Hydrothermal Carbonization of Water Care Material (WCM) and Analysis of Fuel and Soil Amendment Characteristic of Hydrochar

Abstract

As freely available but not yet commercially acquired biomass resource, water care material (WCM) is generated seasonally in the periodic maintenance of surface water bodies and consists of mainly aquatic and/or rural-associated biomass of the water body profile, as well as wood, soil substrate, water or other possible impurities. In addition to other recovery options, such as composting or utilization in biogas production, hydrothermal carbonization (HTC) was selected as a thermochemical process because it is suitable for converting biomass with a high content of carbon into high-quality combustibles. The biomass sample used in this investigation was obtained during a single sampling event from a small stream in the North German lowlands. The material was pretreated by shredding it to a particle size of <0.12 mm. Through a 5 L stirred reactor, hydrothermal treatments were performed under low temperature conditions (200, 220 and 240 °C), residence times (120, 180, 240 min) and solid dry matter of the sample content: 6%. Solid phase was evaluated in terms of calorific value and proximate and ultimate analysis. The results suggested that the hydrothermal carbonization of WCM gave a high heating value of 23.84 MJ/kg for its char after being dried for 24 h at 105 degrees. At the same time, biochar can be used in agriculture to improve soil properties. To understand to what extent the product is suitable for soil amendment, the surface and the nutrient content of the resulting hydrochar were analyzed in detail. As the initial material is rich in fiber contents, process temperatures up to 240 °C have a huge impact on effective particle size. Furthermore, the analysis of selected nutrients, minerals and heavy metals shows the suitability of the produced hydrochar for soils in accordance with current legislation.

1. Introduction

The energetic and material utilization of biomass from waste and residual material streams is an indispensable element in the implementation of the circular bioeconomy [1]. The role of previously unused material flows is becoming increasingly important, especially in the economy [2], in order to be able to counteract the emerging competition to food production [3]. Also, due to rising substrate costs, biomass from maintenance processes (landscape maintenance, water maintenance) are of particular interest [4]. As primary products, these substrates are often not transformed to such an extent and are less contaminated with problematic components. This makes energy and material recovery much attractive [5,6,7,8]. In the case of water care material (WCM), more than 18,000 km of open water bodies in Mecklenburg–Western Pomerania are regularly maintained to prevent hydraulic damage [9]. In a rather uncomplex procedure, the vegetation growing in the water body profile is removed from one side between mid-July and mid-November with the help of heavy technology. The resulting biomass is not put to any further use, either materially or energetically, but remains in the area of the slope or is distributed in the surrounding areas for economic reasons [10]. While the retention is tolerated, it is recommended that the biomass be disposed of and recovered [9,11]. However, despite the ever-increasing demand for alternative biogenic substrates for energy and material recovery [5], there are as yet no area-wide concepts for the consistent acquisition and use of WCM. The material consists of mainly aquatic and/or rural-associated biomass of the water body profile, as well as wood, soil substrate, water or other possible impurities [12] and is spatially very heterogeneous depending on various location factors. This means that there is a wide range of variation in terms of dry matter content, the content of substances that are difficult to degrade and the carbon content [13]. With regard to potential utilization, it is considered harmless and usable for material and energy recovery [14]. In addition, the export of the biomass is a way to avoid the accumulation of nutrients in the slope of water bodies with negative consequences for the ecosystem [15]. The water bodies where the substrate accumulates are distributed over a large area. In the course of the need for the maintenance, they are connected to the local infrastructure and equipped on least on one side with a maintenance trass, which makes management possible [13].

The material is also used as an energy carrier of low-grade coals, as bituminous coal remains of high importance in several European countries. This is one reason why the clean production of coal could become increasingly important [16]. With respect to case studies and literature, this paper determines the suitability of water care material (WCM) for utilization in hydrothermal carbonization (HTC). The natural process of charring inspires this thermochemical process. Compared to the natural process, which takes centuries if not millions of years to create coal, HTC takes much less time (a few hours) to transform the biomass into a material similar to brown coal [17]. We investigated the utilization of WCM at different temperatures and residence times and determined the corresponding yield of hydrochar. The energy input operated was also included. The study showed that the utilization of WCM in hydrothermal carbonization is possible. The product of the studies is a coal with a higher calorific value, similar to lignite. The use of water in the process was high. Further studies are necessary on the utilization of leachate after carbonization. We also investigated the suitability of the coal produced as a soil conditioner. This application is widely applied [18,19] and has a positive effect on agricultural greenhouse gas emissions [20] as well as on the yield balance [21] and offers advantages for both producers and users as a supplement to energy recovery [22]. With respect to possible utilization opportunities in this background, we have analyzed the surface, the particle size and the nutrient and heavy metal content of the emerged hydrochar.

2. Materials and Methods

2.1. Feedstock and Experimental Setup

Water care material (WCM) is the initial biomass obtained from water management activities in a second-order stream situated in the North German lowlands. The biomass was obtained from stream 2/1 Elmenhorst-Lichtenhagen, located in close proximity to Rostock, during the month of July in the year 2021. This stream exemplifies small, suburban water basins, primarily surrounded by agricultural land. The surrounding vegetation exhibited a varied collection of water-associated macrophytes, with grasses and other rural plant species prevailing.

The biomass was hand-picked using a sickle, guaranteeing complete coverage of the whole management site. The biomass sample used in this investigation, identified as 146-GPM-21, was obtained during a single sampling event. In order to inhibit the deterioration of organic components and aid in further pretreatment procedures, the gathered biomass was promptly dehydrated and placed in a hermetically sealed receptacle until it was needed for future purposes.

The WCM was subjected to a comprehensive analysis using an organic elemental analyzer, following the guidelines of EN ISO 16948, 2015 [23]. The proximate analysis was conducted using an LECO Thermogravimetric Analyser (TGA) model TGA701 (LECO Corp., St. Joseph, MI, USA) to measure the moisture content, volatile organic compound, fixed carbon and ash content. The heating value of the sewage sludge and subsequent char was measured using a Parr 6400 calorimeter (Parr Instruments Inc., Moline, IL, USA), according to the procedure outlined in EN 14918, 2018 [24]. The measurements were conducted in duplicate and the average value was given.

WCM is rich in crude fiber (XF), acid detergent fiber (ADF) and neutral detergent fiber (NDF). The WCM was ground to a particle size of <0.12 mm using a sieve mill and treated in a 1 L HTC reactor. The reactor was equipped with an electrical heater and an agitator, made from stainless steel. The parameters XF, ADF and NDF were determined for the initial material. Near-infrared spectroscopy (NIRS) with the Instrument FT-NIR-Spektrometer from the company MPA Bruker (Billerica, MA, USA) was chosen for this purpose. A reliable NIRS estimation model from VDLUFA was used to calibrate the measured values.

2.2. Hydrothermal Carbonization

The processed WCM of particle size < 0.12 mm was subjected to hydrothermal carbonization (HTC) using a Parr 4523 reactor (manufactured by Parr Instrument (Frankfurt am Main, Deutschland) GmbH, located at Zeilweg 15, 60439 Frankfurt, Germany), specifically built to function under autogenic pressure circumstances. The reactor is furnished with a 1 L reaction vessel that has the ability to endure pressures of up to 138 bar. The system comprises a 2 kW heating element, a motorized stirrer and integrated sensors for measuring temperature and pressure. The Parr 4848 PID reactor controller is used to properly control these parameters. HTC tests were conducted under three distinct settings, and the resulting hydrochar and by-products were methodically gathered for subsequent examination. The experimental protocols are thoroughly described in Figure 1.

Following the completion of the WCM slurry’s full homogenization at 6% dry matter content, it was subjected to HTC under the influence of autogenic pressure. Initiation of the HTC process was performed at temperatures of 200 °C, 220 °C and 240 °C, with a regulated heating rate of 4 kelvins per minute. In order to guarantee that the material was processed in a consistent manner, each target temperature was kept at a constant level for defined residence lengths of 120, 180 and 240 min simultaneously, with continuous stirring.

After the HTC reaction was over, the system was allowed to finish cooling down to room temperature on its own, without any more cooling being applied from the outside. The internal pressure of the reactor was carefully vented behind a fume hood once it had achieved the temperature of the surrounding environment. This was performed in order to release any gaseous by-products. Subsequently, the HTC slurry was split into its solid and liquid phases in order to conduct additional analysis. For achieving constant moisture levels, the solid fractions were dried at 105 degrees Celsius for twenty-four hours before the analysis.

2.3. Analysis (Product Recovery and Analysis)

We used a vacuum filtering device to separate the hydrochar and produced process water after the HTC of WCM. We transferred the entire HTC slurry carefully into a Büchner funnel, vacuum pressure was applied via a vacuum pump to facilitate the separation of solids from liquids and the isolated hydrochar was oven-dried at 105 °C for 24 h. These standard operating procedures were followed throughout the solid–liquid separation and subsequent analysis of the hydrochar. After that, we kept the dehydrated hydrochar in airtight receptacles for additional examination.

Similarly to the original WCM feedstock, the hydrochar samples’ higher heating value (HHV) was ascertained using a Parr 6400 calorimeter. Equation (3) was used to compute the hydrochar yield. Furthermore, the same calorimeter was used to test the hydrochar’s lower heating value (LHV) in compliance with EN 15170:2010 methodology [25]. To ensure accuracy and reproducibility, all measurements were made in triplicate and the means are presented.

All of analysis was under supervised by the “technikum” lab of AUF. Mass yield (Equation (1)), energy densification ratio (Equation (2)) and energy yield (Equation (3)) are three parameters that were calculated in the study and are expressed as follows:

$$\mathrm{mass} \mathrm{yield} ={\displaystyle \frac{mass of dried hydrochar}{mass of dried feedstock}}\times 100$$

(1)

$$\mathrm{energy} \mathrm{densification} \mathrm{ratio} (\mathrm{EDR})={\displaystyle \frac{HHV of hydrochar}{HHV of feedstock}}$$

(2)

energy yield = mass yield × energy densification ratio (EDR)

(3)

In order to obtain an extensive understanding of the ultimate and proximal properties of the water care material (WCM), analyses utilizing conventional techniques were carried out. Using an organic elemental analyzer, the elemental composition was ascertained in accordance with EN ISO 16948:2015 requirements [23]. Furthermore, an LECO Thermogravimetric Analyzer (TGA701) was used for proximate analysis in order to evaluate the fixed carbon, ash content, moisture content and volatile organic compounds. Using a Parr 6400 calorimeter, the heating values of the sewage sludge were evaluated in compliance with EN 14918:2018 [24]. These analyses were carried out twice, and the accuracy of the average results was reported.

The resulting hydrochar samples produced after the hydrothermal carbonization of WCM was subjected to the same analytical techniques and procedures. This methodology ensured that any observed changes in the material’s characteristics could be correctly attributed to the HTC process by enabling a direct comparison between the raw WCM and its hydrochar derivatives. A thorough characterization of the material after carbonization was provided by the thorough examination of the hydrochar, which included the determination of elemental composition, proximate analysis and heating values.

The element characterization regarding carbon and sulfur, nutrients like nitrogen and phosphorous, minerals like potassium, calcium, sodium and magnesium and heavy metal contents of copper, arsenic, cadmium and lead were determined for the initial material and the hydrochar sample P9-240-240. These elements play an important role when used as a soil additive or even as a fertilizer with regard to approval restrictions or critical limit values. Carbon, nitrogen and sulfur were determined using Elementar vario Pyro for gas chromatographic volumetric determination of elements oxidized to gases after high-temperature combustion according DIN ISO 10694, 13878 and 15178. The contents of the elements potassium, calcium, sodium, magnesium and copper were determined by the external laboratory LUFA with the protocol VDLUFA III 10.8.2: 2006-01. Arsenic, cadmium and lead contents were determined by the external laboratory LUFA with the protocol VDLUFA III 17.9.1: 2012-01.

The hydrogen content of hydrochar was determined using Near-infrared spectroscopy (NIRS) with the Instrument FT-NIR-Spektrometer from the company MPA Bruker. A reliable NIRS estimation model from VDLUFA was used to calibrate the measured values.

Oxygen content was determined using the following calculation [26]:

Oxygen = Carbon content − Hydrogen content − Nitrogen content − Sulfur content

As the characteristics of the hydrochar are determined by HTC conditions [20], this study also investigates how the different temperatures and retention time influence the surface structure of the produced hydrochar. The surface morphology, along with the particle size of the hydrochar, is an important indicator for soil amendment properties. For example, the water absorption capacity is related to the surface of a material [27]. The analysis was made using the Keyence Digital Microscope VHX-X1. This allows for the finest surface structures to be optimally focused and visualized even at high magnifications. Individual particles can also be measured in a targeted manner.

Determining the particle size distribution is an important part of quality control in many areas and provides information on the surface area of the hydrochar produced in this study. Such morphological characteristics like particle size and surface area play an important role for use as a soil conditioner [20]. The particle size distribution is determined using the sieve shaker RETSCH AS 200, ISO 9001 [28]. Render series is used in selecting the screen sizes: (63 µm, 200 µm, 630 µm, 1000 µm, 2000 µm). The application of vibration sieving with the sieve shaker AS 200 works on the principle of electromagnetic drive. Even with short screening times, it provides high separation precision (Retsch^® 2017). The hydrochar samples are distributed on the top sieve. We carefully removed agglomerations by grinding. The sieving was carried out for one minute periods with an amplitude of 1.5 and an interval time of 10 s. The sieves used in this trial were weighed after the sieving with respect to empty weight. The mass distribution was converted into the percentage particle size distribution (P) by dividing the respective mass fraction (M mass fraction) by the total mass (M mass total) of the used sample.

3. Results and Discussion

3.1. Characteristic of the Feedstock Material

Table 1. Proximate analysis of water care material.

Parameters	Units	Value
Moisture content	%OS	1
Total solids	%OS	99
Volatile solids	%OS	92.5
Ash (815 °C)	%OS	6.9
Ash (550 °C)	%OS	7.5
Fixed carbon (FC)	%OS	0.6
XF	% DM	37.77
NDF	% DM	78.52
ADF	%DM	50.84
HHV	MJ/kg	15.3847
LHV	MJ/kg	14.5827
Carbon	% DM	49.394
Nitrogen	% DM	1.3124
Sulfur	% DM	0.30576
Total phosphorus	Mg/kg	1724.69593
Hydrogen	% DM	6.5
Oxygen	% DM	42.48784
Potassium	g/kg DM	7.7
Calcium	g/kg DM	5.1
Sodium	g/kg DM	0.2
Magnesium	g/kg DM	0.8
Copper	mg/kg DM	5.2
Arsenic	mg/kg DM	0.504
Cadmium	mg/kg DM	0.027
Lead	mg/kg DM	4.26

provides an overview of the findings from the water care material’s (WCM) proximate and ultimate analyses. The WCM showed a 99% total solids concentration and a 1% moisture level. The WCM’s ash concentration was 7.5% at 550 °C and 6.9% on a dry basis at 815 °C, according to the proximate analysis. With a comparatively low fixed carbon (FC) concentration of 0.6% on a dry basis, the volatile solids (VS) were measured at 92.5% on a dry basis. According to these results, WCM is highly organic and has little ash and fixed carbon, which is consistent with the properties of comparable materials that have been found in other studies [13].

For the fiber analytics, the composition of the WCM was determined to be as follows: Raw fiber (XF) content was 37.77%, the acid detergent fiber (ADF) content was 50.84% and the neutral detergent fiber (NDF) content was 78.52%. These values are consistent with the results in previous studies of WCM as feedstock [13].

In the end, the elemental composition of the WCM was determined to be as follows: On a dry basis, the carbon (C) content was 49.39%, the hydrogen (H) content was 6.50%, the nitrogen (N) content was 1.31%, the sulfur (S) content was 0.31% and the oxygen (O) content was 42.48%. With a total phosphorus (P) level of 1724.70 mg/kg, it was found to be a substance with a large nutritional content, indicated by a high concentration of this element.

The WCM had a higher heating value (HHV) of 15.38 MJ/kg and a lower heating value (LHV) of 14.58 MJ/kg in terms of energy content. These figures indicate that, in comparison to many other organic compounds, the WCM has a significant energy content. We also examined the WCM for a number of other elements and compounds, which included trace levels of copper (5.2 mg/kg), arsenic (0.504 mg/kg), cadmium (0.027 mg/kg), lead (4.26 mg/kg), calcium (5.1 g/kg), sodium (0.2 g/kg) and magnesium (0.8 g/kg). These findings offer a thorough analysis of the composition of the WCM, stressing both its possible uses and environmental concerns.

3.2. Effect of Operation Condition

Table 2. Experimental design and results of hydrochar.

Temp.	Residence Time	Mass Yield	High Heating Value (HHV)	Energy Yield
°C		%	MJ/kg	%
200 °C	120	61.22%	20.0734	79.88%
	180	61.75%	20.0903	80.64%
	240	59.69%	20.7969	80.69%
220 °C	120	59.17%	21.6985	83.45%
	180	58.75%	21.1897	80.92%
	240	47.69%	22.0736	68.43%
240 °C	120	43.17%	23.7969	66.77%
	180	53.83%	21.8902	76.60%
	240	41.39%	23.8445	64.15%

shows the experimental design of this study and gives the measured values for the parameters mass yield, high heating value and energy yield. We investigated two kinds of different operating conditions, temperature and residence time, during the HTC process and observed their effect on the calorific properties of the WCM. The results show that both influenced the properties of hydrochar.

The results suggested that even as mass yield decreases, HHV increases as temperature rises. Energy yield increases up to 220 °C, and then decreases with further increases in temperature, especially in connection with increasing residence times. The mass yield ranges from 41.39 to 61.75%. The HHV ranges from 20.07 to 23.89 MJ/kg. The energy yield ranges from 64.15 to 83.45%. The biomass hydrolysis in this process starts at 180 °C. In the following, as the temperature increases, all easily digestible organic compounds are gradually decomposed and transformed. For example, more than 90% of the hemicellulose decomposes in the temperature range 180–220 °C. As expected, the mass yield decreases especially because WCM contains high levels of ADF and NDF, so it has a high content of cellulose and hemicellulose. The crushing and breakdown of hemicellulose via shredding during the mechanic pretreatment of WCM results in a loss of structure in the biomass. The destroyed structure has improved friability and degradability. After these process stages, the residues in the sample are, besides minerals, ligno-cellulose and lignin. WCM has high contents of ligno-cellulose and lignin, which is hard to decompose in hydrothermal carbonization below 250 °C. The complete loss of the decomposable structures can be observed for the samples treated at 240 °C. Here we reach the highest calorific values and carbon contents.

3.3. Effects of HTC Reaction Condition on the Dewatering Potential

The observed variations in dry matter content at varied temperatures and residence times demonstrate the link between hydrothermal carbonization (HTC) conditions and the dewatering capability of water care material (WCM). The given data in

Table 3. Proximate analysis of emerged hydrochar.

Temp.	Residence Time	Proximate Analysis (wt% Fresh Basis)					Lower Heating Value (LHV)
		Dry Matter	Organic Dry Matter	Ash 550 °C	Ash 815 °C	Fixed Carbon
°C	minute	% OS	% OS	% OS	% OS	% OS	MJ/kg
200 °C	120	96.60	84.70	15.30	14.80	0.50	19.03
	180	96.50	87.60	12.40	11.80	0.60	19.04
	240	96.40	91.70	8.30	7.90	0.40	19.71
220 °C	120	96.90	91.00	9.00	8.60	0.40	20.57
	180	96.60	92.70	7.30	6.90	0.40	20.09
	240	97.00	86.90	13.10	12.50	0.60	20.92
240 °C	120	97.80	86.40	13.60	13.20	0.40	22.56
	180	97.10	91.10	8.90	8.50	0.40	20.75
	240	98.10	82.00	18.00	17.60	0.40	22.60

show a direct association between higher HTC temperature and residence time and increased dry matter content of the hydrochar residue, indicating more effective dewatering. The extracellular polymeric components found in water care material (WCM) consist of hydrophilic molecules, such as viscosity proteins and other organic debris [29]. To improve the dewatering performance of WCM, it is necessary to break down the cell walls and disrupt the floc structure in order to release and hydrolyze the organic components. The hydrothermal carbonization (HTC) process successfully facilitates this process through the utilization of greater temperatures and pressures [30]. The HTC procedure decreases the cohesive forces among the particles in the WCM, hence enhancing the efficiency of water removal.

At 200 °C, the dry matter content of the hydrochar remained rather stable across residence durations, ranging from 96.40% to 96.60%. However, as the residence period rose from 120 to 240 min, the organic dry matter content increased significantly from 84.70% to 91.70%, whereas the ash content decreased at both 550 °C and 815 °C. This shows that longer residence periods at this temperature improve organic material conversion, lowering ash content and increasing overall dewatering efficiency. When the temperature was increased to 220 °C, the dry matter content improved somewhat, reaching 97.00% after 240 min. The organic dry matter content remained high, peaking at 92.70%, with a subsequent drop in ash concentration, particularly around 815 °C. The fixed carbon content remained low and stable across all circumstances, demonstrating that the HTC process at this temperature successfully decreases non-combustible fractions, hence improving dewaterability. The greatest improvements were seen at 240 °C, when the dry matter content reached as high as 98.10% after 240 min. Despite a slight reduction in organic dry matter content at this temperature and residence time (82.00%), the increase in ash content at both 550 °C and 815 °C suggests that higher temperatures allow for more complete carbonization of organic material, resulting in a higher concentration of mineral residues. However, the overall high dry matter concentration implies that this temperature is ideal for dewatering.

These findings imply that increasing the HTC temperature and lengthening the residence duration improves WCM’s dewatering capacity. The decrease in organic content, combined with a rise in dry matter, indicates that the material is being broken down and converted more efficiently, making it easier to separate water from the hydrochar residue. This is especially obvious at higher temperatures, where dewatering efficiency is maximized, demonstrating the advantages of running HTC in more demanding conditions.

3.4. Effects of Reaction Condition on Heating Value of Water Care Material

After undergoing hydrothermal carbonization (HTC), the water care material (WCM) was converted into a solid hydrochar with clearly identifiable physical and chemical characteristics. The hydrochar displayed a consistent composition with higher density and has the potential to be transformed into pelletized forms for diverse applications. In order to assess the fuel properties of the hydrochar that was formed, we conducted proximate analysis and measured the lower heating value (LHV) while adjusting the reaction conditions of temperature and residence time. The proximate analysis results, summarized in Table 1, demonstrate the impact of higher temperature and longer residence time on many properties of the hydrochar, including dry matter, organic dry matter, ash content at 550 °C and 815 °C and fixed carbon content. The LHV (lower heating value) was also determined in order to assess the energy capacity of the hydrochar under various process conditions. We detected an evident pattern when the reaction temperature rose from 200 °C to 240 °C.

The dry matter content of the hydrochar rose as the temperature and residence time increased, reaching a maximum of 98.10% at 240 °C for 240 min. This signifies a substantial decrease in the moisture level, resulting in a hydrochar that is more concentrated and has a higher energy density [31]. The proportion of organic dry matter, which includes the volatile components of the hydrochar, first rose as the temperature and residence duration increased, reaching a maximum of 92.70% at 220 °C for 180 min. Nevertheless, when the temperature reached its peak at 240 °C, there was a slight decrease in the amount of organic dry matter, especially when the residence time was longer (82.00% at 240 min). This decline can be attributed to the intensified breakdown of organic material and the subsequent release of volatile substances during the HTC process.

The ash content, which represents the mineral residue remaining after burning, typically reduced as temperature and residence time increased, indicating the efficient volatilization of organic matter. Nevertheless, under the conditions of the greatest temperature and longest duration (240 °C, 240 min), the ash content exhibited an increase (reaching 17.60% at 815 °C). This implies that the process of carbonization became more thorough, resulting in the accumulation of inorganic substances as the organic portion continued to decrease. The fixed carbon (FC) content, defined as the solid combustible residue remaining after volatile matter is released during heating in the absence of air [32], exhibited negligible fluctuation under varied conditions, consistently ranging from 0.40% to 0.60%. This indicates that whereas HTC conditions did influence the overall structure of the hydrochar, the transformation of organic matter into fixed carbon was not substantially affected, potentially because the initial WCM feedstock had inherently low levels of fixed carbon. However, in order to discuss this finding clearly, further investigations would be useful.

The lower heating value (LHV) of the hydrochar, which is linked to the amount of organic dry matter and fixed carbon present, remained consistently stable. This indicates that the energy content of the hydrochar remained constant under diverse hydrothermal carbonization (HTC) settings. The data shown in Figure 2 indicates that subjecting the WCM to HTC results in the production of a hydrochar with energy qualities that make it appropriate for use as a solid fuel. However, the specific application of the hydrochar may vary depending on the required balance between its organic content and ash composition. In summary, the HTC process showed a direct correlation between higher temperature and longer residence time, resulting in improved hydrochar properties, specifically in terms of dry matter concentration and dewatering potential. This indicates that WCM hydrochar has great potential for energy recovery and other possible uses.

3.5. Characteristic of Hydrochar for Its Utilization as Soil Amender

3.5.1. Surface Morphology

The results of the surface morphology analyses show that the different HTC conditions affect the structure of the produced hydrochar. The temperature seems to have a huge impact on the particle size, as it decreases with rising temperature (Figure 3, Figure 4 and Figure 5). Characteristics like surface morphology and structure of hydrochars are largely dependent on the feedstocks [20]. As the initial material is rich in acid detergent fiber (ADF) and neutral detergent fiber (NDF), higher temperatures up to 240 °C decompose fiber structures like hemicellulose and cellulose. Up to 220 °C, the retention time also affects these dynamics. These results align with previous studies [33].

3.5.2. Particle Size

The sieve analysis (Figure 6) shows that the particle size decreases with rising temperature and retention times. The particle size distribution (P) shifts more and more in the direction of particle sizes smaller than 0.200 mm. While this proportion was still 55.51% at 200 °C and 120 min, it is 99.74% at 240 °C and 240 min. The highest distribution here is 0.063 mm with 57.34%. In addition, particle sizes above 2000 mm decrease continuously with increasing temperatures and retention times. These results support the impression from the analysis of the surface morphology. This leads to the assumption that higher temperatures and retention times during the HTC process ensure an overall larger surface area of the hydrochar. This increases the absorption capacity and thus improves the properties when used as a soil additive [20,34].

There are numerous studies on the use of hydrochar in the field of soil improvement. These provide clear indications of the fundamental suitability of hydrochar from plant-based residues [21]. The incorporation of plant-based charcoal into the soil ensures increased microbial activity and the promotion of soil microbial communities [20] as well as soil macrofauna, especially worms [35]. Furthermore, it can be stated that plant-based charcoal can significantly increase the water retention capacity of near-surface soil layers and thus the water use efficiency of plants, especially when applied in the root zone area [21]. Its high adsorption capacity [20], as well as its influence on bulk density and the aggregate stability of the soil [34], creates an agronomically positive effect on water availability for plants. There is also a clear increase in the efficiency of organic fertilizers with biochar. This is partly due to the large surface area of plant-based charcoal [21]. In addition, char-induced changes in soil organic matter content, electrical conductivity, C/N ratio and cation exchange capacity (CEC) have been demonstrated, which were also the main causes of the yield increase [36,37]. In addition, the charcoal itself has a valuable content of plant nutrients and is suitable for use as a slow-release fertilizer [20].

3.5.3. Nutrient Content

For the element analysis, the composition of the WCM and hydrochar was determined to be as follows: The HTC conditions determine the N contents of the produced hydrochars, ranging from 0.89 to 1.62% (

Table 4. Element analysis of hydrochar in comparison to the initial material.

Sample ID	C	N	S	P Total
	%	%	%	mg/kg
P1-200-120	48.72	0.91	0.30	2775.05
P2-200-180	49.16	0.91	0.24	2192.81
P3-200-240	50.50	0.91	0.29	2778.12
P4-220-120	52.00	0.99	0.25	2800.04
P5-220-180	51.17	0.89	0.23	2365.68
P6-220-240	52.97	1.19	0.28	3448.45
P7-240-120	56.76	1.43	0.26	3631.69
P8-240-180	53.42	1.14	0.22	3094.04
P9-240-240	59.67	1.62	0.31	3369.81
146-GPM-21	49.394	1.3124	0.30576	1724.69593

). The N content of the initial material is 1.31%. The S content of the produced hydrochars, ranging from 0.22 to 0.31, seem not to be influenced by temperature or retention time. The HTC process and conditions determine the P contents of the produced hydrochars, ranging from 2192.81 to 3631.39 mg/kg. The P content of the initial material is 1724.7 mg/kg. The S content of the initial material is 0.31% and does not decrease significantly in the process. The sulfur contents in the hydrochar range from 0.22 to 0.31%.

We reported minerals like K, Ca, Na and Mg in the initial material and hydrochar sample P9-240-240 (

Table 5. Content of minerals and heavy metals in initial material compared with hydrochar sample P9-240-240.

Sample ID	K	Ca	Na	Mg	Cu	As	Cd	Pb
	g/kg	g/kg	g/kg	g/kg	mg/kg	mg/kg	mg/kg	mg/kg
146-GPM-21	7.7	5.1	0.2	0.8	5.2	0.504	0.027	4.26
P9-240-240	1.2	5.6	0.1	0.6	26.1	0.57	0.043	7.84

). The calcium (Ca) content in the initial material is 5.1 g/kg and 5.6 g/kg in the hydrochar. The content of sodium (Na) in the initial material is 0.2 g/kg and 0.1 g/kg in the hydrochar. The magnesium (Mg) content in the initial material is 0.8 g/kg and 0.6 g/kg in the hydrochar. The concentration of these elements decrease slightly but do not change significantly compared to the initial material in this experiment. There is only a strong reduction in sodium and potassium in the HTC process. It could not be evaluated in this test, but further studies show that in general, higher temperatures during HTC largely affect hydrochar elemental compositions [20,38].

We have reported heavy metals such as Cu, As, Cd and Pb in the hydrochar sample P9-240-240. The concentrations of these elements are higher compared to the initial material. During the HTC process, the heavy metals contained in the initial material are concentrated through the decomposition of the organic components.

Copper content in the initial material is 5.2 mg/kg and 26.1 mg/kg in the hydrochar. Cadmium content in the initial material is 0.027 mg/kg and 0.043 mg/kg in the hydrochar. Lead content in the initial material is 4.26 mg/kg and 7.84 in the hydrochar. During the HTC process, the heavy metals contained in the initial material are concentrated through the decomposition of the organic components. Only the arsenic (As) content, with 0.504 in the initial material and 0.57 in the hydrochar, has a smaller increase after the HTC process.

As the results show, depending on the initial material, hydrochar contains heavy metals that occur naturally in all biomass. This poses certain risks with regard to increasing accumulation in the soil. The selection of the initial substrates is therefore crucial. However, it should also be noted that the heavy metals bound in coal from plants are practically insoluble and therefore neither pollute the groundwater nor are they available to plants [39]. Adding charcoal into the soil usually increases the pH value, which can reduce the mobility of many metals. The plant absorption of pollutants is reduced by the immobilization on the carbon [40]. In addition, the surface structure of the charcoal can even help to absorb critical pollutants in co-substrates or from the soil after application. Plant-based charcoal is therefore also used in soil restoration [41,42], especially for light to medium polluted soils, in order to counteract long-term ecological problems such as pollutant emissions and negative effects on biodiversity, as well as economic disadvantages such as loss of habitat value and limited utilization options [43]. The addition of charcoal therefore fundamentally reduces the contamination of agricultural soils [21]. The levels of nutrients and heavy metals found in hydrochar made from water care material (WCM) make it suitable for use in soils in accordance with the DüMV [44]. If not used directly in the soil, it also offers advantages as a co-substrate in composting and helps to minimize nutrient losses, increase biological activity and thus sanitize the substrates more quickly and prevent odor pollution [45,46]. After application to the soil, these plantchar-based substrates appear to increase complex formation and thus soil build-up and soil organic matter formation [47,48]. In this way, WCM-hydrochar can become an essential component of sustainable organic material flow management. Overall, the use of hydrochar as a soil conditioner offers high agronomic potential and many positive ecological side effects. This offers a promising perspective, especially for organic farming [21].

The produced hydrochars have different contents of C, ranging from 48.72 to 59.67 (

Table 6. Analysis for the van-Krevelen diagram.

Sample ID	C%	H%	O%	H/C	O/C	C/N
P1-200-120	48.72	5.27	44.81	0.92	0.11	53.83
P2-200-180	49.16	5.41	44.28	0.90	0.11	54.02
P3-200-240	50.50	5.38	42.92	0.85	0.11	55.49
P4-220-120	52.00	5.23	41.54	0.80	0.10	52.79
P5-220-180	51.17	5.62	42.10	0.82	0.11	57.49
P6-220-240	52.97	5.40	40.17	0.76	0.10	44.70
P7-240-120	56.76	5.09	36.47	0.64	0.09	39.83
P8-240-180	53.42	5.60	39.62	0.74	0.10	46.86
P9-240-240	59.67	4.91	33.49	0.56	0.08	36.83
146-GPM	49.39	6.50	42.49	0.13	0.86	37.64

). The relatively higher temperature leads to a higher degree of carbonization. This is further evidenced by the results of the van-Krevelen diagram (Figure 7). Using the atomic ratios of O/C and H/C of the hydrochar we can describe and show the position van-Krevelen diagram and discuss how HTC conditions affect the degree of carbonization. The atomic ratios of O/C and H/C of the produced hydrochars decrease with increasing temperature and retention time. Although the hydrogen content in the samples, ranging from 4.91 to 5.62%, seem to be relatively stable, the oxygen content decreases from 44.81% to 33.49% with higher temperatures and retention times. The O/C ranges from 0.08 to 0.11 and the H/C ranges from 0.56 to 0.92. The analyzed samples are in the range of bituminous coal. These results go hand in hand with data on fiber-containing plant-based residues [20].

4. Conclusions

The results, both in terms of hydrochar yield and chemical as well as thermal properties, show that HTC represents a suitable way to energetically valorize WCM. Furthermore, higher temperatures and extended residence times during hydrothermal carbonization (HTC) greatly improve the efficiency of removing water and increasing the concentration of dry matter in water care material (WCM). The lower heating value (LHV) remains constant, indicating a continuous energy content, which makes the product a feasible solid fuel. HTC successfully transforms WCM into a material that is both more energy-efficient and easier to manage. The positive effects of the use of hydrochar on agricultural greenhouse gas emissions are well known. In addition, the hydrochar produced has useful characteristics for soil amendment, such as water retention capacity, absorption properties and valuable content of plant nutrients. There are ecological and economic interests in the acquisition of new biomass sources and thee development of innovative utilization and recycling concepts. In the context of energy crisis, land use competition and status or ecological potential of water bodies, the approach of harvesting highly productive, freely available biomass from water maintenance and recovery can serve as a model for sustainable water management and sustainable resource management. However, although the process of HTC is technically feasible, due to demanding substrate handling, logistical challenges and a lack of clarity of responsibilities, the economic efficiency of the utilization of WCM is currently controversial in practice. To address these gaps, future works should lead to concrete demonstration projects with approach-specific results. Current research on utilization options for heterogeneous, fiber-containing biomass offers good opportunities in this regard.