tested the HTC of sewage sludge and digested sludge under conditions of 190 °C and 1 to 3 h duration. These authors observed an increment in methane yield when evaluating biochemical methane potential (BMP) tests containing a mixture of the sludge with HTC solids and the liqueur fraction. The COD content of this liqueur fraction was 34 g/L, and the methane yield was 76 ± 20 mL/g COD. A similar study has been carried out by Aragón-Briceño et al. [216], also treating digested sludge and assessing the biogas potential of the process water. Their results are markedly more promising than the previous results, obtaining a biogas yield between 288.2 and 325.6 mL CH₄/g COD added, a value similar to that observed from other wastes, which may indicate that inhibitory conditions were almost absent. However, from the same experimental work reported by Ahmed et al. [217] and with regard to digested sewage sludge, the authors also showed lower biogas yield values from the process water when severe HTC treatment was applied (longer duration but with a temperature of 190 °C for all tests). Values were 235 mL CH₄/g COD_added when HTC duration was 30 min, falling to 84 mL CH₄/g COD_added when the duration was increased to 3 h.

Urbanowska et al. [223] characterized the composition of HTC process water derived from the treatment of digestate. No specific parameters were reported for the digestate. The HTC process was carried out at 200 °C for 4.5 h. Authors found acetic acid, 1-hydroxyacetone, 1,3-propanediol, and 3-pyridinol as the main components of the process water. Several authors have also measured acetic acid as one of the predominant acid constituents, alongside furfural, HMF, and a variety of phenolic and cyclic nitrogen compounds [212,219,224].

Recent alternatives for treating this process water include combining HTC with wet air oxidation (WAO) or SCWG. The approach of using SCWG was studied by Feng et al. [225], but the integration of AD and two additional costly processes does not seem adequate. The second stage of the proposed solution still generates process water with a relatively high COD value, exceeding 5 g COD/L, therefore needing further treatment. The use of WAO was tested by Riedel et al. [226] and Reza et al. [227]. The process is a severe oxidation carried out at high temperatures and pressure in the presence of pure oxygen; it is clear that these extreme conditions can reduce almost all complex and toxic compounds, with authors reporting COD removal of over 55%. Nevertheless, this treatment alternative is also a costly and complex intermediary to attain partial conversion of organics, though it offers the advantage of highly reducing toxicity.

As with the thermal process described above, the outcome expected from HTC depends on the conditions applied and the chemical composition of the raw material used. Cao et al. [228] evaluated three different digestates, one from the digestion of organic household wastes and two others containing cattle manure, one of which had a high lignocellulosic content. The conditions applied (temperatures between 170–250 °C and processing time between 2 and 5 h) led to hydrochars with varying carbon content. The higher the process temperature, the lower the char yield; therefore, the char production trends were similar to those derived from previous thermal processes. At 210 °C, a large fraction of the cellulosic material remained in the hydrochar. In a different experiment, the addition of cheese whey to the HTC processing of manure or digested manure was tested by Belete et al. [229] as a way to increase the carbon content of the reacting mixture. Therefore, the HHV of the hydrochar was 31–38% higher thanks to the addition of whey (temperature conditions at 210 and 240 °C). However, this approach implies the use of costly equipment just to produce a solid fuel-like material. In contrast, cheese whey may be directly used as a co-substrate in the anaerobic reactor, recovering a significant amount of energy in the form of biogas and while using equipment with much lower installation and operating costs.

The amount of HTC process water requiring treatment is expected to be much higher than that derived from the pyrolysis process, given the higher moisture content of the raw material used in the former case. Therefore, if process water is considered an inconvenience when attempting to integrate AD and pyrolysis, treating this byproduct is paramount in the present case with HTC as the second stage. Some authors have explored the recirculation of process water to the feed preparation stage as a feasible option and have also reported an increase in hydrochar production and its HHV when recycling this water [224,230]. However, this approach is only a good option when water needs to be added to adjust the solid content of the feeding material. In the case of treating digestate, excess water is obtained, so recirculation of these streams means its return to the digestion plant head only when the feeding material requires such addition. Electrooxidation is another option that has been tested to remove organic compounds from HTC process water. However, the energy demand would be excessive when considering this approach as a single treatment unit for organics removal [231,232].

Hydrochar is claimed to be a valuable solid fraction considered as either a solid fuel or a carbon material capable, one that is capable of recovering nutrients (P and N) that have several soil benefits when applied to cropland and which are similar to the properties claimed for biochar that is derived from pyrolysis processes. As with other thermal technologies, the process conditions must be carefully evaluated to achieve a specific aim. Stutzenstein et al. [233] have indicated, on the one hand, that phosphorus recovery is maximal at a pH of 8 and 165 °C, whereas maximizing nitrogen recovery requires a pH of 3 at the same temperature. On the other hand, carbonization is maximal at a temperature of 230 °C with a pH of 3 units. Aside from the primary aim of applying this process to treat digestate, integration with anaerobic digestion does not seem to be a feasible option given the stringent conditions required and the bottleneck that process water may create in such an approach.

The energy demand of the HTC process is usually presented as a key advantage when compared with gasification and pyrolysis due to the necessity of drying the organic material. However, these two subsequent processes can attain self-sufficiency under regular operation conditions. If only drying requirements are considered, the values mentioned above can be used to estimate the amount of energy required per mass of dried material, resulting in 8.8–20.2 MJ/kg, based on the previous assumptions regarding the drying process [133]. In consideration of the thermal demand estimated by Metyouy et al. [36] (1.2 MJ/kg wet material, equivalent to 9.8 MJ/kg dry material) and assuming that drying hydrochar requires about 40% of the energy needed in the previous case, the thermal demand of the process may be approximated to 14.4–19.0 MJ/kg of dried material, which effectively negates any initial perceived benefit.

5. Conclusions

Anaerobic digestion is a widely applied technology for treating organic wastes. However, the amount of material remaining at the end of the biological degradation requires searching for additional valorization options. Restrictions regarding the land application of digestates make thermal processes a feasible alternative for producing valuable fuels and greatly reduce the amount of residue remaining. Gasification and pyrolysis require the previous drying of digestate, a feature that adversely affects the energy balance. However, the extra energy derived from gaseous and liquid fuels may partly compensate for the high energy demand of the integrated approach. In contrast, HTC does not require a drying stage, but the excessive amount of process water derived from the treatment and the high pressure required act against this technology, as does the inability to produce valuable fuel byproducts. Gasification and pyrolysis are currently the most suitable processes for converting digestates, although several aspects still need extensive research, such as treating condensates and removing tar from the products.