Novel approach of phosphate-reclamation as struvite from sewage sludge by utilising hydrothermal carbonization

The effect of an acidic environment during the hydrothermal carbonization of sewage sludge on solid and liquid products: The fate of heavy metals, phosphorus and other compounds

The pH of sewage sludge is a crucial factor during the hydrothermal carbonization process that influences the characteristics of the resulting products and migration of certain compounds from the solid to liquid phase. Accordingly, this work is focused on examining the pH impact during the HTC process, in particular, pH equals 2, 3, 4, 5 and 6 on the individual hydrothermally carbonized products generated at 200 °C and 2 h residence time. For this reason, the chemical and physical indicators describing the post-processing liquid and hydrochar were determined. For instance, it was observed that the phosphorus content detected in the liquid, derived at pH2, rose significantly by 80%. Furthermore, decreasing the pH of sewage sludge had a significant impact on the ash content and the calorific value of the hydrochar. Additionally, changes in the specific surface area of hydrochar were noticed: pH = 5 and pH = 6 showed an increase of 20-30%, while for lower pH values a decrease of c.a. 26% was achieved. The distribution of heavy metals between the obtained fractions in the HTC process (solid and liquid) indicated that 92 to almost 100% of the tested heavy metals were transferred to the hydrochar. A significant effect of pH on the distribution between these fractions was observed only for Zn and Ni. For instance, for pH = 2, Zn and Ni in post-processing liquid were 34% and 29%, respectively. In addition, the sequential extraction of heavy metals from hydrochar was also performed in order to identify mobile and non-mobile phases. It was noticed that the acidic environment favours a higher amount of mobile heavy metals in hydrochar. The largest effect was observed for Cd, Pb, Cr and Cu, for which, at pH = 2, their respective amounts in the mobile fraction were 2.7; 3.6; 1.8; 6.2 times higher, compared to the hydrochar without pH correction.

Hydrochar from Pine Needles as a Green Alternative for Catalytic Electrodes in Energy Applications

Hydrothermal carbonization (HTC) serves as a sustainable method to transform pine needle waste into nitrogen-doped (N-doped) hydrochars. The primary focus is on evaluating these hydrochars as catalytic electrodes for the oxygen reduction reaction (ORR) and carbon dioxide reduction reaction (CO2RR), which are pivotal processes with significant environmental implications. Hydrochars were synthesized by varying the parameters such as nitrogen loading, temperature, and residence time. These materials were then thoroughly characterized using diverse analytical techniques, including elemental analysis, density measurements, BET surface area analysis, and spectroscopies like Raman, FTIR, and XPS, along with optical and scanning electron microscopies. The subsequent electrochemical assessment involved preparing electrocatalytic inks by combining hydrochars with an anion exchange ionomer (AEI) to leverage their synergistic effects. To the best of our knowledge, there are no previous reports on catalytic electrodes that simultaneously incorporate both a hydrochar and AEI. Evaluation metrics such as current densities, onset and half-wave potentials, and Koutecky-Levich and Tafel plots provided insights into their electrocatalytic performances. Notably, hydrochars synthesized at 230 °C exhibited an onset potential of 0.92 V vs. RHE, marking the highest reported value for a hydrochar. They also facilitated the exchange of four electrons at 0.26 V vs. RHE in the ORR. Additionally, the CO2RR yielded valuable C2 products like acetaldehyde and acetate. These findings highlight the remarkable electrocatalytic activity of the optimized hydrochars, which could be attributed, at least in part, to their optimal porosity.

Effect of hydrothermal carbonization on pyrolysis behavior, nutrients and metal species distribution in municipal sludge

In this study, the effect of hydrothermal carbonation (HTC) on the pyrolysis behavior and the distribution of nutrients and metal species of waste-activated sludge (WAS) was investigated. Results showed that the pyrolysis activation energy range of WAS decreased from 11 to 57 kJ/mol to 10-36 kJ/mol when the hydrothermal carbonization was at 160 °C. As indicated by thermodynamic parameters, the hydrothermal carbonization process reduces the pyrolysis reaction activity of the hydrochar. The results of the chemical analysis indicate that hydrothermal carbonization significantly enhances the release of phosphorus and nitrogen, with maximum recovery at a temperature of 200 °C. The standard measurement and testing protocol revealed that hydrothermal carbonization increased the content of non-apatite inorganic P fraction in hydrochar and enhanced the availability of P. Heavy metal analysis shows that hydrothermal carbonization can strengthen the stability of heavy metals in WAS.

Study of two approaches for the process water management from hydrothermal carbonization of swine manure: Anaerobic treatment and nutrient recovery

Hydrothermal carbonization (HTC) is a promising alternative to transform biomass waste into a solid carbonaceous material (hydrochar) and a process water with potential for material and energy recovery. In this study, two alternatives for process water treatment by conventional and acid-assisted HTC of swine manure are discussed. Process water from conventional HTC at 180 °C showed high biodegradability (55% COD removal) and methane production (∼290 mL STP CH4 g-1 CODadded) and the treatment in an upflow anaerobic sludge blanket reactor allowed obtaining a high methane production yield (1.3 L CH4 L-1 d-1) and COD removal (∼70%). The analysis of the microbiota showed a high concentration of Synergistota and Firmicutes phyla, with high degradation of organic nitrogen-containing organic compounds. Acid-assisted HTC proved to be a viable option for nutrient recovery (migration of 83% of the P to the process water), which allowed obtaining a solid salt by chemical precipitation with Mg(OH)2 (NPK of 4/4/0.4) and MgCl2 (NPK 8/17/0.5), with a negligible content of heavy metals. The characteristics of the precipitated solid complied with the requirements of European Regulation (2019)/1009 for fertilizers and amendments in agricultural soils, being a suitable alternative for the recycling of nutrients from wastes.

Enhancing phosphorus source apportionment in watersheds through species-specific analysis

Phosphorus (P) is a pivotal element responsible for triggering watershed eutrophication, and accurate source apportionment is a prerequisite for achieving the targeted prevention and control of P pollution. Current research predominantly emphasizes the allocation of total phosphorus (TP) loads from watershed pollution sources, with limited integration of source apportionment considering P species and their specific implications for eutrophication. This article conducts a retrospective analysis of the current state of research on watershed P source apportionment models, providing a comprehensive evaluation of three source apportionment methods, inventory analysis, diffusion models, and receptor models. Furthermore, a quantitative analysis of the impact of P species on watersheds is carried out, followed by the relationship between P species and the P source apportionment being critically clarified within watersheds. The study reveals that the impact of P on watershed eutrophication is highly dependent on P species, rather than absolute concentration of TP. Current research overlooking P species composition of pollution sources may render the acquired results of source apportionment incapable of assessing the impact of P sources on eutrophication accurately. In order to enhance the accuracy of watershed P pollution source apportionment, the following prospectives are recommended: (1) quantifying the P species composition of typical pollution sources; (2) revealing the mechanisms governing the migration and transformation of P species in watersheds; (3) expanding the application of traditional models and introducing novel methods to achieve quantitative source apportionment specifically for P species. Conducting source apportionment of specific species within a watershed contributes to a deeper understanding of P migration and transformation, enhancing the precise of management of P pollution sources and facilitating the targeted recovery of P resources.

Hydrothermal carbonization of Chinese medicine residues: Formation of humic acids and combustion performance of extracted hydrochar

Aiming at the sustainable management of high-moisture Chinese medicine residues (CMR), an alternative way integrating hydrothermal carbonization (HTC), humic acids (HAs) extraction and combustion of remained hydrochar has been proposed in this study. Effect of HTC temperature, HTC duration, and feedwater pH on the mass yield and properties of HAs was examined. The associated formation mechanism of HAs during HTC was proposed. The combustion performance of remained hydrochar after HAs extraction was evaluated. Results show that the positive correlation between hydrochar yield and HAs yield is observed. According to three-dimensional excitation emission matrix (3D EEM) fluorescence intensity, the best quality of HAs is achieved with a yield of 8.17 % at feedwater pH of 13 and HTC temperature of 200 °C. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses show abundant aromatic and aliphatic structure as well as oxygenated functional groups in HAs, which is like commercial HAs (HA-C). Besides, in terms of comprehensive combustion index (CCI), HTC can improve the combustion performance of CMR, while it becomes a bit worse after HAs extraction. Higher weighted mean apparent activation energy (Em) of hydrochar indicating its highly thermal stability. HAs extraction reduces Em and CCI of remained hydrochar. However, it can be regarded a potential renewable energy. This work confirms a more sustainable alternative way for CMR comprehensive utilization in near future.

Two birds with one stone: The multiple roles of hydrothermal treatment in dewatering municipal sludge and producing value-added products

Sludge dewatering and resource recovery are key steps in the sustainable treatment of municipal sludge (MS) owing to the high levels of moisture and nutrients. Among the treatment options available, hydrothermal treatment (HT) is promising to efficiently improve dewaterability and recover biofuels, nutrients, and materials from MS. However, hydrothermal conversion at different HT conditions generates multiple products. Integrating the characteristics of dewaterability and value-added products under different HT conditions facilitates the application of HT for the sustainable management of MS. Therefore, a comprehensive review of HT for its multiple roles in MS dewatering and value-added resource recovery is conducted. First, the impact of HT temperature on sludge dewaterability and key mechanisms are summarized. Then, this study elucidates the characteristics of biofuels produced (combustible gases, hydrochars, biocrudes, and H2-rich gases), nutrient recovery (proteins and phosphorus), and value-added materials under a wide range of HT conditions. Importantly, along with the integration and evaluation of HT product characteristics under different HT temperatures, this work proposes a conceptual sludge treatment system that integrates the different value-added products in different HT stages. Furthermore, a critical evaluation of the knowledge gaps in the HT for sludge deep dewatering, biofuels, nutrients, and materials recovery is provided along with recommendations for further research.

Phosphorus recovery from aqueous product of hydrothermal carbonization of cow manure

The work studies the recovery of nutrients (phosphorus and nitrogen) from the process water of acid-assisted hydrothermal carbonization (HTC) of cow manure. Three organic acids (formic acid, oxalic acid, and citric acid) and sulfuric acid were evaluated as additives in HTC. Using 0.3 M sulfuric acid, more than 99% of phosphorus and 15.6% of nitrogen from manure are extracted and dissolved during HTC at 170 °C with 10 min reaction time in a batch reactor. Nutrients (mainly phosphorus) were recovered through precipitation from process water by raising the ionic strength of the solution by addition of salts of magnesium and ammonia, and by raising the pH to 9.5. Subsequently, phosphorus-rich solids were recovered containing almost all (greater than 95%) of the dissolved phosphorus in the sulfuric and formic acid assisted runs. Morphology and qualitative chemical analysis of the precipitates were determined. It is shown by XRD that the precipitate formed from process water generated by HTC with oxalic acid is crystalline, although the diffraction pattern could not be matched with any expected substance.

Phosphorus recovery from municipal sludge-derived hydrochar: Insights into leaching mechanisms and hydroxyapatite synthesis

Hydrothermal liquefaction has the potential to exploit resources from municipal sewage sludge. It converts most organics into a liquid biofuel (biocrude), concentrates P in the solid residue (hydrochar), and consequently enables its efficient recovery. This study thoroughly evaluated the effects of extraction conditions on P and metal release from hydrochar by nitric acid. Among assessed factors, acid normality (0.02-1 N), liquid-to-solid ratio (5-100 mL/g), and contact time (0-24 h) had positive effects while decreasing eluate pH (0.5-4) improved leaching efficiencies of P and metals. Notably, eluate pH played a dominant role in P leaching and pH < 1.5 was crucial for complete extraction. P and metal leaching from hydrochar have strong interactions and their leaching mechanism was identified as product layer diffusion using the shrinking core model. This suggests that the leaching efficiency is susceptible to agitation and particle size but not temperature. Using 10 mL/g of 0.6 N HNO3 for 2 h was considered the best extraction condition for efficient P leaching (nearly 100%) and minimization of cost and contaminants (heavy metals). Following extraction, adding Ca(OH)2 at a Ca:P molar ratio of 1.7-2 precipitated most P (99-100%) at pH 5-6, while a higher pH (13) synthesized hydroxyapatite. The recovered precipitates had high plant availability (61-100%) of P and satisfactory concentrations of heavy metals as fertilizers in Canada and the US. Overall, this study established reproducible procedures for P recovery from hydrochar and advanced one step closer to wastewater biorefinery.

Life cycle assessment of a novel strategy based on hydrothermal carbonization for nutrient and energy recovery from food waste

In this work, a novel strategy for food waste valorization was evaluated from an environmental life-cycle perspective. A system based on acid-assisted hydrothermal carbonization of food waste combined with the exploitation of hydrochar by combustion and process water through nutrient recovery stage and subsequent anaerobic digestion, was assessed and compared with stand-alone anaerobic digestion as the reference system. This combination of processes aims to recover both nutrients in a stage of struvite precipitation from process water and energy through hydrochar and biogas combustion. Both systems were modeled in Aspen Plus® to identify and quantify their most relevant input and output flows and subsequently evaluate their environmental performance through the life cycle assessment methodology. The novel combined system was found to generally involve a more favorable environmental performance than the reference stand-alone configuration, which would be closely linked to the substitution of hydrochar for fossil fuels. In addition, the impacts associated with soil application of the struvite produced in the integrated process would also be reduced compared to the use of the digestate generated in the stand-alone anaerobic digestion process. Following these results and the evolving regulatory framework for biomass waste management, mainly in the field of nutrient recovery, combined process based on acid-assisted hydrothermal treatment plus nutrient recovery stage and anaerobic digestion is concluded to be a promising circular economy concept for food waste valorization.

Characteristics prediction of hydrothermal biochar using data enhanced interpretable machine learning

Hydrothermal biochar is a promising sustainable soil remediation agent for plant growth. Demands for biochar properties differ due to the diversity of soil environment. In order to achieve accurate biochar properties prediction and overcome the interpretability bottleneck of machine learning models, this study established a series of data-enhanced machine learning models and conducted relevant sensitivity analysis. Compared with traditional support vector machine, artificial neural network, and random forest models, the accuracy after data enhancement increased in average from 5.8% to 15.8%, where the optimal random forest model showed the average of accuracy was 94.89%. According to sensitivity analysis results, the essential factors influencing the predicting results of the models were reaction temperature, reaction pressure, and specific element of biomass feedstock. As a result, data-enhanced interpretable machine learning proved promising for the characteristics prediction of hydrothermal biochar.

Systematical analysis of sludge treatment and disposal technologies for carbon footprint reduction

This study aims to comprehensively analyze the Greenhouse Gases (GHGs) emissions from current sewage sludge treatment and disposal technologies (building material, landfill, land spreading, anaerobic digestion, and thermochemical processes) based on the database of Science Citation Index (SCI) and Social Science Citation Index (SSCI) from 1998 to 2020. The general patterns, spatial distribution, and hotspots were provided by bibliometric analysis. A comparative quantitative analysis based on life cycle assessment (LCA) put forward the current emission situation and the key influencing factors of different technologies. The effective GHG emissions reduction methods were proposed to mitigate climate change. Results showed that incineration or building materials manufacturing of highly dewatered sludge, and land spreading after anaerobic digestion have the best GHG emissions reduction benefits. Biological treatment technologies and thermochemical processes have great potential for reducing GHGs. Enhancement of pretreatment effect, co-digestion, and new technologies (e.g., injection of carbon dioxide, directional acidification) are major approaches to facilitate substitution emissions in sludge anaerobic digestion. The relationship between the quality and efficiency of secondary energy in thermochemical process and GHGs emission still needs further study. Solid sludge products generated by bio-stabilization or thermochemical processes are considered to have a certain carbon sequestration value and can improve the soil environment to control GHG emissions. The findings are useful for future development and processes selection of sludge treatment and disposal facing carbon footprint reduction.

Anaerobic digestion of hydrothermally pretreated dewatered sewage sludge: effects of process conditions on methane production and the fate of phosphorus

The hydrothermal pretreatment (HTP) characteristics and the fate of phosphorus (P) and anaerobic digestion (AD) performance of dewatered sewage sludge (DSS) were investigated at different hydrothermal conditions. The maximum methane yield reached 241 mL CH₄/g COD when the hydrothermal conditions were 200 °C-2 h-10% (A4), and the yield was 78.28% higher than that without pretreatment (A0) and 29.62% higher than that of the initial hydrothermal conditions (A1, 140 °C-1 h-5%). Proteins, polysaccharides, and volatile fatty acids (VFAs) were the main hydrothermal products of DSS. 3D-EEM analysis revealed that tyrosine, tryptophan proteins, and fulvic acids decreased after HTP, but the content of humic acid-like substances increased, and this phenomenon was more noticeable after AD. Solid-organic P was converted into liquid-P during the hydrothermal process, and nonapatite inorganic P was converted into organic P during AD. All samples achieved positive energy balance, and the energy balance of A4 was 10.50 kJ/g VS. Microbial analysis showed that the composition of the anaerobic microbial degradation community changed as the sludge organic composition was altered. Results showed that the HTP improved the anaerobic digestion of DSS.

Phytotoxicity and genotoxicity of agro-industrial digested sludge hydrochar: The role of heavy metals

Hydrochar is a new carbonaceous product obtained via hydrothermal carbonization of wet biomass, such as sludges or digested sludges, which often have disposal problems, also due to the presence of contaminants such as heavy metals. The properties of the hydrochar led to an interest in using it as an amendment, but the agro-environmental properties must be considered for its safe use. Raw hydrochar produced by agro-industrial digestate and relative three acidic post-treated hydrochars (for heavy metals removal) have been assessed considering their effect on phytotoxicity, soil, plant growth, mutagenicity, and genotoxicity. The chemical characterization showed the effect of post-treatment on heavy metals contents reduction, except for Cu content (hydrochar, 650 mg/kg; post-treated hydrochars, 940 mg/kg, 287 mg/kg, and 420 mg/kg). The acidic post-treatment also reduces the phytotoxicity compared to raw hydrochar (the germination index at 16 % of hydrochar concentration was: hydrochar, 61.48 %; post-treated hydrochars, 82.27 %, 58.28 %, and 82.26 %), but the low pH and the impact on N-cycle probably have caused the detrimental effect on plant growth of post-treated hydrochar. No mutagenic activity was observed in bacteria using Ames test, while all the samples induced chromosomal aberrations in plant cells (Allium cepa test). The approach adopted, which considers phytotoxicity, plant growth-soil effects, and mutagenicity/genotoxicity bioassays has been proven effective for a proper evaluation of organic products derived from waste to promote a sustainable and circular recovery of materials.

Management of biological sewage sludge: Fertilizer nitrogen recovery as the solution to fertilizer crisis

In the current situation of a serious raw material crisis related to the disruption of supply chains, the bioeconomy is of particular significance. Rising prices and the problem with the availability of natural gas have made N fertilizers production very expensive. It is expected that due to natural gas shortages, conventional production of nitrogen fertilizers by chemical synthesis will be hindered in the coming season. An important alternative and an opportunity to solve the problems of fertilizer nitrogen availability are biological wastewater treatment plants, which can be treated as a renewable biological nitrogen mines. Sewage sludge (including activated sludge) contains up to 6-8% DM. N. Considering the quantity of sewage sludge generated in wastewater treatment plants, it can become an important raw material for the sustainable production of organic-mineral fertilizers from renewable resources available locally, with a low carbon footprint. Furthermore, the sewage sludge management method should take nitrogen retention into account and should not allow the emission of greenhouse gases containing nitrogen. This article analyzes the technological solutions of nitrogen recovery for fertilization purposes from biological wastewater treatment plants in the context of a new and difficult resource situation. Conventional and new nitrogen recovery methods were analyzed from the perspective of the current legal situation. An attempt was made to evaluate the possibility of implementing the assumptions of the circular economy through the recovery of renewable nitrogen resources from municipal wastewater treatment plants.

Sewage Sludge Management at District Level: Reduction and Nutrients Recovery via Hydrothermal Carbonization

In this study, two scenarios of a municipal wastewater treatment plant (WWTP) are presented, which include the integration of the hydrothermal carbonization (HTC) process into the sludge line as a post-treatment of the anaerobic digestion (AD) process. The objective of the simulation is to investigate the performances of AD + HTC treatment to reduce sludge production and improve nutrient and energy recovery. For this purpose, the scheme of an under-construction WWTP was considered, named Trento 3 (Trento, Italy) and with a treatment capacity of 300,000 PE. In the first scenario, the HTC process was fed with thickened sludge from the Trento 3 WWTP, while in the second scenario, dewatered sludge from other local WWTPs was also used as feedstock for the HTC process. Both scenarios allowed to obtain a considerable sludge reduction ranging from 70 to 75% with a notably increase in the biogas production up to 47%, due to the recycling of HTC liquor (HTCL) to the anaerobic digester. Considering nutrients recovery, all the phosphorus and nitrogen present in the HTCL could be used for struvite precipitation with an average yearly gain of 1 million euros. Moreover, the introduction of HTC in the Trento 3 WWTP could allow a reduction in the sludge management costs of up to 2 M€/year.

Graphical Abstract

Feasibility of using different hydrothermal processes for sewage sludge management in China

Due to its tremendous volume and severe environmental concern, sewage sludge (SS) management and treatment are significant in China. The recent prohibition (June 2021) of reusing SS as organic fertilizers makes it urgent to develop alternative processes. However, there is currently little research analyzing the applicability of using HP for sewage SS treatment in China. The significant difference in SS composition and the much less land supply in urban areas might invalidate most previous localized suggestions. In this paper, the development of emerging hydrothermal processes (HPs) for SS treatment will be reviewed, focusing on their decomposition mechanisms and the benefits of HPs compared with current SS treatment technologies. The SS volume, composition, and regulatory regime in China will also be evaluated. Those efforts could address the potential SS treatment capacity shortage and provide an opportunity to recover nutrients, organics and energy embedded in SS. The results show that HPs' high investment cost is mainly limited by the process scale, while their operating costs are comparable to incineration. Minimizing equipment erosion, ensuring process safety, and designing a more efficient heat recovery system are recommended for the future commercialization of HPs in China.

Acid-mediated hydrothermal treatment of sewage sludge for nutrient recovery

Hydrothermal carbonization allows material valorization and energy recovery from wet biomass waste. In this study, the hydrothermal treatment of dewatered waste-activated sludge (DWAS) was evaluated at several temperatures (170-230 °C) and reaction times (5-60 min) in an acid-free medium or in media such as citric acid or HCl (0.1-0.5 mol/L). Compared with the DWAS, an increase in the fixed carbon content (>45 wt%) and heating value (18.9-22.9 MJ/kg) was observed in the hydrochar; however, their ash content remained high, which is the main drawback hindering their direct use as a biofuel. The addition of acids during hydrothermal treatment favored the solubilization of N and P in the process water, which required strict control of the reaction time to avoid the recrystallization of P in the hydrochar. Under optimum operating conditions (230 °C, 15 min, 0.5 mol/L HCl), 94 % of P (as of PO₄) and almost 100 % of N (14 % as NH₄-N) present in the feedstock were concentrated in the process water.

Phosphorus release from hydrothermally carbonized digested sewage sludge using organic acids

Hydrothermal carbonization (HTC) is a treatment technique with great potential for sanitizing digested sewage sludge (SS) and converting it into valuable products. In particular, phosphorus (P) recovery from hydrothermally carbonized SS has attracted special attention in recent years. This work aims to examine the leaching efficiency of P and the consequent release of metals and heavy metals from SS hydrochars (at 180, 215 and 250 °C) using organic acids (oxalate and citrate) over a range of pH values (0-4) and extraction times (5 min-24 h). Both organic acids triggered P extraction efficiencies exceeding 75 % at the lowest pH, but only oxalate reached a nearly complete P release from hydrochars at pH > 0 and for all carbonization temperatures. Low HTC temperature (180 °C) and short extraction time (5 min) were the optimal conditions treatment for P recovery when reacted in oxalate solutions of maximal pH buffering capacity (pH = 1.4). However, oxalate leaching also transferred metals/heavy metals into the P-leachate, with the exception of Ca being retained in the solid residue from HTC as Ca-oxalate precipitate. Different characterization methods confirmed the presence of this precipitate, and provided information about the surface and morphological changes of the SS hydrochars following acid treatment. The results suggest that HTC not only a promising technique to sanitize and reduce the volume of SS, but also an efficient means for P recovery using oxalic acid, thus contributing to the circular economy of P.

Characterization and nutritional value of hydrothermal liquid products from distillers grains

Hydrothermal liquid products (HLPs) produced by hydrothermal treatment (HTT) contain a large amount of nitrogen, phosphorus and other substances, while the environmental problems caused by arbitrary discharge. This work explored the effects of temperature, reaction time and solid-liquid ratio on the chemistry of HLPs of two different distillers grains, with a focus on nutrient composition. Increased HTT temperature was related to increased HLPs pH, dissolved organic carbon content, and aromaticity, and decreased electrical conductivity. Maximum nutrient extraction efficiencies observed for NH₄⁺-N, NO₃^--N and PO₄^3- were 92.0, 89.9, and 94.3%, respectively. Response surface methodology showed that the release of nutrient extraction efficiency was the greatest at the hydrothermal treatment of 200 °C for 1 h, and using a solid/liquid ratio of 10%. Comparative studies, the nutritional value of HLPs are appropriate for use as an agricultural fertilizer, and its use as a substitute for synthetic fertilizers could increase the sustainability and profitability of farming.

A review on the production of P-enriched hydro/bio-char from solid waste: Transformation of P and applications of hydro/bio-char

Phosphorus (P) is a necessary element for plant growth and animal health. Most P utilized by anthropogenic activities is released within the generation of various solid wastes such as sewage sludge, animal manure, and wetland plant, which increase the risk of water contamination. (Hydro)thermal treatment could be employed for solid waste treatment with the production of value-added hydro/bio-char, and the behavior of P during the thermochemical treatment process is critical for the further utilization of hydro/bio-char. This study provides a systematic review of the migration and transformation mechanisms of P during thermochemical treatment of various solid wastes, and special emphasis is given to the potential applications of P-enriched hydro/bio-char. Future challenges and perspectives in the thermal treatment of P-enriched solid waste are presented as well. The distribution and speciation of P were affected by feedstock properties, thermal technique, and reaction conditions, correspondingly affecting hydro/bio-char applications. The derived P-enriched hydro/bio-char was mainly applied as an agricultural soil amendment, P recovery source, and heavy metal sorbent, which could be adjusted by varying treatment process parameters. Additionally, potentially toxic substances, such as heavy metals in the solid waste, should be addressed during the production and application of hydro/bio-char. Overall, the production of P-enriched hydro/bio-char from solid waste is a promising route to simultaneously achieve P reclamation and solid waste treatment.

Thermal treatment of sewage sludge: A comparative review of the conversion principle, recovery methods and bioavailability-predicting of phosphorus

Phosphorus is a nutrient that is essential to nature and human life and has attracted attention because of its very limited reserves. Dwindling phosphorus reserves and soaring prices have made the recovery of phosphorus from waste biosolids even more urgent. Waste activated sludge, as the final destination of most of the phosphorus in human domestic and industrial water, has been considered as a reliable source of phosphorus recovery. The thermal treatment method of sewage sludge is currently a relatively environmentally friendly disposal method, which mainly includes incineration, pyrolysis and hydrothermal carbonization. This paper reviews the methods for the recovery of different forms of phosphorus (wet chemical, thermochemical and electrodialysis) from solid products obtained from different sludge thermal treatment methods (incinerated sewage sludge ash, pyrolysis of sewage sludge char and hydrochar) and the bioavailability of the recovered phosphorus products. Incineration of sewage sludge is currently the most established and effective method for recovering phosphorus from the thermal treatment products of sewage sludge. One of the wet chemical methods has been applied on a commercial scale and is expected to be further developed for future industrial applications. Pyrolysis and hydrothermal carbonation still have many research gaps in this field. Based on their principles and laboratory performance, both of them have the potential to recover phosphorus and should be further explored.

Preparation of Citric Acid-Sewage Sludge Hydrochar and Its Adsorption Performance for Pb(II) in Aqueous Solution

In order to seek the value-added utilization method of sewage sludge and develop low-cost and high-efficient adsorbents, a hydrochar was prepared by the co-hydrothermal carbonization of sewage sludge and citric acid and then characterized. The differences in Pb(II) adsorption performance between the citric acid–sewage sludge hydrochars (AHC) and the hydrochar prepared solely from sewage sludge (SSHC) were also investigated. When citric acid dose ratio (mass ratio of citric acid to dry sewage sludge) is 0.1, the obtained hydrohcar (AHC0.1) has the highest specific surface area (59.95 m²·g⁻¹), the most abundant oxygen-containing functional groups, the lowest pH_pzc (5.43), and the highest equilibrium adsorption capacity for Pb(II). The maximum adsorption capacity of AHC0.1 for Pb(II) is 60.88 mg·g⁻¹ (298 K), which is approximately 1.3 times that of SSHC. The potential mechanisms can be electrostatic attraction, co-precipitation, complexation, and cation-π interaction. It was demonstrated that by incorporating citric acid into the hydrothermal carbonization, resource utilization of sewage sludge can be accomplished effectively.

Influence of Post- and Pre-Acid Treatment during Hydrothermal Carbonization of Sewage Sludge on P-Transformation and the Characteristics of Hydrochar

Phosphorus (P) recovery from alternative P-rich residues is essential to meet the growing demands of food production globally. Despite sewage sludge being a potential source for P, its direct application on agricultural land is controversial because of the obvious concerns related to heavy metals and organic pollutants. Further, most of the available P recovery and sludge management technologies are cost-intensive as they require mandatory dewatering of sewage sludge. In this regard, hydrothermal carbonization (HTC) has gained great attention as a promising process to effectively treat the wet sewage sludge without it having to be dewatered, and it simultaneously enables the recovery of P. This study was conducted to analyse and compare the influence of acid (H2SO4) addition during and after HTC of sewage sludge on P leaching and the characteristics of hydrochar. The obtained results suggested that despite using the same amount of H2SO4, P leaching from solid to liquid phase was significantly higher when acid was used after the HTC of sewage sludge in comparison with acid utilization during the HTC process. After HTC, the reduction in acid-buffering capacity of sewage sludge and increase in solubility of phosphate precipitating metal ions had a greater influence on the mobilization of P from solid to liquid phase. In contrast, utilization of H2SO4 in different process conditions did not have a great influence on proximate analysis results and calorific value of consequently produced hydrochar.

Fate of nutrients during hydrothermal treatment of food waste

Hydrothermal carbonization was evaluated as a food waste valorization strategy to obtain hydrochar and recover nutrients. In the hydrothermal treatment, the temperature (170-230 °C), reaction time (5-60 min), and addition of HCl (0.1-0.5 M) during the reaction were analyzed. Compared to the feedstock, hydrochar showed an increase in fixed carbon (greater than 45%) and a decrease in ash content (<7%), along with a higher heating value (18.6-26.2 MJ/kg), which would allow for its application as a biofuel for industry according to ISO/TS 17225-8. The hydrochar obtained using plain carbonization showed 75% P and 40% N of the feedstock content, whereas the HCl-mediated treatment (0.5 M) solubilized most of the P, K, and N in the process water (98% P as PO₄-P, 98% K, and the total N content as NH₄-N (16%) and organic-N) operating at 170 °C for 60 min.

Changes and release risk of typical pharmaceuticals and personal care products in sewage sludge during hydrothermal carbonization process

Hydrochars were obtained by hydrothermal carbonization treatment of municipal sewage sludge. Effects of reaction temperature (180-300 °C) and reaction time (2-15 h) on structural characteristics of the hydrochars, and changes and release risk of typical pharmaceuticals and personal care products (PPCPs) in the hydrochars were investigated. Reaction temperature played a more important role than reaction time on hydrochar properties and decarboxylation reaction was the primary process during the converting of sludge to hydrochars. The sludge hydrochars had higher yields, carbon recovery rates, polarity and less aromaticity than biochars. Hydrothermal process reduced PPCPs' load in sludge hydrochars effectively except caffeine and acetaminophen. The hydrochars prepared at intermediate and high temperatures (240 and 300 °C) had higher caffeine concentrations than the original sludge, which can be ascribed to the transformation of N-containing precursors. The highest CaCl₂ extracted caffeine concentration occurred at intermediate temperature of 240 °C (48.1 μg/kg) due to the stronger affinity of caffeine in the high-temperature hydrochars. Caffeine was not detected in hydroxypropyl-β-cyclodextrin (HPCD) extract. Hydrochars prepared at low temperature (180 °C) had a higher acetaminophen concentration than the original sludge, which was attributed to the high thermal stability temperature of acetaminophen. Low- and intermediate-temperature hydrochars had higher CaCl₂ extracted acetaminophen concentrations. The HPCD extracted acetaminophen was low with a range of nd to 6.72 μg/kg. In conclusion, PPCPs are less likely to constitute a limiting factor on the farm application of sludge hydrochar. This study provides theoretical support for the safe application of sludge hydrochar in the farmland.

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.

Nutrient recovery from the digestate obtained by rumen fluid enhanced anaerobic co-digestion of sewage sludge and cattail: Precipitation by MgCl2 and ion exchange using zeolite

The aim of this study was to recover nutrients (NPK and other) from the liquid fraction of digestate obtained by rumen fluid enhanced anaerobic co-digestion of sewage sludge and cattail (Typha latifolia grass). Firstly, anaerobic digestion (AD) studies were performed to examine the biogas potential of selected substrates. The liquid fraction of digestate was then used in nutrient recovery experiments. Four methods were applied to recover nutrients: i) conventional struvite precipitation by MgCl₂, ii) simultaneous precipitation and ion exchange by Na-zeolite, and iii) two-step recovery using precipitation, followed by ion exchange with powdered or iv) granulated Na-zeolite. The products of nutrient recovery were characterised using different chemical methods and the cress seed germination test was performed to evaluate their fertility potential. The results show that co-digestion of sewage sludge with cattail enhanced biogas production by almost 50 vol%. The addition of rumen fluid positively contributed to the degradation of lignocellulosic materials and to biogas production. In all of the recovery methods tested, phosphorus was successfully recovered with efficiency of more than 99 wt%. Nitrogen recovery was less efficient than phosphorus recovery, 85-92 wt%. Simultaneous precipitation and ion exchange lowered nitrogen recovery efficiency compared to classical struvite precipitation, while sequential precipitation and ion exchange resulted in improvement. The most efficient method was two-step recovery using granulated zeolite. The precipitates consisted of different Mg and K-phosphates in quite irregular shapes. The struvite and K-struvite were detected in low quantities. The precipitates contained more than 25 wt% of macronutrients (NPK), exhibited effective utilization of nutrients by plants, and showed good fertility potential. Precipitate mixed with powdered Na-zeolite promises to be interesting for further agricultural use, as zeolite offers several potential improvements for soil. Both zeolites exhibited good performance in the recovery of K⁺ ions.

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.

Sewage Sludge Treatment by Hydrothermal Carbonization: Feasibility Study for Sustainable Nutrient Recovery and Fuel Production

Phosphorus recovery from waste biomass is becoming increasingly important, given that phosphorus is an exhaustible non-renewable resource. For the recovery of plant nutrients and production of climate-neutral fuel from wet waste streams, hydrothermal carbonization (HTC) has been suggested as a promising technology. In this study, digested sewage sludge (DSS) was used as waste material for phosphorus and nitrogen recovery. HTC was conducted at 200 °C for 4 h, followed by phosphorus stripping (PS) or leaching (PL) at room temperature. The results showed that for PS and PL around 84% and 71% of phosphorus, as well as 53% and 54% of nitrogen, respectively, could be recovered in the liquid phase (process water and/or extract). Heavy metals were mainly transferred to the hydrochar and only <1 ppm of Cd and 21–43 ppm of Zn were found to be in the liquid phase of the acid treatments. According to the economic feasibility calculation, the HTC-treatment per dry ton DSS with an industrial-scale plant would cost around 608 USD. Between 349–406 kg of sulfuric acid are required per dry ton DSS to achieve a high yield in phosphorus recovery, which causes additional costs of 96–118 USD. Compared to current sewage sludge treatment costs in Switzerland, which range between 669 USD and 1173 USD, HTC can be an economically feasible process for DSS treatment and nutrient recovery.

Evaluating the Aqueous Phase From Hydrothermal Carbonization of Cow Manure Digestate as Possible Fertilizer Solution for Plant Growth

Improving the agronomic use of recycled nutrients derived from organic waste is one of the priorities within the measures adopted by the European community to reduce environmental issues but remains an unexplored area of research. This study focused on investigating the possibility of using innovative fertilizer solutions in hydroponic systems for the growth of agricultural plants. To this purpose, a liquid fraction [aqueous hydrothermal carbonization (HTC) liquid (AHL)] derived from HTC of cow manure digestate was chemically characterized (pH, electrical conductivity, mineral elements, and organic compounds such as phytotoxins), diluted with distilled water (1:30, 1:60, and 1:90, v/v) to reduce its potential phytotoxicity, and used to grow hydroponic maize (Zea mays L.) plants instead of the classical full-strength nutrient solution. The results indicated that the dilution ratio 1:30 of the AHL solution maintained a high level of toxicity for the plants (phytotoxic substances, especially Na and alkalinity), inducing the arrest of their growth. Differently, the two other dilution ratios (i.e., 1:60 and 1:90) seemed to considerably limit the levels of toxicity, since they allowed the plants to develop. However, these dilution ratios were poor in nutrient elements, inducing alteration in photosynthesis and an onset of deficiency symptoms such as pronounced leaf chlorosis. In view of an eco-friendly approach, future studies are, therefore, needed to identify the correct species-specific dilution ratio to supply both low levels of phytotoxins and adequate content of essential nutrients for appropriate plant growth and development. Furthermore, in order to lower specific Na phytotoxicity, treatments are of utmost importance before using AHL as a fertilizer solution.

Enhanced transformation of phosphorus (P) in sewage sludge to hydroxyapatite via hydrothermal carbonization and calcium-based additive

Phosphorus (P) recovery from sewage sludge is one of the promising substitutes to the phosphate ores to alleviate the shortage of P supply in the future. In this study, how hydrothermal carbonization (HTC) temperature functioned in the migration and transformation of P from sewage sludge during HTC process was clarified. The enhancement effect of Ca-based additive on the transformation of P to hydroxyapatite via HTC was systematically studied. Additionally, leaching characteristics of P in the hydrochars with Ca-based additive were analyzed to assess their P-bioavailability. Results indicated that favoring by increased pH in the process water, HTC temperature played a significant role in the migration and transformation of P during HTC. Therefore, higher HTC temperature was beneficial for P enrichment and transformation to apatite phosphorus in the hydrochars. Both alkaline environment and existence of sufficient calcium ions were essential for enhanced formation of apatite phosphorus during HTC. Adding CaO could stimulate almost complete transformation of non-apatite inorganic phosphorus to apatite phosphorus such as hydroxyapatite, causing a maximum increase of apatite phosphorus by 252%. Concurrently, P-bioavailability by 2% (w/w) formic acid extraction of the hydrochars increased by 233% at CaO = 4%. These findings were confirmed by XRD analyses. Ca-associated apatite phosphorus such as Ca₅(PO₄)₃OH and Ca₂P₂O₇ ∙ 2H₂O, was detected in the hydrochars. With the assistance of thermochemical calculation, transformation pathways of P after CaO addition have been proposed, which coincided with our experimental results well. The stable P-containing minerals of hydroxyapatite (Ca₅(PO₄)₃OH) was calculated at equilibrium state.

Process Waters from Hydrothermal Carbonization of Sludge: Characteristics and Possible Valorization Pathways

Hydrothermal carbonization (HTC) is an innovative process capable of converting wet biodegradable residues into value-added materials, such as hydrochar. HTC has been studied for decades, however, a lack of detailed information on the production and composition of the process water has been highlighted by several authors. In this paper the state of the art of the knowledge on this by-product is analyzed, with attention to HTC applied to municipal and agro-industrial anaerobic digestion digestate. The chemical and physical characteristics of the process water obtained at different HTC conditions are compared along with pH, color, organic matter, nutrients, heavy metals and toxic compounds. The possibility of recovering nutrients and other valorization pathways is analyzed and technical feasibility constraints are reported. Finally, the paper describes the main companies which are investing actively in proposing HTC technology towards improving an effective process water valorization.

Hydrochars as Emerging Biofuels: Recent Advances and Application of Artificial Neural Networks for the Prediction of Heating Values

In this study, the growing scientific field of alternative biofuels was examined, with respect to hydrochars produced from renewable biomasses. Hydrochars are the solid products of hydrothermal carbonization (HTC) and their properties depend on the initial biomass and the temperature and duration of treatment. The basic (Scopus) and advanced (Citespace) analysis of literature showed that this is a dynamic research area, with several sub-fields of intense activity. The focus of researchers on sewage sludge and food waste as hydrochar precursors was highlighted and reviewed. It was established that hydrochars have improved behavior as fuels compared to these feedstocks. Food waste can be particularly useful in co-hydrothermal carbonization with ash-rich materials. In the case of sewage sludge, simultaneous P recovery from the HTC wastewater may add more value to the process. For both feedstocks, results from large-scale HTC are practically non-existent. Following the review, related data from the years 2014–2020 were retrieved and fitted into four different artificial neural networks (ANNs). Based on the elemental content, HTC temperature and time (as inputs), the higher heating values (HHVs) and yields (as outputs) could be successfully predicted, regardless of original biomass used for hydrochar production. ANN3 (based on C, O, H content, and HTC temperature) showed the optimum HHV predicting performance (R2 0.917, root mean square error 1.124), however, hydrochars’ HHVs could also be satisfactorily predicted by the C content alone (ANN1, R2 0.897, root mean square error 1.289).

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.

The impact of hydrothermal carbonization on the surface functionalities of wet waste materials for water treatment applications

Hydrothermal carbonization (HTC) is an energy-efficient thermochemical process for converting wet waste products into value added materials for water treatment. Understanding how HTC influences the physicochemical properties of the resultant materials is critical in optimizing the process for water treatment, where surface functionality and surface area play a major role. In this study, we have examined the HTC of four wet waste streams, sewage sludge, biosludge, fiber sludge, and horse manure at three different temperatures (180 °C, 220 °C, and 260 °C). The physicochemical properties of these materials were examined via FTIR, SEM and BET with their adsorption capacity were assessed using methylene blue. The yield of solid material after hydrothermal carbonization (hydrochar) decreased with increasing temperature for all samples, with the largest impact on horse manure and fiber sludge. These materials also lost the highest degree of oxygen, while HTC had minimal impact on biosludge and sewage sludge. The differences here were due to the varying compositions of each waste material, FTIR identified resonances related to cellulose in horse manure and fiber sludge, which were not detected in biosludge and sewage sludge. Adsorption capacities varied between 9.0 and 68 mg g^-1 with biosludge HTC at 220 °C adsorbing the highest amount. Adsorption also dropped drastically at the highest temperature (260 °C), indicating a correlation between adsorption capacity and HTC conditions. This was attributed to the loss of oxygen functional groups, which can contribute to adsorption. These results suggest that adsorption properties can be tailored both by selection of HTC temperature and feedstock.

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

Conventional activated sludge systems, still widely used to treat wastewater, produce large amounts of solid waste that is commonly landfilled or incinerated. This study addresses the potential use of Hydrothermal Carbonization (HTC) to valorize sewage sludge residues examining the properties of hydrochars depending on HTC process conditions and sewage sludge withdrawal point. With increasing HTC severity (process residence time and temperature), solid yield, total Chemical Oxygen Demand (COD) and solid pH decrease while ash content increases. Hydrochars produced from primary (thickened) and secondary (digested and dewatered) sludge show peculiar distinct properties. Hydrochars produced from thickened sludge show good fuel properties in terms of Higher Heating Value (HHV) and reduced ash content. However, relatively high volatile matter and O:C and H:C ratios result in thermal reactivity significantly higher than typical coals. Both series of carbonized secondary sludges show neutral pH, low COD, enhanced phosphorous content and low heavy metals concentration: as a whole, they show properties compatible with their use as soil amendments.

Distribution and transformation behaviors of heavy metals and phosphorus during hydrothermal carbonization of sewage sludge

This study investigated the distribution and transformation behaviors of heavy metals (HMs) and phosphorus (P) during hydrothermal carbonization (HTC) of sewage sludge. In addition to a large reduction in sludge volume, HTC significantly decreased the exchangeable fraction of HMs (Zn, Cu, Cr, Ni, and Mn) and increased their residual fraction, which resulted in immobilization of HMs accumulated in hydrochar. The ecological toxicity of HMs was greatly reduced, and all HMs exhibited their lowest risk levels after HTC at 220 °C for 1 h in 2% H₂SO₄ solution. Most of the P (~ 97%) in the input sludge remained in the hydrochar after HTC. HTC facilitated transformation of organic P to inorganic P and promoted conversion of apatite P to non-apatite inorganic P under acidic conditions. The feasibility of recovering P from sludge by HTC was verified by an acid extraction experiment utilizing hydrochar, which recovered more than 90% P. Graphical abstract.

Energy and phosphorous recovery through hydrothermal carbonization of digested sewage sludge

This work evaluates the potential of hydrothermal carbonization (HTC) to valorize the digestate derived from the anaerobic digestion of sewage sludge into useful materials for P and energy recovery. The hydrothermal treatment of digestate at 180-240 °C did not lead to high-rank hydrochars. On the other hand, inorganic P concentration did not change with the temperature, while as the carbonization temperature increased, the organic P retention yield in hydrochar became lower, increasing the total P in the process water obtained at the highest temperature, up to 25.3%. P recovery from acid leaching of the hydrochar obtained at 180 °C, via precipitation with CaO at pH up to 9, led to a brown solid precipitate with total P content close to 42 mg g^-1, in the range of low grade phosphorus ores. Moreover, acid leaching reduced by 50% the ash content, yielding lignite-like upgraded hydrochars with higher heating values in the range of 20.5-23.1 MJ kg^-1, fairly interesting as solid fuels. Anaerobic digestion of the process water enabled additional energy recovery in form of biogas (325 and 279 mL CH₄ g^-1 VS -at standard temperature and pressure; STP- from the process water resulting at 180 and 210 °C, respectively).

Moving bed biofilm reactor as an alternative wastewater treatment process for nutrient removal and recovery in the circular economy model

Over the last years, an increasing concern has emerged regarding the eco-friendly management of wastewater. Apart from the role of wastewater treatment plants (WWTPs) for wastewater and sewage sludge treatment, the increasing need of the recovery of the resources contained in wastewater, such as nutrients and water, should be highlighted. This would allow for transforming a wastewater treatment plant (WWTP) into a sustainable technological system. The objective of this review is to propose a moving bed biofilm reactor (MBBR) as a novel technology that contributes to the circularity of the wastewater treatment sector according to the principles of circular economy. In this regard, this paper aims to consider the MBBR process as the initial step for water reuse, and nutrient removal and recovery, within the circular economy model.

Feedstock-Dependent Phosphate Recovery in a Pilot-Scale Hydrothermal Liquefaction Bio-Crude Production

Microalgae (Spirulina) and primary sewage sludge are considerable feedstocks for future fuel-producing biorefinery. These feedstocks have either a high fuel production potential (algae) or a particularly high appearance as waste (sludge). Both feedstocks bring high loads of nutrients (P, N) that must be addressed in sound biorefinery concepts that primarily target specific hydrocarbons, such as liquid fuels. Hydrothermal liquefaction (HTL), which produces bio-crude oil that is ready for catalytic upgrading (e.g., for jet fuel), is a useful starting point for such an approach. As technology advances from small-scale batches to pilot-scale continuous operations, the aspect of nutrient recovery must be reconsidered. This research presents a full analysis of relevant nutrient flows between the product phases of HTL for the two aforementioned feedstocks on the basis of pilot-scale data. From a partial experimentally derived mass balance, initial strategies for recovering the most relevant nutrients (P, N) were developed and proofed in laboratory-scale. The experimental and theoretical data from the pilot and laboratory scales are combined to present the proof of concept and provide the first mass balances of an HTL-based biorefinery modular operation for producing fertilizer (struvite) as a value-added product.

Experimental and thermodynamic studies of phosphate behavior during the hydrothermal carbonization of sewage sludge

Recovered phosphate from sewage sludge is becoming a key product in the fertilizer market. This study investigates the fate of phosphate during the hydrothermal carbonization of digested sewage sludge to support the development of an economic and sustainable solution for dealing with sewage sludge for phosphate recovery. The solid products from the hydrothermal carbonization of digested sewage sludge in a batch reactor (180, 220, and 260 °C; 1, 2, and 4 h; digested sewage sludge-to-water ratios of 0.2 and 0.1 w/w) were analyzed using a sequential chemical extraction procedure to understand and predict the formation of phosphate species and the related extraction behavior of phosphate. The obtained results were compared with the thermochemical equilibrium composition of hydrothermal carbonization products, calculated using the software FactSage 7.2. The majority of phosphate was retained as Al, Ca, and Fe salts in hydrochar. The decomposition of organic phosphates was observed by processing at lower temperatures. Hydrothermal carbonization at temperatures higher than 180 °C resulted in the transformation of the Al-bound phosphate into Ca-bound phosphate. Hydroxyapatite (Ca₅(PO₄)₃(OH)) and Fe₇(PO₄)₆ were calculated as stable phosphate-containing minerals at equilibrium. This study suggests that kinetic constraints inhibit the formation of these minerals in the batch reactor and presents a mechanism of phosphate transformation using the obtained data. The results allow for targeted optimization of phosphate recovery strategy.

A study of solubilization of sewage sludge by hydrothermal treatment

The use of activated sludge process for biological treatment of domestic and industrial wastewaters generates large amounts of sewage sludge, which is regarded as problematic biowaste. Conventional waste treatment methods such as landfilling and ocean pumping have been used to dispose the unwanted sludge, but this practice is no longer recommended due to serious secondary pollution and strict environmental regulations. Hydrothermal treatment represents a promising alternative that has attracted attention in recent years. In this study, batch experiments of hydrothermal treatment of domestic sewage sludge were conducted under varying conditions (temperature of 150-300 °C, reaction time of 0.5-3.0 h, and sludge concentration of 5-30 g/L). A statistical study of the responses, including disintegration degree and concentration of dissolved compounds, was performed using a response surface methodology. Optimal conditions for hydrothermal treatment of sewage sludge were obtained through mathematical modeling.

Transformation of Biomass Waste into Sustainable Organic Fertilizers

The management of solid waste presents a challenge for developing countries as thegeneration of waste is increasing at a rapid and alarming rate. Much awareness towards thesustainability and technological advances for solid waste management has been implemented toreduce the generation of unnecessary waste. The recycling of this waste is being applied to producevaluable organic matter, which can be used as fertilizers or amendments to improve the soil structure.This review studies the sustainable transformation of various types of biomass waste such as animalmanure, sewage sludge, municipal solid waste, and food waste, into organic fertilizers and theirimpact on waste minimization and agricultural enhancement. The side effects of these organicfertilizers towards the soil are evaluated as the characteristics of these fertilizers will differ dependingon the types of waste used, in addition to the varying chemical composition of the organic fertilizers.This work will provide an insight to the potential management of biomass waste to be produced intoorganic fertilizer and the advantages of substituting chemical fertilizer with organic fertilizer derivedfrom the biomass waste.