Hydrothermal Treatment of Digestates: Challenges and Perspectives

1. Introduction

Anaerobic digestion is a mature technology widely recognized as the ideal option for treating organic waste with high biodegradability and high moisture levels. The process has traditionally been applied to the stabilization of sewage sludge, manures, and agronomic residues. In recent years, it has also been proposed as a strategy for providing renewable methane as a substitute for natural gas, using energy crops in this latter case. Anaerobic microflora degrade organics through a sequential set of biological reactions, producing volatile fatty acids as intermediates. Subsequent transformation occurs, resulting in the production of biogas, which primarily consists of methane and carbon dioxide, along with a slurry known as digestate. This digestate is rich in stable organic compounds, minerals, and biological material [1].

Biogas is the primary energy product, with its most common application being the production of energy, either through boilers (to generate heat) or combined heat and power (CHP) engines (to produce both electricity and heat). Other technologies, such as microturbines and fuel cells, have also been proposed, but their higher costs limit their implementation. Biogas upgrading has recently become a preferred option. The REPowerEU communication promotes reducing demand for Russian gas following the Ukraine invasion by doubling the objective of biomethane production established in the Fit for 55 package, which aims to reach 35 billion cubic meters per year by 2030, thereby accelerating the green transition [2]. The process of biogas upgrading involves the removal of contaminants such as water vapor, hydrogen sulfide, oxygen, nitrogen, ammonia, and mercaptans, among others, using different technologies and produces a purified methane gas stream suitable for injection into the natural gas grid [3,4].

However, biogas plants are not free from controversy; a review by Hurtig et al. (2025) indicated that methane fugitive emissions in biogas plants range from 0.5% to 6%, with an average value of approximately 5%. Thus, environmental benefits related to climate change may be negated by the negative impact of biogas leaks. Another relevant issue is associated with the rejection observed in local communities when installing new biogas plants and the final disposal of digested material. Despite the environmental benefits claimed for creating jobs, producing renewable energy, and valorizing waste, which facilitate carbon emission reduction, the local population may still be opposed to this type of investment. Their concerns typically revolve around the annoyance caused by unpleasant odors, as well as problems related to waste transport and storage, including digestate storage and its final disposal [5,6].

1.1. Finding a Solution for Digestate Disposal Is an Urgent Task

Digestate is the by-product obtained from anaerobic digestion. The disposal of this slurry is one of the major drawbacks of the technology if it is not properly stabilized and if no adequate strategy is available for valorization as an organic amendment or for obtaining valuable products; thereby, environmental benefits may be lost, and the material may become a nuisance [7]. Digestate characteristics are dependent on several factors, such as the type of raw materials, digestion time, reactor operating conditions, and reactor solid content. Digestate accumulates complex molecules rich in aromatic carbons and some other compounds with an alkyl carbon structure [8]. Digestates also have a higher mineral content than the raw material from which they originate, due to the biological conversion of labile compounds, the accumulation of minerals, and the breakdown of recalcitrant organics [9]. Therefore, the expected content of sugar and acid intermediaries should be low, and the pH of the material is usually close to neutrality or even higher when the feed contains high levels of proteins.

With the recent interest in biogas plants, the focus of research has shifted to the treatment and valorization of digestate. Applying digestate to land, either right after anaerobic stabilization or following composting, is one of the most common practices. Several benefits have been demonstrated regarding the recycling of nutrients, conservation of soil organic matter, and improvement of soil microbial biomass and enzyme activity [10,11]. However, the high ammonia concentration of the slurry and the specific feature of the stabilization process, which concentrates minerals and metals in digestate solids, may create risks related to uncontrolled emissions of pollutants and soil contamination by copper and zinc [12]. Additionally, pathogen risks associated with inadequate stabilization have also been identified [13,14], but these latter risks can be mitigated by effective monitoring of digestion parameters and implementing post-stabilization stages after the anaerobic treatment.

The scale of a digestion plant greatly influences the feasibility of a specific valorization approach for the digested material. The treatment capacity of the plant is directly related to the amount of digestate produced. The removal of volatile solids from manures, food wastes, agronomic residues, and sewage sludge ranges from 35 to 80% [15,16,17,18]. The presence of lignocellulosic material and the crystallinity of cellulose hinder microbial access to cellulosic compounds, resulting in reduced degradation rates and lower conversions at a given hydraulic retention time. Therefore, pretreatments are recommended to increase digestibility and enhance biogas production [19,20]. Nevertheless, the application of pretreatments brings as a disadvantage an increase in installation and operational costs, either due to the expenses associated with chemical or biological reactive compounds, or with the higher energy demand linked to the auxiliary equipment used to run these pretreatment units.

The land disposal of digestates has its limitations when large areas of agronomic land are required to dispose of all the digested material obtained throughout the year. Land application of organic amendments is a seasonal activity; therefore, sufficient land must be available to absorb the digestate produced continuously over a few months, thereby creating disposal difficulties as the size of the plant increases. The profitability of the biogas plant is influenced not only by its size and the input material to be treated and transported, but also by the plant location, which affects the valorization option selected for digestate. Consequently, digestate transport becomes a crucial factor when land application is the chosen option [21]. Other alternatives for digestate valorization include composting and pelletization, although both options involve land application of the material and therefore experience similar issues to those already mentioned.

Alternatives recently available for valorizing digestate include its use as a medium for bacterial growth, mushroom cultivation, or producing oils from lipid-accumulating microorganisms and microalgae [22,23,24,25]. However, these proposals are at an early experimental stage, and finding a feasible solution for digestate valorization using an already developed technology with a large treatment capacity is urgent. Thermal treatments have been widely studied as suitable alternatives for digestate conversion, recovering energy and valuable products from organics. Among these technologies, combustion, gasification, and pyrolysis have been tested at pilot and large scale, either as a treatment for digestate [26,27,28] or as a co-processing with conventional fuels (coal, biomass) [29,30,31] and other wastes [29,32,33].

Gasification and pyrolysis show a better environmental performance when considering CO₂ process emissions than incineration because of their feature of producing valuable fuels capable of replacing fossil fuels [34], but the energetic balance of the integrated approach between digestion and thermal processes is a critical issue as well as the treatment of by-product streams [35]. Any of the previous technologies involves digestate drying as an intermediary step in the integration between anaerobic digestion and thermal processing, which can adversely affect the energetic balance. Other thermal processes available for treating organics include hydrothermal technologies, which are carried out in a water-rich environment, thereby eliminating the drawbacks associated with drying. However, the process is performed under higher pressure, which may reduce profitability due to higher investment and maintenance costs.

The present manuscript reviews the current state of the art in hydrothermal carbonization and hydrothermal liquefaction when applied to digested material as a solution to the valorization problem of large-scale digestion plants. The document focuses on the challenges encountered when implementing this type of technology in digestion plants and covers the issues associated with treating process water. A review was also conducted regarding techno-economic feasibility studies on the implementation of either hydrothermal carbonization (HTC) or hydrothermal liquefaction (HTL) plants using different biomass materials.

2. Materials and Methods

The databases used for this work included Google Scholar, PubMed, and Scopus. Additionally, a search for grey literature was conducted, focusing on commercial technologies currently available in the field of Hydrothermal Carbonization (HTC) and Hydrothermal Liquefaction (HTL). The literature search emphasized both ongoing development projects and recent initiatives that have received public funding.

The following main keywords were used to select relevant manuscripts: “anaerobic digestion” and “hydrothermal carbonization HTC”, “anaerobic digestion” and “hydrothermal liquefaction HTL”. Keywords related to this subject were also employed, such as: “digestate”, “HTC process-water”, “HTL process-water”, and “techno-economic assessment”. Manuscripts published between 2010 and 2025 were given priority, particularly those in which the energetic performance of the process was considered. Manuscripts containing basic knowledge of the subject were also included, even though the publication date was earlier than the reviewing period.

3. Hydrothermal Processes

The treatment of organics in the presence of water, acting as a solvent due to the increase in temperature and pressure, enables the degradation of molecules by hydroxyl ions present under conditions below the supercritical point, thereby avoiding the thermal penalty associated with phase change [36]. The family of hydrothermal treatments encompasses hydrothermal carbonization, liquefaction, and gasification, which are classified based on the severity of the process conditions that affect the primary outcome [37,38]. However, when considering only process conditions, a broader classification should also involve the traditional hydrothermal treatment as the less aggressive category. HTC operates at milder temperatures, in the range of 180 – 260 °C and pressures of 20 – 50 bars when compared with its homologous [39]. However, below these temperatures the process can be classified as thermal hydrolysis, with optimum temperatures between 160 and 180 ºC and processing times of 30 to 60 min [40].

The application of thermal hydrolysis as a post-treatment stage offers the advantage of lower capital investment and energy demand, as the biological process has already removed easily degradable material, and the total amount to be heated is significantly reduced [41,42]. As the severity of the thermal hydrolysis process is increased, the occurrence of inhibitory conditions in the biological degradation has been reported due to Maillard reactions, involving the combination of carbohydrates and amino acids to form colored compounds (melanoidins) which are difficult to degrade [43,44]. The occurrence of these reactions allows for foreseeing the challenges that will be encountered when attempting the hydrothermal treatment of organic waste, which takes place under conditions more severe than those applied during thermal hydrolysis. The carbonization process yields a hydrochar material as its main product, along with by-products: a gaseous stream containing mainly CO₂ and a liquid stream known as process water. Figure 1 illustrates a simplified scheme of the main reactions produced, process conditions, and main products obtained from hydrothermal processes.

Increasing the temperature of the treatment leads to the second category, which is hydrothermal liquefaction (HTL), with operating conditions ranging from 200 to 370 °C and pressures between 40 and 220 bars, yielding biocrude as the primary valuable product. In contrast, in the HTC process, it is hydrochar [45]. Just as in the previous case, hydrochar is also formed, along with a gaseous stream of low energetic value; however, the optimum operating conditions are dedicated to maximizing biocrude yields. This yield is highly dependent on the characteristics of the raw material, as well as the operating conditions, with values ranging from 21% to 53% [46]. Both processes produce hydrochar. The thermal stability of this material and its carbon content increase with process severity (increase in time and temperature) [47].

The severity of hydrothermal treatments is defined in terms of the temperature and reaction time applied. Different authors use factors to describe the combinatorial effect of these two main parameters, and others may also include the heating and cooling stages. Refer to Table 1, which provides different definitions for calculating the severity factor. The severity factor helps analyze the extent of the reaction and to establish a correlation between process outcomes and operating conditions, allowing for the inclusion of pH effects when necessary [48].

Reactions involved in HTC and HTL follow a similar structure, including hydrolysis, dehydration, decarboxylation, condensation, polymerization and aromatization, with the formation of 5-HMF (5-(hydroxymethyl)furfural), aldehydes and ketones, which are distributed in the aqueous phase, but the less severe conditions of HTC make lignin relatively inert [56,57,58]. It is also considered that the biocrude formed during the HTC process acts as a precursor of hydrochar [59]. Thereby, two routes are differentiated during char formation, considering primary hydrochar that one derived from solid-solid reactions of biomass, having thus a similar structure with the original raw material, and secondary char as that one derived from the condensation and polymerization reactions of compounds found in the liquid phase, showing a microsphere type structure [60]. Under subcritical conditions, ionic reactions are dominant (involving protons and hydroxyl ions derived from water), whereas free radicals are the main species involved in the conversion of organics into gaseous species when reaching supercritical conditions [61,62]. Gas production increases above 300 °C because water density decreases at this point, resulting in the disruption of ionic product formation and an enhancement of the free radical mechanism. Consequently, higher gas yields are expected as the temperature continues to increase [63].

Another related process to these conversion technologies is wet oxidation. Operating temperatures also fall within a similar range to that of HTL. In this case, an oxidant, typically air or oxygen, is added, thereby facilitating the solubilization of organic matter and its conversion into small, soluble compounds and volatile fatty acids (VFA) [64,65]. The last process carried out at the highest temperature is hydrothermal gasification. This process involves conditions close to or above the supercritical point and even higher, receiving the name of supercritical water gasification when operating above the critical point. Around the critical point (374 °C) organics are gasified yielding mainly methane and CO2 (but also other components as CO, H₂ and C₂ gases are generated), whereas increasing temperature above 600 °C favor hydrogen formation, having as main feature the catalytic effect of salts already contained in the raw material which promote the water-gas shift reaction [66,67].

Table 2 provides an example of experiments carried out using typical biomass, detailing the main operating conditions of each process and the yields obtained when using similar raw materials. Many studies consider the hydrothermal process a convenient treatment based on the evaluation of energy ratios between the products obtained and the energy input of the raw materials, without considering the energy demand of the entire global process [68,69,70]. On the contrary, McGaughy and Toufiq Reza [71] evaluated the energy ratio by also including the electricity demand associated with pumping the material, the heating demand of the process, the energy demand of hydrochar dewatering, and subsequent drying. These authors reported a ratio between 2.94 and 4.91 for the HTC conditions tested (200 – 260 °C), with these values being higher than those obtained when considering waste incineration (an energy ratio of 2.2). However, in their case, hydrochar yields were high and characterized by a low ash content, which is not typically the case when dealing with digested material. Therefore, any detriment in char yields and its characteristics approximates the process to conventional energy ratios obtained from other thermal processing technologies. A similar case is that of HTL, Aierzhati et al. [72] assessed the energy ratio of food waste HTL. These authors reported similar values, with a ratio of 1.88 (the reported value was 0.53 because ratio terms were inverted), after including the heat demand of the process.

When considering the HTL process, biocrude is the product of interest, but it is far from being a commercial product. Biocrude requires further processing to reduce its content in heteroatoms (N, O, and S). The upgrading is typically performed under supercritical conditions in a H₂ atmosphere, in the presence of catalysts. Several authors have tested different catalysts, reporting a significant removal of O but finding difficulties in reducing the nitrogen content [83,84,85]. The concentration of heteroatoms in biocrudes is related to the composition of the raw materials; thus, lignocellulosic biomass yields bio-oils with a higher oxygen content and, therefore, lower energetic density. On the contrary, algal biomass and sewage sludge yield bio-oils with higher nitrogen levels, associated with a higher presence of proteins [86,87,88]. Wu et al. [89] reported that 3 – 35% nitrogen present in sludge is transferred into biocrude. The presence of nitrogen in bio-oil poses a significant challenge due to the production of NOx emissions when considering its use as a biofuel.

3.1. Large Demonstration Projects Are Needed

Most of the experiences found in the scientific literature involve small-scale assays in the range of a few milliliters, and a few experiments report the use of small pilot plants under batch or continuous conditions. Despite the great amount of scientific work related to the subject, the large-scale application of technology is far from becoming a reality. The review performed by Castello et al. [90] presents an interesting historical overview of the process. It highlights the limitations encountered when operating under continuous HTL conditions and the research gaps that require a feasible solution. Chen et al. [91] also list the main challenges encountered when dealing with the hydrothermal process, which coincide in aspects such as issues related to high-pressure pumping of the feed, maintaining flowability and avoiding clogging, and corrosion of the reactor and auxiliary equipment.

There is a lack of pilot-plant derived data suitable for developing models for large-scale reaction units [92]. The work of Zaccariello et al. [93] is one of the few working with a batch reactor of 100 L (working volume 70 – 90 L) and the commercial plant of Terranova Energy [94] located in Jining, China with the process of TerraNova®ultra and the development of the project in Uşak, Turkey also involving a commercial plant are the few of their type. Ingelia [95] also built a commercial plant, which has been in operation since 2022, and has participated in different research projects. HTCycle (Murchin, Germany) claims to be the first company to introduce an industrial HTC plant in 2010 [96]. The company built an industrial demonstration plant in Relzow (Germany), receiving funding from Horizon 2020 (grant agreement No 823124) [97]. Data regarding plant performance and product characteristics are available on the Cordis project webpage [98]. The plant was dedicated to treating sewage sludge, transforming this material into a coal-like solid with high phosphorus content. The process water was treated by membrane separation, with the pH having a significant relevance on membrane retention performance. The review performed by Ferrentino et al. [99] focuses on technologies for reducing sewage sludge volume and describes different HTC plants built at a demonstration commercial scale. However, current gaps regarding hydrochar heavy metal content and the treatment of contaminants in process water may probably delay the widespread application of this technology.

An industrial demonstration plant (continuous operation mode) was built by CarboRem Srl (Rovereto, Trento, Italy) with a treatment capacity of 5000 t/year of residues with a solid content between 2 and 10%TS (operation temperature of 200 °C and retention time applied of less than 60 min). The process water was treated by the digestion process in the case of adapted to sewage sludge treatment [100,101]. The review manuscript of Farru et al. [39] lists different companies providing HTC technology, but many of them only have demonstration plants. In the case of HTL, only a few companies have built or are commissioned to construct commercial plants in the short term. The Licella Holdings Ltd. (North Sydney, Australia) is one of them with the Cat-HTRTM technology [102], and Arbios Biotech [103], which is a joint venture between Canfor and Licella, is participating in the construction of a liquefaction plant in Prince George, British Columbia (Canada). Mura Technology offers the HydroPRSTM process, which is capable of treating plastic waste and transforming it into a crude similar to petroleum crude [104]. Just as in the case of HTC technology, data regarding industrial plant performance are scarce.

4. Hydrothermal Conversion of Digested Material

The biological degradation of biomass leads to the mineralization and stabilization of organic compounds, accumulating minerals, metals and recalcitrant materials in the slurry derived from the process. In the case of aerobic processes, such as composting, a higher aromatization is achieved, whereas in anaerobic digestion, long-chain aliphatic compounds are also reported as part of the stabilized components [105,106,107]. The anaerobic stabilization enables the production of an organic material with a lower energy density than the raw input, due to its lower volatile content and higher ash content [35]. Therefore, any thermal valorization attempt of digestate must be confronted with the lower energetic value of digestate compared to the feeding material, and the subsequent higher ash content of hydrochar, which would limit its use as fuel [75], thereby compromising the profitability balance of any hypothetical plant. Applications of hydrochar as adsorbents, as part of fertilizer formulation or catalysts have also been proposed, but the low value of these products hurts the economic balance [108,109,110,111,112,113].

Although the reactions involved in hydrothermal processes are highly complex, comprising several parallel and serial stages, the significance of feed composition and operating conditions is well recognized. Process yields have been successfully predicted using statistical models that integrate multiple factors [114,115]. The empirical knowledge gained from a wide variety of experiments performed using different types of biomass and reactor conditions enables the approximation of expected yields. In the case of digestate hydrothermal processing, the results are less optimistic due to its higher mineral content, particularly when considering digestate derived from manure and sewage sludge. Table 3 presents a list of results reported regarding HTC and HTL processing of digestates.

Digestate derived from agricultural materials or with a relatively high lignocellulose content is expected to show a better performance for the whole integrated process (AD+HTC) due to the higher hydrochar yield from this type of biomass and the higher HHV obtained as well [130]. For this reason, some authors considered the co-processing of digestates and lignocellulosic biomass as a suitable option for improving process performance [131]. However, in the case of HTL, the high lignocellulosic content results in lower yields unless catalysts are added. Sudibyo et al. [132] proposed changing operating conditions to improve biocrude yield. These authors tested acidic conditions to catalyze biocrude production and inhibit Maillard reactions. The application of higher temperatures and longer reaction times enabled hydro-denitrogenation and deoxygenation reactions, thereby improving biocrude quality.

In general, lower HTC temperatures favor hydrochar yields characterized by lower energetic density, whereas in the case of HTL, an optimum temperature is found where biocrude formation is maximized. In both treatments, the effect of temperature is usually more relevant than the effect of reaction time, although comparison of results performed by different authors is not easy because the heating ramp, reactor size, configuration, and cooling conditions applied are usually not clearly described, despite having a great impact on the process outcome [115,133,134,135]. The heating rate applied and its duration may favor the initiation of carbonization-type reactions at temperatures around 180 °C [60], thus conditioning the yields obtained once the desired temperature is reached.

Belete et al. [120] compared the integration of AD and HTC with the single HTC treatment of fresh manure. They reported similar energy ratios for both approaches, although they indicated higher energy gains for the HTC process. However, when considering the energy required to run any of the processes (input energy), the energetic performance was quite similar, with energy ratio values ranging from 1.6 to 2.7. Regarding HTL, the clear benefit of considering this technology as an integral part of a biogas plant is hard to discern, given the complexity of the compounds forming biocrudes and the fact that not only the yield obtained matters, but also its heteroatom content. Actions dedicated to reducing nitrogen and sulfur content in biocrude may enhance global process performance by lowering the energetic requirements in the subsequent bio-oil upgrading stage, as well as the hydrogen demand for this process (approximately 20–50 g H₂/kg bio-oil [136,137]). Sewage sludge and manures are characterized by producing biocrude with high nitrogen levels [54,76,133]. Therefore, any strategy intended to decrease N concentration results in significant economic savings. This is the case of the study by Sudibyo et al. [138], which proposes the co-liquefaction of digested sewage sludge and polyphenolic extracts derived from apple, olive pomace, spent coffee grounds, and sweet orange peels. Their experiment was performed at 370 °C and with a 60-minute holding time (under acidic conditions, pH = 3.0), resulting in a biocrude yield of 30% with nitrogen contents ranging from 0.04 to 0.27%. These values are extremely low compared to those obtained from other high nitrogen-containing biomass, which can lead to bio-oils having nitrogen concentrations between 6.5% and 8.1% [139].

5. Aqueous Phase

The treatment of by-products involves the oxidation of organics found in the aqueous phase. The severity of the treatment and the composition of the raw material determine their distribution in this phase, as well as the yield of the different products of interest. Regardless of the selected technology (either HTC or HTL), the treatment of process water is still a pending assignment. The review performed by Ender et al. [140] indicated that none of the current HTC technology providers report on any data regarding the efficiency of organic removal for the treatment alternative selected when dealing with this polluted stream. Table S1 presents the list of common compounds found in this process water.

Hydrothermal process water is characterized by a high concentration of organic compounds, which can create toxic effects, as evidenced by the toxicity tests performed by Kulikova et al. [141] using Artemia salina and Paremecium caudatum. However, the toxicity of this effluent is highly related to the process conditions. The toxicity test performed by Melo et al. [142] using Eisenia fetida and Artemia salina showed non-toxic or minimal effects when the process water pH was adjusted to 8.5 units, using process water derived from sewage sludge HTC processing at 190 °C in this case. In contrast, the study by Kulikova et al. [141] utilized process water derived from hydrothermal treatment at 240 – 280 °C (sewage sludge hydrothermal liquefaction). Similarly, the results reported by Pham et al. [143] from testing HTL process water derived from the processing of Spirulina, indicate high toxicity of nitrogen-containing organic compounds when using Chinese hamster ovary cells in their toxicity tests.

Anaerobic digestion has been proposed as a feasible solution for treating process water; however, many components contained in the aqueous solution are recalcitrant to biological degradation. Farru et al. [124] studied the digestion of HTC process water, reporting yields ranging from 50.9 to 75.8 mL CH₄/g COD (chemical oxygen demand). They indicated that lower processing temperatures and higher residence times reduced the toxicity of the process water. The low methane yield reported by these authors was also associated with the high recalcitrance of the organics forming the process water. Therefore, any attempt to recirculate this water back to the digestion reactor may dilute the degradable organic content of the feed, adding the risk of also inhibiting the process.

Digestion of HTC process-water was reported by Cao et al. [119], yielding better results with biochemical methane potential (BMP) values ranging from 200.3 to 211.0 mL CH₄/g COD. After considering all energy inputs of the process (HTC heat demand, dewatering, and hydrochar thermal drying), these authors reported an energy gain of 48.5%, which is obtained on average after including the energy derived from anaerobic digestion of the process water in this estimation. When considering the energy inputs of this process (excluding the energy contained in the raw material) and the reported energy outputs, a ratio between 0.96 and 1.025 was estimated (energy ratio: output/input).

Regarding the digestion of HTL process water, inhibitory issues are to be expected, given the previous difficulties encountered when treating HTC-derived water (obtained at a lower severity). In general, authors have reported increased inhibitory conditions with an increase in HTL process temperature, with phenolic and nitrogen compounds being the main sources of toxicity [144,145]. Another essential requirement is the need to remove nutrients before submitting the stream to anaerobic digestion. Shanmugam et al. [146] tested the precipitation of struvite as a method for removing ammonia, enabling the recovery of a product with commercial value in this case.

Given the difficulties associated with the biological treatment of process water, alternative treatment methods have been proposed, including the combination of chemical and biological processes or the application of severe oxidation conditions. Table 4 lists different approaches reported in the scientific literature. Electro-oxidation has proven successful; however, excessive energy demand renders this option unfeasible [147]. Wet oxidation offers the advantage of significantly reducing organic content by transforming complex organics into short-chain acids at a lower energetic cost [148]. Matayeva & Biller [147] estimated the energy ratio of an HTL plant, considering either wheat straw or sludge as raw input, along with the treatment of process water by wet oxidation. The sewage sludge plant had a higher energy ratio due to its higher total solids content and lower processing temperature, which also resulted in the production of more biocrude. Therefore, the energy ratio was 3.4 without considering process water treatment, but reduced to 2.17 when this latter treatment was included. In contrast, the values for wheat straw were 2.2 and reduced to 1.06 after considering process water treatment.

Marrone et al. [79] tested the liquefaction of sewage sludge and subsequent biocrude catalytic upgrading, as well as catalytic hydrothermal gasification of the process water, in a bench-scale plant under continuous conditions. The integration of numerous stages involving severe experimental conditions highlights the high up-front cost a plant of this type would incur, thereby making the process only feasible for large-scale systems, which may be in contradiction with local populations who are more likely to reject large-scale biogas plant projects. In addition, given that any type of treatment applied to the water phase will increase plant operating costs, alternatives that allow for the recovery of fuel-type products or chemical molecules may become areas of interest. This is particularly the case of the catalytic treatment proposed by Davidson et al. [154], which yields olefins as the main product. Another option is to avoid releasing process water by reusing it within the same process.

The recycling of process water is a more feasible option for the HTC than for the HTL process. Since water recirculation may aid in reducing water demand when treating low-moisture-content biomass and increase carbon yields [155], however, when treating digestate—a material with a high water content —the main benefit may be associated with avoiding the need to treat this stream. Recirculation of process water can increase process efficiency by enhancing hydrochar yield and its energy content thanks to the conversion of water-soluble compounds like furfural and its derivatives, which participate in condensation reactions [156,157]. However, in the specific case of sewage sludge, recirculation did not affect hydrochar yields due to its different composition, which was rich in proteins and saccharide hydrolizates. Recirculation of process water in this case resulted in an increase in process water pH due to the accumulation of ammonia [158]. On the contrary, in the case of HTL treatment, the recirculation of process water increases the nitrogen content of bio-oils when dealing with rich protein substrates, thereby hindering subsequent biocrude hydrotreatment [159,160,161]. Therefore, water recirculation is recommended when treating lignocellulosic biomass, but not for digestates, which are typically characterized by a relatively high nitrogen content.

6. Techno-Economic Assessment

The process conditions of HTC and HTL require high-pressure systems, which result in high investment and operating costs. Many scientific reports assess different scenarios using lignocellulosic biomass or waste materials, reporting profitability. However, after several years of experience on a laboratory scale and with small demonstration plants, HTC and HTL technologies have not yet reached commercial deployment. In the case of HTC, the technology appears to have a more promising future, given its lower thermal demand compared to HTL and the fact that hydrochar, as the main product, does not require excessive additional post-treatment. This is not the case when considering HTL and the subsequent stages required for biocrude upgrading.

The high costs of the plant relegates the technology to large scale systems where biomass can be obtained from high productivity crops and small transport distances, like poplar crops, the aggregation of several livestock farms, industries with high waste production such as paper industries (pulp mill waste) or from WWTPs or large anaerobic digester with high annual digestate production [162]. Most of the experimental work found in the scientific literature deals with small-batch reactors, and the few experiments reporting on the use of pilot plants are characterized by low throughput. Some demonstration plants have been built thanks to the availability of public funding, with the HTCycle AG plant in Relzow, Germany, still in operation and capable of producing commercial products. However, there is a lack of information regarding aging, abrasion, erosion, and corrosion of materials subjected to high-temperature and high-pressure stress, thus making it difficult to approximate operational costs. Precipitation of secondary char on reactor walls and inner parts may create maintenance issues and reduce heat transfer efficiency, thereby increasing maintenance costs.

The review conducted by Ho et al. [163] suggested that continuous HTC plants may encounter more operating difficulties associated with high-pressure systems when continuously feeding the reactor. However, the commercial experience of thermal hydrolysis plants have demonstrated a better performance of continuous processes, such as the case of the technologies commercialized by Veolia, Exelys^TM a continuous process with better energy recovery, and BioThelys^TM a batch process finds applicability in cases where high flexibility is needed, or when the process is intended for small installations [164,165]; therefore, the final decision of choosing from continuous or batch processes also rests on aspects regarding the annual treatment capacity expected for the plant and its capital investment. Table 5 presents a list of various technoeconomic assessment studies on HTC and HTL plants. Many of these studies lack some important stages regarding the treatment of by-products or upgrading.

Energy recovery is crucial for achieving a net positive balance when considering the integration of anaerobic digestion and hydrothermal processes. Any increase in biogas production expected from the digestion of hydrothermal process water or the application of hydrochar to enhance digestion performance may not compensate for the additional energetic requirements of the auxiliary units and thermal demand of the hydrothermal unit [177]. However, as the scale increases and restrictions regarding digestate application become a severe constraint, the additional cost of reducing digestate volume to a fraction of the expected solid material may appear as an interesting alternative. The analysis carried out by Bacci di Capaci et al. [178] shows that implementing an HTC unit in a WWTP (250,000 m3/d, 600,000 population equivalent) can reduce the amount of sludge disposal from 20,346 ± 308 t/year (mean value of three cases studied) to 4700 ± 420 t/year of hydrochar. Results reported by the HTC Ingelia plant [171] indicated that from 9 t of sludge (80% water content), 0.624 t of hydrochar and 0.14 t of an inert material were obtained after the HTC process. It is often claimed as an advantage that hydrochar can be dewatered to a much higher value than the raw original feeding material, thereby requiring less energy to evaporate the remaining water [179,180]. The functional groups present in hydrochar composition significantly affect the energy demand for water removal, with carboxyl groups creating strong hydrogen bonding interactions that adversely impact water removal; therefore, the O/C ratio of hydrochar dictates the energy required during drying [181]. Even if the energy required for drying would be the same, drying a much lower mass of material directly translates into energy savings.

The common practice is to consider hydrochar as a valuable resource. Nevertheless, metals are reported to accumulate in this solid material, thereby becoming a concern when using it as an agronomic amendment, as they may increase the total content and bioavailable metal content in soils [182]. Wang et al. [183] reported that although most heavy metals accumulated in hydrochar, the carbonization process reduced their bioavailability, except for arsenic, thereby recommending the application of hydrochar to non-farming soils. If revenues from selling hydrochar are eliminated, the feasibility of these technologies is greatly reduced. Adding this fact to the delicate economic balance of digestion plants, the introduction of a highly costly process can only be justified when digestate land application encounters significant rejection from the local population. Figure 2 illustrates a schematic representation of the primary products generated by combining anaerobic digestion and hydrothermal treatment technologies.

Given the large investment costs of the technology and the high ash content of digestates, in addition to the high metal content of sewage sludge, the co-processing of digested sludge has been lately proposed as an alternative for improving hydrochar quality in the case of HTC [184,185,186] and biocrude quality in the case of HTL processes [187,188,189]. The report by the Pacific Northwest National Laboratory [175] considered the treatment of sewage sludge by HTL in individual plants with a treatment capacity of 110 t/day (dry sludge), as well as the centralized upgrading of biocrude to increase process feasibility and the catalytic treatment of the aqueous phase. The report indicated that, thanks to advancements in technological knowledge, the minimum fuel sale price (MFSP) has been rapidly decreasing from 9.66 USD/GGE (GGE, gasoline gallon equivalent) in 2016 to 2.77-2.79 USD/GGE in 2022. The complexity associated with upgrading biocrude, along with the high costs of the technology, makes the best solution the integration of the HTL process into current crude refining facilities. This can be achieved by introducing either the whole treatment stage or the final upgrading stage, aiming to decrease transport costs. In the case of HTC, the lower technical requirements of the technology make it the best solution for medium-scale plants or coupling to centralized municipal centers.

Whenever possible, the land application of digestate should be considered the best alternative for recovering nutrients and taking advantage of its particular features regarding crop yield enhancement and improved soil organic characteristics [190]. The application of organic amendments, either as composted materials or digestates, offers the added benefit of reducing the use of mineral fertilizers [191], with savings associated with energy demand and CO₂ emissions. Hydrothermal treatment becomes a viable solution only in cases where excessive amounts of digestate need to be handled, making it unfeasible to have enough available agronomic land in the plant's surroundings.

7. Conclusions

Hydrothermal treatments may be considered as a feasible alternative for reducing digestate volume. HTC offers the advantage of operating at a lower temperature and producing, as a byproduct, process water with less toxicity, thereby facilitating its subsequent post-treatment through conventional biological processes or allowing its use as a liquid fertilizer. Depending on the characteristics of hydrochar, this material can be used as fuel, a soil amendment, or just discarded as an inert material when heavy metal content restricts its agronomic use. On the contrary, HTL yields a biocrude as its main product, which requires further processing to obtain a gasoline-type fuel. This feature implies the installation of a more complex process structure to achieve the complete valorization of the raw material, with the added disadvantage of requiring more extreme conditions to handle the process-water derived in this case. The effective use of these technologies should take into account the scale of the digestion plant and the feasibility of processing co-substrates to improve product characteristics and benefit from economies of scale. A deeper understanding of hydrochar characteristics, toxicity, and the effects of process operating conditions and raw material composition is necessary to achieve a more accurate assessment of the process, enabling a more realistic analysis of the technology's implementation and suitability for valorizing digestates. Given the high investment costs of these technologies, their current application may be limited to large-scale digestion plants or as a centralized unit receiving waste materials from different locations.