Hydrothermal Carbonization as a Strategy for Sewage Sludge Management: Influence of Process Withdrawal Point on Hydrochar Properties

Anaerobic valorization of sewage sludge pretreated through hydrothermal carbonization: Volatile fatty acids and biomethane production

Hydrothermal carbonization (HTC) emerged as an effective technology for the treatment of various types of wet biomass and organic residues, including sewage sludge, offering the potential for sludge reduction and resource recovery. HTC pretreatment impact on downstream sludge fermentation is investigated. Results obtained at optimal conditions for HTC pretreatment (170 °C for 30 min) indicated that soluble carbon was significantly increased in the liquid fraction, enhancing feedstock availability for fermentation. Semi-continuous fermentation of HTC-treated sludge resulted in a stable process in which a mixed microbial community produced volatile fatty acids (VFAs) with longer chain acids content, acidification yield of 0.59 ± 0.05 g COD-VFA g-1 CODin and volumetric productivity of 1.6 ± 0.5 g COD-VFA L-1 d-1. Biomethane Potential tests evidenced high values for hydrochar. Overall, the HTC pretreatment enables improved conversion efficiencies, in the view of valorizing the liquid for VFA synthesis and the hydrochar for biomethane production.

Effect of thermochemical treatment of sewage sludge on its phosphorus leaching efficiency: Insights into leaching behavior and mechanism

Thermochemical conversion, including hydrothermal processing, pyrolysis and incineration, has become a promising technology for sewage sludge (SS) treatment and disposal. Furthermore, acid leaching is considered as an effective method to recover phosphorus (P) from SS and its thermochemical treatment products. This study has investigated the potential of P reclamation from SS and its thermochemical derivatives, including hydrochar (HC), biochar (BC), and SS incinerated ash (SA). Comparative analyses of physicochemical properties of these derivatives revealed a decrease in hydroxyl and aromatic groups and an increase in aliphatic and oxygen-containing functional groups in HC and BC. Leaching experiments using 1 M sulfuric acid (H2SO4) and 1 M oxalic acid (C2H2O4) suggested that H2SO4 slightly outperformed C2H2O4 in terms of P leaching efficiency. HC achieved 79.1 % optimal leaching efficiency in 60 min using H2SO4, while BC, SS, and SA required 360 min to achieve comparable efficiency. SS and BC reached optimal leaching efficiency at 74.1 % and 76.2 % in H2SO4, while SA achieved 80.9 % in C2H2O4. Importantly, HC and SA are more favorable for P extraction using acid leaching, whereas BC tends to be a potential P carrier. Time-dependent kinetics revealed a two-stage leaching process, i.e., fast and slow reaction stages. Shrinking core model indicates product layer diffusion as the primary rate-limiting step in both stages. Overall, these fundamental insights play an important role in practical P recovery through acid leaching of SS derived residues after thermochemical treatment.

Co-hydrothermal carbonization of municipal sludge and agricultural waste to reduce plant growth inhibition by aqueous phase products: Molecular level analysis of organic matter

The organic matter molecular mechanism by which combined hydrothermal carbonization (co-HTC) of municipal sludge (MS) and agricultural wastes (rice husk, spent mushroom substrate, and wheat straw) reduces the inhibitory effects of aqueous phase (AP) products on pak choi (Brassica campestris L.) growth compared to HTC of MS alone is not clear. Fourier-transform ion cyclotron resonance mass spectrometry was used to characterize the differences in organic matter at the molecular level between AP from MS HTC alone (AP-MS) and AP from co-HTC of MS and agricultural waste (co-Aps). The results showed that N-bearing molecules of AP-MS and co-Aps account for 70.6 % and 54.2 %-64.1 % of all molecules, respectively. Lignins were present in the highest proportion (56.3 %-78.5 %) in all APs, followed by proteins and lipids. The dry weight of co-APs hydroponically grown pak choi was 31.6 %-47.6 % higher than that of the AP-MS. Molecules that were poorly saturated and with low aromaticity were preferentially consumed during hydroponic treatment. Molecules present before and after hydroponics were defined as resistant molecules; molecules present before hydroponics but absent after hydroponics were defined as removed molecules; and molecules absent before hydroponics but present after hydroponics were defined as produced molecules. Large lignin molecules were broken down into more unsaturated molecules, but lignins were the most commonly resistant, removed, and produced molecules. Correlation analysis revealed that N- or S-bearing molecules were phytotoxic in the AP. Tannins positively influenced the growth of pak choi. These results provide new insights into potential implementation strategies for liquid fertilizers produced from AP arising from HTC of MS and agricultural wastes.

Enhanced energy and nutrient recovery via hydrothermal carbonisation of sewage sludge: Effect of process parameters

Integration of waste management with energy and resource recovery is being widely explored to achieve sustainability. To achieve this, sewage sludge was treated with hydrothermal carbonisation (HTC) at temperatures ranging from 180 °C-260 °C with an increment of 20 °C for three different duration of 1 h, 3 h, and 5 h. The energy and resource recovery potential of the HTC treatment was evaluated through of hydrochar (HC) and process water (PW) properties. Dehydration and decarboxylation reactions resulted in reduced H/C and O/C atomic ratios of 1.35 and 0.45 respectively in HC-260-3, exhibiting peat-like propertied. The calorific value of HC-260-5 was enhanced to 5.9 MJ/kg (increase of 25.8 %) due to the combined effect of H/C and O/C atomic ratios, increased volatile organics and fixed carbon. A maximum energy recovery efficiency of 82.44 % was realised at 240 °C for 3 h rendering it the optimal process condition to ensure energy enrichment. Thermogravimetric analysis (TGA) of HC samples indicated an enhanced combustion behaviour with an increased HTC severity. The elevated levels of volatile fatty acids (VFAs) in PW (maximum 2296 mg/L) made it viable for energy recovery in anaerobic digestion units. Additionally, the PW contains significant concentrations of N and P (2091.68 mg/L and 40.51 mg/L, respectively), indicating enhanced resource/nutrient recovery potential.

Recent Advances in LDH/g-C3N4 Heterojunction Photocatalysts for Organic Pollutant Removal

Environmental pollution has been decreased by using photocatalytic technology in conjunction with solar energy. An efficient method to obtain highly efficient photocatalysts is to build heterojunction photocatalysts by combining graphitic carbon nitride (g-C3N4) with layered double hydroxides (LDHs). In this review, recent developments in LDH/g-C3N4 heterojunctions and their applications for organic pollutant removal are systematically exhibited. The advantages of LDH/g-C3N4 heterojunction are first summarized to provide some overall understanding of them. Then, a variety of approaches to successfully assembling LDH and g-C3N4 are simply illustrated. Last but not least, certain unmet research needs for the LDH/g-C3N4 heterojunction are suggested. This review can provide some new insights for the development of high-performance LDH/g-C3N4 heterojunction photocatalysts. It is indisputable that the LDH/g-C3N4 heterojunctions can serve as high-performance photocatalysts to make new progress in organic pollutant removal.

Introducing hydrothermal carbonization to sewage sludge treatment systems-a way of improving energy recovery and economic performance?

Hydrothermal carbonization (HTC) can mitigate the disposal costs of sewage sludge in a wastewater treatment plant. This study analyzes the impact of integrating HTC with anaerobic digestion (AD) and combustion from a combined energy and economic performance perspective. Net energy balance and investment opportunity are investigated for a number of technical scenarios considering i) different combinations of the technologies: AD + HTC, AD + thermal dryer + combustion, and AD + HTC + combustion, ii) different options for HTC process water treatment: wet oxidation (WO) + AD, and direct return to AD, and iii) different products: heat-only, heat and electricity, hydrochar, and phosphorus. The results show trade-offs between investment cost, self-supplement of heat, and output electricity when WO is used. In AD + HTC, net heat output decreases compared to the reference plant, but avoided disposal costs and hydrochar revenue result in profitable investment when the process water is directly returned to the AD. Although HTC has a lower heat demand than the thermal dryer, replacing the thermal dryer with HTC is only possible when AD, HTC, and combustion are connected, or when WO covers HTC's heat demand. HTC may impair the electricity production because of the necessity for a high-temperature heat source, whereas the thermal dryer can utilize a low-temperature heat source. In conclusion, energy advantages of HTC in AD + HTC + combustion are insufficient to provide a promising investment opportunity due to high investment costs of HTC. The investment opportunity improves by co-combustion of hydrochar and external sludge.

Speciation and transformation of nitrogen for sewage sludge hydrothermal carbonization-influence of temperature and carbonization time

Hydrothermal carbonization (HTC) is an effective means of energizing high-water-content biomass that can be used to convert sewage sludge (SS) into hydrochar and reduce nitrogen content. To further reduce the emission of NO_x during the combustion of hydrochar and seek proper disposal method of liquid product, the mechanism of nitrogen conversion was studied in the range of 180-320 °C and 30-90 min. At 180-220 °C, 42.15-52.91% of the nitrogen in SS was transferred to liquid by hydrolysis of proteins and inorganic salts. At 240-280 °C, the nitrogen in hydrochar was mainly in the form of heterocyclic -N (quaternary-N, pyrrole-N, and pyridine-N). The concentration of NH₄⁺-N increased from 6.82 mg/L (180 °C) to 26.58 mg/L (280 °C) due to the enhancement of the deamination reaction. At 300-320 °C, pyrrole-N (from 15.92% to 9.38%) and pyridine-N (from 5.52% to 3.73%) in the hydrochar were converted to the more stable quaternary-N (from 0.31% to 4.28%). Meanwhile, the NH₄⁺-N and amino-N in the liquid decomposed into NH₃. Prolonging the carbonization time promoted the hydrolysis of proteins, the conversion of heterocyclic -N, and the production of NH₃. Under optimal reaction conditions (280 °C and 60 min), the nitrogen in the SS is converted to stable forms and the energy balance meets the requirements of circular-economy. The results show that temperature determines the nitrogen form and the carbonization time affects the nitrogen distribution. So HTC has the potential to reduce NO_x emissions from SS energy utilization processes.

Co-hydrothermal carbonization of agricultural waste and sewage sludge for product quality improvement: Fuel properties of hydrochar and fertilizer quality of aqueous phase

Hydrothermal carbonization (HTC) is a promising carbon-neutral technology for converting sewage sludge (SS) and agricultural waste into energy. However, HTC-generated aqueous phase (AP) impedes the development of the former. This study investigated the potential of SS with rice husk (RH) and wheat straw (WS) co-HTC to form hydrochar and AP as substitutes for fuel and chemical fertilizer, respectively. Compared with single SS hydrochar, the yield of co-HTC-based hydrochar and higher heating value significantly increased by 10.9%-21.6% and 4.2%-182.7%, reaching a maximum of 72.6% and 14.7 MJ/kg, respectively. Co-HTC improves the safe handling, storage and transportation, and combustion performance of hydrochar. The total nitrogen concentration in AP-SS was 2575 mg/L, accounting for 67.7% of that found in SS. Co-HTC decreased and increased the amine and phenolic components of AP, respectively. AP-SS-RH and AP-SS-WS significantly increased pakchoi dry weight by 45.5% and 49.4%, respectively, compared with AP-SS. The results of the hydroponic experiments with AP instead of chemical fertilizers revealed that AP-SS did not reduce pakchoi dry weight by replacing <20% chemical fertilizers. However, AP-SS-RH or AP-SS-WS replaced 60% chemical fertilizers. Therefore, the co-HTC of SS and agricultural waste increased the AP substitution of chemical fertilizer from 20% to 60%. These findings suggest that the co-HTC of agricultural waste with SS is a promising technology for converting SS into renewable resource products for fuels and N-rich liquid fertilizer while significantly improving fuel and fertilizer quality.

Hydrothermal Carbonization

Over the past decade, hydrothermal carbonization (HTC) has emerged as a promising thermochemical pathway for treating and converting wet wastes into fuel, materials, and chemicals [...]

Hydrochar: A Promising Step Towards Achieving a Circular Economy and Sustainable Development Goals

The United Nations 17 Sustainable Development Goals (SDGs) are a universal call to action to end poverty, protect the environment, and improve the lives and prospects of everyone on this planet. However, progress on SDGs is currently lagging behind its 2030 target. The availability of water of adequate quality and quantity is considered as one of the most significant challenges in reaching that target. The concept of the ‘Circular Economy’ has been termed as a potential solution to fasten the rate of progress in achieving SDGs. One of the promising engineering solutions with applications in water treatment and promoting the concept of the circular economy is hydrochar. Compared to biochar, hydrochar research is still in its infancy in terms of optimization of production processes, custom design for specific applications, and knowledge of its water treatment potential. In this context, this paper critically reviews the role of hydrochar in contributing to achieving the SDGs and promoting a circular economy through water treatment and incorporating a waste-to-value approach. Additionally, key knowledge gaps in the production and utilization of engineered hydrochar are identified, and possible strategies are suggested to further enhance its water remediation potential and circular economy in the context of better natural resource management using hydrochar. Research on converting different waste biomass to valuable hydrochar based products need further development and optimization of parameters to fulfil its potential. Critical knowledge gaps also exist in the area of utilizing hydrochar for large-scale drinking water treatment to address SDG-6.

Indicators for resource recovery monitoring within the circular economy model implementation in the wastewater sector

The European Union is currently in the process of transformation toward a circular economy model in which different areas of activity should be integrated for more efficient management of raw materials and waste. The wastewater sector has a great potential in this regard and therefore is an important element of the transformation process to the circular economy model. The targets of the circular economy policy framework such as resource recovery are tightly connected with the wastewater treatment processes and sewage sludge management. With this in view, the present study aims to review existing indicators on resource recovery that can enable efficient monitoring of the sustainable and circular solutions implemented in the wastewater sector. Within the reviewed indicators, most of them were focused on technological aspects of resource recovery processes such as nutrient removal efficiency, sewage sludge processing methods and environmental aspects as the pollutant share in the sewage sludge or its ashes. Moreover, other wide-scope indicators such as the wastewater service coverage or the production of bio-based fertilizers and hydrochar within the wastewater sector were analyzed. The results were used for the development of recommendations for improving the resources recovery monitoring framework in the wastewater sector and a proposal of a circularity indicator for a wastewater treatment plant highlighting new challenges for further researches and wastewater professionals.

Catalytic co-hydrothermal carbonization of food waste digestate and yard waste for energy application and nutrient recovery

Hydrothermal carbonization (HTC) provides a promising alternative to valorize food waste digestate (FWD) and avoid disposal issues. Although hydrochar derived from FWD alone had a low calorific content (HHV of 13.9 MJ kg^-1), catalytic co-HTC of FWD with wet lignocellulosic biomass (e.g., wet yard waste; YW) and 0.5 M HCl exhibited overall superior attributes in terms of energy recovery (22.7 MJ kg^-1), stable and comprehensive combustion behaviour, potential nutrient recovery from process water (2-fold higher N retention and 129-fold higher P extraction), and a high C utilization efficiency (only 2.4% C loss). In contrast, co-HTC with citric acid provided ∼3-fold higher autogenous pressure, resulting in a superior energy content of 25.0 MJ kg^-1, but the high C loss (∼74%) compromised the overall environmental benefits. The results of this study established a foundation to fully utilize FWD and YW hydrochar for bioenergy application and resource recovery from the process water.

Industrial-Scale Hydrothermal Carbonization of Agro-Industrial Digested Sludge: Filterability Enhancement and Phosphorus Recovery

Hydrothermal carbonization (HTC) provides an attractive alternative method for the treatment of high-moisture waste and, in particular, digested sludge. HTC could reduce the costs and environmental risks associated with sludge handling and management. Although it is recognized that the dewaterability of hydrochars produced from digested sludge, even at mild temperatures (180–190 °C), is highly improved with respect to the starting material, the filterability of HTC slurries for the recovery of the solid material (hydrochar) still represents a challenge. This study presents the results of an investigation into the filterability of agro-industrial digested sludge HTC slurries produced by a C-700 CarboremTM HTC industrial-scale plant. The filterability of HTC slurries, produced at 190 °C for 1 h, with the use of acid solutions of hydrochloric acid, sulfuric acid or citric acids, was investigated by using a semi-industrial filter press. The use of sulfuric acid or citric acid solutions, in particular, significantly improved the filterability of HTC slurries, reducing the time of filtration and residual moisture content. The acid treatment also promoted the migration of heavy metals and phosphorus (P) in the HTC filtrate solution. This study demonstrates that P can be recovered via the precipitation of struvite in high yields, recovering up to 85 wt% by mass of its initial P content.

Hydrothermal carbonisation of mechanically dewatered digested sewage sludge-Energy and nutrient recovery in centralised biogas plant

This study aimed to assess the role of hydrothermal carbonisation (HTC) in digestate processing in centralised biogas plants receiving dewatered sludge from regional wastewater treatment plants and producing biomethane and fertilisers. Chemically conditioned and mechanically dewatered sludge was used as such (total solids (TS) 25%) or as diluted (15% TS) with reject water in 30 min or 120 min HTC treatments at 210 °C, 230 °C or 250 °C, and the produced slurry was filtered to produce hydrochars and filtrates. The different hydrochars contributed to 20-55% of the original mass, 72-88% of the TS, 74-87% of the energy content, 71-92% of the carbon, above 86% of phosphorous and 38-64% of the nitrogen present in the original digestates. The hydrochars' energy content (higher heating values were 11.3-12.2 MJ/kg-TS) were similar to that of the digestates, while the ash contents increased (from 43% up to 57%). HTC treatments produced filtrates in volumes of 42-76% of the dewatered digestate, having a soluble chemical oxygen demand (SCOD) of 28-44 g/L, of which volatile fatty acids (VFAs) contributed 10-34%, and methane potentials of 182-206 mL-CH₄/g-SCOD without any major indication of inhibition. All 32 pharmaceuticals detected in the digestates were below the detection limit in hydrochars and filtrates, save for ibuprofen and benzotriazole in filtrate, while heavy metals were concentrated in the hydrochars but below the national limits for fertiliser use, save for mercury. The integration of HTC to a centralised biogas plant was extrapolated to enhance the annual biogas production by 5% and ammonium recovery by 25%, and the hydrochar was estimated to produce 83 GJ upon combustion or to direct 350 t phosphorous to agriculture annually.

Hydrochar derived from municipal sludge through hydrothermal processing: A critical review on its formation, characterization, and valorization

Additional options for the sustainable treatment of municipal sludge are required due to the significant amounts of sludge, high levels of nutrients (e.g., C, N, and P), and trace constituents it contains. Hydrothermal processing of municipal sludge has recently been recognized as a promising technology to efficiently reduce waste volume, recover bioenergy, destroy organic contaminants, and eliminate pathogens. However, a considerable amount of solid residue, called hydrochar, could remain after hydrothermal treatment. This hydrochar can contain abundant amounts of energy (with a higher heating value up to 24 MJ/kg, dry basis), nutrients, and trace elements, as well as surface functional groups. The valorization of sludge-derived hydrochar can facilitate the development and application of hydrothermal technologies. This review summarizes the formation pathways from municipal sludge to hydrochar, specifically, the impact of hydrothermal conditions on reaction mechanisms and product distribution. Moreover, this study comprehensively encapsulates the described characteristics of hydrochar produced under a wide range of conditions: Yield, energy density, physicochemical properties, elemental distribution, contaminants of concern, surface functionality, and morphology. More importantly, this review compares and evaluates the current state of applications of hydrochar: Energy production, agricultural application, adsorption, heterogeneous catalysis, and nutrient recovery. Ultimately, along with the identified challenges and prospects of valorization approaches for sludge-derived hydrochar, conceptual designs of sustainable municipal sludge management are proposed.

Process Water Recirculation during Hydrothermal Carbonization of Waste Biomass: Current Knowledge and Challenges

Hydrothermal carbonization (HTC) is considered as an efficient and constantly expanding eco-friendly methodology for thermochemical processing of high moisture waste biomass into solid biofuels and valuable carbonaceous materials. However, during HTC, a considerable amount of organics, initially present in the feedstock, are found in the process water (PW). PW recirculation is attracting an increasing interest in the hydrothermal process field as it offers the potential to increase the carbon recovery yield while increasing hydrochar energy density. PW recirculation can be considered as a viable method for the valorization and reuse of the HTC aqueous phase, both by reducing the amount of additional water used for the process and maximizing energy recovery from the HTC liquid residual fraction. In this work, the effects of PW recirculation, for different starting waste biomasses, on the properties of hydrochars and liquid phase products are reviewed. The mechanism of production and evolution of hydrochar during recirculation steps are discussed, highlighting the possible pathways which could enhance energy and carbon recovery. Challenges of PW recirculation are presented and research opportunities proposed, showing how PW recirculation could increase the economic viability of the process while contributing in mitigating environmental impacts.

Hydrothermal Carbonization of Lemon Peel Waste: Preliminary Results on the Effects of Temperature during Process Water Recirculation

Hydrothermal carbonization (HTC) is a promising thermochemical pre-treatment to convert waste biomass into solid biofuels. However, the process yields large amounts of organic process water (PW), which must be properly disposed of or reused. In this study, the PW produced from the hydrothermal carbonization of lemon peel waste (LP) was recycled into HTC process of LP with the aim of maximize energy recovery from the aqueous phase while saving water resources and mitigating the overall environmental impact of the process. The effects of HTC temperature on the properties of solid and liquid products were investigated during PW recirculation. Experiments were carried out at three different operating temperatures (180, 220, 250 °C), fixed residence times of 60 min, and solid to liquid load of 20 wt%, on a dry basis. Hydrochars were characterized in terms of proximate analysis and higher heating values while liquid phases were analyzed in terms of pH and total organic carbon content (TOC). PW recirculation led to a solid mass yield increase and the effect was more pronounced at lower HTC temperature. The increase of solid mass yield, after recirculation steps (maximum increase of about 6% at 180 °C), also led to a significant energy yield enhancement. Results showed that PW recirculation is a viable strategy for a reduction of water consumption and further carbon recovery; moreover preliminary results encourage for an in-depth analysis of the effects of the PW recirculation for different biomasses and at various operating conditions.

Coupling hydrothermal carbonization and anaerobic digestion for sewage digestate management: Influence of hydrothermal treatment time on dewaterability and bio-methane production

Hydrothermal carbonization (HTC) technology is addressed in the framework of sewage digestate management. HTC converts digestate into a stabilized and sterilized solid (the hydrochar) and a liquor (HTCL) rich in organic carbon. This study aims to optimize the HTC operating parameters, namely the treatment time, in terms of hydrochar production, HTC slurry dewaterability, HTCL bio-methane yields in anaerobic digestion (AD), and process energy consumption. Digestate slurry was processed through HTC at different treatment times (0.5, 1, 2 and 3 h) at 190 °C, and the dewaterability of the treated slurries was addressed through capillary suction time and centrifuge lab-testing. In addition, biochemical methane potential (BMP) tests were conducted for HTCL under mesophilic conditions. Results show that by increasing the HTC treatment time the dewaterability was further improved, ammonium concentration in HTCL increased, and methane potential of HTCL decreased. 0.5 h HTCL had the highest bio-methane potential of 142 ± 3 mL CH₄/g COD yet the treatment time was not sufficient for improving the slurry's dewaterability. HTC treatment time of 1 h at 190 °C was identified as the optimum trade-off for improved dewaterability and utilisation of HTCL for biogas production. 1 h HTCL bio-methane potential can cover around 25% of the HTC and AD thermal and electrical energy needs without considering the eventual use of the hydrochar as a biofuel.

Coupling Hydrothermal Carbonization with Anaerobic Digestion for Sewage Sludge Treatment: Influence of HTC Liquor and Hydrochar on Biomethane Production

The present study addresses the coupling of hydrothermal carbonization (HTC) with anaerobic digestion (AD) in wastewater treatment plants. The improvement in biomethane production due to the recycling back to the anaerobic digester of HTC liquor and hydrochar generated from digested sludge is investigated and proved. Mixtures of different compositions of HTC liquor and hydrochar, as well as individual substrates, were tested. The biomethane yield reached 102 ± 3 mL CH4 g−1 COD when the HTC liquor was cycled back to the AD and treated together with primary and secondary sludge. Thus, the biomethane production was almost doubled compared to that of the AD of primary and secondary sludge (55 ± 20 mL CH4 g−1 COD). The benefit is even more significant when both the HTC liquor and the hydrochar were fed to the AD of primary and secondary sludge. The biomethane yield increased up to 187 ± 18 mL CH4 g−1 COD when 45% of hydrochar, with respect to the total feedstock, was added. These results highlight the improvement that the HTC process can bring to AD, enhancing biomethane production and promoting a sustainable solution for the treatment of the HTC liquor and possibly the hydrochar itself.

Hydrothermal Carbonization as a Valuable Tool for Energy and Environmental Applications: A Review

Hydrothermal carbonization (HTC) represents an efficient and valuable pre-treatment technology to convert waste biomass into highly dense carbonaceous materials that could be used in a wide range of applications between energy, environment, soil improvement and nutrients recovery fields. HTC converts residual organic materials into a solid high energy dense material (hydrochar) and a liquid residue where the most volatile and oxygenated compounds (mainly furans and organic acids) concentrate during reaction. Pristine hydrochar is mainly used for direct combustion, to generate heat or electricity, but highly porous carbonaceous media for energy storage or for adsorption of pollutants applications can be also obtained through a further activation stage. HTC process can be used to enhance recovery of nutrients as nitrogen and phosphorous in particular and can be used as soil conditioner, to favor plant growth and mitigate desertification of soils. The present review proposes an outlook of the several possible applications of hydrochar produced from any sort of waste biomass sources. For each of the applications proposed, the main operative parameters that mostly affect the hydrochar properties and characteristics are highlighted, in order to match the needs for the specific application.

Potential Use of Waste Activated Sludge Hydrothermally Treated as a Renewable Fuel or Activated Carbon Precursor

In this work, dewatered waste activated sludge (DWAS) was subjected to hydrothermal carbonization to obtain hydrochars that can be used as renewable solid fuels or activated carbon precursors. A central composite rotatable design was used to analyze the effect of temperature (140–220 °C) and reaction time (0.5–4 h) on the physicochemical properties of the products. The hydrochars exhibited increased heating values (up to 22.3 MJ/kg) and their air-activation provided carbons with a low BET area (100 m²/g). By contrast, chemical activation with K₂CO₃, KOH, FeCl₃ and ZnCl₂ gave carbons with a well-developed porous network (BET areas of 410–1030 m²/g) and substantial contents in mesopores (0.079–0.271 cm³/g) and micropores (0.136–0.398 cm³/g). The chemically activated carbons had a fairly good potential to adsorb emerging pollutants such as sulfamethoxazole, antipyrine and desipramine from the liquid phase. This was especially the case with KOH-activated hydrochars, which exhibited a maximum adsorption capacity of 412, 198 and 146 mg/g, respectively, for the previous pollutants.