1. Introduction
Worldwide, wastewater treatment plants (WWTPs) operate at full steam, serving countless people and producing thousands of tons of sewage sludge daily, which demands adequate disposal [1]. According to global statistics, it is estimated that more than 45 million dry tons of sewage sludge are produced per year, with Europe, Eastern Asia, and North America appearing as the most prominent producers [2,3].
The treatment of sewage sludge is considered a crucial issue in terms of quality of life and is closely aligned with United Nation’s Sustainable Development Goals [4]. In addition to its high volume, another concern regarding this type of residue is due to its significant content of embedded pollutants, such as heavy metals, arsenic, pharmaceuticals, an excessive nutrient load (nitrogen and phosphorus), and massive presence of pathogenic microorganisms, e.g. thermostable coliforms and salmonella [5]. Unfortunately, there is a worst-case scenario that is quite common in underdeveloped and developing countries: a significant portion of citizens are not covered by a sanitation program and have their domestic sewage released directly into the environment, which becomes a relevant problem of public health, demanding consistent actions from decision makers [6].
1.1. Hydrothermal Carbonization
Hydrothermal carbonization (HTC) has emerged as an innovative technology for sewage treatment, simplifying conventional processes used in many countries. It is a thermo-chemical process that operates with water under subcritical conditions, that is, temperature below 374°C and pressure below 221 bar, usually ranging from 180 to 250°C and 6 to 12 bar [7,8]. HTC is considered a robust process due to its flexibility regarding the vast types of wet biomass used as raw material. In addition to sewage sludge, HTC is capable of processing other types of biomass in a single or combined arrangement, such as municipal solid waste [9], manure [10], vinasse [11], waste food [12], algae [13], olive bagasse [14], sawdust [15], organic residue [16], and lignocellulosic biomass [17]. The possibility of co-processing of different sources is a striking feature of this technology, and this modality has been named co-hydrothermal carbonization (co-HTC), which allows it to exploit new horizons through the study of synergistic effects resulting from the combination of, for example, biomass and non-renewable carbon-based sources [18,20].
HTC process can even be applied in the treatment of primary sewage [21–23], which could contribute to a decrease in demand for conventional processes based on anaerobic and aerobic digestion, allowing the waste to be valued and reinforcing aspects of the circular economy that involves this type of biomass. Both typical primary and secondary sewage can be considered a rich renewable source of biomass for different application possibilities, including nutrient recovery and a starting point for clean energy production [3,9–11,24]. Currently, the most commonly used methods for domestic sewage treatment include a long and expensive route within WWTPs, followed by sludge disposal in landfills. Sludge incineration, as well as the use of stabilized sludge in soil amendment, have also been adopted in some countries (Fig. S1) [7,25,26]. However, these methods may present unsustainable costs primarily owing to the high moisture content (80–85wt.%) remaining after mechanical filtration, which implies stabilization procedures, thickening, dehydration and drying, aiming at increasing its calorific value, lower transport costs, among others [27,28].
According to the literature, sewage sludge management is responsible for approximately 50 to 60% of the total operational costs in a WWTP [29]. Therefore, the investigation and development of innovative, efficient, and sustainable technologies have an important role in increasing the accessibility of vulnerable communities to a minimum basic sanitation condition.
One of the reasons that makes the HTC process a promising technology for sewage sludge treatment is that the water is a necessary input; therefore, previous drying, dehydration, or other thermo-chemical steps are disregarded [30]. Under these conditions, lignin, cellulose, and hemicellulose that comprise the biomass source undergo degradation to produce the hydrochar through a series of chemical reactions, such as, hydrolysis, dehydration, decarboxylation, condensation, polymerization, and aromatization [31]. These reactions are intimately related to temperature and time, in which the increase in the temperature usually speeds up the degradation process and the polymerization of the sewage sludge, while lower temperatures activate the depolymerization process. The variation in reaction time impacts the distribution of the products formed during condensation, polymerization, and aromatization [32,33]. Fig. 1 depicts the main chemical reactions involved in HTC of a cellulose-based biomass.
In addition to water friendliness, another reason that favors HTC as a potential technology for domestic sewage treatment is its ability to generate different bioproducts in solid, liquid and gaseous phases, which is a very welcome feature to connect HTC to other industrial processes and improve sustainability aspects. Some of the potential applications of these products are detailed in the following section.
1.2. Potential Applications of HTC Products
In recent years, interest in HTC products has increased significantly among researchers and some industrial sectors. The specific conditions of the HTC process give hydrochar unique characteristics, such as high surface area, pore volume and size, stability, and the presence of reactive functional groups on its surface. In addition, the resulting aqueous phase is rich in nutrients, but may contain significant levels of pollutants; whereas the gaseous phase has a characteristic composition based primarily on carbon dioxide. These products have been widely studied for nutrient recovery for agricultural use and soil amendment, for alternative adsorbents, and for biofuels production. Table S1 shows other potential uses for these products as reported in literature.
1.2.1. Nutrient source and soil amendment
Hydrochars generally contain essential nutrients such as nitrogen, phosphorus, potassium, calcium and magnesium, and can be used as slow-release biofertilizers, especially on infertile soils. However, the effectiveness of hydrochars in agriculture depends significantly on the type of raw material and the experimental conditions of the HTC process [34]. Hydrochars can increase the plant-available water capacity (AWC) due to its ability to retain water in their pores [35]. Moreover, it can improve highly weathered soils with unfavorable chemical properties, such as high electrical conductivity, low cation exchange capacity, and low organic carbon [36,37]. Nevertheless, the use of hydrochars in agriculture presents challenges that must be considered.
In some cases, hydrochars can decrease plant growth due to adverse effects on soil properties, such as an increase in the C/N ratio and a decrease in pH, resulting in greater microbial immobilization of nitrogen and lower nitrogen absorption by plants. These adverse effects can be intensified by inherent contaminants, such as heavy metals, polycyclic aromatic hydrocarbons (PAHs), phenols, and furfurals [34]. Hydrochars produced at high HTC temperature (>260°C) can be phytotoxic, reducing shoot growth by up to 30%, although it does not affect seedling emergence. Conversely, hydrochars produced at low HTC temperature (170–200°C) can induce nutrient deficiency due to the immobilization of essential ones [38]. In this sense, some alternatives have been explored and discussed to mitigate the phytotoxicity of the solid HTC product [39], such as natural aging of the hydrochar, allowing microbial decomposition of germination-inhibiting substances [40], washing with water or organic solvents, which can remove some of these inhibiting substances [41], and modification of the hydrochar to immobilize heavy metals, which although it does not improve the germination rate, can indirectly promote crop growth [42].
The valorization of the liquid phase on HTC process presents considerable challenges, although recent research has indicated its potential as a liquid biofertilizer [43,44]. However, detailed chemical characterization and exploration of possible applications for the HTC liquid phase are still scarce in the literature [45]. This liquid phase contains a high concentration of soluble organic compounds and nutrients, particularly ammonia-nitrogen and potassium, showing its potential use in producing organic liquid fertilizers [46,47].
Due to the high concentration of potassium and nitrogen (especially in the form of ammonium ion), but with lower amounts of phosphorus and heavy metals, the liquid phase presents itself as a potential nitrogen and potassium compound fertilizer, which can be adjusted to meet the specific needs of various plants and types of soil [46]. However, as the severity of the HTC reaction increases (mainly temperature and time), the presence of toxic intermediates, such as aldehydes, phenols, and furans, as well as heterocyclic aromatic hydrocarbons, can restrict its direct application to the soil due to its phytotoxicity and offensive odor [48]. In this sense, some post-treatments can be used to remove these undesirable substances, such as washing and fermentation [46].
Although fewer studies have directly evaluated the impact of applying the liquid phase of HTC on plant growth, their results suggest that it is necessary to identify the correct dilution ratio. The dilution ratio must balance the prevention of phytotoxicity with the adequate supply of essential nutrients for plant germination, growth, and development [44]. Consequently, continued research, as well as development of effective methods for the recovery of the liquid and solid phases of HTC are crucial to maximizing the use of the resources present in sewage sludge, promoting more sustainable agricultural practices or soil amendment, and reducing the environmental impacts associated with the inappropriate disposal of this waste.
1.2.2. Adsorption
HTC and co-HTC of sewage sludge have become viable techniques for producing efficient adsorbents. However, process conditions such as temperature, reaction time, pH, mass ratio, and adding additives play a fundamental role in modifying physical and chemical properties of the hydrochars, directly affecting their adsorption performance [49]. Several studies have reported findings in the production of adsorbents. For instance, Montaño et al. [50] presented an innovative approach, producing highly effective materials for biogas desulfurization. Optimal conditions, such as pH 2.6, reaction time of 180 min, and temperature of 230°C, were identified as the most favorable for maximizing the adsorption performance of the produced hydrochars. Huang et al. [60] observed that pH was one of the most determining factors influencing the surface charge of the adsorbent, the degree of ionization, and the types of adsorbed ions. In addition, they found that introducing citric acid during the HTC experiments using sewage sludge can significantly increase the porosity, the number of functional groups on the surface, and the thermal and chemical stability of the resulting material, increasing adsorption efficiency.
The effects of temperature and residence time of the HTC process on the production of these adsorbents have also been reported. Wang et al. [49] demonstrated that the amount of adsorption of pollutants such as tetracycline and methylene blue decreases with increasing HTC temperature (180–260°C), and only a slight decrease was observed when prolonged reaction times (2–6 h) were used. This result is consistent with those reported by Cheng et al. [51], which showed that increasing the reaction temperature is associated with an increase in the specific surface area and pore volume of hydrochars, improving adsorption capacities.
The water/biomass ratio has also been highlighted in the literature as an influence on the textural and surface properties of hydrochars. Studies have highlighted that this ratio significantly influences the formation of a hydrophilic surface, which is essential for the adsorption of iron ions in acidic conditions [52]. In addition, Ebrahim et al. [53] identified optimum parameters, such as 180°C, 9.6 h, water/biomass mass ratio of 3/1, for co-HTC of sewage sludge and sugarcane bagasse, resulting in useful adsorbents for lead. It was also observed that alkaline modification of the hydrochar significantly improved its adsorption capacity.
Hydrochar has also been evaluated as an adsorbent in sand plants for the removal of Escherichia coli during water treatment; the researchers performed activation of the hydrochar using an alkaline solution, increasing the adsorptive performance of the hydrochar by removing hydrophilic substances on the surface, resulting in an increase in hydrophobicity [54]. Similarly, Wang et al. [55] reported that sewage sludge-derived hydrochar achieved 2 to >3 log removal of human pathogenic rotaviruses and adenoviruses. These studies highlight the potential of hydrothermal carbonization of sewage sludge to produce hydrochars with superior adsorption properties, offering a sustainable solution for waste treatment and the adsorption of pollutants and biological agents. However, the optimum HTC conditions to remove these pollutants can vary for each one, indicating that further research should be carried out to better understand and optimize these processes for different applications.
1.2.3. Biofuels
The production of hydrochar from the HTC of sewage sludge has attracted growing interest due to its potential to generate high value-added carbon materials, especially carbon-rich hydrochars with low ash content, something that the literature shows is not very feasible through direct hydrothermal carbonization [24,55]. Recent studies have shown that co-HTC, which involves the hydrothermal carbonization of mixtures of sewage sludge with other biomasses, can improve the higher heating value (HHV), thermal behavior, and overall quality of hydrochars compared to using a single raw material [20,56,57].
According to the literature, pH is one of the critical parameters influencing the ideal chemical conditions for producing hydrochars used as biofuels. Studies show that acidification improves the quality of hydrochar, reducing the ash content and increasing the HHV [55]. This adjustment promotes condensation-polymerization of dissolved organic compounds and inhibits the incorporation of inorganic products. Similar results have been observed by research indicating that manipulating the pH of the feed water using acetic acid can reduce the ash content and increase the fixed carbon in the hydrothermal carbonization (HTC) process. However, acetic acid favored the leaching of significant amounts of mercury, iron, calcium, and aluminum into the liquid phase [58].
The improvement in hydrochars due to its production under an acidic pH can also be attributed to the catalytic role of hydronium ions (H3O+), which accelerate hydrolysis and intensifies the dehydration of organic materials [57]. In non-catalyzed HTC processes, temperature is an even more crucial factor. For example, observations indicated that temperature was the most determining parameter in controlling the process, with higher temperature accelerating the carbonization reactions, resulting in hydrochars with lower H/C and O/C ratios [59]. These results highlight the relevance of optimizing the parameters in the HTC to maximize the efficiency and quality of the hydrochars. The continuity of this research is essential to improve the applications of hydrochars as alternative biofuels, contributing to more sustainable energy solutions.
1.3. Economic Viability of HTC of Sewage Sludge
The economic viability of the sewage sludge treatment through HTC is a critical issue that can limit its development. Such treatment has been considered unprofitable without public subsidies [60]. In this context, although HTC technology is a potential alternative for sewage sludge treatment, there are challenges to be overcome.
Hydrochar has a high energy density, is transportable, and is potentially renewable, which can be used to feed the same hydrocarbonization process. However, its potential for use decreases when it is produced under unsuitable conditions, such as low temperature, resulting in a hydrochar with high nitrogen, sulfur, and ash contents. Therefore, for HTC to be economically viable, it is necessary to reduce hydrochar production costs through combined processes using catalysts and energy recovery, as reported in the literature [27]. For example, one study evaluated the economic benefits of HTC based on the potential revenues from heat, power, hydrochar and struvite in an integrated system. In addition, it highlighted a potential economic benefit, emphasizing the process as a critical component of the entire feasibility assessment [61].
Integrating HTC with anaerobic digestion has also improved energy recovery and reduced environmental impact. One study showed that energy recovery from sewage sludge increased from 14% to 28%, while global warming potential was reduced from 72 to 18 kg CO2-eq per ton of sludge [60]. Although HTC uses less energy than a thermal dryer, its energy advantages result in negligible investment opportunities due to the high initial investment cost [32].
Thus, hydrothermal carbonization technology offers a promising route for sustainable sewage sludge management, combining environmental and energy benefits. However, economic viability still faces critical challenges, as demonstrated by the high costs and market value of hydrochar. To overcome these barriers, it is necessary to adopt strategies to reduce production costs and improve the quality of hydrochar, such as an integrated approach, combining HTC with other technologies such as anaerobic digestion, the use of catalysts for the process, and the implementation of financial support policies that can offset the high initial investment costs. Scientific advances in HTC and its economic viability in sewage sludge treatment will be essential to maximize the economic and environmental benefits of this innovative technology.
In view of the above, the main goals of this study are: (1) To confirm critical research points in HTC; (2) To collect the main results and findings in the HTC process variables; (3) To assess countries’ contributions in the area of HTC research; (4) To present future outlooks on HTC applied to sewage sludge treatment. The results obtained in this study provide information on the large area of research involving HTC.
2. Methods
Concerning the bibliometric aspects of the research, a systematic review of the literature guided based on PRISMA guidelines (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) was performed [62].
2.1. Review Implementation
2.1.1. Data Collection
Scopus metadata corresponding to 8,116 documents were retrieved using the keyword combination “Hydrothermal Carbonization” + “Sewage Sludge”. The types of documents retrieved were articles (6,102), analyses (1,495), book chapters (319), conference papers (133), books (43), editorials (13), short communications (3), observations (3), retractions (2), errata (2), and data papers (1). The study period for metadata surveying was limited from January 1, 2013, to August 3, 2023. After this first overview, 47 studies were selected considering the most significant “results” and “conclusions” from the literature, taking into account the process parameters (temperature and time). For this second step, the studies were retrieved from the CAPES Platform, the main Brazilian virtual library [63]. The criteria used for this were (a) studies published in the last 10 years, and (b) studies containing empirical data. In addition, the Skimmimg method was used to review the content of each article retrieved to align the information extracted based on (a) focus of study (gas phase, aqueous phase, solid phase); (b) parameters evaluated by the studies (temperature, time, pH, phosphorus recovery, addition of catalysts, mineral behavior, organic behavior, Co-HTC, combustion or energy potential); (c) abstracts; (d) main results; (e) conclusions; (f) access links.
2.1.2. Data Analyses
The bibliometric analysis process was based on five stages: study design, data collection, data analysis, data visualization, and interpretation [64]. Two pieces of software were used to this: “Vosviewer”, which was developed in the Java programming language and is specialized for creating maps based on network data [65]; the second piece of software is “Biblioshiny”, which runs on a package based on the “R” programming language named “Bibliometrics”. The latter allows the two packages to be combined in an environment capable of creating an online data analysis structure generated on the website.
Vosviewer was used to create networks to specifically analyze word competition (titles, abstracts), using the abstracts and keywords of the 8,116 documents found as input data, with the aim of understanding which are the critical research points and topics of most significant interest to researchers in the period analyzed. On the other hand, Biblioshiny was used to extract information for the supplementary material and carry out analyses and graphs such as the keyword competition tree. In addition to the Bibliometrix package in the R programming environment, other packages, such as ggplot2, were used to visualize the information.
2.2. Disseminate and Report the Results of the Analysis
The results of this study have been grouped and presented under the following headings: Results of the bibliometric analysis; Focus of study in sewage sludge HTC research; findings on the main parameters in the sewage sludge HTC process; additives for the sewage sludge HTC process.
3. Results and Discussion
3.1. Bibliometric Analysis
3.1.1. Publications on the HTC
The data considered in the following discussion was obtained from 8,116 documents extracted from the Scopus database corresponding to 2013 – 2023. Fig. S2a shows the annual number of publications follows an increasing trend, which indicates high levels of scientific knowledge research about HTC. From 2020 to 2023, there has been more significant growth than in previous years; in 2022, the rate of research and publication of scientific works increased by almost 50% compared to 2020. By August 2023, the number of publications had already reached 2021, which shows a promising perspective for maintaining the growth trend in the number of publications on the HTC topic. In addition, Fig. S2a shows that most publications are scientific articles, followed by reviews and book chapters, among the other documents, representing a small part of the total number of documents published. Fig. S2b shows the number of documents per year by language, in which English has been suppressed to allow a more precise visualization of the behavior of other languages on the HTC process. Therefore, this study showed that between 90 and 96% of all published studies were written in English. However, the second most published language is Mandarin, and it can also be seen that there was a remarkable increase in publications in Mandarin in 2021 compared to other years, increasing from 30 publications in 2020 to almost 60 publications in 2021. Furthermore, since 2018, studies on HTC have begun to be published in alternative languages, such as Korean, Portuguese, and Spanish.
3.1.2. Global network of research on HTC
In terms of contribution per country (Fig. 2), the publications on the HTC topic show a remarkable predominance of Chinese works. The collaborations with the most vital links in the case of China are specifically with the United States, Singapore, Hong Kong, and South Korea, generating cluster number 1 in this research network. The United Kingdom forms cluster 2, representing a solid connection with most European countries, America, and the Middle East. Cluster 3 is generated by Australia, forming a solid connection with Pakistan, Canada, and Bangladesh. Finally, India generates Cluster number 4 with solid links, especially with Asian countries such as Japan, Malaysia, Egypt, and Taiwan.
3.1.3. Cluster and competition analysis
Cluster and competition analysis of keywords, titles, and abstracts reflect the critical points of HTC research. This study collected 32,380 keywords and 111,883 words between titles and abstracts to build trees and competition networks. Fig. S3 (Supplementary Materials) was created by the Biblioshiny software, considering only the keywords of the documents; the algorithm was configured to show only the 25 most frequently used keywords. The five words with the most competition in the HTC search field are “Adsorption”, “Pyrolysis”, “Biomass”, “Biochar” and “Carbonization” with 9%, 8%, 6%, 6%, respectively. This shows that the phenomenon of adsorption of elements in the solid phase of the HTC process is the most researched part of the studies, corroborated by the keyword biochar, which is the solid product of the HTC process. Although pyrolysis is a different technology regarding HTC, it is the second word of most interest to researchers, showing a tendency to compare these two processes. Pyrolysis and hydrothermal carbonization have factors in common, such as temperature, that play an essential role in chemical reactions. However, the former is carried out under anoxic conditions, and the second in water. In addition, pyrolysis uses higher temperatures to create valuable products by partial loss of carbon; conversely, hydrothermal carbonization aims to increase the carbon content in the material [24,66]. Biomass is the third most cited word, but the competing keyword shows that researchers are interested in studying, for example, its transformation over the reaction time and the solid, liquid, and gaseous composition derived from the transformation of the biomass input into the HTC reactor. As mentioned above, biochar and carbonization appear as the fourth and fifth keywords, respectively, and they also show a high level of interest among researchers in solid products derived from the HTC process to study their composition and assess their potential for energy recovery. Curiously, the keyword hydrochar is not present in TreeMap (Fig. S3), suggesting that researchers have considered biochar to be a keyword equivalent to hydrochar and representative of the solid product produced in the HTC process.
Fig. 3 shows a competition analysis of the words in the titles and abstracts of the 8,116 documents in the Scopus database. It shows 4 clusters identified by the different colors on the map. The algorithm applied to this network was based on normalization using a strength of association method. This method is proportional to the ratio between the observed number of co-occurrences of objects and the expected number of co-occurrences of objects under the assumption that the occurrences are statistically independent [67].
The first green cluster is dominated by research into “Biochar”, a situation in which this topic is associated with removal mechanisms, adsorption of pollutants, pesticides, minerals, and antibiotics, and the study of the morphology of the biochar product through spectroscopy and SEM images, among others. The second cluster, the one pink, has high competition in the word “Hydrochar” associated with other fields of research, such as the kinetics and yield of hydrochar, carbon content as well as some process variables, such as temperature, input mass ratio, pressure, and combustion characteristics of hydrochar. This result also suggests that the word hydrochar has been used in titles and abstracts, which differs from what was found for keyword preferences. Cluster 3 (brown) has a predominance of the word “Technology”, thereby associating other lines of research such as energy recovery and conversion, co-digestion process, additional processes to the HTC process, such as anaerobic digestion, the study of life cycle analysis, also involving the industrial component with a link to development, applications of biochar and the relationship with climate change. Finally, the smallest yellow cluster shows a predominance of research into heavy metals about environmental risks, the soil, and the bioavailability of nutrients, among others.
Fig. 3 shows a notable interest in the solid product resulting from the hydrothermal carbonization (hydrochar) and pyrolysis (biochar) processes, highlighting its potential as materials useful for fuel and adsorption. However, it is observed that the liquid phase of HTC has received less attention than solid product. Since most of the product obtained after the hydrothermal process is liquid, it is crucial to investigate its potential use and applications. Studies focused on evaluating the environmental impact and toxicity of this liquid phase are essential for its acceptance as a biofertilizer, for instance. Approaching these areas of research will not only contribute to a better understanding of the liquid phase of HTC, but also it will promote its integration into sustainable waste management and agricultural practices.
3.2. Studies Focused on HTC of Sewage Sludge
Table 1 shows that among the 47 studies selected in this systematic review, 70.21, 91.49, and 17.02% are related to the aqueous, solid, and gaseous phases, respectively, resulting from the HTC process. For the variables addressed by the researchers, 70.21, 34.04, and 29.79% considered the variation of temperature, residence time, and pH, respectively. In addition, researchers studied the behavior of phosphorus, minerals (including heavy metals in some studies), and organic composition in 29.79 and 46.81%, respectively, of the total works considered. Furthermore, other parameters are recurrent, such as adding catalysts to the reaction and raw materials other than sewage sludge to the process (Co-HTC), with a percentage of 36.17 and 23.40%, respectively. Finally, it was found that among the 76.6% of the studies focused on the solid product, only 51.06% evaluated the energy potential of the hydrochars obtained.
Although several studies have explored the individual effects of temperature, time, pH, and solid/liquid ratio on hydrothermal carbonization (HTC), few investigations analyzed the synergistic interaction between these parameters. Future research employing multifactorial experimental designs or sensitivity analysis to optimize operating conditions more efficiently could provide more accurate results, achieving optimal characteristics for solid, liquid, and gas phases, promoting a more practical application of this technology.
3.3. Findings on Key Process Parameters in HTC of Sewage Sludge
Table 2 provides information on the temperature values and residence time evaluated in each of the 47 selected studies. The highest and lowest temperatures evaluated by the studies were 120 and 350ºC, respectively. The highest and lowest residence times were 0.25 and 24 h, respectively. The moisture content of the sewage sludge used in the studies was also presented. In comparison, 21.3% of the studies used Co-HTC, and 78.7% approached the HTC process.
The composition of biomass derived from sewage sludge and other organic wastes is highly dependent on several variables ranging from the intrinsic characteristics of the feedstock to the specific operating conditions during the hydrothermal carbonization process. Variables such as temperature, reaction time, feed water pH, and solid/liquid ratio play a crucial role in determining the physicochemical properties of the resulting products. In this context, Table 3 summarizes the critical parameters and their effects on the biomass based on the results found in the scientific literature.
3.3.2. HTC-temperature
This review highlights temperature as a critical variable in the HTC process and proposes a critical discussion on their effects. Fig. 4 presents an overview on process temperature and some parameters relevant to the quality of hydrochars. The following subsections detail each topic covered in Fig. 4.
3.3.2.1. Temperature effects on hydrochar
Several effects have been found on the hydrochars with transient increases in temperature up to the critical point that allows the formation of the optimum hydrochar in terms of composition, moisture content, and energy availability. Wang et al. [83] showed that increasing the temperature can be more effective than increasing the residence time to maximize the coalification of the hydrochar. They found an optimum reaction range between 190–220ºC with residence times between 1 and 2 h. Other studies coincide in the ranges of operating variables. Other studies coincide in the ranges of operating variables. Several studies have aligned concerning operational parameter ranges. For example, Wang et al. [56] adopted a milder approach, employing conditions of 160ºC and 2 h. However, some studies emphasize that the optimal temperature for the process is related to the chemical composition of the sewage sludge. This was reported by Zhai et al. [108], who found that achieving hydrochar with a high energy yield, even from sewage sludge with substantial ash content, required temperatures exceeding 220ºC. This observation was supported by Wang et al. [88], who determined 230ºC as the optimal temperature for hydrochar production using municipal sewage sludge with elevated ash content. Although some studies have identified even higher optimal temperatures, they have also integrated additional technical considerations to enhance comprehension of the hydrothermal carbonization process. Cavali et al. [18], studied co-HTC of residual pine sawdust and sewage sludge and found that the hydrochar presented a higher calorific value and a greater surface area at 250ºC. Nevertheless, they emphasized that 260 ºC is the ideal temperature for hydrochar production.
The reaction temperature significantly influences the hydrochar yield and its moisture content. Most studies report a decrease in hydrochar yield with increasing temperature. However, several researchers have monitored the effects of increasing the temperature above the optimum points on the content of fixed and total carbon, phosphates, functional groups, and ions, among others. Wang et al. [56] and Yang et al. [98] studied the effects of increasing the temperature in the range of 160–280ºC. They observed a decrease in the hydrochar yield from 91.23 to 57.92% and from 77.5 to 65.64%, respectively, and a decrease in volatile compounds from 59.38 to 37.27% and from 45.59 to 12.74%, respectively. In addition, the hydrochars showed an increase in the fixed carbon content from 9.20 to 17.25% and from 1.89 to 4.95%, respectively. The difference between these two studies is that the former implemented the co-hydrocarbonization technique by adding pine sawdust, and the second did not use blends, just sewage sludge. Notably, the first study obtained a more significant decrease in the hydrochar yield due to the high amount of organic matter in the process. Other studies have extended the temperature evaluation range up to 300ºC [68,85,108]. Obviously, the temperature increase must be limited in order to prevent unwanted chemical reactions, loss of hydrochar quality, and pushing the HTC process into a condition of energy unfeasibility.
3.3.2.2. Temperature effects on organic and nitrogen content
The main components of organic matter, that is, protein, lipid, lignin, cellulose, and hemicellulose, have often been studied concerning temperature variations in the HTC process. Xu et al. [27] and Yang et al. [98] showed a significant decrease in the protein content with increasing temperature up to 240ºC, which displayed a correlation with the increase in ammonium ion (NH4+) content. Critical temperatures for the degradation of biomass proteins have also been found to increase the content of soluble and hydrolyzed proteins in the aqueous phase; Zheng et al. [79] found an optimal decomposition for proteins into amino acids at 230°C. Xie et al. [103] reported that lipids could be a potential substance for improving the yield of hydrochar above 210ºC, a temperature in which lipids are less prone to carbonization. In addition, Zheng et al. [79] associated a significant decrease in the carbon content of a hydrochar produced at 280ºC with the degradation of lipids into gases, alcohols, and insoluble aldehydes. Regarding lignin, it was found that it begins to decompose at a temperature above 250ºC, which may be the limiting temperature for this approach, contrary to cellulose, which initiates dissolution at 180ºC, at 230ºC, it swiftly undergoes decomposition, giving rise to refractory aromatic hydrocarbons [18,114]. Subsequently, at 330ºC, complete solubilization occurs. It was also found that cellulose can capture and retain soluble nitrogen compounds in the range of 210–240ºC. It was found that hemicellulose begins to decompose above 160ºC and has an outstanding contribution to hydrochar in the range of 210–240ºC [18,103].
Usually, nitrogen compounds are studied because they are present in sewage in different ways and show dynamic behavior throughout chemical reactions. Most studies have observed significant changes in nitrogen content in the range of 210–240ºC, increasing the proportion between soluble and insoluble nitrogen species. Marin-Batista et al. [97] showed a significant increase in nitrogen-bearing species in the aqueous phase, such as amines, pyrimidines, and pyrazines, formed from the hydrolysis of proteins and carbohydrates. Yang et al. [98] reported a slight decrease in pyrrole content from 31.71 to 27.32% and an increase in pyridine-based compounds in the range of 200–240ºC. Similarly, Wang et al. [83] and Zheng et al. [101] found that the yield of pyridine increased significantly, while pyrrole decreased in the range of 190–220°C, probably because part of the pyrrole was converted into pyridine. However, pyrrole content began to increase in the range of 220–250°C due to the carbonyl-ammonia reaction. In contrast, research indicates that with elevated temperatures, the translocation of nitrogen to the solid phase becomes more pronounced, leading to complete hydrolysis of inorganic nitrogen within the hydrochar in the liquid phase.
3.3.2.3. Temperature effects on mineral content
The increase in temperature for the HTC process promotes a series of transformations in the biomass, generating free radicals, new chemical species, and minerals. Several authors have studied these changes for minerals, such as phosphorus and heavy metals, and pharmaceutical compounds that can become part of conventional sewage sludge [84]. According to Zhu et al. [102], environmentally persistent free radicals (EPFRs) can occur at relatively low temperatures compared to pyrolysis. Due to the process of hydrolysis and cleavage of biomass by subcritical water supported by temperature increase, EPFRs can be produced from the breaking of surface bonds of biomass. The formation of free radicals can also oxidize minerals such as aliphatic and aromatic sulfur, among others [88].
Additionally, the increase in temperature is crucial for the redistribution of heavy metals in the solid-liquid phases [46,115]. Under high temperatures and pressure, heavy metals in the ionic form leach easily into the liquid phase, but many factors reduce leaching and promote the immobilization of heavy metals in the solid phase [68,116]. Some minerals containing calcium and magnesium, as well as alkaline metals and alkaline-iron metals, can be adsorbed on hydrochar under high temperatures (260°C) [68,88]. Additionally, Wang et al. [88] found that above 290ºC, alkaline or alkaline-earth minerals such as albite and dolomite can be removed from hydrochar and can also form new mineral species. Other minerals, such as those containing phosphorus, are commonly studied due to their high content in sewage sludge, appreciable economic value, and potential to be applied in fertilizer formulation. Several studies have shown that increasing the HTC process temperature leads to the enrichment of inorganic phosphorus in the hydrochar due to the presence of free radicals that increase the decomposition of phosphorus-based compounds [18,117,118]. It was reported that HTC can exert a dephosphorylating effect on water, fixing phosphorus in the solid phase in the range of 180–240ºC. However, when the temperature rises to 270–300ºC, the phosphorus concentration in the hydrothermal liquid increases [68]. It has also been found that at very high temperatures, some of the phosphorus from the reaction is released as phosphorus oxide [119].
3.3.3. HTC residence time
Residence time, or reaction time as it is commonly designated by several authors, is another critical parameter on hydrothermal carbonization of sewage sludge. Fig. S4 highlights the main effects of the residence time on hydrochars. The following subsections detail each topic covered in Fig. S4.
3.3.3.1. Residence time effects on hydrochar
Regarding the effects of the residence time on the hydrochar characteristics, it has been reported that a short reaction time associated with a moderate temperature is recommended to achieve maximum carbon yields in the hydrochar [48,90,120]. Conversely, other studies have found that a long reaction time at a high temperature (>260ºC) leads to the production of a hydrochar with higher ash content [56,69], which represents a disadvantage for generating an efficient combustion material [95]. However, longer times at higher temperatures allow intense dehydration of the biomass during the HTC process, which decreases the availability of the hydroxyl (-OH) group [71]. Due to the combination of long time and low temperature, strong effects were found on the partitioning nutrients in hydrochar [121].
3.3.3.2. Residence time effects on mineral content
Literature reports that extending the reaction time and increasing the temperature can disintegrate organic phosphorus, thereby confirming that this procedure was the most influential factor for the transformation of this element [95]. Tangredi et al. [122] evaluated whether a low temperature with a longer time can have the same effect on phosphorus as a shorter time associated with a higher temperature. However, they were unable to affirm this hypothesis based on their results. The studies showed a high dependence between reaction time and behavior of the heavy metals, including discrepancies between different metals. It was reported that the concentrations of cadmium and chromium in the hydrochar increased almost twofold with increasing reaction time but decreased for copper and zinc [123]. A more favorable increase in zinc content in hydrochar was reported due to the increase of reaction time. On the other hand, for lead and cadmium contents, it was found a solid correlation with residence time [124].
3.3.3.3. Residence time effects on nitrogen content
Changes in the nitrogen content of the sewage sludge due to variations in carbonization residence time have also been studied. In the literature, it is possible to note that increases in residence time are associated with increases in nitrogen content in the hydrochar due to the continuous fixation of this element, in addition to the transformation of nitrogen into more stable forms. However, Chen et al. [7] concluded that carbonization time mainly affects the distribution of nitrogen, different from temperature, which mainly determines the form of nitrogen. They also reported similar effects between increasing temperature and increasing carbonization time, as both increased the concentration of ammonium ion (NH4+). Sarrion et al. [125] showed that the increase in NH4+ content occurs mainly due to organic nitrogen transformation and to the increase in reaction time, probably due to the gradual hydrolysis of dissolved protein. The increases in nitrogen content and nitrogen retention rates in the hydrochar can occur due to prolongation of residence time, mainly reflecting an increase in the formation of pyrrole-N species [83]. It was also found that short carbonization times and low temperatures resulted in an effluent with a high cumulative methane yield due to the low rate of formation of toxic components for methanogenic bacteria [56], and for a long time and a low temperature, the nitrogen in the hydrochar is transferred to the liquid phase, due to the slow hydrolysis of proteins [7].
3.4. Additives for the HTC Process in Sewage Sludge
The addition of catalysts in the HTC process not only increases the conversion efficiency of the raw material, but their use can also help to immobilize even more diverse products in the HTC process [123]. This review highlights the use of additives or catalysts in the HTC process for immobilizing heavy metals, process accelerators (hydrolysis, dehydration, and decarboxylation reactions), and the transformation of phosphorus and nitrogen (Fig. 5). Xu and Jiang [68] reported that adding FeCl3 and Al(OH)3 increased the exchangeable states of zinc, lead, chromium, and cadmium and showed a decrease in their residual forms. Furthermore, they observed that adding these catalysts promoted macromolecular and solid organic matter decomposition and hydrolysis.
The addition of sodium chloride (NaCl) promoted hydrolysis, dehydration, and decarboxylation, affecting the properties and yield of the hydrochar [82]. Similarly, the use of layered double oxides (LDO) promoted the decarboxylation, dehydration, and denitrification of the hydrochar, thus removing the nitrogen content in the hydrochar by approximately 65%. In addition, it was shown that with the increase in temperature and the addition of LDO, the organic nitrogen in the hydrochar decreased, suggesting that this addition can release and promote the decomposition of proteins [77]. Likewise, Huang et al. [91] also showed increases in the removal of nitrogen in the hydrochar with the addition of acetates. However, some researchers have studied the retention of nitrogen in hydrochar; for example, Wang et al. found that the addition of starches and xylans favored nitrogen retention between 36.8 and 50.9%, respectively, concluding that the synergy between sewage sludge, food waste and the addition of starches and xylans could prepare nitrogen-rich materials or clean-carbon-rich fuels [93].
The transformation of phosphorus through the addition of calcium (CaCl2 and CaO) was studied, thus showing retention and bioavailability of phosphorus in the hydrochar, stimulating an almost complete transformation of non-apatite inorganic phosphorus into apatite phosphorus [75,106]. Likewise, the addition of nitric acid amplified phosphate recovery, in addition to cleaning exhaust gases by indirectly removing hydrogen sulfide [78]. Similarly, Malhotra and Garg [107], who studied the recovery of resources from the liquid phase and the thermal behavior of hydrochar obtained from the HTC process, found that the addition of ammonium sulfate as a salting agent would influence the recovery of phosphates in the liquid phase as struvite; in addition, they showed that the recovery of humic acid with the addition of ammonium sulfate improved plant growth after its application.
In summary, the retention or reduction of nutrients in the solid or liquid phases resulting from the hydrothermal carbonization reaction of sewage sludge can be improved by adding different compounds, thus allowing further use and reducing different environmental impacts.
4. Future Outlooks of HTC Applied to Sewage Sludge Treatment
The hydrothermal carbonization process appears as a powerful green technology for the management of sewage sludge based on a range of possibilities directly related to the UN’s sustainable development goals. Indeed, a deeper understanding of the mechanisms that rule its complex chemistry and its dissemination across the world will contribute to its competitiveness concerning the conventional sewage treatment used in WTTPs and also in comparison to incineration, which is a destructive way of treating residual biomass.
From a more optimistic point of view, the HTC technology could bring disruptive innovation to wastewater and sewage sludge treatment, considering a typical WWTP as an open-loop recycling station capable of generating important sustainable products. This novel business model will be fundamental to boosting the economy of developing countries with high domestic sewage generation, which should also favor public and private investments for the construction of industrial plants and increase access for communities in vulnerable conditions to environmental sanitation.
In this sense, an essential reason for the eventual success of HTC technology in the coming years lies in its flexibility for coupling to other industrial processes in a circular economy context. For instance, HTC technology could directly favor the viability of biorefineries due to its potential to support the energy supply or even the production of biofertilizers and valuable precursors.
Biorefineries face the challenge of overcoming highly endothermic routes, which require high energy (heat) to satisfy energy demand. The availability of renewable solid fuel with a high calorific value, such as hydrochar, gives HTC technology significant relevance within the energy transition in the next few years.
Some examples of this type of perspective have been reported in the literature. Important synergies can be achieved when an HTC plant is employed as a central hub for a regional biorefinery, making available an alternative solid fuel (hydrochar), which is less energy-demanding than drying of the raw biomass and pressed into pellets or briquettes; the ashes from the burning of fuel can be a source of nutrients as phosphorus [126]. Likewise, Cavali et al. [8] reviewed the hydrothermal carbonization of potential biomass waste, including sewage sludge, the characterization and environmental utilization of hydrochar, and the biorefinery potential of this process. Among the various works cited, there is a consensus on the preference for using primary sewage sludge instead of secondary one if the focus is the use of hydrochar as a solid biofuel because the ash content increases throughout the organic matter digestion, providing a lower energy amount per mass unit.
The use of primary sewage sludge corroborates the perspective of replacement of a conventional WWTP for an innovative business model even more robustly than well-stablished technologies such as incineration, which, although it is an alternative to WWTP, does not allow the reuse of the various compounds contained in residual biomass.
Finally, the choice of a specific route, focused on the optimized production of just one of the three phases generated by HTC, depends on a set of information, such as type of biomass available, flexibility of local environmental regulations, market trends, political scenario, which can generate exceptional demands due to warfare or armed conflicts, among others.
5. Conclusions
This review has compiled important information on hydrothermal carbonization research and its main parameters. According to the bibliometric results, China has the most research on HTC; the prominent global research focuses are centered on the solid product and the mechanisms of removing and absorbing nutrients, minerals, and pollutants. The systematic review of the 47 selected studies showed that liquid, solid, and gaseous products were studied in 70.21, 91.49, and 17.02%, respectively; in addition, the variables most studied by researchers in HTC were changes in temperature, residence time and pH with a percentage of 70.21, 34.04, and 29.79% respectively. This review detailed the effects and findings about reaction temperature and residence time in HTC with hydrochar yield, organic content, and mineral content. The hydrochar yield and organic matter content consistently decreased with increasing temperature and residence time. At the same time, minerals such as heavy metals showed that increasing temperature and residence time are crucial for redistributing heavy metals in the solid-liquid phases. The incorporation of various compounds has been extensively investigated in the literature, primarily targeting the immobilization of heavy metals and the retention and recovery of nitrogen and phosphorus. However, the hydrothermal carbonization of sewage sludge, particularly Co-HTC, merits increased attention due to its substantial potential for product utilization. This review endeavors to provide a comprehensive overview of the current state of HTC research and the key process variables involved.
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