Hydrothermal carbonization of wet biomass from nitrogen and phosphorus approach: A review

With increasing energy and resource consumption due to population growth, the biorefinery concept is becoming popular. This concept aims to harness all the properties of biomass by producing energy and recovering useful chemical products. Nutrients such as nitrogen and phosphorus play a key role in the world’s food production because they are the main elements used in fertilizer production. Hydrothermal carbonization (HTC) has been presented as a suitable option for energy recovery that can also be used as a pre-treatment for enhanced nutrient recovery. During the HTC process, part of the nitrogen and phosphorus are solubilized into the process water and the other part remains within the hydrochar. Hydrochars are mainly used as soil amendments due to their high content of phosphorus and nitrogen, but in this process, water still contains a considerable concentration of these compounds making it a potential source for their recovery. Therefore, HTC may boost the nutrient recovery strategy by extraction (process water) or densification (hydrochar) from biomass if it is coupled with another nutrient recovery process. This review presents an overview of the phosphorus and nitrogen fate during the HTC process from a perspective of nutrient recovery, presenting existing technologies and future trends. © 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).


Importance of phosphorus and nitrogen recovery
Nutrients such as phosphorus (P) and nitrogen (N) are vital for modern agriculture and fertilizer production and their utilization is closely related to population growth. At the same time, these nutrients represent a significant threat to the environment due to the volume that is disposed of on land and in water. Most of the nutrients disposed of in nature originate from manure production of cattle, pigs, and poultry, as well as the wastewater and organic residuals produced from human activities [1]. The traditional disposal method for manure is application in farmlands, however, the unequal portion of P and N in this waste exceed the nutrient demands of the crops and pasture land resulting in nutrient saturation in the soils [2,3]. Rainwater washes these excess nutrients into soils, taking them into rivers, lakes, aquifers, and oceans, causing algae growth in the water bodies. Algae consume the oxygen in the water bodies affecting the surrounding environment by killing the flora and fauna [4]. Furthermore, P is considered a nonrenewable resource because its extraction depends on the mining of phosphate rock. The importance of P and N application, especially in agriculture, gives an extra added value to wastes with high P and N content and makes them a target for nutrient recovery strategies [5]. Some researchers have proposed different potential scenarios for the production of P and N rich products from different wastes, especially from manure: 1) production of elemental P or N from the solid fraction for the industrial market, such as the food and detergent industries, 2) production of a PeN rich fertilizer from the solid fraction (P and N salts) and 3) production of struvite from the liquid fraction for the agricultural market [5]. Therefore, the scientific community is under enormous pressure to develop technologies that can reutilize the nutrients, especially P, from the waste stream [6]. These will help to mitigate the environmental problems and alleviate the dependence on exhaustible resources like P.

Phosphorus and nitrogen recovery strategies
N and P removal and recovery strategies are mainly focused on sewage sludge, manure, and digestates due to their high nutrient concentration and the high quantities of waste produced [4,7,8]. However, there are other high concentration P and N sources such as food waste, algae, or other industrial waste that should be considered within these strategies. P and N removal and recovery strategies are shown in Table 1. The most common strategies for P and N removal are biological nitrification and de-nitrification, and chemical precipitation. Nevertheless, in these processes, N and P cannot be recovered [8]. On the other hand, there are existing technologies for P and N recovery, such as struvite precipitation, air stripping, or acid washing, among others [8].
The most used P recovery technologies are based on the precipitation of phosphoric minerals from liquid wastes through the formation of struvite, hydroxyapatites, or calcium phosphates [7]. High-quality phosphoric minerals can be obtained from this process with recovery rates of 99e100% and can be applied directly in agriculture [9,10]. For instance, hydroxyapatite precipitate materials are considered safe for the environment due to their low concentration of heavy metals [11]. Moreover, the poor solubility of the struvite can be considered as an advantage because, in the case of over-applying the amount of fertilizer to the ground, there is less likelihood of P filtration to underground water and promotion of eutrophication [12]. Furthermore, P precipitation processes using struvite also remove N, but only in low quantities (12.5%) [7,13]. Another common strategy is P recovery from the ashes of the co- Table 1 -Phosphorus and nitrogen removal and recovery strategies.

Method
Focus Description Recovery -Removal Reference Air stripping process Nitrogen The physicochemical process is carried out at high pH and is applied in a liquid mixture. The high pH converts ammonium ions into ammonia in solution and the air injected into the liquid mixture is used to volatize the ammonia. Then, the ammonia is absorbed by a strongly acidic solution (i.e. H 2 SO 4 ) to form mineral fertilizers. 70e92% Liu et al. [16] and Sengupta et al. [8] Anaerobic ammonium oxidation (anammox) Nitrogen A biological process that converts the NH 4 and NO 2 À to N 2 gas under anaerobic conditions. However, it is important to ensure an appropriate nitrite concentration within the liquid mixture to be able to carry out the anammox process. 70e99% Ge et al. [18], Zekker, et al. [19] and Li et al. [20] Struvite precipitation Phosphorus and Nitrogen The process consists of precipitation of magnesium ammonium phosphate (MAP or Struvite) by balancing the magnesium, phosphate, and ammonium ions (ratio 1:1:1) at pH around 9. The precipitate (struvite) can be recovered by centrifugation or filtration. 99e100% Le Corre et al. [9] and Yu et al. [10] Microalgae cultivation

Phosphorus and Nitrogen
A liquid with high P and N concentrations is removed by growing algae. Algae harvest is mainly through coagulation/flocculation, centrifugation, flotation, etc. Algae can be used for biofuel production.
62e91% of Total phosphorus 48e83% of Total nitrogen Cai et al. [14] and Mulbry et al. [15] Acid wash Phosphorus Application of acids (i.e. HCl or H 2 SO 4 ) to a dry biomass (ash, biochar, or hydrochar) in order to solubilize the P as soluble phosphate. The P is recovered from the solution through an isolation process. 80e90% Heilmann et al. [4] Washing method with extract Phosphorus Consists of extracting P from the solid sample with an extraction solution (i.e. CaCl 2 , Olsen solution, LiCl, etc.) considering as variables the pH, solid to solution ratio, and extraction time. The solution that should be used for extraction depends on the species of P.

Not reported as percentage
Wuenscher et al. [21] Co-combustion Phosphorus Thermochemical conversion of the biomass into ashes at high temperatures (850e1250 C). Most of the P-organic from the biomass is retained in ashes in the form of P-metal consortiums. The P is recovered through mineral or acid extraction.
Up to 100% Cie slik and Konieczka [7] HTC Phosphorus and Nitrogen Solubilization of N and P from biomass through a high temperature (180 e250 C) and pressure (5e45 bar) process in the presence of water. The solid product also retains some of the P and N making it suitable as a potential fertilizer.
Up to 49% of P and up to 41% of N solubilization Arag on-Briceño et al. [22], Heilmann, et al. [4] and Ekpo et al. [2] Acid-supported HTC Phosphorus and Nitrogen Solubilization of N and P through HTC in acidic conditions. The acidic conditions promote the solubilization of the N and P into the process water during the HTC process. The solid product also retains some of the P and N but in a lower concentration.
Up to 100% and 63.4% of P and N solubilization, respectively Dai et al. [1] Alkali-supported HTC Phosphorus and Nitrogen Solubilization of N and P through HTC in alkali conditions. The alkali conditions promote the solubilization of the N and P into the process water during the HTC process.
Up to 16% of P and up to 49% of N solubilization Ekpo et al. [2] Electro dialytic process (ED) Phosphorus This process was developed by the Technical University of Denmark in 1992 and was patented in 1995 (PCT/DK95/00209). ED is applied to the ashes (dried solids) in stationary cells for P recovery. 32e84% Cie slik and Konieczka [7] and Guedes et al. [23] Enhanced biological phosphorus removal (EBPR) Phosphorus Removal of P by microbes after producing a solid stream (sludge) that is suitable for P recovery. The solid product can contain from 5 to 7% of P. 90e99% Yuan et al. [24] Ion exchange and adsorption-based methods

Nitrogen
This process is based on the recovery of the cation NH 4 þ from liquids by using an ion exchanger/adsorbent such as zeolite.
65e80% Sengupta et al. [8] Bioelectrical systems Nitrogen This process aims at ammonium recovery in liquids. It uses low-grade substrates (organic matter) as an electron source in order to produce electricity and recover ammonia. The process is carried out at high pH conditions that allow ammonia recovery. combustion of biomass. In this process, the biomass is subject to thermochemical conversion (incineration) at high temperatures (850e1250 C) and most of the organic P is retained in ashes from the biomass in the form of P-metal consortiums. The P is extracted through mineral or organic acids (H 2 SO 4 , HNO 3 , HCl, H 3 PO 4 , citric, or oxalic acids), but is considered an expensive process [7]. Another issue is that extraction with some acids, such as H 2 SO 4 , can result in leaching of minor amounts of heavy metals. For this process, recovery rates have been reported from 80 to 100% [4,7]. P recovery through algae is also a well-studied and common strategy that has been developed in recent years. Liquid wastes with high P and N concentrations are treated by growing algae in the media via P and N uptake. Algae harvesting is mainly carried out through coagulation/flocculation, centrifugation, flotation, and other approaches [14,15]. Nevertheless, the conditions needed for growing algae (installation space, temperature, light conditions) makes this strategy expensive or complicated to implement. Other less common reported strategies are a washing method with extract, electro dialytic process (ED), and enhanced biological P removal (EBPR) (see Table 1). N is not as valuable as P in the chemical market. However, it is still important to design effective disposal strategies to avoid negative environmental consequences. Most of the N recovery technologies such as air stripping, anammox, struvite precipitation, microalgae cultivation, ion exchange and adsorption methods, and bioelectrical systems are based on the transformation of N into ammonia form for the recovery process [8,10,16,17]. For instance, the ion exchange adsorption uses an ion exchanger/adsorbent (i.e. zeolite) to recover the cation NH 4 þ as a salt precipitate [8]. Ammonia stripping is carried out at a high pH that converts the ammonium ions into ammonia (solubilization) followed by air injection into the liquid mixture to volatize the ammonia for recovery through an acidic solution [8,16]. Bioelectrical systems use low-grade substrates (organic matter) as an electron source to produce electricity and recover ammonia [8]. Lastly, anaerobic ammonium oxidation (anammox) is a biological process that converts NH 4 þ and NO 2 to N 2 gas under anaerobic conditions [18]. N and P strategies have been developed and adapted depending on the recovery source, resources, and environmental conditions. The recovery source (biomass) is one of the main factors to be considered when selecting a recovery strategy due to the potential hazardous organic and inorganic pollutants (heavy metals, hydrocarbons, pesticides, etc.) or/and pathogens that might be contained in it or derived from it that could result in damage to the environment [7]. For this reason, the current strategies should consider the integration of pollutant removal and hygienization processes.

Hydrothermal carbonization
Energy production from biomass, although it has limitations, can be considered one of the main alternative energy sources to complement or substitute for fossil fuels along with wind, solar, and wave power. There is a wide range of processes used nowadays for biomass transformation, e.g. thermochemical, biological, and mechanical [25]. In the past years, thermochemical conversion of biomass was one of the most studied and developed fields worldwide and includes different processes such as combustion, pyrolysis, torrefaction, and hydrothermal treatments (carbonization, gasification, liquefaction) [25,26]. Table 2 shows the different thermal treatments for biomass conversion. Energy production from biomass via combustion processes can present some drawbacks that could affect the system's operation. These include increasing the cost of transportation, storage, and process efficiency [27]. These disadvantages are related to the high moisture content, fast biological degradation, low bulk density, low energy properties, heterogeneous chemical properties, and the milling of raw biomass, as well as fouling and corrosion problems caused by inorganics present in biomass [27]. On the other hand, hydrothermal treatments (HT) present an advantage over the other thermochemical treatments (combustion) as they are carried out in the presence of water, avoiding the drying pre-treatment step and reducing the energy requirements of the system [22,26]. The main target of HT is energy densification through the concentration of carbon and oxygen removal in the solid fraction. HT by-products and their characteristics will depend on the severity of the process (pressure, temperature, and reaction time), resulting in either a solid hydrochar, a bio-crude, or a syngas. However, one of the main disadvantages is the high setup requirements (energy and installation costs) for the equipment [28].
HTC is also known as wet torrefaction and it is carried out at temperatures ranging from 200 C to 250 C. HTC involves the application of high temperature and pressure to convert high moisture biomass into carbonaceous biofuel, gas (mainly CO 2 ), and process water rich in organic and inorganic compounds [22,26,28,29]. During the HTC process, a series of physicochemical reactions occur associated with hydrolysis, decarboxylation, and dehydration reactions [27,30e32]. Characteristics of by-products are strongly related to the severity of the process, which is ruled by the process conditions such as residence time and temperature but also related to the feedstock used for the treatment [33e35]. The solid product resulting from the HTC process is called hydrochar, which presents superior properties in comparison to the raw biomass in terms of higher mass and energy density. Moreover, the HTC process improves the dewaterability and the combustion performance as solid fuel [36]. The main applications for hydrochar are soil improvement, carbon sequestration, bioenergy production, and wastewater pollution remediation [28,37]. Furthermore, it has been demonstrated that the HTC process is not only used for the production of a solid by-product (hydrochar), but also produces a liquid product that is rich in organic and inorganic compounds [38e40]. This process water is considered as a product with no biological activity, but its main drawback is related to the presence and/or high concentration of recalcitrant products such as phenols, furfural, 5-HMF, their derivatives, and nutrients (ammonia), that are strongly related to the type of biomass treated and process parameters. Although the liquid products can contain complex compounds, the suitability of the process water for biogas production has been proven [22,34,38,40e50]. This is because the process water can contain up to 15% of the total carbon, mainly in the form of acetic and formic acid that is highly biodegradable [44].
HTC has grown up fast in the past years to the point that HTC technology has been developed on a commercial scale by companies based mainly in Europe and Asia (see Table 3). Every company presents its own design, process conditions, and additives, as summarized in Table 3. Most of these companies have focused on the HTC of sewage sludge and its integration within wastewater treatment plants in order to reduce the waste volume, obtain a pathogen-free solid product, and to reduce the energy consumption by improving the dewaterability of the solid fraction and by increasing the biogas production. Nonetheless, companies are still investing in technology to make more self-sustaining processes to better harness the properties of the biomass feedstock.

Importance of HTC integration with phosphorus and nitrogen recovery
There are some problems derived from HTC by-products: 1) the combustion of the solid by-products from HTC (hydrochars) releases NOx into the environment and 2) eutrophication can be caused by liquid or solid wastes with high N and P concentration, resulting in permanent damage to the underground water. Therefore, it is important to create new strategies by combining or modifying the existing technologies to achieve higher P and N recoveries. Introducing the HTC process coupled with other technologies can enhance the P and N recovery strategies and, at the same time, minimize the risk of pathogens in the final products. In this regard, the potential advantages of the integration of HTC into a nutrient recovery strategy can be summarized as 1) the wide range of biowastes that can be treated despite their different characteristics, 2) the valuable by-products that can be obtained from the HTC (hydrochars, bio-oil, organic products), 3) the pathogen and organic decomposition during the HTC process, 4) the waste volume reduction, and 5) the potential as a P and N reclamation process [51].
Some authors have adopted the approach of using HTC as a process for P and N extraction from the solid fraction to the process water [1e3,39,52e54]. As a result, it has been found that the HTC process is very efficient in solubilizing and converting organic N into ammonium N, turning it into a good alternative for N solubilization for further recovery [1,51,54,55]. P conversion is similar to N conversion (from organic to inorganic), but its solubilization through HTC is not efficient if the conditions are not acidic [2,4,52].  Hence, N and P recovery from the liquid fraction can be improved if acidic conditions are promoted because this in turn promotes the hydrolysis of the organic-N species and the solubilization of the inorganic P [1]. On the other hand, HTC also promotes P precipitation and crystallization that allows P extraction in the solid fraction [4,51,52,56]. Therefore, HTC has the potential to boost the nutrient extraction (process water) or densification (hydrochar) from biomass for further nutrient recovery [51,57]. These two approaches do not seem to be mutually exclusive. Nonetheless, only a few studies have included a small section discussing the fate of N or P during and after the HTC process [26,36,51,55,58e62]. HTC review papers are mainly focused on the type of feedstock, biomass chemical reactions, operation conditions, energy balance, product characteristics (hydrochar and process water), and perspectives for future research opportunities, applications, and gaps. For example, Funke and Ziegler [32] presented a review that summarized the knowledge about the chemical nature of the HTC process from a process design point of view and described the most important parameters qualitatively. Libra et al. [63] discussed the conversion process and chemistry involved in hydrochar production from different feedstocks (biomass residues and waste materials). Reza et al. [64] reviewed the HTC development considering process parameters, chemical reactions, and by-products characteristics for energy and crop production. Zhao et al. [36] focused on biofuel production from bio-waste (sewage sludge, municipal solid waste, and palm oil empty fruit bunches) by using hydrothermal treatments as pre-treatments, including HTC, highlighting advantages and disadvantages of the process and the economic variability of HT solid biofuel production. Kambo and Dutta [65] discussed the advantages of hydrochar over biochar. Jain et al. [66] presented a review of the conversion of biomass using HTC to activated carbon. Rom an et al. [67] focused on experimental and modeling studies of the HTC experimental parameters and hydrochar applications, with special emphasis on its use as a material for electrodes in supercapacitors. Li and Shahbazi [68] wrote a review about the carbon spheres formed through HTC. Zhou et al. [69] considered integration of HTC and anaerobic digestion technologies using food waste as a biomass source. Wang et al. [28] discussed hydrochar formation and characteristics during the HTC process for lignocellulosic biomass and sewage sludge, and Biller and Ross [26] summarized the state of the art of HT (including carbonization) in algal biomass. However, the biomass is not limited to the production of biofuels, and can also be used as a feedstock to recover or produce new sustainable chemical compounds such as N and P. Therefore, the challenge is to create new strategies by combining or modifying the existing technologies to achieve higher P and N recovery efficiencies from biomass waste. Introducing the HTC process, coupled with other technologies, could enhance the current P and N recovery strategies and, at the same time, minimize the risk of pathogens in the final products. In this review, the variables that influence the fate of P and N during HTC of biomass are discussed from a perspective of nutrient recovery, potential applications and future challenges, and trends.

Variables that influence N and P solubilization in HTC
When N and P are considered as targets in a nutrient recovery strategy, the understanding of how the variables affect the fate of both nutrients, especially their solubilization, becomes a key factor for the recovery efficiency of the process (see Table 3). During the HTC process, N and part of the P from the biomass is solubilized within the process water and this process is influenced by the reaction temperature, retention time, feedstock, solids loading, and pH, among other factors [22,51,53,55,59,62,85]. For instance, according to Kruse and Dahmen [86], around 80% of the phosphate content within the solid fraction of sewage sludge and digestate can be recovered by using HTC. The extracted phosphate can be used to produce fertilizers and, at the same time, will avoid overfertilization, by using the digestate for land spreading. Nonetheless, the fate of P during hydrothermal treatment not only depends on the process conditions but also on the feedstock source which is strongly linked to the levels of metals present in the solid fraction [39]. For this reason, it is important to determine the main factors that affect the behavior of P and N during the HTC process in order to define better recovery strategies.
The first factor that should be considered in the recovery strategy that plays a key role in the N and P fate is the source of biomass treated. The N and P species are strongly related to the source of biomass and their determination could help to predict the potential products and the different pathways that N and P can take during the HTC process. Moreover, the N and P speciation could help to determine their mobility and bioavailability [51,59]. Another factor that could help to determine the fate of N and P during HTC, is the presence and concentration of heavy metals such as Ca, Mg, Al, and Fe, because they can promote precipitation of N and P through mineralization or adsorption [51,54,59]. In Table 4, the characteristics that influence the fate of N and P for different biomass sources are presented. These biowastes are categorized into agricultural wastes, animal manures, sewage sludges, algae, food waste, and municipal solid waste. Elemental analysis (CHNS), proteins, total P (TP) and metals such as Ca, Mg, Al, and Fe and ash content can give an insight into the potential fate-transformations during the HTC process [51,53,59,87,88]. Elemental and total P analyses can provide information about the amount of P and N contained in the biomass. Nevertheless, it is necessary to consider more specific analyses to determine the organic and inorganic fractions of the N and P species of a given biomass. Ash content can provide an estimation of the total amount of metal contained in the biomass but only the concentration of specific metals such as Ca, Mg, Al, and Fe will provide more information about the potential salts that can be formed during the HTC process [51,62].
HTC process conditions such as process temperature, retention time, pH, and solids loading also influence the N and P solubilization. In the literature many studies can be found related to N and P extraction and recovery from HTC [2,4,10,22,39,52,55e57,89e96]. Ekpo et al. [2] stated that an acid medium could promote the hydrolysis of N and enhance the N recovery efficiency. This statement is supported by the research of Dai et al. [1] where they achieved the release of higher concentrations of total N and ammonia at low pH conditions using the acid-supported HTC. However, Ekpo, et al. [2] also stated that the N solubilization is mostly influenced by the severity of the reaction (temperature and retention time) rather than the pH conditions. He et al. [54] found that pressure is another factor that influences N dissolution. They concluded that higher pressures might result in a dramatic cracking or dissolution of N from the solid fraction. Furthermore, it was found that pressure also influences the N species distribution in the liquid fraction affecting mainly NO3 --N and CN À -N. Another factor that has a great influence on N solubilization is the type of feedstock treated because their specific properties influence the quality of the HTC byproducts [34]. Some biomass possesses more nitrogenous compounds than others, making them more susceptible to this N solubilization. The raw material is closely related to the N species that will be formed during the HTC [93]. Thus, N solubilization increases as the temperature and retention time increases but reduces as the solids loading increases [22,39,41,53,85,97].
The reaction temperature is a factor that affects the fate of P in the HTC process. Some studies have found that higher reaction temperatures favor P densification within the hydrochar and reduce its solubilization in the liquid fraction [90,92,93]. In addition, it has been suggested that this immobilization of P during the HTC process results in a stable form such as P-apatite [90]. The solubilization of the P species increases at low temperatures and longer residence times [92]. This can be related to the presence of multivalent metal ions, especially metal cations, that could be responsible for the formation of the insoluble phosphates at high temperatures during the HTC process [2]. The resident time is also a key factor that may have some influence on the nutrient behavior during the HTC process. Ghanim et al. [90] suggested that most of the insoluble P forms occur quickly, highlighting the importance of the residence time during the HTC treatment. The retention time might indirectly influence the P immobilization or solubilization. This is because the residence time influences the solubilization of the cations that are closely related to the P behavior, such as Ca, Mg, K, and Na. In general, the residence time may have a strong effect on the partitioning of P, but the effect is less pronounced at high temperatures. This may be due to precipitation of P occurring quickly at high temperatures [90]. The role of pH in P extractionrecovery is important. Regardless of the acid used (mineral or organic), the acidity favors the formation of soluble metal compounds [32]. Ekpo et al. [2] concluded that the pH, especially in acidic conditions, enhanced both N and P solubilization during the HTC process.
Hence, the main governing factors that influence the fate of N and P during the HTC process can be divided into two: HTC process conditions and biomass characteristics (see Fig. 1).

Nitrogen reaction pathways during HTC
A simplified scheme of the potential N transformation pathways during the HTC process is shown in Fig. 2 and was based on the studies carried out by He et al. [54] and Wang et al. [59]. The hydrothermal process leads the nitrogenous compounds to form several inorganic N ions such as NH 4 þ , CN À ,NO 2 À and NO 3 À [54].
Furthermore, many studies agree that the N solubilized into the process water is mainly the ammonia form [2,22,39,53,54,59,85,154]. The increase of temperature suggests a transfer of solid-N into the liquid fraction whereas the inorganic species of N (including ammonium and nitrate salts) suffer hydrolysis to mainly form NH 4 þ -N and NO 3 À -N [54,59,60]. The organic N, generally composed of proteins and pyridine-N compounds, also suffers a transformation through hydrolysis and deamination reactions into inorganic N (NH 4 þ -N). However, the hydrolysis of organic N (mainly proteins) is slow when the reaction temperature  is below 230 C, suggesting that higher temperatures may reduce the HTC process time if the target is the N solubilization [59,100]. Despite most of the N dissolving during the HTC process, some N still remains in the solid fraction due to chemical, precipitation, and crystallization reactions. Compounds such as proteins, pyrrole-N, pyridine-N, and quaternary-N have been found in the hydrochar [54,59,60]. The presence of sugars has also been found to promote the incorporation of N into heterocycles resulting in more stable N species (quaternary-N and pyridine-N) as the reaction temperature in the HTC process increases [59]. As a result, these stable species can be fixed within the hydrochar. Wang et al. [59] found that pyridine-N increased within the hydrochar as the temperature increased (180e240 C), but decreased at temperatures above 260 C. The presence of metal ions such as Ca þ2 , Mg þ2 , and PO 4 þ2 can promote N precipitation due to salt formation (struvite) [54,56,59,60]. For instance, CaCO 3 can promote precipitation via ureolysis accelerated deamination [59].
Few studies have extensively studied the effects of temperature and heating rate on N transformation and migration behaviors in thermal treatments [53,54,59,60]. He et al. [54] proposed eight routes for the N transformation during the HTC process of digestate from sewage sludge, including the addition of CaCO 3 as a N solubilization booster. They indicated that the majority of labile Ncontaining substances may be decomposed and released into liquid fraction at 220 C. The novelty in this study was the integration of HTC þ Air stripping for N recovery from the solid fraction which achieved a global N recovery of 62%. Zhuang et al. [53] studied the transformation pathways of N in sewage sludge during hydrothermal treatment. In this study, the detailed transformation of the N species during different treatment temperatures was reported for the liquid and solid by-products. Xiao et al. [60] and Wang et al. [59] studied the effect of temperature in the N speciation and transformation during the HTC process of spirulina and food waste, respectively. They found that the N that remained in the solid fraction (hydrochar) after thermal treatment was composed of pyridine-N, pyrrole-N, quaternary-N, amino-N, and inorganic-N. It was suggested that amino-N was mostly converted into quaternary-N and the formation of the heterocyclic-N species was caused by crystallization and ring condensation of N-containing intermediates via the Dies-Alder reaction. The study also reported that N was solubilized during the thermal process due to the hydrolysis of the protein-N and inorganic-N that were transformed to a stable amide-N by the cleavage of peptide bonds that were subsequently transformed into NH 4 þ -N, via deamination and a ringopening reaction.

Phosphorus reaction pathways during HTC
Compared with the dry thermochemical transformation where the P migrates into only the solid fraction, the P in hydrothermal treatments can migrate into both the solid and liquid fraction. The potential transformation pathways of P during HTC are summarized in Fig. 3. During the HTC process all the P species generally transform into orthophosphates [51]. Species such as pyrophosphates, polyphosphates, phytic acids, phosphate diesters, and others suffer hydrolysis, breaking down the molecular bonds and converting them into soluble orthophosphates that migrate into the liquid fraction. Nevertheless, the amount of metal ion content in biomass waste also defines the fate of P during the thermochemical reaction. Metals such as Ca, Mg, Cu, and Zn can react with P forming insoluble precipitates (phosphate salts) and adsorption can be promoted by Fe and Al hydroxides due to their high affinity with P [51,58,62].
Several studies have researched P transformation during the HTC process in order to get a better understanding of the transformation pathways. Ghanim et al. [90] and Dai et al. [52] have studied the speciation of P during HTC using poultry litter and cattle manure, respectively. Both concluded that identification of the different P forms in the feedstock is the key factor for controlling the solubility of P before treatment. Therefore, knowing the molar ratios of Ca, K, and Mg compared to P helps to identify if the mineral content is a sufficient amount to form precipitates that will result in the densification of the P within the hydrochars [22,90]. According to Huang et al. [51], P minerals such as Ca, K, and Mg phosphates are the most common found in biomass. For this reason, it is important to know the metal cation profile and the P forms contained within the biomass before thermal treatment to predict the P transformation pathways during the thermal treatment and design better strategies for P recovery. Furthermore, the P species within a char will determine its potential as a soil amender because there are species that are more chemically and biologically available for the soil [155]. Several studies state that multivalent metal cations (for example, Ca, K, Mg, Na, Al, Fe) are strongly bonded with the P transformation pathways [52,90,92,93,155]. During thermal treatment there is a transformation of the P species, with the non-soluble P being the most dominant. The transformation to non-soluble P phosphates is attributed to the presence of multivalent metal elements (such as Ca, K and Mg) reacting and forming precipitates that are mainly contained within the hydrochar [2,4,52,90]. It has been found that the organic P species break down as the process temperature increases, forming phosphates with the metal cations. Ghanim et al. [90] found that during the HTC process the P is immobilized into inorganic and apatite forms because of its stability. Although pyrolysis is a different thermal treatment from HTC, some studies might give an insight into the transformations of P when undergoing high-temperature treatments. For instance, Sun, et al. [155] suggested that the organic P in biomass, such as phytates and lipids, suffers a transformation (the breakdown of phytate) during the thermal process (pyrolysis) forming inorganic metal-P consortiums. These consortiums are formed and chemo-adsorbed on the surface of the hydrochar during the HTC process as insoluble phosphates such as Ca 3 (PO 4 ) 2 and Mg 3 (PO4) 2 [4,93,156].

Studies related to P and N recovery
The HTC studies related to P and N are mainly focused on the extraction and solubilization in the process water or nutrient densification in the hydrochar. In general, biomass feedstocks such as digestates and animal manure have been widely studied because of their high nutrient content (see Table 4). However, there are some other valuable and interesting biomass feedstocks that have been studied as well, such as food waste, microalgae, and agricultural waste [57,94,157].
The high N and P content in sewage sludge, especially digestate, makes this type of biomass attractive for the integration of the HTC process within wastewater treatment systems. Arag on-Briceño et al. [22] introduced an approach coupling HTC with anaerobic digestion within a wastewater treatment plant. They applied the HTC process for sewage sludge digestate at three different reaction temperatures, 180, 220, and 250 C, followed by the anaerobic digestion of the HTC by-products. They obtained solubilizations of P and N up to 27 and 58%, respectively. They concluded that the N extraction is related to the reaction severity of the HTC process but they did not find a correlation with the P solubilization. Yu et al. [91] analyzed the HTC products from the granular digestate from an up flow anaerobic sludge blanket reactor (UASB). A reduction of N in the hydrochar from 9.58% to 5.49% was reported and an almost total immobilization of the P with a bioavailability >80%. Zhao et al. [6] worked with the HTC process of digestate and evaluated the P recovery and the feasibility of produce activated carbon from the hydrochar. The study reported a recovery of 92.6% of the P within the hydrochar, mostly as calcium phosphate. The hydrochar was acid washed to recover the P, achieving 88.9e94.3% recovery from the total P of the digestate. Zhuang et al. [53] obtained N recoveries from 36.9% to 75.5% in the process water from the HTC of sewage sludge by applying reaction temperatures up to 300 C. He et al. [54] had an interesting approach, integrating the HTC process and air stripping process to recover N from dewatered sewage sludge. They obtained solubilization of 84% by the addition of CaCO 3 and reported recovery of 62% of the total N. Huang et al. [55] did a similar study with chicken manure, obtaining a recovery of 57% of the total N of the initial biomass. Another study was carried out on septic tank waste and the HTC process at different reaction temperatures and retention times in which 70% of the N was solubilized in the process water [88].
Unlike sewage sludge that is treated in a continuous system, animal manure is usually treated in semi-continuous or noncontinuous systems. Moreover, large amounts of different animal manure is generated around the world due to intensive farming to meet global food needs. Therefore, the interest in recycling or harnessing the different manures, especially for their high nutrient content, has led some researchers to investigate the benefits of applying HTC to these types of biomass. Heilmann et al. [4] examined the capture of P in the hydrochar of the HTC of the manure of three different animals (poultry, pigs, and cattle) under various reaction conditions (temperature, solids loading, and time). It was found that hydrochars coming from poultry litter retained 83e90% of the total P, 64e100% for hydrochars coming from swine manure, and 88e100% for hydrochars coming from cattle manure. The study concluded that the feedstock source determines the behavior and fate of the P. For example, for hydrochars coming from poultry litter, no correlation was found between the reaction conditions and the P retained within the hydrochar, but for cattle and swine manure the process temperature and solids loading (retention times above 1hr) enhanced the retention of P within the hydrochar. Reza et al. [110] found that around 50% of the N is solubilized during the HTC processing of cow manure.
There are few studies related to P and N regarding lignocellulosic biomass (mostly agricultural waste) because of its low P and N content and, for that reason, they are mainly focused on carbon densification and fuel properties. Nevertheless, some lignocellulosic biomass still contains sufficient amounts of N and P that can be recovered (see Table 4). Funke et al. [158] performed HTC of wheat straw and anaerobically digested wheat straw. They focused on the N and P retained within the hydrochar at different temperatures and retention times. It was found that between 55 and 65% of the N was retained within the hydrochar from digestate and 48e64% from wheat straw. P retention was higher compared to N. Between 77-80% and 36e78% was retained within the hydrochar of digestate and wheat straw, respectively. Gronwald et al. [159] investigated the nutrient adsorption capacity of the hydrochars from digestate, Miscanthus Andersson, and woodchips. It was concluded that hydrochars coming from 200 to 250 C treatments presented a poor or negligible sorptive effect on NO 3 À , NH 4 þ , and PO 4

3À
. However, they found that hydrochars coming from digestates released PO 4 3À into the aqueous solutions when the soils were washed. This was attributed to the high P content in the digestate. However, they stated that the nutrient retention potential of a hydrochar depends on the feedstock carbonized, process conditions, surface area obtained, and the interaction with the cations and anions of the material. Parmar and Ross [41] applied HTC in agricultural and sewage sludge digestates, municipal solid waste (organic fraction), and vegetable-garden-fruit waste. According to the results reported, they obtained better recovery rates (74e84%) of N in the hydrochar and process water for the lower reaction temperature (200 C) rather than higher temperature (250 C) using 20% of solids of each feedstock. Chen et al. [92] investigated the HTC of watermelon peel. They found densification of P in the hydrochar from 53 to 154% compared with the original feedstock. Moreover, they found that lower temperatures promoted P solubilization. On the other hand, the ammonia in the hydrochar reduced up to 11%, although the total N increased from 36 to 99%. For the process water, they obtained no difference in the total N concentration compared with the raw biomass. Vinasse and sugar cane bagasse as a biomass feedstock was studied by Silva et al. [93]. They investigated the addition of acid, base, and salts as promoters in order to immobilize nutrients within the hydrochars at different reaction temperatures. They concluded that the addition of H 2 SO 4, H 3 PO 4 , and (NH 4 ) 2 SO 4 promoted the immobilization of N and P.
Other studies have focused on the HTC of micro and macro algae [94,96,157,160]. Heilmann et al. [157] evaluated the nutrient solubilization through the HTC process using microalgae (Chlamydomonas reinhardtii P.A.Dang.) as a feedstock, achieving extractions of 80% of the total N and 100% of the P. Levine et al. [94] obtained retention of up to 48.7% and 43% for N and P, respectively, within the hydrochar by using HTC in algal biomass (Nannochloris Naumann and Synechocystis Sauv.). The work carried out by Du et al. [96] studied the feasibility of growing algae (Chlorella vulgaris Beij.) in the process water coming from the HTC process of Nannochloropsis oculata (Droop) D.J.Hibberd in which removals of 45.5e59.9% of total N and 85.8e94.6% of total P were achieved.
Another promising biomass waste for nutrient reclamation by the HTC process is food waste. Idowu et al. [57] evaluated the fate of nutrients resulting from the HTC of (restaurant) food waste, focused on the usage of the solid product as fertilizer. It was concluded that the majority of the N remains in the hydrochar and the P fate is dependent on the reaction time and temperature. Moreover, they estimated that up to 0.96% and 2.3% of the N and Pbased fertilizers, respectively, can be replaced in the US with hydrochar and process water from restaurant food waste.
Compared with the traditional HTC process as a strategy for P and N extraction, the acid-supported HTC has shown higher efficiencies, especially for P [2,4]. The addition of acid (H 2 SO 4 or HCl) can not only increase the extraction of cations (promoting the formation of soluble phosphates from Ca, K, Na, and Mg), but can also catalyze the conversion of organic acids through esterification and promote dehydration and decarboxylation [2]. Ekpo et al. [2] analyzed the effect of adding three types of acids, H 2 SO 4 , acetic and formic acids, into the HTC process for swine manure. Reaction temperatures of 120,170,200, and 250 C were tested, achieving P recoveries of 79, 94, 80, and 60%, respectively. They found that sulfuric acid promotes better solubilization of P. Furthermore, 60e70% of the total N concentration determined in the process water corresponded to organic N and the rest to NH 4 þ . Nonetheless, the addition of acids to the sample before thermal treatment had not significant effect. Dai et al. [1] achieved solubilization of up to almost 100% and 63% of the total P and N, respectively, in cattle manure with acid-supported HTC using HCl (2%) to lower the pH. As well as this, it was found that the acid-based HTC in corn stover led to N solubilization of 83e97% from the hydrochar to the liquid fraction [95]. Heilmann et al. [4] used HCl (4 M) to extract P from swine manure hydrochar, achieving 89% P extraction.
Some authors have proposed integrating HTC with the struvite process to recover P from the process water. Becker et al. [56] proposed a novel approach integrating HTC with PeN reclamation via struvite precipitation. The sewage sludge was acidified with nitric acid prior to HTC treatment in order to improve the P solubilization. Furthermore, the ammonium formation due to hydrolysis and deamination reactions during the HTC process, the addition of magnesium salts, and pH increase, all promoted struvite precipitation achieving P recovery up to 82.5% from the native sludge. Yu et al. [10] recovered up to 91.6% and 54.88% of P and N, respectively, from the process water of a carbonized sewage sludge digestate using struvite precipitation. Zhang et al. [89] performed hydrothermal treatment using HCl and H 2 O 2 and reported recovery of 99.3% (Mg 2þ :PO 4 3À 1.84:1, pH 9.98) through struvite crystallization with a modest energy requirement reaching values as low as 768 kWh/kg of P. Overall, 16.6% of total P was recovered after P was solubilized, captured and made available.

Existing technology for nitrogen and phosphorus recovery
Many technologies have been developed for nutrient recovery in the last decades. Most of them were mainly focused on P recovery rather than N recovery. In addition, these technologies have been predominantly designed for wastewater, sewage sludge, and manure [161]. The final products from the P recovery process are mostly used for agricultural purposes as fertilizers given their important value in the market [5].
The selection of a P recovery process in a recovery strategy is strongly dependent on the properties of the biomass. In general, P recovery technologies consist of extraction (in acidic conditions) and precipitation (mainly in struvite form) with some differences depending on the properties and state of the biomass (solid or liquid). According to Cie slik and Konieczka [7] there are several kinds of technologies and approaches for P recovery, especially for sewage sludge, but most of them lack management of the waste materials produced during the recovery processes or the operational costs are high. For instance, processes such as precipitation and crystallization are used for wastewater, Seaborne, KREPO, and Aqua-Reci processes for sludge, and BioCon process for biomass ashes. Therefore, it can be said that P recovery technologies for biomass (especially for sewage sludge) have been designed either for aqueous fraction, sludge, or ash [162,163]. On the other hand, Nfocused recovery technologies are fewer compared with P and are mainly focused on recovery from liquid waste streams with high concentrations of NH 4 þ -N. These technologies are based on N conversion through biological processes (i.e. anammox) or stripping, from ammonia to N 2 , in combination with a fixation process, N 2 and H 2 to form NH 3 (i.e. Haber-Bosh) [161]. In Table 4, the N and P recovery technologies developed at a large scale are presented. Paques Technology B.V [164]. has developed the PHOSPAQ™ and the ANAMMOX® processes for P and N recovery, respectively. The PHOSPAQ™ process is mainly based on the recovery of phosphate as struvite, and ANAMMOX® is a biological process that converts ammonium and nitrite into N gas for its recovery. Another process that has been developed and patented for P extraction from animal manure, is the "Quick Wash" process, which consists of the use of mineral or organic acid solutions to extract P and the addition of liquid lime and an organic polyelectrolyte to precipitate P to recover it. This process claims a recovery rate of 90% of P from pig manure [3]. Ostara [165] has developed PEARL® and WASSTRIP® processes for P recovery that produce a premium slow-release fertilizer called Crystal Green®. The Ostara PEARL® reactor employs the same chemical principle for the formation of struvite: in continuous aeration, high pH, and with magnesium dosage (Mg 2 þ ). The company claims that their struvite recovery process decreases the operating cost in a wastewater treatment plant and, at the same time, its Crystal Green® fertilizer meets the relevant environmental regulations. GMB [166] Bioenergie has developed a technology to recover ammonia from compost using drying tunnels and recovering the ammonia through a sulfuric acid scrubber. Other processes that are being used for P recovery are BioCon and Cambi/Kempro [4]. The first process recovers P from the P-rich ashes of the previously incinerated sewage sludge through sulfuric acid. The second process combines the thermal hydrolysis of the Cambi technology and the Kepro process. Some fraction of P is solubilized with a high temperature (around 150 C) and extracted by addition of ferric chloride solution to produce ferric phosphate. Nonetheless, technologies presented in Table 5 require either maximization of the soluble fraction of P and N or maximization of the retention of P within the solid fraction for subsequent recovery. For instance, in the route of P recovery from ashes, the moisture content of biomass should not be overlooked as this can exhibit a detrimental effect on the energy efficiency of the incineration process. In this scenario, the potential synergy offered by HTC, in terms of enhanced dewatering, could be beneficial and this is a reason why careful selection is the key to finding a middle ground between maximization of dewatering and minimization of the loss of P to liquid [167,168].

Future challenges and trends
As reviewed in previous sections, there is some information about the N and P transformation fate during the HTC process, but a better understanding is still needed, especially due to the wide range of biomass that can be subject to P and N recovery. Information about N and P speciation is one of the key factors that is needed to obtain a better picture of N and P behavior during the thermal process. However, this is not an easy task since every type of biomass has different properties and the HTC conditions are not always the same. Previous studies have demonstrated that the HTC Table 5 Phosphorus and Nitrogen recovery technologies.

Technology
Process % of Recovery Reference BioCon process A drying-incineration process of sewage sludge. The first process consists of spreading the sludge uniformly in a laying belt and the drying is achieved by hot air (180 and 80e100 C). The incineration process is carried out at 850 C. The P extraction process is carried out with sulfuric acid-producing soluble phosphate. This phosphate is extracted by ion exchange followed by acidification and evaporation.

Cambi/Krepro
The process consists of using the Cambi process to hydrolyze the sewage sludge at high temperatures (~150 C) and low pH (~1) to solubilize the P. This is followed by the separation of the solid and liquid fractions. The P is recovered from the liquid fraction using ferric chloride to produce insoluble ferric phosphate (Kepro process).
Up to 70% phosphorus recovery. [4,171] "Quick Wash" Technology that uses mineral or organic solutions to extract P. The P recovery is through the addition of liquid lime and organic poly-electrolyte to the liquid extract. The P is recovered as a calcium-P precipitate. This process produces a manure low in P and a P precipitate.
Claims to recover 90% of phosphorus in the pig manure. [3] PHOSPAQ™ Technology based on the struvite precipitation process. The P is extracted by the addition of magnesium oxide, causing precipitation of the ammonium and phosphate as struvite. In addition, this process removes chemical oxygen demand (COD) from wastewater.
Removal efficiency of 70e95% of phosphate. [164] ANAMMOX® The biological process that removes ammonia from liquid wastes converting it into gas N. This process reduces the C0 2 emissions and reports to have cheaper operation costs (up to 60%) than the nitrification/denitrification process.
Over 95% of ammonia removal.
PEARL® Technology based on the struvite precipitation process. The process consists of injecting magnesium salts into a controlled pH reactor. This produces crystal granules called Crystal Green®. Crystal Green® is a highly pure fertilizer that is commercialized.
Claims to remove more that 85% of the phosphorus and 40% of nitrogen. [162,165] WASSTRIP® Technology that works as a complementary technology for PEARL®, removing the internal P from the waste of activated sludge via stripping. This process is commonly placed before an anaerobic digester. The extra added value is the protection of the anaerobic digesters from the formation of struvite that could block the pipelines.
Up to 85e95% of the formerly dissolved P can be recovered.
Seaborne process A process created by German Seaborne Environmental Research Laboratory Gmbh for sewage sludge treatment that recovers P and N as struvite and purifies the biogas, harnessing the sewage sludge's biogas, ashes, and biomass.
Not reported. [169] Ammonium sulfate from process air Technology for N recovery from compost or solid biomass that releases gases with a high concentration of ammonia. The process uses biological drying tunnels to volatilize the ammonia that is recovered with sulfuric acid (air washer) in ammonium sulfate form.
Recovery of 80 kg N and 90 kg sulfur per ton of sludge treated. [166] RAVITA process Technology for N and P recovery developed by Helsinki Region Environmental Services Authority (HSY) for a wastewater treatment plant. The process combines struvite precipitation and ammonia stripping. The N and P recovered as phosphoric acid are used in the air washer to form ammonium phosphate.
Potential phosphorus recovery is around 70% of the total phosphorus inlet. [161] ANITA TM MOX This process combines an aerobic and an anoxic process into one moving bed biofilm reactor. The bacteria used are nitrate producers and specific Anammox biomass. The advantages of this technology are that it does not use external carbon sources and has a very low energy cost.
Up to 85% total nitrogen removal and up to 95% ammonia removal. [170] ANITA TM Shunt This process integrates the nitrate-shunt principle into the traditional activated sludge system. The objective is to stop N oxidation at the nitrate stage. A sequential batch reactor is used to facilitate the process operation for the nitrate shunt. The advantage is that the quantity of biological sludge produced is reduced as well as the oxygen and external carbon demand.
Up to 95% of total nitrogen removal.

Airprex®
The process consists of the increase of pH through CO 2 stripping. The process is followed by the addition of MgCl 3 in order to precipitate the P as struvite. This process was developed to prevent struvite incrustation at EBPR WWTP.
Up to 7% of the sludge input. [172,173] Ashdec® The process is aimed at recovering P from sewage sludge ash (SSA). The SSA is used as a reducing agent and is mixed with Na salts to form NaCaPO 4 . The reaction is carried out at 900 e1000 C for 20min in a rotary kiln.
98% of phosphorus recovery from the sewage sludge ash. [172] Gifhorn® This process is the modified-optimized version of the seaborne process. The first step consists of low pH (4.5) with sulfuric acid to extract the P from the sewage sludge. The second step aims to precipitate heavy metals by the addition of Na 2 S at 5.6 pH (adjusted with NaOH). The P precipitation is carried out by the addition of Mg(OH) at high pH (9) to form struvite.
49% of the total phosphorus from the sludge.

Mephrec®
The sludge is dried to 80% DM and pressed into briquettes. The briquettes or ash from sludge are treated with gasification at 1450 C. During gasification, the phosphates presented in SS are transformed into silico-phosphates and recovered as slag. The advantage of this process is the production of energy as electricity and heat.
81% of the total phosphorus content in the sludge/ash input in the process.
process is a feasible process to be included in the N and P recovery strategies. Nevertheless, more studies related to the combination of HTC with more nutrient recovery processes are needed, focused on the recovery efficiency and the reduction of operational costs. Furthermore, it is important to take into account the environmental implications of the disposal of by-products. The process water not only contains organic matter and nutrients that can be harnessed and recovered, but also contains chemical compounds that can damage the environment or affect other subsequent treatment processes. Therefore, more studies related to the process of water purification are needed. Moreover, there are not many studies related to hydrochars and their applications in soils. For that reason, it is important to continue research into this area, to tackle the negative implications, and to ensure the harnessing of the benefits of using hydrochars as soil amenders. Thus, one of the challenges is the standardization of the processes involved with the nutrient recovery strategies to be able to design and construct commercial units.
The current trend is focused on the integration of HTC into wastewater treatment systems to improve energy production and recovery. The approach taken by most of the companies and current research projects is the design of a portable and flexible unit for the HTC process that is able to be integrated within wastewater treatment plants (see Table 3). Another interesting proposed approach is focused on the design of a portable HTC unit for integration into a municipal solid waste treatment plant, but with the aim of recovering water for agricultural purposes [174]. Indeed, all the previously mentioned approaches are novel, but they are focused on waste reduction, energy, and water recovery.
The existing HTC technology has the potential to be coupled with the N and P recovery process, especially because the HTC process has proved to be a good option for solubilizing N and P or densify it within the solid fraction. The concepts of biorefinery and waste reduction are leading to a new trend in which HTC is coupled with other N and P reclamation technologies in order to better harness the properties of the biomass and, at the same time, make the whole N and P reclamation process energy self-sustainable (see Fig. 4). The first steps have been taken towards designing these new N and P reclamation strategies. CarboRem has developed C700, a portable HTC unit integrated with a nutrient recovery system (N, P, and magnesium) [175]. This product is aimed at integration into wastewater treatment plants and claims benefits such as waste volume reduction, enhanced biogas production (increase up to 50%), nutrient recovery, and self-sustained energy process.

Conclusions
There is a wide range of methods and technologies developed for P and N recovery and the HTC process is an attractive process that can be used as a step to complement the N and P reclamation strategies. However, a better understanding of the factors that affect the N and P transformations and fate during the HTC process is necessary. This would help to predict the resulting chemical byproducts and help to assertively select a recovery process. Therefore, in order to implement better nutrient recovery strategies, potential routes for integrating P and N reclamation and recycling into the treatment of nutrient-rich biowastes via HTC are needed.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.