Advances in Hydrothermal Carbonization for Biomass Wastewater Valorization: Optimizing Nitrogen and Phosphorus Nutrient Management to Enhance Agricultural and Ecological Outcomes

Abstract

This study presents a novel approach that integrates hydrothermal carbonization (HTC) technology with circular economy principles to optimize the management of nitrogen and phosphorus in agricultural wastewater. Given the increasing global resource scarcity and continuous ecological degradation, the valorization of biomass wastewater has become a critical pathway for the promotion of sustainable development. Biomass wastewater, which contains crop residues, forestry leftovers, and food processing byproducts, has long been regarded as useless waste. However, this wastewater contains abundant organic matter and possesses significant renewable energy potential. The valorization of biomass wastewater can significantly reduce environmental pollution. Through the optimization of the HTC process parameters, we achieved an improvement in the quality and yield of carbonized products, facilitating the efficient recycling and utilization of resources. This research demonstrates that HTC technology can transform agricultural wastewater into valuable biofertilizers, biomass energy, and organic feed, while simultaneously reducing the reliance on fossil fuels, decreasing greenhouse gas emissions, and mitigating the environmental impact of agricultural activities. This paper provides a comprehensive exploration of the application of HTC technology in agricultural ecosystems, highlighting its beneficial role in nitrogen and phosphorus management, resource utilization efficiency, and environmental pollution reduction. The findings of this study suggest that HTC technology holds significant potential in optimizing agricultural wastewater treatment, promoting resource recycling, and advancing sustainable agricultural development. Furthermore, this research offers theoretical support and practical guidance for the implementation of HTC technology in agricultural ecosystems, which is of paramount importance in fostering circular economic development and achieving sustainable agriculture.

1. Introduction

With the increasing global resource scarcity and continuous ecological degradation, the valorization of biomass wastewater has become a critical pathway for the promotion of sustainable development [1,2,3]. Biomass wastewater, which contains crop residues, forestry leftovers, and food processing byproducts, has long been regarded as useless waste [4,5,6]. However, this wastewater contains abundant organic matter and possesses significant renewable energy potential [1,7]. The valorization of biomass wastewater can significantly reduce environmental pollution [3,8]. Traditional methods, such as incineration or landfill disposal, not only waste resources but also release harmful gases, exacerbating climate change and environmental pollution [1]. Through valorization, these waste products can be transformed into biofertilizers, biomass energy, and organic feed, thereby reducing the dependence on fossil fuels, decreasing greenhouse gas emissions [2,9], and promoting agricultural sustainability [8,10].

The agro-ecological environment faces multiple challenges, the most significant among which are nitrogen and phosphorus loss and environmental pollution [11,12,13,14,15,16,17,18]. Nitrogen and phosphorus are essential nutrients for plant growth; however, excessive or improper fertilization results in their leaching into aquatic systems, causing eutrophication and disrupting aquatic ecosystems [19,20,21,22]. Moreover, the overuse of chemical fertilizers and pesticides leads to significant contamination in soil and aquatic environments [23,24,25,26], while also degrading the soil structure [27] and contributing to biodiversity loss [28]. These issues not only threaten the sustainability of agricultural production but also pose risks to human health.

To address these challenges, there is a strong need to develop efficient and environmentally friendly technologies to mitigate nitrogen and phosphorus loss and achieve wastewater valorization. Hydrothermal carbonization (HTC) is an emerging thermochemical conversion technology that transforms nitrogen- and phosphorus-containing organic wastewater into stable hydrochar, under relatively low temperatures (typically in the range of 180 °C to 260 °C) [29,30] and autogenous pressure conditions [30,31]. This process not only effectively immobilizes nitrogen and phosphorus, reducing their migration into the environment, but also avoids the secondary pollution that may arise from traditional treatment methods. Furthermore, HTC is characterized by high processing efficiency, low energy consumption, and minimal pollution, enabling the effective conversion of the organic matter in biomass wastewater into valuable resources while achieving waste reduction [31,32,33]. The byproducts generated during HTC, such as bio-oil and gas, can be further valorized, providing technical support for the sustainable development of agro-ecosystems. Therefore, HTC demonstrates significant potential in biomass wastewater valorization and is expected to serve as a crucial technological approach in advancing the circular economy [34].

In recent years, HTC, an emerging environmentally friendly technology, has achieved significant progress in the research into and application of biomass wastewater treatment [34,35]. This technology uses water as the reaction medium to process biomass wastewater under anaerobic or microaerobic conditions by controlling parameters such as the temperature, pressure, and catalysts, thereby converting it into hydrochar [36,37,38,39,40,41,42,43,44,45]. Using experimental methods, researchers have successfully optimized the process parameters of HTC, including the temperature, pressure, duration, and feedstock pretreatment, significantly enhancing the yield and quality of hydrochar [37,39,40,44,45]. Furthermore, significant progress has been made in understanding the transformation and immobilization mechanisms of nutrients such as nitrogen and phosphorus during the carbonization process [46,47,48,49,50,51,52]. However, despite its enormous potential, the large-scale application of HTC technology still faces several challenges, including high equipment costs, significant energy consumption, and an immature product market [34,53,54]. These issues hinder the widespread adoption and application of this technology, necessitating further research and technological innovations to overcome them. Figure 1 illustrates the number of research reports published on HTC technology in biomass wastewater applications from 2013 to 2024, based on data retrieved from ScienceDirect, highlighting its increasing importance in biomass wastewater research. Figure 2 shows a keyword co-occurrence map generated using VOSviewer 1.6.31, indicating high frequencies of terms such as “hydrothermal carbonization”, “biochar”, “water”, and “biomass” and demonstrating the broad applications of HTC technology in biomass valorization.

Nitrogen and phosphorus are essential nutrients for plant growth, and their effective management is critical for agricultural production [8,55,56,57]. The proper application of nitrogen and phosphorus fertilizers can enhance crop yields and quality, promoting sustainable agricultural development [21,58,59,60,61]. However, improper nitrogen and phosphorus management can result in nutrient loss, leading to environmental issues such as aquatic eutrophication, soil degradation, and air pollution [58,59,61]. Therefore, the development of optimized and effective nitrogen and phosphorus nutrient management strategies can improve agricultural productivity, reduce resource waste, and alleviate environmental pressures [55,62]. During the hydrothermal carbonization process, nitrogen and phosphorus nutrients are stabilized and transformed, helping to mitigate environmental pollution while enhancing the value of hydrochar.

The circular economy is an economic model centered on the efficient use and recycling of resources, aiming to reduce resource consumption and environmental pollution [63,64,65,66,67,68,69,70,71]. Within this framework, HTC technology can convert biomass wastewater into valuable hydrochar, achieving the full-value utilization of resources and improving the resource use efficiency [66]. Nutrients such as nitrogen and phosphorus in hydrochar are stabilized and transformed during the carbonization process, reducing nutrient loss and contributing to environmental protection [32,62,72,73,74]. Additionally, as a multifunctional material, hydrochar demonstrates broad application potential in fields such as soil improvement, wastewater treatment, and adsorbent preparation, contributing to the establishment of an industrial chain for the valorization of agricultural wastewater [30,38,39,42,43,44,45,47,49,51,52].

This study aims to systematically elucidate the applications of HTC technology in biomass wastewater valorization, with a particular focus on its agricultural and ecological benefits in terms of nitrogen and phosphorus nutrient management within the framework of the circular economy. The objective of this study is to highlight the environmental friendliness and economic feasibility of HTC technology and note its role in sustainable agricultural development. The main contents include the principles of HTC technology, strategies for the optimization of nitrogen and phosphorus nutrient management, theories and practices of the circular economy, and case studies of HTC applications in agricultural and ecological contexts. By integrating HTC technology with modern agricultural practices, this research provides a comprehensive framework aimed at enhancing the resource efficiency, reducing the environmental impacts, and promoting sustainable development in agricultural ecosystems.

2. Hydrothermal Carbonization Technology

2.1. The Basic Principle of Hydrothermal Carbonization

2.1.1. Physical Chemistry Process

The fundamental process of HTC is a physicochemical transformation that relies on the conditions of high temperatures, high pressure, and water [75,76]. High-temperature conditions, typically ranging from 180 °C to 350 °C, are essential in initiating and accelerating physicochemical reactions in HTC. These temperatures provide sufficient energy to break the strong chemical bonds in biomass macromolecules, facilitating hydrolysis by enabling water molecules to interact with polymers such as cellulose, hemicellulose, and lignin, thereby breaking them into smaller molecular substances. High-pressure conditions, usually between 2 MPa and 25 MPa, maintain water in a subcritical or supercritical state, enhancing its properties as both a reaction medium and a reactant. In this state, water exhibits increased ionic products and reduced viscosity, promoting reactant diffusion and reaction efficiency. Additionally, high pressure suppresses the vaporization of water and volatile substances, ensuring a stable reaction environment. Water plays a dual role in HTC, directly participating in hydrolysis by breaking biomass bonds and influencing the solubility and diffusion of reactants and products, which ultimately affects the reaction rates and product distribution. Under anaerobic or microaerobic conditions, biomass wastewater is fed into a reactor and processed under a high temperature and high pressure [76,77,78,79]. During this process, macromolecular organic substances in the biomass, such as cellulose, hemicellulose, and lignin, undergo thermal decomposition under the influence of heat and water [77,79,80,81,82].

Hydrolysis: At high temperatures, water molecules can break the chemical bonds within biomass macromolecules, converting them into smaller molecular substances. This process primarily targets carbohydrates in biomass wastewater, such as cellulose, hemicellulose, and lignin, which decompose into sugars and other small molecular compounds, including pentoses, hexoses, furfural, and phenols, in the presence of hot water [49,75]. The temperature range that favors hydrolysis in hydrothermal carbonization (HTC) is typically between 180 °C and 250 °C. Within this range, the reaction rate significantly increases, leading to the more efficient conversion of biomass into valuable products [83].

Dehydration: During the hydrothermal carbonization process, dehydration reactions involving biomacromolecules occur following hydrolysis. At this stage, water molecules are removed from the biomacromolecules, leading to chain shortening and the production of various intermediates, such as furfural and organic acids. Dehydration reactions are critical for the formation of the final carbon network, as they provide additional radicals and reaction sites for subsequent aromatization reactions [33,35].

Aromatization and Decarboxylation: Aromatization refers to the process by which the bonds between carbon atoms in the dehydrated intermediates are rearranged and reorganized to form more stable and ordered structures. This process primarily occurs in intermediates such as furfural and organic acids, produced during dehydration, as well as phenolic compounds generated during hydrolysis. Through aromatization and decarboxylation reactions, a series of macromolecular liquid fragments, CO₂, and H₂O can be produced [32,84,85,86,87].

Polymerization: Under high-temperature conditions, the organic molecules in biomass wastewater undergo condensation reactions to form new, larger molecular structures. These newly formed molecular structures typically exhibit higher carbon content and thermal stability, constituting a significant component of hydrochar [88,89].

Through this series of physicochemical reactions, biomass wastewater is transformed into new materials with distinct chemical and physical properties. Hydrochar, with its large specific surface area, porous structure, and stability, has significant potential for applications in fields such as soil improvement, wastewater treatment, and energy recovery [29,39,41,47,50,51,52]. Figure 3 summarizes the fundamental process of hydrothermal carbonization.

2.1.2. Influencing Factors

During the hydrothermal carbonization process, the distribution and final properties of the products are influenced by various factors; among them, the temperature, reaction time, pressure, and pH play critical roles [40,88,90,91,92,93,94,95].

Table 1. Mechanisms and impacts of key factors on hydrothermal carbonization efficiency.

Influencing Factor	Influence Mechanism	Large Degree of Influence	Small Degree of Influence	References
Temperature	The key factor affecting the degree of carbonization, pore structure, and surface functional group distribution of products	Increases the reaction rate and promotes the thermal decomposition of macromolecular organic matter	Hydrolysis and degradation reactions are incomplete, the carbon content of the product is low, and the pore structure is underdeveloped	[92]
Time	Affects the complete degree of hydrolysis, polymerization, and the carbonization of biomass	Excessive carbonization, pore structure changes, surface area, and decreased adsorption capacity	Results in the incomplete thermal decomposition of biomass, affecting the carbon yield and carbon quality	[91,96]
Pressure	Affects the degree of decomposition of biomass and the structural characteristics of carbonized products	Enhances the absorption of CO2 and increases the surface functional groups and porosity	Low pressure is not conducive to the destruction of the molecular structure of biomass, decreasing the degree of carbonization	[40]
pH	Affects the surface properties and pore structures of carbon materials	Under alkaline conditions, lignin degradation and carbonization are promoted, which is conducive to the formation of hydrochar	Under acidic conditions, it is beneficial to the hydrolysis of cellulose and hemicellulose and produces hydrochar with high calorific value	[88,93,94]

summarizes the key influencing factors of the hydrothermal carbonization process.

Temperature: Increasing the temperature can accelerate the reaction rate, promoting the thermal decomposition of the macromolecular organic substances in the biomass. Within a specific temperature range, elevated temperatures typically promote the more extensive thermal decomposition of biomass into char, thereby potentially augmenting the hydrochar yield [92]. However, the correlation between the temperature and hydrochar yield is non-linear and is also contingent upon the physicochemical characteristics of the feedstock. For example, although increased temperatures can facilitate the conversion of biomass to char, excessively high temperatures may induce the volatilization of organic matter, which could consequently diminish the overall solid yield.

Time: Insufficient reaction times may lead to the incomplete thermal decomposition of the biomass, thereby reducing the char yield and quality. Conversely, excessively prolonged reaction times can result in overcarbonization, altering the pore structure, decreasing the surface area and adsorption capacity, and ultimately reducing the mass yield [91,96].

Pressure: Increasing the pressure can elevate the boiling point of water, raise the reaction temperature, and thereby enhance both the yield and quality of the hydrochar. Under high-pressure conditions, CO₂ absorption is enhanced, facilitating the formation of surface functional groups and increasing the porosity, which in turn improves both the physical and chemical adsorption mechanisms [40].

pH: The hydrolysis of cellulose and hemicellulose is facilitated under acidic conditions, resulting in the increased production of liquid and gaseous products and the formation of high-calorific-value, deoxygenated hydrochar [93,94]. In contrast, alkaline conditions promote the degradation and carbonization of lignin, favoring the formation of hydrochar. The influence of the pH on hydrochar’s properties is not limited to the acidity levels; it also affects the reaction pathways and intermediate products. For instance, acidic conditions can accelerate the hydrolysis and dehydration reactions, while alkaline conditions can enhance the polymerization and aromatization processes. By adjusting the pH, the carbonization process can be optimized to meet various application demands [88].

2.1.3. Products

Hydrothermal carbonization technology can convert biomass wastewater into a variety of valuable products, including hydrochar, liquid byproducts [74,97,98,99], and gaseous products. Each of these products possesses distinct physical and chemical properties and holds extensive application potential. Their high-value utilization provides robust technological support to advance the circular economy and achieve sustainable development [100,101,102].

Hydrochar: Hydrochar is a solid product formed during the hydrothermal carbonization process. It is a carbon-rich, porous material characterized by a large specific surface area and stable chemical properties [36,38,52]. Its porous structure leads to excellent performance in pollutant adsorption and nutrient storage, resulting in wide applicability in environmental remediation, soil improvement, and water resource management [44,45,49,50]. In addition, hydrochar can serve as a catalyst support, improving catalysts’ dispersion and activity [103,104,105,106,107].

Liquid byproducts: The liquid byproducts produced during the hydrothermal carbonization process serve as critical intermediates in biomass conversion. These liquid byproducts typically contain organic acids, sugars, phenolic compounds, and other water-soluble organic compounds [74,97,98,99].

Gaseous products: The primary gaseous products produced during the hydrothermal carbonization process are hydrogen, methane, carbon dioxide, and other light hydrocarbons [100,101,102]. According to the literature, after hydrothermal carbonization, carbon dioxide constitutes approximately 90–95% of the gaseous products [108]. These gases hold significant value for energy recovery and the production of chemical feedstocks. The addition of hydrochar to municipal solid waste alters its gaseous emissions profile, reducing the release of CO, NO_x, and hydrochlorides [109].

2.2. Application Status of Hydrothermal Carbonization Technology

2.2.1. Application to Different Types of Biomass Wastewater

Hydrothermal carbonization (HTC) technology has garnered significant interest due to its versatile applications in treating diverse biomass-derived wastewaters. This section comprehensively examines the broad utilization of HTC across distinct biomass categories (e.g., agricultural residues, livestock manure, and municipal organic waste), emphasizing its mechanistic efficacy, environmental and economic advantages, and multifaceted research potential. Through representative case studies, we systematically outline the implementation strategies and outcomes of HTC in tailored biomass wastewater treatment scenarios, thereby providing a consolidated framework for future innovation in sustainable waste valorization.

In the treatment of agricultural residues (e.g., rice husks and straw), hydrothermal carbonization can effectively produce hydrochar for soil improvement and moisture retention [110,111]. Using the microwave-assisted hydrothermal conversion of rice straw and rapeseed stalk, Li et al. [112] produced hydrochar, noting that increasing the temperature and the concentrations of phosphoric acid and magnesium acetate promoted higher concentrations of organic matter and nutrient ions in the liquid products. The hydrochar yields of crop straws ranged from 43.25 wt% to 72.77 wt%. Wang et al. [44] produced hydrochar using the microwave-assisted hydrothermal treatment of dried rice straw under conditions of 200 °C for 2 h. Their study suggested that the resulting hydrochar has the potential to increase the soil organic carbon (SOC) stocks in rice production without adversely affecting the rice yields or carbon emissions. Ding et al. [113] hydrothermally carbonized Phragmites australis (RS) at 200 °C for 24 h and then activated the resulting hydrochar with peroxymonosulfate (PMS). The hydrochar exhibited excellent PMS catalytic activity, achieving nearly complete quinoline acid (QC) degradation under optimal conditions.

In the treatment of livestock and poultry manure (e.g., swine and cattle manure), HTC not only reduces odors and pathogens but also enables the recovery of nutrients such as nitrogen and phosphorus. Goldfarb et al. [114] performed HTC on cattle manure at 190 °C for 1 h and 230 °C for 3 h, producing hydrochar with mass yields of 76 and 57 wt%, respectively. They concluded that lower temperatures and shorter residence times enhanced the hydrochar’s adsorption capacity and improved the purity of the recovered hydroxyapatite. Using the hydrothermal conversion of swine manure, Xie et al. [115] produced hydrochar by varying the temperature (180, 220, 260 °C) and time (1, 2, 3 h) and adding FeCl₃. The hydrochar yields ranged from 54.7 wt% to 89.8% wt%. The results showed that FeCl₃ impregnation improved the carbon stability and phosphorus availability in the hydrochar, particularly under a lower pH (4), 220 °C, and 2 h of reaction time. Using swine manure as feedstock, Ding et al. [116] produced hydrochar at 180 °C for 1 h, achieving a solid yield of 53.3 wt%. They concluded that the application of HTC reduced the residual phosphorus in soil by 23.8–26.3%, while increasing the proportions of HCl-P and orthophosphate by 116.2–158.6% and 6.1–6.8%, respectively. Increasing HCl-P and orthophosphate is beneficial because HCl-P is more available to plants and can enhance phosphorus uptake by plants, while orthophosphate is a stable form of phosphorus that can reduce the risk of phosphorus loss in the soil.

In the treatment of urban organic waste (e.g., kitchen refuse and yard debris), HTC reduces the volume of waste while generating usable resources. By hydrothermally carbonizing sludge, McIntosh et al. [117] found that mild conditions (180 °C) and short residence times (0.5 h) yielded the highest phosphorus (P) recovery rates, favoring commercial operations and continuous processing. The hydrochar produced under these conditions had a mass yield of 75.7 wt%. Recovering P in fertilizer form from sewage sludge supports the sustainability of the global P cycle. Zhou et al. [118] prepared hydrochar from kitchen waste at 225 °C for 1.5–9.0 h using recycled process water. The hydrochar produced at 1.5 h had a solid yield of 67.42 wt%, while the hydrochar produced at 9.0 h had a solid yield of 66.86 wt%. Their findings indicated that, when using hydrochar as a cathodic catalyst, the hydrochar produced at 1.5 h exhibited superior electrochemical performance compared to that produced at 9.0 h. Ali Khan et al. [119] simulated hydrochar production from food waste by performing HTC at 180, 200, and 220 °C for 2 h. The hydrochar produced at 220 °C exhibited the highest heating value (HHV: 23.61 MJ/kg), compared to the raw food waste HHV of 18.17 MJ/kg. The hydrochar produced at 180 °C had a mass yield of 69.46 wt%, while the hydrochar produced at 200 °C had a mass yield of 68.5 wt%, and the hydrochar produced at 220 °C had a mass yield of 65.35 wt%.

Additionally, in the treatment of aquatic biomass, HTC has been applied to microalgae and macroalgae. Zhou et al. [120] co-hydrothermally carbonized microalgae (Chlorella pyrenoidosa) and macroalgae (Undaria pinnatifida) at 180–260 °C for 1–4 h, producing hydrochar. They observed that the synergies between microalgae and macroalgae significantly influenced the product distribution, properties, and nitrogen transformation pathways, which were dependent on the temperature and time. The hydrochar yields ranged from 12% to 35%. Ansah et al. [121] conducted the hydrothermal carbonization of Chlamydomonas debaryana at 200 °C for 6 h, producing hydrochar with a yield of 28.3% wt. The process resulted in increased carbon content, decreased nitrogen content, and enhanced HHV in the hydrochar. A comparative analysis of different raw materials shows that agricultural residues and livestock manure generally produce higher hydrochar yields (43.25 wt% to 89.8 wt%) compared to aquatic biomass (12 wt% to 35 wt%). This suggests that the type of raw material significantly influences the hydrochar yield and properties.

Table 2. Application of hydrothermal carbonization technology in different types of biomass wastewater.

Type of Raw Material	Raw Material	Temperature (°C)	Time (h)	Modified Condition	Yield of Hydrochar	Results	References
Agricultural residues	Straw and rape stalks	150, 180, and 210	1	Microwave assisted	43.25 wt%–72.77 wt%	Increased concentrations of organic matter and nutrient ions in liquid products	[112]
Agricultural residues	Dried rice straw	200	2	Microwave assisted	-	Hydrochar has the potential to increase SOC stocks in rice without adverse effects on rice production or carbon emissions	[44]
Agricultural residues	Phragmites australis	200	24	PMS activation	-	Excellent PMS catalytic activity: under the best conditions, it can achieve almost total QC degradation efficiency	[113]
Livestock and poultry manure	Cattle manure	190, 230	1, 3	-	76 and 57 wt%	The adsorption capacity of hydrochar and recovery purity of hydroxyapatite were improved	[114]
Livestock and poultry manure	Swine manure	180, 220, 260	1, 2, 3	FeCl3 impregnation	54.7 wt%–89.8 wt%	Improved C stability and P availability in HC-Fe, especially at low pH (4), 220 °C, and 2 h	[115]
Livestock and poultry manure	Pig manure	180	1	-	53.3 wt%	The proportion of residual phosphorus in soil decreased by 23.8–26.3%	[116]
Urban organic waste	Sludge material	180	0.5	Acid treatment	75.7 wt%	The P recovery rate is the highest under mild conditions, and the holding time is short	[117]
Urban organic waste	Kitchen waste	225	1.5–9.0	Liquid phase cycle	67.42 wt%, 66.86 wt%	The hydrochar prepared at 1.5 h showed better electrochemical properties than that at 9.0 h	[118]
Urban organic waste	Food waste	180, 200, and 220	2	-	69.46 wt%, 68.5 wt%, 65.35 wt%	The hydrochar prepared at 220 °C had the highest calorific value (HHV: 23.61 MJ/kg)	[119]
Algae	Chlorella pyrenoidosa, Undaria pinnatifida	180–260	1–4	-	12 wt%–35 wt%	Microalgae–macroalgae synergies impact product aspects and nitrogen transformations, being temperature- and time-dependent	[120]
Algae	Chlamydomonas debaryana	200	6	-	28.3 wt%	Increased carbon content, decreased nitrogen content, and improved HHV in hydrochar	[121]

summarizes the applications of HTC across various types of biomass wastewater.

2.2.2. Comparison of the Hydrothermal Carbonization Effects of Different Types of Biomass Wastewater

Different types of biomass wastewater exhibit varying degrees of carbonization during the hydrothermal carbonization process.

Lignin-rich biomass wastewater, such as wastewater rich in crop stalks, contains a large proportion of lignin [45,122,123,124]. During the hydrothermal carbonization process, lignin readily undergoes polymerization reactions, resulting in hydrochar with a porous structure. Such hydrochar is often suitable for use as an adsorbent, catalyst support, or water treatment material [122,123].

Protein-rich biomass wastewater, such as wastewater rich in livestock and poultry manure (e.g., swine, cattle, and chicken manure), contains substantial amounts of protein and other organic substances [114,115,116]. During the hydrothermal carbonization process, the thermal decomposition of proteins generates abundant liquid and gaseous byproducts, including ammonia, methane, and other volatile organic compounds.

The chemical composition of biomass wastewater also influences the carbonization outcomes. For instance, lipid-rich and highly volatile biomass may yield greater quantities of gaseous byproducts, including ammonia, methane, and light hydrocarbons, during the carbonization process [125].

By comparing the structures, properties, and compositions of various biomass materials, researchers can optimize the process parameters to enhance both the quality and yields of carbonization products. This approach enables the production of high-value-added products, improving the practicality and cost-effectiveness of hydrothermal carbonization technology and facilitating its commercial application in agriculture, environmental remediation, and the energy sector within the framework of the circular economy.

3. Nitrogen–Phosphorus Nutrient Management

3.1. The Roles and Challenges Associated with Nitrogen and Phosphorus Nutrients in Agriculture

Nitrogen and phosphorus are indispensable nutrients for plant growth, as they are essential components of proteins, nucleic acids, and chlorophyll and play a critical role in photosynthesis, energy transfer, and cell division. They also play a critical role in promoting crop development, increasing yields, and improving the quality of agricultural products [58,60,61]. In agricultural production, the proper application of nitrogen and phosphorus fertilizers can significantly enhance crop yields, growth efficiency, and the market competitiveness of agricultural products [20,21,126]. However, the improper use and management of nitrogen and phosphorus, especially their excessive application, can lead to a series of environmental challenges and issues [20].

3.1.1. Environmental Problems Due to the Overuse of Nitrogen–Phosphorus Nutrients

The excessive application of nitrogen and phosphorus fertilizers exerts multiple negative impacts on the environment. Excess nitrogen and phosphorus readily enter water bodies via surface runoff or soil infiltration, triggering eutrophication. Under these conditions, algae and other aquatic plants proliferate due to the surplus nutrient supply, resulting in algal blooms [127]. This phenomenon not only disrupts the original ecological balance of the water body but can also lead to a sharp decrease in the dissolved oxygen levels, affecting the survival of fish and other aquatic organisms and disturbing the equilibrium of aquatic ecosystems [127,128,129,130,131]. When algae and other aquatic plants die and decompose, they consume a significant amount of dissolved oxygen in the water, leading to oxygen depletion. This process is further exacerbated by the high respiration rates of dense algal populations, which consume oxygen during the night when photosynthesis is not occurring. In severe cases, eutrophication may even lead to the collapse of aquatic ecosystems, causing irreversible environmental damage [130,131].

The excessive use of nitrogen and phosphorus nutrients also leads to soil quality issues. Conventional nitrogen fertilizers, such as ammonium sulfate and ammonium chloride, contain a high level of nitrogen, some of which is present in the form of nitrates. When these fertilizers are applied to the soil, plants absorb a portion of the nitrogen, but the unabsorbed nitrogen and the fertilizer’s secondary components (such as sulfate and chloride ions) remain in the soil. This can lead to soil salinization, which in turn damages the soil structure, impairs its aeration and water retention abilities, and ultimately reduces the soil fertility [132,133]. Additionally, nitrate nitrogen from fertilizers can accumulate in the soil and infiltrate into groundwater, leading to the nitrate contamination of aquifers. This process is not without exceptions, as nitrate nitrogen can be converted to other forms of nitrogen through denitrification, which is a common phenomenon in many regions. Denitrification can reduce the accumulation of nitrate nitrogen in the soil, thereby mitigating the risk of groundwater pollution [134]. The excessive application of nitrogen and phosphorus may also contribute to air pollution. During the soil conversion process, a portion of the nitrogen is released into the atmosphere in the form of nitrogen oxides and related compounds, which are significant contributors to air pollution and the greenhouse effect [135,136]. Figure 4 summarizes the environmental issues resulting from the excessive use of nitrogen and phosphorus nutrients.

3.1.2. Resource Utilization Potential of Nitrogen and Phosphorus Nutrients in Biomass Wastewater

Biomass wastewater is rich in nitrogen and phosphorus nutrients, exhibiting substantial potential for resource utilization [137,138,139]. Through scientifically sound treatment and utilization, not only can environmental pollution be reduced, but the resource use efficiency can also be improved. Nitrogen and phosphorus in biomass wastewater can be converted into organic fertilizers using methods such as hydrothermal carbonization [138] and biological fermentation, providing a sustainable nutrient source for agricultural production.

3.2. Nitrogen and Phosphorus Migration and Transformation During the Hydrothermal Carbonization Process

3.2.1. Mechanism of Hydrothermal Carbonization Regarding Nitrogen and Phosphorus

During hydrothermal carbonization, nitrogen- and phosphorus-containing compounds in biomass wastewater undergo complex chemical reactions under the conditions of high temperatures, high pressure, and water saturation. Studies have shown that HTC effectively extracts nitrogen and phosphorus from solid phases into process water, dissolving organic ammonia and converting it into ammonium nitrogen. This makes HTC a favorable alternative for nitrogen solubilization and subsequent recovery [140,141,142]. Phosphorus transformation is similar to that of nitrogen, both involving conversion from organic to inorganic forms.

During the measurement process, NaOH-P is used to detect unstable phosphorus bound to Al, Fe, and Mn (hydroxides), while HCl-P typically refers to phosphate bound to apatite-P or Ca-bound P. Organic-P (OP) is a complex component that includes phospholipids, DNA, and simple phosphate monesters, which are highly mobile in soil. During hydrothermal carbonization, the content of NaOH-P and OP in the hydrochar decreases, while the content of HCl-P increases, indicating that phosphorus is being fixed into a more stable form. This process is influenced by the dissolution efficiency of hydrothermal carbonization and varies in effectiveness under alkaline and acidic conditions [143]. Therefore, creating an acidic environment during the process helps to improve nitrogen and phosphorus recovery in the liquid fraction. Acidic conditions accelerate the decomposition of organic nitrogen compounds and the release of inorganic phosphorus, thereby enhancing their dissolution and subsequent recovery in the liquid phase [140,144]. The hydrothermal carbonization process not only promotes phosphorus precipitation and crystallization, enabling its separation from solid products [142,145], but also facilitates the extraction of nutrients from biomass. These nutrients can be retained in the process water or incorporated into dense hydrochar, thus enabling their recovery [146].

3.2.2. Fate of Nitrogen and Phosphorus Under Different Conditions

The migration and transformation of nitrogen and phosphorus during the hydrothermal carbonization process are complex and influenced by multiple factors (e.g., temperature, time, and pressure) [140]. In solid products, nitrogen primarily exists in organic forms. These organic nitrogen compounds in hydrochar are highly bioactive and can be decomposed by soil microorganisms, gradually releasing nitrogen for plant uptake. In contrast, phosphorus is immobilized in hydrochar as inorganic phosphates. These phosphates exhibit high stability in soil, are less prone to leaching, and can release phosphorus slowly to meet plant requirements.

In liquid byproducts, nitrogen primarily occurs in dissolved forms, such as ammonium salts (NH₄⁺), nitrates (NO₃⁻), and amino compounds. These dissolved nitrogen species can be further processed chemically or biologically, enabling their recovery and subsequent utilization, such as fertilizers or chemical feedstocks [62,73]. Phosphorus in liquid byproducts also remains in dissolved form, primarily as phosphates [32]. The nitrogen and phosphorus in liquid byproducts can be concentrated and purified to produce soluble or slow-release fertilizers [35]. In gaseous products, nitrogen primarily appears as NH₃ and N₂, whereas phosphorus release in gaseous form is comparatively minimal. Ammonia can be captured and converted into nitrogen fertilizer, whereas nitrogen gas is either vented or utilized as an inert gas. Since phosphorus is not readily volatilized at high temperatures, its concentration in gaseous products remains low. Figure 5 summarizes the pathways of nitrogen and phosphorus in hydrothermal carbonization products.

3.3. Management of Nitrogen and Phosphorus Nutrients in Hydrothermal Carbonization Products

3.3.1. Nitrogen and Phosphorus Content and Their Fertilizer Efficiency in Hydrochar

The nitrogen and phosphorus content in hydrochar serves as an indicator of its fertilizer efficiency, originating from both the feedstock and the carbonization process. The feedstock and carbonization conditions influence the nitrogen and phosphorus levels. Appropriate thermal treatment can promote the conversion of N and P compounds, enhancing their availability to plant roots. High-temperature treatment decomposes organic nitrogen into soluble forms such as ammonium nitrogen, thereby improving the nitrogen availability [117]. Similarly, phosphorus speciation varies with the carbonization conditions, and different forms exhibit varying degrees of bioavailability to plants. The porous structure of hydrochar improves soil water and nutrient retention, minimizing nutrient loss and thereby enhancing the fertilizer efficiency [147]. By optimizing the carbonization conditions and feedstock selection, it is possible to produce hydrochar with greater fertilizer efficiency, offering more environmentally friendly and sustainable fertilizer options for agriculture, as well as valuable insights for the refinement of fertilization management strategies [21].

McIntosh et al. [117] investigated the recovery and transformation of phosphorus in sewage-sludge-derived hydrochar. Under mild conditions (180 °C, pH 8.0, 30 min), they achieved the maximum recovery rates of phosphorus (99%), along with carbon (62%) and nitrogen (43%). Zhao et al. [52] conducted the co-hydrothermal carbonization of rice straw and acidic whey, significantly enhancing the hydrochar’s performance. The higher heating value (HHV) increased by 53.6%, the yield improved by 20.0%, and the carbon content rose by 42.7%. Kalderis et al. [147] produced hydrochar from aloe leaves under hydrothermal conditions of 180–220 °C and 1–8 h. Under alkaline conditions, after 8 h of treatment, the calcium and phosphorus concentrations increased to 10.4% and 7382 mg·kg⁻¹, respectively, making this hydrochar a suitable substrate for the development of biofertilizers in alkaline soils.

3.3.2. Utilization of Nitrogen and Phosphorus in Liquid Byproducts

HTC technology valorizes biomass wastewater into solid products with substantial economic and resource value. Compared to the original feedstock, these solids exhibit enhanced heating value and yields and specialized properties (e.g., catalytic, adsorptive, and combustion characteristics). However, in conventional HTC processes, far less attention is given to the aqueous phase than to the hydrochar. The direct discharge of the aqueous phase poses environmental concerns that are often overlooked, as the nitrogen and phosphorus within it can lead to eutrophication [148,149]. By effectively recovering nitrogen and phosphorus from the liquid byproducts, the nutrient losses and environmental pollution risks can be reduced. This approach also decreases the reliance on chemical fertilizers, providing a sustainable nutrient source for agriculture and promoting sustainable agricultural development.

The HTC process is typically conducted in water. After the reaction with sewage sludge, the aqueous phase contains significant amounts of organic carbon and ammonium nitrogen (NH₄⁺-N), primarily derived from protein hydrolysis. Additionally, the aqueous phase contains other recalcitrant organics, such as humic substances, melanoidin-like compounds, nitrogenous heterocycles, and phenolics [148]. Common treatments for the aqueous phase produced during HTC include membrane filtration, aqueous-phase reforming, anaerobic digestion, aqueous-phase recycling, and magnesium ammonium phosphate (MAP) crystallization. The crystallization of MAP, also referred to by its mineral name, struvite, involves adding magnesium salts to an NH₄⁺-N and phosphate-containing aqueous phase. This process simultaneously recovers NH₄⁺-N and phosphorus, producing MAP, a valuable slow-release fertilizer [148,149]. Gou et al. [149] coupled struvite crystallization with aqueous-phase recycling to recover nitrogenous nutrients and enrich organic components in the aqueous phase. This integrated approach enabled the comprehensive utilization of all components in the sewage sludge HTC aqueous phase. Köchermann et al. [150] recycled process water during the hydrothermal treatment of urban green waste. Their results indicated that, as the aqueous-phase recycling proceeded, the mass yield of hydrochar increased, and higher temperatures amplified this effect. Picone et al. [151] performed the hydrothermal carbonization of orange peel waste and recycled the resulting process water. Their study revealed strong synergistic interactions between the biomass and the recovered solvent, depending on the reaction conditions, increasing the hydrochar mass yield on a dry basis by 0.5 to 11 wt%.

Table 3. Application of nitrogen and phosphorus nutrient utilization in hydrothermal carbonization products.

Raw Material	Temperature (°C)	Time (h)	Modified Condition	Utilization Type	Results	References
Sludge	180	0.5	Acid treatment	Solid-phase product	Maximized recovery of P (99%), as well as carbon (62%) and nitrogen (43%)	[117]
Straw and acid whey	250	1	Co-hydrothermal carbonization	Solid-phase product	HHV increased by 53.6%, yield increased by 20.0%, and carbon content increased by 42.7%	[52]
Aloe leaf	180, 220	1–8	KOH treatment	Solid-phase product	After 8 h of treatment under alkaline conditions, Ca and P concentrations increased to 10.4% and 7382 mg·kg−1, respectively	[147]
Sewage sludge	200, 230, and 260	1	Struvite crystallization and aqueous-phase recycling	Liquid-phase product	Recovery and reuse of nitrogenous nutrients in the water phase	[149]
Urban green waste	180, 220	5	Water-phase recirculation	Liquid-phase product	With water recirculation, the product’s mass yield increases	[150]
Orange peel waste	180–260	0.33–4	Water-phase recirculation	Liquid-phase product	The mass yield of hydrochar was increased by 0.5 to 11 wt% on the dry basis	[151]

summarizes the applications of nitrogen and phosphorus nutrient utilization in hydrothermal carbonization products.

3.3.3. Nitrogen and Phosphorus Recovery Technologies

To effectively recover nitrogen and phosphorus nutrients from hydrothermal carbonization products, researchers have developed a range of advanced recovery technologies [74,152]. These approaches not only enhance the efficiency of nutrient recovery but also enable the conversion of nutrients into fertilizer products that are readily applicable in agriculture.

Table 4. Nitrogen and phosphorus nutrient recovery technology.

Technology Type	Method	Application	Advantages	Disadvantages	References
Chemical Precipitation	A precipitating agent was added to the liquid byproduct to transform the dissolved nitrogen and phosphorus compounds into less soluble precipitates	Process into solid fertilizer or apply directly to soil	Simple operations and low costs	A large amount of sediment is produced, requiring subsequent treatment	[153,154,155,156,157,158]
Ion Exchange	Ion-exchange resin and other materials were used to selectively adsorb nitrogen and phosphorus ions from a hydrothermal solution	Made into fertilizer	High recovery efficiency	The need for the regular regeneration of resin and the relatively high cost	[159,160,161,162,163]
Reverse Osmosis	The selective permeability of a semi-permeable membrane is used to separate nitrogen and phosphorus from water in a hydrothermal solution	Made into fertilizer	High-efficiency separation	High equipment investment and operating costs	[164,165,166,167,168,169]
Electrodialysis	A DC voltage is applied in the electrolyzer, using the difference in ion migration in the selectively permeable film	Separation of liquid-phase product ions	Automatic control, high separation efficiency	Requires a stable power supply and high equipment maintenance	[170,171,172,173]
Biological Methods	Nitrogen and phosphorus nutrients are converted into microbial biomass by microbial metabolism	Nitrogen and phosphorus are absorbed and converted into biomass	Environmental protection, low costs	The treatment cycle is long, and the culture and management of microorganisms are required	[174,175,176,177]
Membrane Bioreactor	The metabolic function of microorganisms is combined with the physical screening function of membranes	Achieves biotransformation and solid–liquid separation	High treatment efficiency, less pollution	Membrane material and operation and maintenance costs are relatively high	[178,179,180,181,182,183,184]

summarizes several such nitrogen and phosphorus recovery technologies, as described below.

Chemical precipitation: By adding precipitants (e.g., lime or sodium hydroxide) to the aqueous byproducts of HTC, dissolved nitrogen and phosphorus compounds can be converted into insoluble precipitates [153,154,155,156,157,158]. These precipitates can then be processed into solid fertilizers or applied directly to soil. Although chemical precipitation is simple and cost-effective, it may produce substantial precipitate volumes, requiring further treatment.

Ion exchange: Ion-exchange resins or similar materials selectively adsorb nitrogen and phosphorus ions from the HTC aqueous phase. Once the resin is saturated, adjusting the pH or introducing competing ions facilitates the elution of the adsorbed nutrients [159,160,161,162,163]. The eluted nitrogen- and phosphorus-rich solution can be concentrated and solidified into fertilizer products. Although ion exchange offers high recovery efficiency, periodic resin regeneration increases the overall costs.

Reverse osmosis (RO): Reverse osmosis employs semi-permeable membranes to separate nitrogen and phosphorus nutrients from water in the HTC aqueous phase. Water passes through the membrane, while N and P are retained [164,165,166,167,168,169], thereby concentrating these nutrients for subsequent fertilizer production. Although RO provides efficient separation, its equipment investment and operational costs are relatively high.

Electrodialysis: Electrodialysis applies a direct current voltage in an electrolytic cell, exploiting differences in ion mobility through selective membranes to separate ions in the solution [170,171,172,173]. In HTC aqueous products, N and P exist as ions, which can be efficiently separated and collected at the cathode and anode, respectively. This method allows for automated control and high separation efficiency but requires a stable power supply and involves higher maintenance costs.

Biological methods: Biological processes employ microbial metabolism to transform nitrogen and phosphorus in the liquid phase into microbial biomass. Typically, the aqueous product is mixed with a culture medium containing specific microorganisms under suitable conditions, enabling them to absorb and incorporate N and P into their biomass [174,175,176,177]. While biological methods are environmentally friendly and cost-effective, they require longer processing times and the careful cultivation and management of microorganisms.

Membrane bioreactor (MBR) method: The MBR approach combines biological processes with membrane filtration, integrating microbial metabolism and physical membrane screening. This allows for the biological transformation of N and P nutrients and the simultaneous separation of solid and liquid products [178,179,180,181,182,183,184]. MBRs offer high treatment efficiency, a small footprint, and reduced pollution, but the cost of membrane materials and maintenance remains relatively high.

4. Applications Within the Circular Economic Framework

4.1. The Concept of the Circular Economy and Its Application in Agriculture

The circular economy is a development model centered on the efficient use and cyclical reuse of resources, guided by the principles of “reduce, reuse, and recycle” [185]. The circular economy emphasizes minimizing waste while maintaining the value of products for as long as possible. Although a product may have reached the end of its useful life, the circular economy seeks to preserve resource utility within the economy, primarily by repeatedly using resources efficiently and judiciously to create additional value [130]. In contrast, the traditional linear economy—characterized by a “take, make, dispose” model—shows limited concern for products after their end of life, focusing instead on handling and eliminating byproducts and relying predominantly on the extraction of virgin raw materials [130].

As a closed-loop system, the circular economy focuses on optimizing the resource use efficiency, reducing waste generation, and achieving sustainable resource utilization [65,66,67]. This concept is particularly critical in agriculture, which is both a major consumer of natural resources and a primary generator of agricultural wastewater, including straw, livestock manure, spent agricultural film, discarded packaging, and household refuse [186]. The application of the circular economy in agriculture not only reforms traditional production methods but also optimizes and upgrades the entire agricultural ecosystem.

Sustainable agricultural development requires improvements in production efficiency and benefits while preserving the ecological environment. The practice of the circular economy aligns seamlessly with these objectives. Within the circular economic framework, sustainable agricultural development also involves integrating and optimizing the agricultural value chain. By establishing a lifecycle management system for agricultural products [187,188,189], it is possible to achieve efficient resource utilization and waste reduction at every stage, from cultivation to sales.

The economic feasibility of HTC technology is limited. A study comparing HTC and traditional wastewater treatment found that HTC had energy advantages over pyrolysis and therefore lower emissions [111]. The global warming potential (GWP) of using hydrochar ranged from −71.4 to 7.7 g CO₂-eq. kg⁻¹ sewage sludge, while, for pyrolysis, the GWP was between −11.7 and 9.1 g CO₂-eq. kg⁻¹ sewage sludge [111]. Additionally, hydrochar production requires less energy consumption compared to biochar production, making it a less costly process [190]. For example, the production cost of hydrochar from residual plant biomass was found to be 158.72 USD/t, similar to that of hydrochar from wood waste (218 USD/t) [190].

The application of the circular economy in agriculture is a critical pathway to achieving sustainable agricultural development [191]. It not only addresses resource constraints and environmental challenges in agricultural production but also enhances the economic, social, and ecological benefits, laying a solid foundation for the healthy advancement of agriculture.

4.2. The Role of Hydrothermal Carbonization Technology in the Circular Economy

Within the circular economic framework, HTC technology plays a crucial role in treating biomass wastewater and recovering resources. By converting biomass wastewater into high-value hydrochar, aqueous products, and gaseous byproducts, HTC not only alleviates the environmental pressures associated with waste disposal but also provides new resource streams for agriculture and other sectors [192,193,194]. Xu et al. [192] utilized hydrochar produced from waste wood via HTC to reduce asphalt consumption and enhance its performance. Their results indicate that HTC significantly improves the high-temperature and fatigue properties of asphalt. Silvestri et al. [193] investigated the preparation of hydrochar-based composites using agricultural organic residues (rice husks, spent coffee grounds) combined with ZnO or ZnFe₂O₄. They confirmed the efficacy of the composites in degrading conjugated estrogens, achieving 100% photocatalytic potential within 30 min. This study proposes converting biomass wastewater into photocatalytic materials to advance the circular economy. Pan et al. [194] produced hydrochar from wild almond shells and demonstrated that it achieved maximum adsorption capacities of 85.37 mg/g for norfloxacin (NOR), 153.46 mg/g for methylene blue (MB), and 93.35 mg/g for sunset yellow (SY), with excellent fitting to both the Langmuir and Freundlich models.

Hydrochar, the primary solid product of HTC, features a porous structure and a large specific surface area, making it an effective soil amendment. Hydrochar enhances soil water and nutrient retention, increases the microbial diversity, and promotes plant growth [195]. Additionally, the nitrogen and phosphorus nutrients in hydrochar are released slowly, providing long-term nutrition for plants [152,195]. Lawa et al. [195] produced hydrochar from wild herbs (Amaranthus sp.) at 200 °C for 8 h. Analyses revealed that the hydrochar-amended soil exhibited a 32% expansion capability and possessed excellent nutrient storage and slow-release properties. Ding et al. [196] prepared hydrochar from cattle manure at 180 °C and 260 °C for 1 h. Their findings indicated that the resultant hydrochar increased phosphorus’ stability, and its application to soil significantly elevated the soil organic carbon (SOC) and dissolved the organic carbon (DOC) in soil extracts. Kravchenko et al. [197] produced hydrochar from wood and peanut shells under hydrothermal conditions at 250 °C for 1 h. Their study showed that adding wood- and peanut-shell-derived hydrochar reduced the crack strength factors by 43% and 51%, respectively, as well as increasing the crack length density in amended soils. Zhang et al. [198] prepared hydrochar from Sedum plumbizincicola at 260 °C for 2 h. Their experiments showed that, at 45 °C, the hydrochar’s maximum phosphate and ammonium adsorption capacities reached 52.46 and 27.56 mg/g, respectively. Using this hydrochar as a fertilizer in in situ phytoremediation minimized the environmental risks and advanced the circular economy.

HTC provides multiple pathways for the reuse of biomass wastewater in agricultural production. Hydrochar can be applied directly as a soil amendment [195,196,197], the liquid byproducts can be converted into organic fertilizers through chemical precipitation and struvite crystallization, and the gaseous byproducts can be collected and utilized as energy sources [148,149]. These reuse pathways not only reduce the environmental impact of waste but also enhance the sustainability of agricultural production.

Table 5. Applications of hydrothermal carbonization technology in the circular economy.

Raw Material	Temperature (°C)	Time (h)	Modified Condition	Results	Application Aspect	References
Waste wood	180	8	Asphalt modification	HTC significantly improves the high-temperature and fatigue properties of asphalt	Resource recycling	[192]
Agricultural organic residues	250	4	ZnO or ZnFe2O4 composite	Hydrochar-based composites have higher photocatalytic potential in conjugated estrogens	Resource recycling	[193]
Wild almond shell	160–240	4–12	Chemothermal activation	The maximum adsorption capacities of NOR, MB, and SY were 85.37, 153.46, and 93.35 mg/g, respectively	Resource recycling	[194]
Weeds	200	8	-	The swelling capacity of hydrochar-synthesized soil is 32%, and it has good fertilizer storage and slow-release characteristics	Soil amendment	[195]
Cattle manure	180, 260	1	-	The content of SOC and DOC in soil extracts was significantly increased by hydrochar returning to the field	Soil amendment	[196]
Wood and peanut shells	250	1	-	The addition of hydrochar reduced the fracture strength factor by 43% and 51%	Soil amendment	[197]
S. plumbizincicola	260	2	-	At 45 °C, the maximum phosphate and ammonium adsorption capacities reached 52.46 and 27.56 mg/g, respectively	Soil amendment	[198]

summarizes the applications of HTC technology in the circular economy.

4.3. Nitrogen and Phosphorus Cycling and Their Agro-Environmental Benefits

HTC produces hydrochar and aqueous products that recover and reuse nitrogen and phosphorus nutrients from biomass wastewater. These nutrient-rich byproducts, once treated, can be applied as organic fertilizers to cropland. This approach reduces the reliance on chemical fertilizers, mitigates nonpoint source pollution risks, improves the soil conditions, and enhances the quality of agricultural products [59,60,61,199,200]. Shan et al. [200] performed the co-hydrothermal carbonization of agricultural waste and sewage sludge at 220 °C for 1 h, producing hydrochar and liquid byproducts. Their experiments showed that using the co-HTC liquid phase as a liquid fertilizer to replace 60% of chemical fertilizers enhanced cabbage growth. Additionally, 40.1–67.7% of the nitrogen resources from sewage sludge could be utilized in agriculture, underscoring the significance of solid waste valorization in agricultural recycling.

Applying hydrochar to soil exerts multiple positive effects. Hydrochar increases soil organic matter, enhances soil water and nutrient retention, reduces the need for chemical fertilizers, and supports both ecological conservation and economic development [195,196,197,198,201]. Increased microbial diversity within hydrochar fosters nutrient transformation and cycling in the soil [201]. These microorganisms decompose soil organic matter, releasing plant-available nutrients [48]. Additionally, hydrochar improves soil aeration and permeability, providing a favorable environment for root growth [201]. Xue et al. [201] prepared hydrochar from sewage sludge at 250 °C for 2 h. Their findings indicated that hydrochar restored the soil bacterial abundance, pH, and urease activity, which were previously reduced by peroxydisulfate (PDS) induction. Feng et al. [48] produced hydrochar from swine and cattle manure at 180 °C, 220 °C, and 260 °C for 1 h. Their study revealed that hydrochar modified the N and P compositions in soil–water systems under flooding conditions by inhibiting soil urease and acid phosphatase activity, while increasing the nitrate and nitrite reductase activity.

The application of HTC technology helps to reduce environmental pollution by transforming biomass wastewater into valuable resources, thereby alleviating environmental pressures. Additionally, hydrochar applications can mitigate soil erosion and runoff, lowering the risk of eutrophication. Moreover, hydrochar absorbs heavy metals and organic pollutants in soil, reducing their environmental and health hazards and contributing to remediation efforts [48,124,195,196,197,201]. Li et al. [124] used plant fibers mixed with sodium metasilicate and conducted hydrothermal reactions at 160 °C for 4 h, achieving the 93.8% removal efficiency of Cd²⁺ from water. Feng et al. [48] produced manure-derived hydrochar from swine and cattle manure at 180 °C, 220 °C, and 260 °C for 1 h. The hydrochar treatment reduced the flood-induced ammonium nitrogen and total phosphorus concentrations by 12.9–36.9% and 11.7–20.7%, respectively, helping to mitigate nonpoint source pollution risks.

Table 6. Applications of nitrogen and phosphorus cycles in the agro-ecological environment.

Raw Material	Temperature (°C)	Time (h)	Modified Condition	Results	Application Effect	References
Agricultural waste and sewage sludge	220	1	Co-hydrothermal carbonization	The liquid phase replaces 60% of the chemical fertilizer with liquid fertilizer to promote the growth of cabbage	Reduced fertilizer use	[200]
Sewage sludge	250	2	-	Hydrochar restores the abundance, pH, and urease activity of soil bacteria induced by PDS	Improved soil fertility	[201]
Swine and cattle manure	180, 220, and 260	1	-	Hydrochar changes the composition of N and P in soil–water systems by inhibiting the activity of soil urease and acid phosphatase	Improved soil fertility	[48]
Plant fibers	160	4	Silicon modification	The Cd2+ removal rate of hydrochar in actual water is 93.8%	Reduced environmental pollution	[124]
Swine and cattle manure	180, 220, and 260	1	-	Hydrochar treatment reduces the concentrations of ammonia nitrogen and total phosphorus by 12.9–36.9% and 11.7–20.7%	Reduced environmental pollution	[48]

summarizes the applications of nitrogen and phosphorus cycling in the agricultural ecological environment.

5. Analysis of Agro-Environmental Benefits

5.1. Agricultural Benefits of Nitrogen and Phosphorus Management

Nitrogen and phosphorus are essential plant nutrients, yet, in conventional agricultural practices, large amounts are lost or immobilized in the soil, limiting their effective utilization by crops. HTC improves the nutrient utilization efficiency by recovering and converting the high nitrogen and phosphorus content in biomass wastewater [202]. Ebrahimi et al. [47] co-hydrothermally carbonized sludge and biomass, demonstrating that the resulting hydrochar significantly increased the soil content of these essential nutrients. Additionally, Fei et al. [203] conducted a suitability study for the valorization of hydrochar derived from municipal sludge, finding that the nitrogen content in the hydrochar increased significantly from 1.58 to 6.87 g/kg, phosphorus from 0.270 to 0.901 g/kg, and potassium from 0 to 0.873 g/kg. Baronti et al. [204] applied maize-silage-derived hydrochar as a substrate component for poplar growth, increasing both carbon and nitrogen in field soils and thus promoting plant development. Mahmood Al-Nuaimy et al. [199] found that incorporating hydrochar effectively maintained optimal plant moisture levels, enhancing the water use efficiency and nutrient retention capacity.

HTC technology fosters an agro-ecological balance, reducing nitrogen and phosphorus losses caused by excessive fertilizer use and thereby decreasing the risk of eutrophication and safeguarding water resources [62,152]. Concurrently, it enhances the quality and safety of agricultural products by scientifically managing nitrogen and phosphorus, thus reducing the accumulation of heavy metals and other harmful substances in produce [33,84]. It strengthens agricultural adaptability and resilience, utilizing circular economic approaches to improve the sector’s capacity to cope with climate change and resource scarcities.

5.2. Contributions of Hydrothermal Carbonization Technology to Carbon Mitigation

Hydrochar sequesters carbon from biomass wastewater through the carbonization process. This sequestration is a long-term process, as hydrochar exhibits substantially greater structural stability than the original biomass [205]. Under natural conditions, biomass decomposition releases CO₂. In contrast, hydrochar’s chemical stability and resistance to biodegradation allow it to persist in soils for decades or even centuries. Hydrothermal carbonization is a process that stabilizes the carbon in the soil carbon pool in a secure form, and this efficient carbon sink pathway plays a significant role in carbon sequestration and emission reduction [205,206]. Notably, understanding how the feedstock composition influences hydrochar stability aids in designing sustainable hydrochar production methods. In co-HTC processes, interactions between two or more biomass types can affect these stability characteristics [205]. Dinjus et al. [207] carbonized mixtures of straw and beech, finding that lignin may influence the release of carbonization intermediates, potentially inhibiting carbonization by forming protective layers around the feedstock.

Agricultural carbon emissions ultimately stem from agricultural activities. Varying degrees of mechanization and field management practices can lead to different emission levels, influencing agricultural outputs and energy consumption [199,208]. As an environmentally beneficial material, hydrochar helps to sequester carbon in soil and reduce the greenhouse gas emissions arising from hydroponic or related cultivation practices [199,209]. Under natural conditions, decomposing biomass wastewater (e.g., manure, crop residues) emits substantial amounts of methane. Through thermochemical conversion, HTC transforms these biomasses into hydrochar and other byproducts, thereby preventing methane formation and release. Methane [210,211] is a potent greenhouse gas with a global warming potential that is significantly higher than that of CO₂, making methane emission reductions critical in mitigating climate change [211]. When applied to soil, hydrochar’s stability can lower the CO₂ emissions that result from soil respiration. Al-Naqeb et al. [212] evaluated the effects of hydrochar on nitrogen dynamics and soil respiration by measuring the soil characteristics and greenhouse gas emissions (CH₄, CO₂, N₂O, and NH₃). Using hydrochar produced at 200 °C, they found that the hydrochar significantly influenced the CO₂ and N₂O emissions, while markedly reducing the total Kjeldahl nitrogen (TKN) by 46% and total organic carbon (TOC) by 49%.

6. Conclusions and Agro-Environmental Implications

This study provides an in-depth analysis of the research progress regarding hydrothermal carbonization (HTC) in valorizing biomass wastewater, with a particular emphasis on optimizing nitrogen and phosphorus nutrient management within a circular economic framework. The findings indicate that HTC, as an innovative resource conversion method, holds significant practical value and promising prospects to advance the resource utilization of biomass wastewater.

HTC demonstrates high processing efficiency, converting biomass wastewater into hydrochar, aqueous products, and gaseous byproducts with high added value, thereby offering new pathways for the utilization of biomass wastewater resources. The application of nitrogen and phosphorus management techniques significantly enhances the nutrient recycling efficiency, effectively reducing chemical fertilizer use and mitigating environmental pressures in agriculture, thereby promoting an agro-ecological balance. Within the framework of the circular economy, the integration of HTC and nitrogen–phosphorus nutrient management not only drives resource efficiency and environmental friendliness but also provides technical support for the sustainability of agricultural production systems, enhancing the agro-environmental benefits.

• The application of HTC changes conventional biomass wastewater management by converting residues such as straw, rice husks, and fruit shells into hydrochar, aqueous products, and gaseous products with high added value. This approach reduces the environmental pollution from waste accumulation and burning, provides organic fertilizers and renewable energy, and promotes cyclical utilization and sustainable development in agriculture, thereby contributing to a more stable and resilient agricultural production system.
• As a soil amendment, hydrochar enhances soil water and nutrient retention, improves the soil structure, and increases the soil organic matter content, creating favorable conditions for crop growth. Moreover, its long-term stability in soil supports carbon sequestration, reduces the atmospheric CO₂ concentrations, lowers greenhouse gas emissions, and mitigates climate change.
• By reducing the use of chemical fertilizers and pesticides, HTC technology enhances the ecological adaptability and market competitiveness of agriculture, laying a solid foundation for its long-term healthy development.
• HTC technology offers new economic growth opportunities for rural areas. By developing high-value-added carbonized products, it diversifies rural economies, increases farmers’ income streams, and improves the living standards of rural populations.
• Integrating HTC technology with modern agricultural production techniques facilitates the advancement of agricultural modernization, elevates the overall technological level of agriculture, and establishes a solid foundation for long-term agricultural development.