Process Waters from Hydrothermal Carbonization of Waste Biomasses like Sewage Sludge: Challenges, Legal Aspects, and Opportunities in EU and Germany

Abstract

Hydrothermal carbonization (HTC) has developed considerably over the last 15 years and offers a viable alternative for the utilization of municipal and industrial organic waste such as sewage sludge. However, the technology has yet to establish itself as a valorization process for waste biomasses (2024) and is not yet a recognized state of the art. Nevertheless, the HTC technology could gain greater relevance in the future, especially as an alternative valorization pathway for sewage sludge. During HTC, significant amounts of HTC process water (PW) are produced as a byproduct. The process water is inorganically and organically polluted and has to be treated, as it would be a burden on water bodies and thus on the environment if left untreated. In the EU and specifically Germany, industrial wastewater producers like HTC-plant operators are obliged to treat their industrial wastewater before discharging it into the environment. In addition to a large amount of PW and its treatment to the required limits, the organic load and possible persistent and toxic substances pose major challenges for plant operators. Many proven processes from industrial wastewater treatment were transferred for the treatment of PW. Treatment of the PW in a manner that is industrially viable, economically viable, and efficient is crucial for the effective commercialization of HTC technology. In this, the challenges and opportunities of PW composition, management, and treatment, including legal aspects, are mainly discussed. Therefore, the legal framework in the European Union and specifically for Germany will be elaborated. Furthermore, different treatment pathways are also highlighted.

1. Introduction

Due to increasing population growth and growing industrialization, the amounts of municipal and industrial organic waste are increasing. A sustainable and efficient circular economy requires innovative technologies that valorize and convert residual and waste materials in a climate-neutral way. In addition to the well-known biochemical conversion processes, which convert waste biomasses, e.g., into methane, hydrogen, ethanol, or methanol, the thermo-chemical conversions become interesting as they can generate climate-neutral bioenergy. Thermo-chemical processes include, on one side dry processes such as combustion, technical gasification, pyrolysis, and torrefaction, and on the other side hydrothermal processes (HTP) [1,2].

With the increasing population and urbanization, the amount of sewage sludge that needs to be managed and disposed of is also rising. Sewage sludge is waste that is produced as a byproduct during wastewater treatment. This waste has to be treated and disposed of. On the other hand, sewage sludge contains nutrients such as phosphorus that can be recovered [3,4]. The disposal of sewage sludge is country specific and differs significantly from country to country. In Europe (EurEau) in 2021, approximately 47.5% of sewage sludge was land-based utilized. Another 27.5% was recycled thermally, 8.3% agriculturally, 5.6% via landfill, and 11.4% in other ways [5].

At the European level, Directive 86/278/EEC is the directive on the use of sewage sludge in agriculture and the related protection of the environment. In Germany, the German Sewage Sludge Ordinance (AbfKlärV 2017) provides the legal framework for sewage sludge disposal. The ordinance originally transposed the regulations of Directive 86/278/EEC into national law. According to this law, sewage sludge is waste from the finished treatment of wastewater in WWTP, but it excludes screen, sieve, and grit trap residues [6]. In 2017, an amended version of the Sewage Sludge Ordinance was published, replacing the old one from 1992. Based on data from the Federal Statistical Office of Germany, about 1667,083 tons of dry matter (DM) of sewage sludge were generated in Germany in 2022 [7]. With an approximate European (EU27) sewage sludge volume of 10 million t DM [8], this equates to 16.67%. Utilization in agriculture and farming has been a widespread method of recycling sewage sludge. But in addition to the mentioned nutrients, sewage sludge also contains heavy metals and organic pollutants, making its direct application for agriculture purposes difficult [3]. According to the German Sewage Sludge Ordinance (AbfKlärV 2017), sewage treatment facilities larger than a 100,000 population equivalent (PE) and 50,000 PE will no longer be allowed to recycle sewage sludge for land-based utilization starting in 2029 and 2032, respectively. At the European level, there is not yet an adapted and updated version of Directive 86/278/EEC. As the regulation still applies, the latest technologies and alternative options for sewage sludge utilization are not considered [8]. However, the trend in sewage sludge disposal in Germany is now towards thermal disposal, also due to the new regulations of the German Sewage Sludge Ordinance (AbfKlärV 2017) [9,10].

Hydrothermal carbonization (HTC) is a promising alternative for the utilization of particularly moisture biomasses and municipal and industrial organic waste like sewage sludge or manure. HTC converts the feedstock into lignite-like hydrochar [11]. Friedrich Bergius first presented the process in 1913 as an imitation of the natural generation of hard coal in controlled process conditions [12]. The process takes place in an aqueous environment without the need for the pre-drying of waste biomasses, making direct processing a major advantage of the technology. For example, mechanically dewatered sewage sludge produced in WWTP could thus be processed directly without further drying [13]. The sewage sludge is converted to a carbonaceous product under temperatures (180–280 °C) and pressures below the saturation vapor pressure in several parallel or serial chemical-reaction mechanisms [14]. Figure 1 below shows the basic reaction pathways for the HTC of waste biomasses and the distribution into hydrochar and process water. The main reaction mechanisms of hydrothermal carbonization are hydrolysis, dehydration, decarboxylation, condensate polymerization, and aromatization [15]. Water acts as a reaction environment, reactant, and solvent [2]. During the process, a series of reactions results in numerous dissolved intermediates. Polymers like carbohydrates first break down into simpler components (e.g., di- and monosaccharides) by hydrolysis. The hydrolysis of cellulose produces glucose and fructose. Lignin, on the other hand, is converted to phenol [2]. According to Funke and Ziegler [15], the retention temperature for hydrolysis has to be above 180 °C. As the HTC process continues, various platform chemicals, such as 5-HMF (5-(Hydroxymethyl)-2-furaldehyde), furfural, or levulinic acid, are formed. Furthermore, Maillard reactions take place at certain reaction temperatures and retention times, which contribute to the formation of hydrochar but also lead to high contents of total organic carbon (TOC) and total nitrogen (TN) in the resulting PW [16,17]. It has been widely reported that 5-HMF and furfural are crucial compounds for char formation [18]. The key reaction that leads to the formation of hydrochar is aldol condensation [18,19], which results in hydrochar.

HTC generally produces two product phases, i.e., solids (hydrochar) and liquid (process water). The resulting hydrochar is essentially hygienic, easy to dewater, and expected to exhibit moderate energy density [20,21]. The yield of hydrochar produced depends on various reaction parameters, of which temperature and retention time play a decisive role [17,22,23]. Applications of hydrochar include nutrient recovery, energy recovery, remediation of wastewater pollution, or possible use as a soil conditioner [3,4]. The process water is inorganically and organically polluted and has to be treated, as it would be a burden on water bodies and thus on the environment if left untreated [24]. In Section 2 of this review paper, the characterization and influences on the composition of PW are mentioned. Through the hydrothermal treatment of moist waste biomasses, the HTC process produces significant amounts of PW. The quantities/yields depend on the process parameters as well as on the input material itself [25,26]. A well-functioning, reliable, and economical disposal, respectively, and treatment of the PW is essential for the successful commercialization of the HTC technology [27].

At the European level, the legal framework for water protection and the associated obligations for wastewater disposal are regulated by Directive 2000/60/EC (Water Framework Directive). This directive aims to standardize European water protection law. In the context of wastewater treatment and disposal, both Directive 91/271/EEC (Urban Waste Water Treatment Directive) and the Industrial Emissions Directive (IED, 2010/75/EU) are worth mentioning. Directive 91/271/EEC concerns the collection, treatment, and discharge of urban wastewater and the treatment and discharge of wastewater from certain industrial sectors. EU member states are obliged to report regularly to the EU Commission on the status of implementation of the Directive’s requirements [28]. In combination, these directives regulate the handling of industrial wastewater such as discharge into municipal wastewater treatment plants [28]. Annex 1 of the Urban Waste Water Treatment Directive specifies the requirements for discharges of industrial wastewater into sewers and municipal WWTPs. Furthermore, requirements for discharges from municipal WWTPs are listed in the form of concentration values. The European Commission has announced that it will revise the directive and would like to reorganize European law on wastewater disposal with a long-term perspective [29].

Wastewater handling and its treatment in Germany is regulated by the Federal Water Act (WHG), which is, among other aspects, a translation of the Water Framework Directive into national law [30]. A definition of wastewater is given in § 54 of the Act and PW from the hydrothermal treatment of waste biomasses is wastewater as defined by this act. It is therefore subject to the treatment obligation (§ 56 WHG). This takes on a key role, the principles of which are discussed in section 55 WHG. According to WHG, wastewater has to be disposed of in such a way that the public welfare is not impaired. Wastewater can be discharged either directly into a body of water (§ 57 WHG) or indirectly into a municipal WWTP (§ 58 WHG). For both cases, legal requirements and respective permission by the responsible authority have to be fulfilled. Direct discharge requires a water law permit from the authority (§ 8 WHG). In the context of PW from hydrothermal treatment, this means that the PW has to be treated either to direct discharge quality or to indirect discharge quality. Reaching and keeping the minimum required values is a prerequisite for this. These are listed in the Waste Water Ordinance, AbwV) [31]. Among other things, the ordinance regulates the discharge of wastewater into water bodies and implements the guidelines of the above-mentioned Urban Waste Water Treatment Directive and Industrial Emissions Directive. According to Londong and Rosenwinkel, 2013, the best available techniques (BAT) must also be taken into account [32]. The German Waste Water Ordinance’s Annex 27 “Treatment of waste by chemical and physical processes (CP plants) and waste oil regeneration” applies to process waters resulting from hydrothermal treatment [31]. A decision tree regarding the disposal of process water from hydrothermal carbonization is shown in Figure 2. This is largely based on the legal requirements for industrial wastewater.

Process waters and their treatment/disposal are one of the biggest challenges for the commercial implementation of HTC technology. Efficient, economical, and industrially suitable disposal of PW is essential for the successful commercialization of HTC technology. The technology has yet to establish itself as a valorization process for waste biomasses (2024) and is not yet a recognized state of the art. The technology was temporarily tested on a sewage treatment plant in Düsseldorf (Germany) but was discontinued after a test phase [33]. Currently, there are various approaches to the industrial implementation of the technology as a process, optionally as a multi-batch process (quasi-continuous) or as a process in tubular reactors [26,34]. There are some providers of commercial HTC solutions, like TerraNova Energy, HTCycle, or SunCoal Industries [3]. However, there are no data on the efficiency of these technologies, and there is a lack of continuous operation on a large scale.

2. Methodology

This review presents HTC as a utilization technology for sewage sludge and other municipal and industrial waste biomasses. The challenges and opportunities of process water composition, management, and treatment, including legal aspects for disposal and treatment, are mainly discussed. Therefore, the legal framework in the European Union and specifically for Germany will be elaborated. Furthermore, different treatment pathways are also highlighted.

A literature search was carried out to obtain a comprehensive picture of process waters from hydrothermal carbonization. Google Scholar and Web of Science were chosen as the search engine and the database for this work. The search rule is that both “process water” or “aqueous phase” or “liquid phase” and “hydrothermal carbonization” appear at least in the title. The focus was on contributions to process water, various treatment technologies, and the relevant specific literature about industrial wastewater treatment and anaerobic technology. Approximately 144 studies were retrieved, of which 105 were used for this review. For a fundamental evaluation, 85 relevant manuscripts were carefully reviewed and evaluated. These formed the basis for this review paper. In addition to the legal aspects, the relevant European and German laws and directives relating to water law and pollution control law were consulted.

3. Process Water Characterization and Influences on the Composition

3.1. Overview

The PW is an aromatic-smelling, brownish-to-black-looking liquid. The composition of PW is complex and has been the subject of many scientific investigations [25]. Xu and Jiang [35], performed HTC trials at temperatures between 180–300 °C. By increasing the reaction temperature, the color of the PW lightened from black–brown at 180 °C to yellow–brownish at 300 °C. Depending on the technology, fresh process waters are filtrated. Therefore, the amount of filterable substances can be ignored [36].

In general, the characterization depends on various general conditions, which then affect the specific composition. These include primarily the type of waste/biomass used as input material (lignocellulosic biomass or non-lignocellulosic biomass), and reaction parameters such as temperature, residence time, pressure, etc. For the characterization of the PW, important analytical parameters of wastewater analysis are used, among others. These include pH value, chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), TOC, dissolved organic carbon (DOC), total combined nitrogen (TN_b), ammonium nitrogen (NH₄-N), total phosphorus (TP), and ortho-phosphate (PO₄-P). However, these can be extended by specific parameters, such as organic acids (VFA), hydrocarbons, furfural, 5-HMF (5-(Hydroxymethyl)-2-furaldehyde), phenols, and some other anions like chloride (Cl⁻) or sulfate (SO₄²⁻) [36,37,38]. To characterize the PW, it is also helpful to look at the legally required limit values for certain parameters. Annex 27 of the German Waste Water Ordinance (AbwV), which applies to PW in Germany, as mentioned in Section 1, also lists the following parameters that have to be determined in the context of treatment to indirect discharge quality: adsorbable organic halides (AOX), arsenic, lead, cadmium, total chromium, chromium VI, copper, nickel, mercury, zinc, cyanide, sulfide, chlorine, benzene and its derivatives, and total hydrocarbons. Annex 27 also contains additional parameters (like COD, nitrite-nitrogen, and total nitrogen) and requirements for direct discharges that deviate from these [31].

PW is inorganically and organically contaminated. The organic load of PW is generally high and depends, among other things, on the mentioned input material as well as on the HTC process parameters [25,39]. Short-chain organic acids (VFA) represent a majority of the organic load. The main acids are acetic acid, propionic acid, butyric acid, valeric acid, and levulinic acid. The VFAs are formed by the hydrolysis of the input substances [40,41]. According to Reza et al. [14], the formation of organic acids is probably independent of the input materials but rather a result of the degradation of sugars during the HTC reaction. It was observed by Hoekman et al. [42], and also by Becker et al. [43], that the process conditions affect the concentrations of lower fatty acids (C2–C6) and these acids contribute to the acidic character of the PW.

Usman et al. [39] wrote a critical review of PW and its characterization and utilization. They gave an overview of lignocellulosic and non-lignocellulosic biomass and the major components that can be found in PW. Food waste and sewage sludge are examples of non-lignocellulosic biomasses, whereas crop straw and woody biomass contain lignocellulose. They mentioned volatile fatty acids, benzene, acetic acid, carbonic acid, alkenes, phenolic, aromatic compounds, and pyrazines as major components of PW from sewage sludge [39]. Danso-Boateng et al. [41], came to similar results and found that these compounds occur at all reaction temperatures and retention times. According to Hoekman et al. [42], PW from lignocellulosic biomasses contains many sugars, sugar alcohols, carbohydrate compounds, furfural, 5-HMF, and organic acids.

Danso-Boateng et al. [41], performed various HTC experiments with primary sludge from wastewater treatment. For this, different reaction temperatures (140–200 °C) and residence times (15–240 min) were chosen. The obtained PW was analyzed and indicates that TOC, COD, and BOD values are much higher than those of common domestic wastewater [41]. Quicker and Weber, 2016, give ranges of values for COD from 4900 to 78,000 mg L⁻¹, for BOD₅ 1700–42,000 mg L^−1, and TOC 4000–31,700 mg L⁻¹ [34]. TOC includes mainly organic acids, sugars, and fatty acids [44]. For PW from the HTC of sewage sludge, it has been observed that organic precipitation occurs when PW is stored for a longer time. It was also found that the concentrations of both dissolved COD and TOC decreased because of the storage [36,45]. The COD–TOC ratio can vary between 2.49 (sewage sludge) [23] and 4.92 (mixed MSW) [46]. The COD–BOD₅ ratio also varies from 2.00 (anaerobically digested waste) [46] to 2.70 (mixed MSW) [47]. Higher COD–BOD ratios are also found in the literature. COD–BOD₅ ratios of 2 to ≤1 indicate good biodegradability according to Stiefel, 2020, whereas a ratio of five or higher indicates poor degradability. The COD–BOD₅ ratio can, therefore, provide initial information on the biodegradability of PW. However, it should be noted that an activated sludge adapted to the PW may be used for the BOD₅ determination to be able to make statements on the degradability [37].

Table 1. Overview of the various literature findings about hydrothermal carbonization (HTC) of wastes/biomasses and the corresponding characterization of the process water. If not marked, these are actual values and not average values with statistical error.

Input Material	HTC Reaction Parameters		HTC Process Water							Source
	Temperature [°C]	Retention Time [min]	pH-Value	Chemical Oxygen Demand [mg L−1]	Total Organic Carbon [mg L−1]	Dissolved Organic Carbon [mg L−1]	Volatile Fatty Acids [mg L−1]	Ammonium-Nitrogen [mg L−1]	Total Phosphorus [mg L−1]
Mixed municipal solid waste	250	10		20,500.00	6700.00					[48]
	250	60		22,500.00	7100.00
	250	360		21,000.00	6100.00
	280	10		25,500.00	8160.00
	280	60		24,000.00	7800.00
	280	360		30,500.00	6200.00
	310	10		25,000.00	7700.00
	310	60		23,500.00	6750.00
Sewage sludge as raw sludge	220	135				8510.00		975.00		[23] 1
	190	30				9160.00		575.00
	190	240				9395.00		730.00
	250	30				8055.00		980.00
	250	240				7640.00		1223.00
	220	30				9160.00		783.00
	220	240				8520.00		1045.00
	220	30				8650.00		858.00
	220	240				8335.00		995.00
Anaerobic digested sewage sludge	160	30	9.15	12,642.00	4686.77		191.10	1258.00	94.03	[46]
	220	30	7.14	12,992.00	4583.71		406.00	1704.00	72.60
	250	30	8.08	12,164.00	4879.33		715.00	1685.00	103.83
Cow manure	180	5	6.4 ± 0.1		4400.00			0		[49]
	220	5	5.3 ± 0.1		10,000.00			100
	260	5	5.1 ± 0.1		9200.00			100
	180	30	5.8 ± 0.3		4600.00			100
	220	30	4.7 ± 0.2		6100.00			100
	260	30	4.4 ± 0.3		7100.00			100
Poultry litter	180	60	5.1 ± 0.01			25,151.00 ± 1064			2301.00 ± 23	[50]
	200	60	5.5 ± 0.02			19,352.00 ± 516			1896.00 ± 36
	220	60	5.7 ± 0.03			19,242.00 ± 492			1303.00 ± 46
	250	60	5.3 ± 0.05			19,999.00 ± 916			856.00 ± 29
Anaerobic digested sewage sludge (addition of citric acid to HTC process)	200	360	4.7	34,300.00	13,400.00					[45]
Paper	250	1200	5	76,000.00	27,000.00					[47]
Food waste	250	1200	5.3	52,000.00	18,000.00
Mixed municipal solid waste	250	1200	4.8	62,000.00	19,000.00
Anaerobic digested waste (dried)	250	1200	8	10,000.00	4000.00

Note: ¹ The values are only an excerpt of the results. For full results, see [23].

below summarizes some of the literature findings about HTC of various wastes/biomasses and the corresponding characterization of PW. Different sources were taken into account in order to represent a broad diversity.

It can be seen from the literature that PW is mostly acidic [36], although there are also exceptions where the pH value is neutral or even basic. However, the pH value is, again, dependent on the input material used, the process conditions, and the possible addition of acids or bases during the HTC process. Blöhse, 2017, gives a pH value range of 3.7–5.6 [26]. Nevertheless, higher pH values may occur (see Table 1). As shown by Marin-Batista et al. 2020, anaerobically digested sewage sludge can have a basic pH value [51]. The sewage sludge’s natural buffering capacity is a major contributing factor in deciding the pH, among other aspects [26].

With regard to the composition of PW, there could be both harmful (e.g., phenols, phenolic compounds, furfural, and 5-HMF) and beneficial (nutrients, e.g., for a possible recovery) components. The nutrient load of the PW primarily depends on the input waste biomass for HTC. Sewage sludge, for example, contains nitrogen (2–10%) and phosphorus (2–55 g/kg (raw)) [52], so these nutrients also end up in the PW [25,26]. Manure from poultry, cattle, and pig farming also contains high nitrogen levels of 3.94–5.03% and phosphorus levels of 0.71–1.86%. The high nitrogen levels result from the high levels of crude proteins in the fresh manure (18.1–31.6%) [53]. Kruse et al. [54] performed HTC trials with different biomasses (carrot green, algae Chlorella pyrenoidosa, and straw). The focus was on nitrogen in the resulting hydrochar and process water. The authors found that the three biomasses showed significant differences with regard to the fate of nitrogen. The reason for this was due to the different compositions of the input materials [54]. Nitrogen-containing compounds in the PW results, among other things, from the hydrolysis of proteins to amino acids and the Maillard reaction (see also Figure 1) [16,39,55]. According to Chen et al. [55], amino acids are further converted to ammonium and short-chain fatty acids [55]. The literature also gives ranges for the concentrations for this. For total nitrogen, these are 98–9300 mg L⁻¹ and NH₄-N 0.1–4400 mg L⁻¹ [34].

The transfer of phosphorus from the waste biomass to the process water during hydrothermal treatment has been the subject of various studies. Ekpo et al. [44] investigated the influence of pH on the HTC of swine manure. The extraction of phosphorus from the manure into the process water during HTC was trialed either with water alone or with the addition of sulfuric acid, sodium hydroxide, acetic acid, and amino acid. The results showed that water only had a minor effect on extraction, whereas the addition of sulfuric acid can leach significant amounts of phosphorus into the process water during HTC [44]. Shettigondahalli et al. [56] again conducted HTC trials with sewage sludge and the addition of sulfuric acid before and after HTC. The authors observed that, in contrast to acid leaching after HTC, a significantly lower concentration of phosphorus was converted from the solid phase to the liquid phase by the addition of sulfuric acid before HTC [56].

As mentioned above, 5-HMF and furfural are the main intermediates of HTC and contribute significantly to hydrochar formation [18,43]. Both compounds are known as toxicants that can be found, e.g., in food like coffee, fruits, fruit juice, vegetables, or bakery products [57]. It has been widely reported that 5-HMF is present in the PW [14,39,40]. At this point, it should be mentioned again that the composition of the PW can be very complex and can vary greatly depending on the input substance and reaction parameters [14]. 5-HMF can have a value range of 0.2–1.23 g L⁻¹, depending on the input material and reaction conditions [26], but other studies determined lower concentrations. Huezo et al., 2021, performed various HTC trials with anaerobically stabilized sewage sludge at different pH values, retention times, and reaction temperatures followed by a detailed investigation also of the produced PW. The HTC experiments with unchanged pH showed that the concentration of 5-HMF in the PW increases with an increase in the reaction severity (3.0 mg L⁻¹ at 163 °C and 50 min and 16.8 mg L⁻¹ at 277 °C and 50 min). Similar results were observed with furfural, but the resulting data are not sufficient for conclusive statements to be made. On the contrary, the concentrations of the also-measured cellobiose, xylose, and succinic acid decreased with higher HTC temperatures [22]. It can be assumed here that the carbohydrates undergo a dehydration reaction and are converted to 5-HMF and furfural [19]. Ghosh et al. [57] report that HMF has profound toxicological effects, such as cytotoxicity, mutagenicity, genotoxicity, and carcinogenicity in rats and mice due to its volatile behavior and associated uptake. Furthermore, furfural is said to have ecotoxic effects. Water pollution would be a major problem [57]. For the aforementioned reasons, the treatment of wastewater containing furfural, such as PW from the HTC, is essential. Other relevant organic compounds in HTC process water are phenols. These are aromatic compounds with one or more hydroxy groups (-OH). Phenolic components like phenol, guaiacol, and cresol are toxic pollutants that can be found in certain industrial wastewaters, for example, and pose, even in low concentrations, an environmental risk [58]. Therefore, phenols have to be removed from wastewater before discharge into the environment. Phenols can occur in varying concentrations in process waters from the hydrothermal carbonization of waste biomasses. Blöhse [26] gives a value range of 0.29–0.8 g L⁻¹ for phenol in PW. In addition to the organic components listed, Chen et al. [55] detected further compounds such as pyrazines, ketones, and pyrimidines in PW from HTC of dewatered sewage sludge by GC-MS. An increase in concentrations with increasing HTC reaction temperatures (200 °C, 260 °C, 320 °C) was observed. Decisive for the load of the PW is the carbonized input material and the process conditions. Hydrothermal carbonization of lignin-containing biomasses leads to the formation of phenols in the aqueous phase so that these substances end up in the PW [39,58,59].

3.2. Influences on Process Water Composition

Various studies report the important influence of the temperature and retention time on the process liquid [25,26,39]. Both have an influence on the organic composition, the pH value, and the formation of organic acids. Danso-Boateng et al. [41] observed from the HTC of sewage sludge that TOC- and NH₄-N- concentrations increased in the PW with an increasing reaction temperature and retention time. A slight increase was observed also for COD and BOD. In contrast, the authors noted that organic acids decrease with increasing reaction temperature and retention time. Here, it is assumed that the decomposition of organic acids from the decomposition of glucose and fructose to intermediate products occurs [41]. On the other hand, Erdogan et al. [60] noted that, for the HTC of orange pomace, both TOC and COD concentrations in the PW decreased with increasing temperature and time. They assume that the intermediates formed during HTC are degraded to gaseous products [60]. Li et al. [61] came to different results after performing HTC trials with chicken, cow, and pig manure at 200 °C, 250 °C and 300 °C. For the PW from cow manure, the COD concentration first decreased and then increased with temperature, whereas for the PW from chicken manure and pig manure, it first increased and then decreased. The BOD was largely the same for all PW and independent of the reaction temperature [61]. Furthermore, the influences of the HTC reaction conditions on certain organic compounds in the process water are interesting. Based on HTC trials with various lignocellulosic (woody and herbaceous) biomasses, Hoekman et al. [42] discovered that the concentration of organic acids in the PW increased with increasing the HTC temperature. This affected especially acetic acid and lactic acid, while formic acid, however, decreased with the increasing temperature. Furthermore, it was observed that sugars only increased up to a temperature of 235 °C and then decreased. The authors assume that the sugars are degraded into other degradation products, such as furfural and 5-HMF [62]. Based on studies like these, Becker et al. [43] studied various process waters from the HTCs of several lignocellulosic biomasses. In addition, cellulose and xylose were carbonized as pure substances. At a reaction temperature of 190 °C, furfural reached its highest concentration and decreased with the increasing reaction temperature, whereas the concentrations of 5-HMF differed with the increasing temperature for the different input substances [43]. Erdogan et al. [60] also concluded that furfural could degrade at high temperatures [60]. Becker et al. [43] observed that 5-HMF formed more slowly than furfural and decreased from temperatures of 270 °C in the PW [43]. Hoekman et al. [42] discovered high decreases of furfural and 5-HMF at a reaction temperature of 275 °C [62]. Reza et al. [14] performed similarly to Hoekman et al. [42] and Becker et al. [43] in HTC experiments of grassy and woody biomass and cellulose as well and also studied the process liquid [14]. The PW contains sugars such as sucrose, glucose, fructose, 5-HMF, and furfural, which were most strongly detected at lower HTC temperatures, as well as retention times. Similar to Hoekman et al. [42] the authors found that, above a temperature of 230 °C, the sugars decreased. However, glucose was an exception because it dehydrated to furfural and 5-HMF [14,15]. These observations were partially confirmed by Kambo et al. [40]. When studying the HTC of Miscanthus × giganteus at three different operating temperatures (190, 225, and 260 °C) with residence times of 5 min, Kambo et al. [40] found that the concentration of intermediates in the PW increased with increasing the HTC reaction temperature. In addition to the reaction temperature, the retention time also plays a role in the degradation of intermediates such as 5-HMF. Reza et al. [14] noted that 5-HMF is degraded with retention time at a reaction temperature of 230 °C. Furthermore, the residence time can affect the concentrations of sugars and organic acids in the PW, as shown by Hoekman et al. [14,62].

As mentioned in Section 3.1, the process conditions can affect the pH value. Hoekman et al. [42] observed a slight decrease in pH with an increasing reaction temperature. However, Becker et al. [43] discovered that process temperature did not affect the pH of the respective PW, but the feedstock did [43]. A major influence of the retention time on pH is not assumed by Hoekman et al. [42]. Usman et al. [39] also state that the pH value of the process water depends on the input material used. Therefore, treated input materials should result in an acidic PW for lignocellulosic materials, as stated by Becker et al. [43]. For feedstocks with a high protein content, the formation of ammonia results, on the other hand, in a basic PW [39]. Becker et al. [43] also found that the TOC load was largely independent of the feedstocks used. In contrast, a difference in the TOC load about the input substances can be found in Berge et al. [47], who examined the HTCs of various municipal waste streams at 250 °C for 20 h in each case [47] (see Table 1).

4. Possible Utilization Pathways

4.1. Reuse of Process Water/Recirculation

The first approach to managing the significant amounts of PW from HTC is to reuse a part-flow or complete PW back in the process. The reuse or recirculation of the HTC process water has several advantages. First of all, recirculation does not affect the aforementioned obligation to dispose of the wastewater and discharge it into a body of water or municipal WWTP, as the process water is reused internally. This reduces the operating costs for the HTC process. Furthermore, recirculation can reduce the amount of freshwater (if needed) for the HTC process, which also has an impact on operating costs [40]. In addition, the reuse of the PW can achieve various impacts on the HTC process, as evidenced by many scientific studies on this topic. For example, increases in carbon yield, energy density, and nutrient contents are often listed. The effects of reuse are closely dependent on the process conditions and the waste biomass to be converted [63,64]. Sewage sludge, e.g., can contain different amounts of water (65–75%) depending on the dewatering technology so that the dry matter content behaves accordingly [52].

With regard to the effects mentioned, it was found, for example, that the reaction temperature can influence the effectiveness of PW recirculation. Köchermann et al. [64] performed HTC trials with municipal green waste and completely reused the gained PW for subsequent trials. They also investigated the influence of the reaction temperature. A slight increase in hydrochar yield was observed due to recirculation. This effect was enhanced when the reaction temperature was increased in the subsequent HTC experiments with PW recirculation. This additionally led to an increase in energy density, although the higher heating value (HHV) did not increase. Furthermore, increasing the reaction temperature could favor the increase of organic acids in the PW such as acetic acid, lactic acid, and propionic acid [65]. The organic acids can catalyze certain reactions and thus accelerate the HTC process [63,65].

Picone et al. [66] evaluated the current scientific knowledge of PW recirculation from the HTC of different waste biomasses. They concluded that the PW produced in each case contained organic acids that could enhance the hydrolysis reactions during recirculation. In addition, reuse could increase the concentration of reactive organic compounds. Arauzo et al. [63] also noted in this context that the enrichment of organic acids in the HTC process water through recirculation may catalyze some reactions in the HTC process. This was confirmed by Köchermann et al. [64] as described above. According to Picone et al. [66] soluble organic compounds could be involved in various reactions in the course of char formation and, thus, increase the yield of hydrochar. Furthermore, the reuse of the PW and the organic acids present in it promote the dehydration and decarboxylation reactions occurring in the HTC reaction mechanism [40,63,67]. According to Stemann et al. [67] this leads to a decrease in functional groups in the hydrochar but also improves the dewatering behavior [68].

Similar results were obtained by Boutaieb et al. [68] based on experiments with the HTC of Tunisian pine cones. Recirculation of the PW improved the yield of hydrochar and led to an increase in char. Furthermore, the reuse of the process water seems to influence the surface structure of the char formed [69]. However, Boutaieb [68] also describe that the effects of recirculation are largest for the first reuse of the PW and significantly lower for subsequent cycles [69]. Weiner et al. [69] analyzed the recirculation of PW from the HTC of paper. Reuse led to a significant increase in mass yield after initial recirculation, but carbon yield did not increase [70]. Arauzo et al. [63] came to similar results from the HTC of brewer’s grains and found that the first PW recirculation was most effective. Reuse continues to polymerize the intermediates present, which affects the surface structure of the hydrochar formed. Recirculation of the PW not only leads to an increase in carbon yield but also has effects on the distribution of nitrogen, as Wüst et al. [70] also observed from the HTC of brewer’s grains. The trials and analyses revealed that the reuse of PW supports the formation of nitrogen-doped hydrochars. Up to about 90% of the dissolved nitrogen in the PW can be incorporated into the hydrocarbons when reused. Maillard-type reactions enable that. Especially at reaction temperatures of 240 °C, recirculation can be used to produce nitrogen-doped hydrochars for use as precursors for the production of electrode materials [70].

Heidari et al. [71] noted a 12% increase in the mass yield of hydrochar formed and a 2% increase in the HHV after the first recirculation of PW from the HTC of sawdust. Here, the connection between the organic acids present in the PW and the dehydration and decarboxylation reactions taking place in the HTC process, already described above, was found. Like Köchermann et al. [64] they found that an increase in HTC temperature leads to an increase in volatile organic acids, which increase the mass yield during recirculation [71]. Mau et al. [72] converted poultry litter using HTC and investigated the reuse of PW for nutrient concentration and application in agriculture. Recirculation increased N, P, and K when reused up to three times. Regarding a correlation between recirculation and treatment of HTC process water, Weiner et al. [69] concluded that recirculation reduced biodegradability. They assume that substances that are difficult to biodegrade, such as phenols and benzenediols, accumulate due to the reuse of the PW. In general, the scientific evidence obtained so far concludes that PW recirculation reduces wastewater volume and, thus, operating costs and has various positive effects on the HTC process.

4.2. Treatment Concepts

In addition to the frequently mentioned legal requirements that necessitate treatment of the HTC process water, the ecological aspects of treatment should not be ignored. The hydrothermal treatment of a wide variety of municipal and industrial wastes, most notably sewage sludge, probably brings, inevitably, micropollutants of anthropogenic origin into PW. The aforementioned recirculation is one option to handle PW and leads, e.g., to a reduction in wastewater treatment costs. In addition to the reuse, there are currently a few options for the treatment of PW. Anaerobic technology is known for the treatment of industrial organically polluted wastewater. In addition, however, the elimination of trace substances and the further development of resource-saving and cost-effective processes must also be pursued [73]. Rosenwinkel et al. [73] describe sole anaerobic technology as a process for treating industrial wastewater to indirect discharger quality. For direct discharge of the treated wastewater, aerobic treatment (such as the activated sludge process) must be added after anaerobic treatment for further COD reduction and the removal of possible nutrients like nitrogen [73]. On the other hand, Blach et al. [74] have found from various experiments that aerobic treatment of PW alone is not sufficient for discharging into a municipal WWTP. According to Fettig et al. [75], this should also be viewed critically, as limited denitrification capacity was found in trials with PW. Nitrogen backloads would have to be feared. Another problem would be refractory organic substances (measured as refractory COD), which could exceed the discharge limits for treated wastewater [76].

Aragón-Briceño et al. [3] evaluated the current scientific knowledge about nitrogen and phosphorus recovery from HTC of wet biomasses. They concluded that HTC is an attractive technology for nutrient recovery and mentioned PW as a source for these nutrients [3]. However, it should also be noted that high nitrogen loads (as can occur in the PW) must be removed from the effluent of an anaerobic treatment in a subsequent treatment step. Possible innovative aerobic technologies like nitritation/denitration or deammonification may be appropriate [74]. Fettig et al. [36] presented a concept for process water treatment including anaerobic treatment as a first step, followed by aerobic treatment as the second step, and treatment with activated charcoal as the last step. They used brewer’s grains, sugar beet pulp, and food scraps as input material for HTC and generated a PW with COD ranges from 33.6 to 84.4 g L⁻¹. However, a direct implementation of the described concept was not yet possible, as there were open questions regarding anaerobic treatment at the time [36]. Anaerobic technology as a possible treatment option for PW is presented in the following Section 4.3. In a follow-up project, it was shown that, in the case of nutrient-rich PW, nutrient elimination must take place before anaerobic treatment, as otherwise, mineral precipitation such as struvite can occur [75].

Various providers of HTC solutions have their treatment technologies, like nanofiltration and reverse osmosis [75,76] or aerobic treatment in a constructed wetland [76]. However, no data were available on the efficiency of these technologies, and continuous large-scale operation is lacking.

Table 2. Treatment concepts of some commercial HTC providers.

Provider	Treatment Concept	Source
AVA-CO2 Schweiz AG	nanofiltration and reverse osmosis, recirculation of concentrate into HTC process	[76]
CS Carbon Solutions GmbH	nanofiltration and reverse osmosis, discharge of permeate into wastewater treatment plant	[75]
Grenol GmbH	aerobic treatment in a constructed wetland	[75]
SunCoal Industries GmbH	thermal processing by evaporation and condensation	[77]

lists some of these treatment technologies. Other commercial providers of HTC solutions are not listed in this table.

According to Londong and Rosenwinkel [32], the legal requirements for the treatment of wastewater in the mentioned German Wastewater Ordinance (AbwV 1997) are regularly reviewed for their state of the art. BATs are also included for this purpose. This must be taken into account in the treatment concepts for PW.

4.3. Anaerobic Treatment

A well-known and successful option for industrial wastewater treatment is anaerobic digestion (AD). Anaerobic wastewater treatment processes have been used for many years for the treatment of organically highly polluted industrial wastewater from the food, beverage, pulp, and paper industries [78]. The advantages of AD are slow sludge production and low energy costs, as the process takes place in the absence of oxygen and, therefore, requires no aeration technology. The biogas generated by the AD of wastewater can be used for energetic purposes [73,79]. In 2008, there were 250 industrial-scale anaerobic plants for industrial wastewater treatment [80]. Over time, a variety of reactor types have been developed for the treatment of such wastewater. According to Austermann-Haun et al. [80], anaerobic reactors can optionally be differentiated according to the type of biomass retention and the type of biomass (flocculent or granular). The most important reactor types are the anaerobic activated sludge process, UASB reactors (Upflow Anaerobic Sludge Blanket), EGSB reactors (Expanded Granular Sludge Bed), fixed-bed reactors (anaerobic filter, AF), and fluidized bed reactors. In addition, there are also new developments such as membrane-supported reactors [80].

Process water from the hydrothermal carbonization of municipal and industrial organic waste such as sewage sludge may be suitable for anaerobic digestion due to the described composition (high COD loads, high concentrations of volatile organic acids like acetic acid, etc.). Numerous researchers like [38,45] or [81] have already focused on this topic. Many influencing parameters can affect the AD of the PW. One influence is the relationship between the reaction temperature of hydrothermal conversion of sewage sludge and methane yield in PW AD, as reported by Chen et al. [55]. The HTC experiments were performed at temperatures ranging from 140 to 320 °C and times ranging from 0.5 to 6 h. The authors noted that the methane yield decreased with an increasing reaction temperature as well as with an increasing retention time. Hardly degradable compounds, such as humic acids and humic compounds, melanoidins, nitrogen heterocycles, and phenols are suspected to lead to a lower methane yield at higher HTC reaction temperatures. Furthermore, reference is also made to the microbial composition of the anaerobic reactor, which varies depending on the PW [55]. The toxicity of phenols for anaerobic microorganisms, and the possible resulting disruption of biogas production, was also observed by Wirth et al. [59]. However, phenols in wastewater can be degraded up to an initial concentration of up to 2000 mg L⁻¹ by slowly adapted microorganisms [59]. Other important parameters are the organic load and inoculation-to-sludge ratio. Their importance was researched by Villamil et al. [81] for the AD of PW from the HTC of sewage sludge. The PW had high COD (95.5 g L⁻¹) and TKN (total kjeldahl nitrogen) values (8.7 g N L⁻¹). Methanogenesis was inhibited due to a low inoculation-to-sludge ratio (≤0.5), high COD loading of 25 g COD L⁻¹, and high NH₄-N and VFA release. As a result, only less than 15% of the COD in the PW could be degraded. Therefore, for efficient operation, the organic load and the inoculation-to-sludge ratio must be chosen appropriately [81]. Different anaerobic reactor types and the composition of PW can affect the degradation of COD, as shown by Wirth and Mumme [38]. The authors investigated the AD of PW from the HTC of maize silage. The PW was treated both in a continuous stirred-tank reactor (CSTR) and a fixed-bed reactor (AF). The PW had a COD of 41.35 g L⁻¹ and TOC of 15.66 g L⁻¹. Nutrients were measured as TKN = 685 mg L⁻¹, NH₄-N = 229.50 mg L⁻¹, and phosphorus = 197.40 mg L⁻¹. Both anaerobic reactors were operated mesophilically (37 °C) for 91 d with an organic loading rate of 1 g COD L d⁻¹. During the first five weeks, the removal efficiency was high (80%) and methane production of up to 0.25 L CH₄ d⁻¹ was measured. Subsequently, the removal rates of both reactors decreased due to a lack of phosphorus and sulfur. After 1–5 weeks, the COD removal in the CSTR was 75% and, after 6–13 weeks, only 52%. The fixed-bed reactor proved to be more stable, with 69% in weeks 1–5 and 56% after 6–13 weeks [82]. The influence of digestion temperature and organic loading rate on the continuous AD of PW from the HTC of sewage sludge (COD = 28.2 g L⁻¹) was reported by Wirth et al. [45]. The PW was digested mesophilically (37 °C) and thermophilically (53 °C) in two identical anaerobic fixed-bed reactors. During the experiments, the organic loading rate was increased from 1 g COD L d⁻¹ to 5 g COD L d⁻¹, which reduced the hydraulic retention time (HRT) from 34 to 5 d. A COD removal efficiency of 75% in the mesophilic test and 68% in the thermophilic test was achieved. The methane yield was a maximum of 0.18 L CH₄ g COD⁻¹. No significant differences were found between the two reactor systems. Interestingly, sufficient nutrients were available for the AD of the PW to conduct the experiments [45].

It should be noted that, after the anaerobic treatment, another treatment (e.g., aerobic) is required to directly discharge the PW [73]. In summary, the AD of PW can be considered a possible and promising treatment option. However, the above-mentioned aspects should be carefully considered for the AD of PW to work effectively.

4.4. Co-Digestion of PW

In addition to the anaerobic wastewater treatment with the associated reactor types, it is also possible to co-digest the PW with other substrates. The principle is already known from wastewater technology, where sewage sludge and organic industrial waste, commercial waste, or biogenic waste are fermented together in digesters at municipal WWTPs [83]. There are various approaches to co-digestion of the PW, such as with primary sludge from WWTP [82], biomasses [84], or organic fraction of municipal solid waste (OF-MSW) [85,86]. Different mixtures of PW and the co-substrate were performed for AD, respectively, here. Co-digestion of PW and primary sludge, researched by Villamil et al. [82] resulted in mixtures of 10% PW and 90% primary sludge, a 1.15 times higher methane yield in contrast to mono-digestion of the sludge. However, beneficial effects were not always observed. De la Rubia et al. [85] tested the co-digestion of PW from the HTC of sewage sludge with the organic fraction of municipal solid waste. Mixtures with high PW contents achieved lower methane yields due to organic compounds like phenols or aromatics. Co-digestion did not achieve higher methane yields than sole digestion of OF-MSW [85]. The same results were also reached by Villamil et al. [86]. Wang et al. [84] performed the co-digestion of PW from the HTC of maize straw with maize straw as a co-substrate. In contrast to mono-digestion, co-digestion increases methane production. However, this depends on some factors [84]. The influence of the HTC reaction temperature on the PW already noted by Chen et al. [55] was also observed. Wang et al. [84] observed inhibition of AD by toxic organic compounds such as phenols in the PW but also during high PW rates. The mentioned influence of poorly degradable or toxic organic compounds such as phenols in the PW for the inhibition of AD was also observed by Villamil et al. [82,86]. They also noted an accumulation of organic acids, lower degradation of organic substances, and lower methane production at higher PW percentages from 50% [86]. In summary, co-digestion of PW at low mixing ratios like 10% PW is a viable method for PW valorization, even if it does not necessarily lead to an increase in methane yield.

4.5. Wet Oxidation

Wet oxidation (WO) is a physicochemical technology primarily used to treat industrial wastewater [87,88,89]. The process can be performed with air or molecular oxygen. In this context, it is also referred to as wet air oxidation (WAO). Under high temperatures (180–315 °C) and pressures (2–15 MPa), air dissolves and the existing organic compounds are oxidized. For example, refractory organics are degraded into more easily degradable intermediates such as organic acids or directly to CO₂ [90,91]. There are several reactor designs available for WO, of which the bubble column reactor is the simplest design [92]. The retention time varies from 15 to 120 min. Using catalysts (catalytic wet oxidation), which may vary depending on the wastewater to be treated, the reaction conditions are lowered, and the treating time becomes shorter [93]. Therefore, wet oxidation is becoming increasingly attractive for the treatment of highly concentrated, low-biodegradable wastewater [87,89]. The removal of organic compounds is primarily dependent on the type of wastewater, pH, and reaction conditions [88]. COD removal rates of 75–90% have been reported [88,94] and various scientific studies have been conducted with, for example, wastewater containing pesticides or phenols [89]. COD concentrations of >20 g L⁻¹ should allow for a thermally self-sustaining reaction [88]. In particular, for the treatment of HTC process waters, wet oxidation could be used for pre-treatment before subsequent biological treatment. Reza et al. [91] performed the WAO of PW from HTC of dairy manure and digested sewage sludge. The produced PW had a TOC of 5.6 g L⁻¹ (manure) and 6.5 g L⁻¹ (sewage sludge). Under reaction conditions of 260 °C and 30 min, the TOC in PW could be reduced by up to 60%. The process water was a clear liquid after the process. Short-chain organic acids such as formic acid, acetic acid, propionic acid, and lactic acid were formed as final products [91].

Weiner et al. [88] explored wet oxidation with air to treat PW from the HTC of sewage sludge and straw. The HTC with sewage sludge produced PW with COD values of 24.4–80.4 g L⁻¹ and DOC values of 0.7–28.1 g L⁻¹. Retention temperatures of 120 °C to 200 °C were applied for the WO. The retention times ranged from 30 min up to 6 h and yielded good results in terms of COD reduction (50–70%). A subcritical WO led to the formation of various organic intermediates such as acetic acid. The pre-treated PW was well suited for subsequent anaerobic treatment [88]. Stutzenstein et al. [92], use iron (III) nitrate as an alternative oxidant to treat PW from HTC of sewage sludge instead of oxygen [95]. After HTC, PW was collected with a COD of 70.00 g L⁻¹ and TOC of 30.00 g L⁻¹. The results show that, even when the reaction temperature was reduced from 200 °C to 120 °C, COD and TOC were reduced by 50% and 30%, respectively. The degradation of WO occurred in the first hour, and extending the retention time achieved only minimal improvements [92]. In addition, WO can also be used to treat process waters from hydrothermal liquefaction (HTL), as shown, e.g., by Thomsen et al. [96] from the HTL of sewage sludge. The treated HTL-PW initially had a COD of 28,300.00 mg L⁻¹ and a TOC of 11,900.00 mg L⁻¹. At a temperature of 350 °C and a retention time of 180 min, COD was removed by 97.6% and TOC by 96.1%. The energy consumption was 9.6 kilowatt-hour (kWh) per kg COD at a reaction temperature of 200 °C and a reaction time of 180 min. They also concluded that WO is suitable as a pre-treatment step for treating PW [96].

4.6. Nutrient Recovery

During the hydrothermal carbonization of municipal and industrial waste streams like sewage sludge, nutrients are transferred into the hydrochar and process water. This enables nutrient recovery. Many relevant scientific studies already exist on the topic, such as those by [4,44,95]. However, most studies focus on the recovery of nutrients from hydrochar. Known technologies for nutrient recovery include precipitation and crystallization as struvite (magnesium ammonium phosphate, MAP) or ammonium stripping (e.g., recovery of ammonium sulfate, (NH₄)₂SO₄). Based on the hydrothermal treatment of pig slurry, Ekpo et al. [44] found that significant amounts of organic nitrogen are transferred to the PW. The addition of acids such as sulfuric acid (H₂SO₄) can enhance the extraction of nitrogen into the liquid phase [44]. Similar effects could be observed with the HTC of sewage sludge. Phosphates were transferred and accumulated into hydrochar alongside several heavy metals [56], whereas nitrogen is transferred to the process water as ammonium nitrogen and organic nitrogen [97,98]. If PW contains high nutrient loads, recovery may be worthwhile. The mentioned ammonia stripping could be a possible recovery technology for PW with high nitrogen concentrations. During this process, nitrogen is recovered as ammonium by adsorption on sulfuric acid. The produced ammonium sulfate could be used as fertilizer [99]. However, the yield of ammonium sulfate is limited by the amount of nitrogen in the PW. In some cases, nutrient recovery is necessary as a pre-treatment part of the PW treatment. Fettig et al. [75] conclude that, for nutrient-rich PW, nutrient removal should precede further treatment steps such as anaerobic treatment. This would prevent the precipitation of salts such as struvite or possible inhibition by ammonia. The authors also found that MAP precipitation was proven to be a suitable process for the recovery of ammonium and phosphorus in PW from the HTC of brewer’s spent grains as well as fermentation residues. High efficiency at already neutral pH values was especially positive [75].

4.7. Use as a Nutrient Source for Plant and Fungi Growth

Another utilization path for process waters from hydrothermal carbonization could be direct use as liquid fertilizer for the development and growth of plants and fungi. The fact that many process waters contain the plant-relevant nutrients nitrogen (N), phosphorus (P), and potassium (K) allows for this assumption. The PW is then used as a nutrient source for cultivation analogous to Section 4.6. However, the mentioned poorly degradable and toxic substances could have an impact on plant growth.

Mau et al. [50] performed the HTC of poultry litter and subsequently also examined the PW. Relatively high concentrations of the macronutrients N, P, and K were found in the liquid (N from 1356 to 2292 mg L⁻¹, P from 940 to 2250 mg L⁻¹, and K from 5870 to 6330 mg L⁻¹). However, the authors point out that the PW would have to be diluted so that the nutrients could be present in suitable concentrations for the plants. Furthermore, reference is also made to the need to test organic ingredients such as 5-HMF for inhibition or plant toxicity [46]. The use of PW in hydroponic systems for growing maize was investigated by Celletti et al. [100]. Cow manure was hydrothermally converted, and the produced PW was tested as a possible fertilizer for plant growth. For PW use, three dilution levels were tested with distilled water: 1:30, 1:60, and 1:90. The authors found that a dilution of 1:30 had too high a level of toxicity for plant growth, whereas the other two dilutions were less toxic and allowed the plants to grow. However, the maize plants then showed deficiency symptoms and a change in photosynthesis. The authors also mention phytotoxic substances in the PW [100]. More research is needed in this area. Another application could be the use of PW as a nutrient source for the cultivation and growing of algae and fungi. Tsarpali et al. [101] conducted HTC experiments with algal biomass and used PW as a cultivation medium. The resulting PW showed high concentrations of COD of about 58.00 g L⁻¹, TOC of about 20.90 g L⁻¹, and NH₃ of about 2.20 g L⁻¹. Microalgae cells cultivated in 5% PW in distilled water proved to be an optimal concentration. The algae removed, after 10 days, 95% of the nitrate, 6% of the ammonia, 45% of the phosphate, and 30% of the TOC. The trials also showed that a dilution to 5% PW produced a maximum biomass yield for algae cultivation [101]. Similar results were found by Chen et al. [102] in using PW from the HTC of microalgae–fungi pellets to cultivate the same microalgae and fungi. The PW was diluted 20-fold (5% PW) with deionized water and tested stepwise to cultivate fungi and then microalgae. Among other things, a degradation of NH₄-N, TN, TP, and COD was observed with simultaneous biomass cultivation, so the dilution of the PW is suitable for sustainable cultivation [102].

5. Challenges and Opportunities

In this review, the process water (PW) from hydrothermal carbonization (HTC) and its possible recycling processes were presented and explained. As mentioned at the beginning, the PW is organically as well as inorganically polluted and would lead to immense environmental pollution if left untreated. In particular, persistent and toxic organic compounds (e.g., phenols, phenolic compounds, furfural, and 5-HMF) remain a challenge, as they are formed during HTC and can leach into the PW. These substances are occasionally difficult to degrade biologically (high COD–BOD₅-ratios) and may complicate the process water treatment. The composition of the PW depends primarily on the waste biomass used at the beginning and the reaction parameters. Here, a detailed chemical analysis should be performed to determine the differences between the PWs in terms of the composition and persistence of the organic compounds. This also helps in the evaluation and classification of harmful and useful (nutrients, e.g., for possible recovery) compounds. A major challenge of PW management, in addition to the actual contamination of the PW, is the large quantities that would be produced during the industrial, continuous process, of an HTC plant. An additional challenge for HTC and process water management is still the lack of legal regulations or a legal framework in Europe, specifically in Germany, to which the HTC technology can refer. For example, at the European level, there is not yet an adapted and updated version of Directive 86/278/EEC regarding sewage sludge utilization. Because of this, the latest technologies and alternative options for sewage sludge utilization like HTC are not considered [8]. Here, an overview of the existing and necessary legal conditions should be drafted. Only through education and legal certainty can HTC technology establish itself as an alternative [27].

At the European level, Directive 91/271/EEC applies to the collection, treatment, and discharge of urban wastewater and the treatment and discharge of wastewater from certain industrial sectors. An assessment of the directive is presently underway, since it was first adopted in 1991 and has not been put into practice due to, among other reasons, technological advancements [28]. Annex 1 of the Urban Waste Water Treatment Directive specifies the requirements for discharges of industrial wastewater into sewers and municipal WWTPs. However, the industrial sectors listed in appendix three are primarily focused on the food and beverage industry, meaning that further industrial sectors need to be added. At the European level (and thus also for Germany), either a direct discharge quality, which allows direct discharge into the environment, or an indirect discharge quality, which allows discharge into the municipal WWTP, is legally required. Londong and Rosenwinkel [32] point out that the pollutant load to be discharged must be kept as low as possible and must correspond to the current state of the art. For this reason, the requirements for discharging PW may become more stringent over time, which requires a continuous critical review of the current process water treatment technology. So far, however, there is no industrial implementation of HTC technology in continuous operation that also includes solutions for PW. The missing industrial readiness, including a technical implementation of an intelligent and efficient process water management, as well as the actual treatment plants, still has to be implemented. There are many ways to utilize PW. One opportunity to valorize the PW, save fresh water, and thus reduce costs, is to reuse it in the HTC process [40]. Continuous operation of an HTC plant not only produces large amounts of PW but also requires large amounts of fresh water for the process. Reusing the PW is therefore an obvious option. This step should be chosen to reduce costs and to benefit from the positive effects of recirculation. Various providers of HTC solutions have their treatment technologies, but continuous large-scale operation is lacking. The mentioned missing industrial implementation should be remedied to unleash the potential of the technology. A combination of different treatment processes, as described e.g., by Fettig et al. [36], could be a solution.

The HTC technology offers the opportunity to process moist biomasses and waste without prior dewatering and drying and to recover nutrients. This would make the process an interesting alternative to other thermo-chemical conversion processes, such as incineration and pyrolysis. Especially for nutrient recovery, HTC could be an opportunity to close loops. In addition, for the utilization of wet wastes such as sewage sludge, HTC could be an interesting alternative to sewage sludge mono-incineration, as these do not require drying of the sewage sludge and processing it directly. According to Gerner et al. [103], HTC can currently compete with the current costs of sewage sludge disposal. However, Reißmann et al. [27] point out that HTC for sewage sludge utilization can only compete with other processes with the integrated recovery of nutrients such as phosphorus as well as with efficient process water treatment and other aspects. For the example of sewage sludge utilization with HTC, the plant with subsequent process water treatment and nutrient recovery facilities should be built on the site of a municipal WWTP to enable indirect discharge of pre-treated PW. Reference could be made here to the concept of process water treatment proposed by Fettig et al. [75] (recirculation of PW should be integrated to increase the efficiency of the whole system [40]). Nutrient-rich PW should be subjected to recovery before treatment. The recovered nutrients could be marketed. Following this, anaerobic treatment could take place to degrade organic compounds and generate biogas at the same time. Anaerobic technology would have the advantage that there would be no costs for aeration. The determination and evaluation of the costs incurred should be the subject of further consideration. In summary, there are still a few challenges regarding process water from hydrothermal carbonization, but also some possible solutions. Future research and the next step should focus on a technical and economic evaluation of all available pathways for the treatment and utilization of PW.

6. Conclusions

Process water from hydrothermal carbonization of sewage sludge and other municipal and industrial waste needs to be treated considering the environmental burden if left untreated and the legal requirements. In addition to environmental reasons, there are legal requirements. There are a number of choices available for process water management. In any case, the PW should first be reused to reduce the amount of PW to be treated and to allow for side effects of recirculation such as increasing mass yield and the concentration of nutrients. There are many applications for subsequent valorization, which have been listed and explained. The treatment of PW should be safe, reliable, and cost-effective, which is why anaerobic digestion is a popular method. Other treatment techniques such as wet oxidation or nutrient recovery can be processes that pre-treat the PW and allow discharge to a municipal WWTP. Combining different methods might be sensible. However, it should be pointed out that many of the presented utilization methods still need a lot of research. Moreover, utilizations, such as for the growth of plants and fungi, are not industrial processes. Anaerobic treatment as well as co-digestion of PW, on the other hand, could be industrially applicable, as there has been experience with the anaerobic treatment of industrial wastewater for many years. However, persistent and toxic organic compounds such as phenols can make treatment and recovery difficult, so further research is needed in this area. Finally, there is still no definitive management and treatment of PW, considering the environmental and legal requirements. In any case, the treatment of PW is a mandatory measure to further promote the dissemination and commercialization of HTC technology. Then, HTC could be a way to utilize municipal and industrial waste and provide an alternative to processes such as incineration and pyrolysis.