Nutrient Behavior in Hydrothermal Carbonization Aqueous Phase Following Recirculation and Reuse

Unraveling the photochemical behavior of dissolved organic matter derived from hydrothermal carbonization process water: Insights from molecular transformation and photoactive species

Hydrothermal carbonization process water (HTPW) has been utilized as a substitute for chemical fertilizers in agricultural applications. However, the input of HTPW into paddy water, particularly the significant proportion of dissolved organic matter (DOM) in HTPW (DOM-HTPW), directly engages in photochemical transformations, a phenomenon often overlooked. This study observed a consistent decrease in humification (SUVA280, 7.7-53.9%) and aromaticity (SUVA254, 6.1-40.0%) of DOM-HTPW after irradiation. The primary active photobleaching components of DOM-HTPW varied depending on the feedstock, such as protein for chicken manure DOM-HTPW and lignin for rice straw DOM-HTPW. The photochemical activity of DOM-HTPW was augmented by its lower molecular weight and higher hydrophilic composition, particularly evident in chicken manure DOM-HTPW, which exhibited higher generation rates for 1O2 (35.1-37.1%), 3DOM* (32.8-43.9%), and O2•- (28.6-48.8%) as measured by molecular probes. DOM-HTPW effectively facilitated the phototransformation of tetracycline, with the contribution of O2•- being more significant than 3DOM* and 1O2. These findings shed new light on the understanding the photochemical processes of DOM-HTPW as exogenous DOM and the interconnected fate of contaminants in aquatic environments.

Fate of antibiotics and hormones during hydrothermal carbonization of poultry litter: degradation kinetics and toxicity assessment of filtrates and hydrochars

This study investigated degradation kinetics of five selected organic micropollutants (OMPs) present in poultry litter (namely: sulfadiazine, tetracycline, and doxycycline hyclate (antibiotics); estrone and 17-β-estradiol (hormones)) during hydrothermal carbonization (HTC) treatment as the temperature stepwise increased to 250 °C. All five pure OMPs were completely degraded before 250 °C was reached during the HTC process. Nevertheless, presence of poultry litter slowed down the degradation of OMPs. Through elemental mass balance calculation, it is noted that after 15 min (temperature less than 137 °C), 69-82% of organic carbon and 50-66% of organic nitrogen initially consisting part of the target antibiotics were fully mineralized. Both HTC filtrates and hydrochars obtained from poultry litter inhibited Escherichia coli and Bacillus subtilis growth. A combination of high salinity, high nutrients, dissolved organic carbon, and other ions in the filtrate as well as the adsorption of OMPs on hydrochars were probably the reason for the high toxicity.

The dynamic features and microbial mechanism of nitrogen transformation for hydrothermal aqueous phase as fertilizer in dryland soil

Hydrothermal aqueous phase (HAP) contains abundant organics and nutrients, which have potential to partially replace chemical fertilizers for enhancing plant growth and soil quality. However, the underlying reasons for low available nitrogen (N) and high N loss in dryland soil remain unclear. A cultivation experiment was conducted using HAP or urea to supply 160 mg N kg-1 in dryland soil. The dynamic changes of soil organic matters (SOMs), pH, N forms, and N cycling genes were investigated. Results showed that SOMs from HAP stimulated urease activity and ureC, which enhanced ammonification in turn. The high-molecular-weight SOMs relatively increased during 5-30 d and then biodegraded during 30-90 d, which SUV254 changed from 0.51 to 1.47 to 0.29 L-1 m-1. This affected ureC that changed from 5.58 to 5.34 to 5.75 lg copies g-1. Relative to urea, addition HAP enhanced ON mineralization by 8.40 times during 30-90 d due to higher ureC. It decreased NO3-N by 65.35%-77.32% but increased AOB and AOA by 0.25 and 0.90 lg copies g-1 at 5 d and 90 d, respectively. It little affected nirK and increased nosZ by 0.41 lg copies g-1 at 90 d. It increased N loss by 4.59 times. The soil pH for HAP was higher than that for urea after 11 d. The comprehensive effects of high SOMs and pH, including ammonification enhancement and nitrification activity inhibition, were the primary causes of high N loss. The core idea for developing high-efficiency HAP fertilizer is to moderately inhibit ammonification and promote nitrification.

Hydrothermal carbonization of corncob for hydrochar production and its combustion reactivity in a blast furnace

A key factor restricting the application of biochar in the steel industry is its high-quality upgrading. This paper evaluated the characteristics of hydrochar produced by HTC (hydrothermal carbonization) process of corncob to be used as a solid fuel. HTC temperatures (240-300 °C) and HTC water-reused times (1-3 times) were examined for their effects on hydrochar yield, physicochemical characteristics, and combustion properties. The results showed hydrochar yields, O/C, and H/C parameters decreased as HTC temperature and water-reused times increased, while its high heating value increased. Due to dehydration and decarboxylation, hydrochar showed similar characteristics to those in bituminous coal. The removal efficiency of alkali metal K reached 99% after HTC treatment. Carbonaceous hydrochar had become more compact, orderly, and stable with increasing amounts of aromatic functional groups, C = C, and C = O. Hydrochar, as a biofuel, has higher ignition energy and is more stable than corncob due to its high carbonaceous order degree. To calculate combustion kinetic parameters, the Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods were applied. The results revealed that Eα (average activation energy) was quite similar between the two models. HC-300 had an Eα of 262 kJ/mol. HTC could be an efficient way to reutilize corncob biomass into clean biofuels with high calorific value.

Effects of persulfate-assisted hydrothermal treatment of municipal sludge on aqueous phase characteristics and phytotoxicity

Hydrothermal technology (HT) has received much attention in recent years as a process to convert wet organic waste into hydrochar. The aqueous phase (HTAP) produced by this process is still a burden and has become a bottleneck issue for HT process development. In this study, we provide the first investigation of the HTAP characteristics, phytotoxicity, and their correlation with persulfate (PS) (PS, 2.0 mmol/g TS)-assisted municipal sludge HT. The results showed that PS accelerated the hydrolysis of protein substances and increased the concentration of NH₄⁺ by 13.4% to 190.5% and that of PO₄^3- by 24.2% to 1103.7% in HTAP at hydrothermal temperatures of 120 to 240 °C. PS can reduce the phytotoxicity of HTAP by reducing aldehydes, ketones, N heterocyclic compounds, and particle size and by increasing its humification index. The maximum values of the root length and biomass of pakchoi (Brassica chinensis L.) seedlings occurred when electrical conductivity was 0.2 mS/cm of HTAP. This work provided a new strategy for the selection and design of HTAP management strategies.

Co-hydrothermal carbonization of agricultural waste and sewage sludge for product quality improvement: Fuel properties of hydrochar and fertilizer quality of aqueous phase

Hydrothermal carbonization (HTC) is a promising carbon-neutral technology for converting sewage sludge (SS) and agricultural waste into energy. However, HTC-generated aqueous phase (AP) impedes the development of the former. This study investigated the potential of SS with rice husk (RH) and wheat straw (WS) co-HTC to form hydrochar and AP as substitutes for fuel and chemical fertilizer, respectively. Compared with single SS hydrochar, the yield of co-HTC-based hydrochar and higher heating value significantly increased by 10.9%-21.6% and 4.2%-182.7%, reaching a maximum of 72.6% and 14.7 MJ/kg, respectively. Co-HTC improves the safe handling, storage and transportation, and combustion performance of hydrochar. The total nitrogen concentration in AP-SS was 2575 mg/L, accounting for 67.7% of that found in SS. Co-HTC decreased and increased the amine and phenolic components of AP, respectively. AP-SS-RH and AP-SS-WS significantly increased pakchoi dry weight by 45.5% and 49.4%, respectively, compared with AP-SS. The results of the hydroponic experiments with AP instead of chemical fertilizers revealed that AP-SS did not reduce pakchoi dry weight by replacing <20% chemical fertilizers. However, AP-SS-RH or AP-SS-WS replaced 60% chemical fertilizers. Therefore, the co-HTC of SS and agricultural waste increased the AP substitution of chemical fertilizer from 20% to 60%. These findings suggest that the co-HTC of agricultural waste with SS is a promising technology for converting SS into renewable resource products for fuels and N-rich liquid fertilizer while significantly improving fuel and fertilizer quality.

Energy and nutrient recovery by spent mushroom substrate-assisted hydrothermal carbonization of sewage sludge

Hydrothermal carbonization (HTC) has been recognized as a promising sewage sludge (SS) treatment technology for effective pathogen elimination, bioenergy recovery, organic contaminant destruction and volume reduction. However, the solid product (hydrochar) of SS after HTC as fuel has the problems of high ash content, high nitrogen content and low calorific value. The aqueous phase (AP) produced is still considered a burden and has become a bottleneck in the development of HTC. In this study, co-HTC of SS with spent mushroom substrate (SMS) is conducted, and the fuel properties of hydrochar and the quality of the AP as a liquid fertilizer are investigated. In comparison with hydrochar of single SS, the energy yield and higher heating value of the hydrochar from co-HTC were significantly increased by 12.1-44.8 % and 33.2-137.8 %, respectively, reaching their maximum of 72.75 % and14.98 MJ/kg, respectively. Co-HTC can improve safe handling, storage and transportation, and combustion performance of hydrochar. Furthermore, the AP of co-HTC could significantly increase the biomass of pakchoi, which was 140.9 % and 90.7 % of AP from single SS and Hoagland nutrition solution (represents commercial fertilizer), respectively. The AP of co-HTC as fertilizer can recover 62.03-64.65 % nitrogen from SS and SMS. These findings suggest that co-HTC of SMS with SS is a promising technology for the conversion SS into renewable resource products for fuels and N-rich liquid fertilizer while also significantly improving fuel and fertilizer quality.

Effects of process water obtained from hydrothermal carbonization of poultry litter on soil microbial community, nitrogen transformation, and plant nitrogen uptake

Process water (PW) obtained from hydrothermal carbonization of nitrogen-rich (N-rich) biowaste is proposed to be a renewable resource utilized as a liquid N fertilizer. However, its effects on soil microbial community, N transformation, and plant N uptake are unclear or controversial. In this study, fertilizers were prepared with different percentages of PW (poultry litter, 220 °C 1 or 8 h, PW-S or -L) and urea to supply 160 mg kg^-1 total N in a barren alkali soil. Results showed that the addition of PW relative to pure urea decreased organic N mineralization by low bio-accessibility, increased N loss by high soil pH, and decreased NO₃^--N by low nitrification substrate. It supported the lettuce in health but decreased plant N uptake by low NO₃^--N. It significantly increased the gram-positive bacteria that responded to resistant organic matter, changed the bacterial community to enhance decomposition, detoxification, ureolysis, and denitrification, and to decrease nitrification. Its inhibition effect on nitrification activity was stronger than that on nitrifiers growth. Different from PW-S, the addition of PW-L seriously and significantly decreased seed germination index and fungal biomass that responded to N retaining capacity, respectively. The best fertilizer was 50% urea +50% PW-S that supported the seed germination and seedling growth, and mildly affected microbial community.

Enhanced electrochemical performance of porous carbon from wheat straw as remolded by hydrothermal processing

To improve the electrochemical properties of lignocellulose-derived carbon, wheat straw was hydrothermally processed at different temperatures followed by KOH activation for the preparation of porous carbons. Their physical, chemical, and electrochemical properties were analyzed to clarify the effects of hydrothermal processing. The results indicated that high-temperature hydrothermal processing fragmented the wheat straw and increased the heteroatoms content to make the hydrochars more conducive to activation, thereby improving the specific surface area, N-heteroatoms and phenolic hydroxyl groups of activated carbons. A maximum specific surface area of 2034.4 m² g^-1 was achieved by HAC-300 (the activated carbon derived from hydrothermally processed wheat straw at 300 °C) with more N-heteroatoms and phenolic hydroxyl groups. Correspondingly, the excellent electrochemical performance of the three-electrode supercapacitor device assembled by HAC-300 showed a specific capacitance of 286.95 F g^-1 at 0.5 A g^-1, representing an improvement of 89.5 % over than that of the original wheat straw without hydrothermally processing. Its symmetric supercapacitor also realized a good capacitance retention of 95.8 % after 10,000 cycles at 5 A g^-1, suggesting the excellent cycling stability of the porous carbon from the hydrothermally processed wheat straw.

Process water recirculation for catalytic hydrothermal carbonization of anaerobic digestate: Water-Energy-Nutrient Nexus

The process water (PW) from acid-catalyzed hydrothermal carbonization (HTC) is still an environmental burden due to the enriched organics, nutrients, and salts. This study proposed a novel strategy to valorize food waste digestate into multifunctional hydrochar by recirculating the PW in the HCl-catalyzed HTC process. The produced multifunctional hydrochar could be utilized as a high-quality solid fuel with HHV of 27.9 MJ kg^-1 (hydrochar without PW recirculation) and a slow-release fertilizer by converting the complex Ca and P compounds from the food waste digestate into a Ca-P deposit (hydroxyapatite) with more than a 93 % P recovery rate (hydrochar with PW recirculation). Adding fresh HCl in the HTC PW recirculation system only displayed a marginal catalytic impact on the hydrochar properties after two cycles of recirculation. This study demonstrated the importance of inherent Ca in the feedstocks and the dual role of HCl in the HTC with PW recirculation.

Preparation of solid organic fertilizer by co-hydrothermal carbonization of peanut residue and corn cob: A study on nutrient conversion

With continuous recognition of green, organic and pollution-free products, the organic fertilizer plays an increasingly important role in agricultural production. Hydrothermal carbonization (HTC) is an efficient and environmentally friendly biomass treatment technology that can achieve value-added utilization of solid wastes. This study evaluated the potential of two typical agricultural and forestry wastes (corn cob and peanut residue) in preparing as solid organic fertilizers through HTC. The effects of reaction temperature, residence time, and the raw material composition on hydrochar yield, total nutrient content (TNC), nitrogen recovery, and nutrient elements transformation in HTC were investigated. Corn cob was proven to be not an ideal raw material for the preparation of organic fertilizers because of the low TNC and the high C/N ratio of its hydrochar. On the contrary, peanut residue was suitable for preparing organic fertilizers due to its high TNC and appropriate C/N ratio. The co-HTC of corn cob and peanut residue could further improve the N recovery rate from 8.52% (for peanut residue only) to 19.51% due to the synergistic effect between them. Under the optimal hydrothermal conditions of 240 °C, 120 min, and mixing ratio of 1:1, the hydrochar yield was as high as 27.86%, and the C/N value (11.98) and TNC (6.331%) were both appropriate as fertilizer. Furthermore, the potential migration and transformation paths of nutrients including N, P, K and metal elements in the co-HTC were analyzed. The thermodynamic conditions and raw materials composition significantly affect the migration and transformation of N, P and K between solid and liquid. N dissolved into process water (mainly ammonia) would migrate into hydrochar and bio-oil with increasing of reaction temperature. P was fixed in hydrochar through precipitation and adsorption reaction with metal ions. Further, adjusting pH or adding metal salts can promote the fixation of N and P in solid.

Characterization and nutritional value of hydrothermal liquid products from distillers grains

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

Occurrence of Antibiotic-Resistant Genes and Bacteria in Household Greywater Treated in Constructed Wetlands

There is a growing body of knowledge on the persistence of antibiotic-resistant genes (ARGs) and antibiotic-resistant bacteria (ARB) in greywater and greywater treatment systems such as constructed wetlands (CWs). Our research quantified ARGs (sul1, qnrS, and blaCTXM32), class one integron (intI1), and bacterial marker (16S) in four recirculating vertical flow CWs in a small community in the Negev desert, Israel, using quantitative polymerase chain reaction (qPCR). The greywater microbial community was characterized using 16S rRNA amplicon sequencing. Results show that CWs can reduce ARG in greywater by 1–3 log, depending on the gene and the quality of the raw greywater. Community sequencing results showed that the bacterial community composition was not significantly altered after treatment and that Proteobacteria, Epsilonbacteraeota, and Bacteroidetes were the most dominant phyla before and after treatment. Pseudomonas, Citrobacter, Enterobacter, and Aeromonas were the most commonly identified genera of the extended spectrum beta lactamase (ESBL) colonies. Some of the ESBL bacteria identified have been linked to clinical infections (Acinetobacter nosocomialis, Pseudomonas fulva, Pseudomonas putida, Pseudomonas monteilii, and Roseomonas cervicalis). It is important to monitor intI1 for the potential transfer of ARGs to pathogenic bacteria.

Hydrothermal treatment as a complementary tool to control the invasive Pampas grass (Cortaderia selloana)

The rapid spread of invasive Pampas grass (PG) is having not only ecosystems impact, but also significant economic and social effects. The tonnes of bulky waste from the plant disposal require proper treatment to avoid seed dispersal, greenhouse gas emissions and landscape damage. In the pursuit of zero-waste management, hydrothermal treatment (HT) appears as a challenging alternative. The possibility of mobile HT systems offers an alternative to accomplish on-site both the PG waste management and the application of the resulting by-products within a circular framework. As a first step, this research shows that, without a prior drying step, the hydrothermal treatment at 100-230 °C under autogenous water vapor pressure for only 30 min allows safe seeds inertization, while a stable carbon-enriched solid and an aqueous stream are generated. Prolonging the process for 2 h has no profitable effects. As the reaction temperature increases, the PG residue is converted into a material with 49-58 wt% of carbon, 41-32 wt% of oxygen and 3-4 wt% of ash. The pH (~6.3), low electrical conductivity (1.21-0.86 dS/m), high carbon content, open porosity (5-8 m²/g) and improved performance in seed germination and in the early growth test suggest the potential of HT-solids derived at 100-120 °C as amendment to sequester carbon in the soil and improve its physico-biological properties. The phytotoxicity detected in the peat/lignite-like solids obtained at 200-230 °C limits its application in soil, but calorific values of 22-24 MJ/kg indicate their suitability as CO₂-neutral fuel. The agrochemical analysis of the liquid by-products indicates poor value on their own, but their use supplemented with compost may be an option.

Study on the process wastewater reuse and valorisation during hydrothermal co-carbonization of food and yard waste

The commercial success of hydrothermal carbonization (HTC) is contingent on seeking solutions for the downstream wastewater (process water) generated during the process which is still regarded largely as a nuisance. In the present study, the reusability and valorization strategy of process wastewater generated during co-HTC of organic fraction of municipal solid waste (food and yard waste) at 220 °C for 1 h was established. The process wastewater was anaerobically digested in the first part to determine its methane-generating capacity; and in the second part, it was recirculated up to five times to understand the evolution of physicochemical and fuel characteristics of hydrochar. The process water was characterized by the presence of high total organic carbon (up to 40 g/L) and chemical oxygen demand (up to 96 g/L). The decreasing trend of heavy metals with increasing recirculation suggested possible adsorption/immobilization mechanism taking place to the hydrochar surface. The process water generated from co-HTC condition has anaerobic biodegradability of 72% while experimental and theoretical methane yield observed were 224 mL/g COD and 308 mL/g COD, respectively. The presence of high organic and ionic species in recirculated process water promoted the overall carbonization process which was evident from the increased energy yield (86 to 92%), carbon content (68 to 71%) and calorific value (20 to 27 MJ/kg). The recirculation also enhanced overall combustion characteristics of hydrochar as analyzed by thermogravimetric analysis. The recirculation strategy enhanced fuel properties of hydrochar while making sure upstream and downstream water related burden is reduced (as illustrated by life cycle analysis) to create a cleaner production system for renewable solid biofuels production.

Phytotoxicity and nutrition behavior of process water obtained from the hydrothermal carbonization of poultry litter and its effect on lettuce germination and growth

Nine hydrothermal carbonization process waters (PWs) of poultry litter were prepared at 180, 220, and 260 °C for 1, 4, and 8 h, respectively. They were characterized with pH, EC (electric conductivity), DOC (dissolved organic carbon), TN (total nitrogen), NH4+-N, NO3--N, etc. After diluted according to TN, the PWs were supplied as liquid nitrogen fertilizers and their phytotoxic and nutrition effects on lettuce germination and growth were studied. The results showed that the PWs from short time (1 h) were with low DOC/TN and DOC/NH4+-N and high NH4+-N/TN. Compared with the PWs from long time 4 and 8 h, they provided more NH4+-N and less DOC and resulted in lettuce with relatively high germination index (GI), dry biomass, and low antioxidant enzyme activities. Especially, the PW from 220 °C and 1 h significantly enhanced the dry weight by 196.3% relative to negative control of nitrogen deficiency. However, all the PWs led lettuce to an unhealthy condition, which decreased GI and the chlorophyll content and increased antioxidant enzyme activities. Furthermore, it was confirmed by linear regression that the ratios of DOC/TN, NH4+-N/TN, and DOC/NH4+-N were the determining indexes for evaluating the phytotoxicity and nutrition behavior of the PWs as liquid nitrogen fertilizers.

Hydrothermal carbonization of anaerobic digestate and manure from a dairy farm on energy recovery and the fate of nutrients

Hydrothermal carbonization (HTC) of raw and anaerobically digested (AD) manure with either water or whey was studied, with the goal of recovering energy and nutrients. Specifically, the impacts of HTC reaction temperature (180-240 °C), solid feedstock, and type of liquid on hydrochar quality and aqueous phase properties were tested. Of the hydrochars produced, the calorific value of whey-based hydrochar was the highest, (19.4 and 16.0 MJ/kg for manure and digestate, respectively). Overall, the net energy gain was higher for HTC of manure with whey (7.4-8.3 MJ/kg dry feedstock) and water (4.4-5.1 MJ/kg) compared to the combined AD-HTC process with whey (4.4-5.3 MJ/kg) and water (2.3-2.9 MJ/kg). Digestate-derived hydrochar contained up to 1.8% P, higher than manure-derived hydrochar (≤1.5%). Using whey as a liquid for HTC increased the aqueous-phase N-P-K concentrations up to 3,200, 410, and 7,900 mg/L, respectively, suggesting its potential use as a liquid fertilizer.

Advanced near-zero waste treatment of food processing wastewater with water, carbon, and nutrient recovery

A near-zero waste treatment system for food processing wastewater was developed and studied. The wastewater was treated using an anaerobic membrane bioreactor (AnMBR), polished using an outdoor photobioreactor for microalgae cultivation (three species were studied), and excess sludge was treated using hydrothermal carbonization. The study was conducted under arid climate conditions for one year (four seasons). The AnMBR reduced the total organic carbon by 97%, which was mostly recovered as methane (~57%) and hydrochar (~4%). Microalgal biomass productivity in the AnMBR effluent ranged from 0.25 to 0.8 g·L^-1·day^-1. Nitrogen (N) and phosphorous (P) uptake varied seasonally, from 18 to 45 mg·L^-1·day^-1 and up to 5 mg·L^-1·day^-1, respectively. N and P mass balance analysis demonstrated that the process was highly efficient in the recovery of nitrogen (~77%), and phosphorus (~91%). The performance of the microalgal culture changed among seasons because of climatic variation, as a result of variation in the wastewater chemistry, and possibly due to differences among the microalgal species. Effluent standards for irrigation use were met throughout the year and were achieved within two days in summer and 4.5 days in winter. Overall, the study demonstrated a near-zero waste discharge system capable of producing high-quality effluent, achieving nutrient and carbon recovery into microalgae biomass, and energy production as biogas and hydrochar.

Facile activation of sludge-based hydrochar by Fenton oxidation for ammonium adsorption in aqueous media

Lately, there has been a growing interest in converting low-cost biomass residuals, including wastewater sludge, into char-like materials for various applications. In this research, ammonium (NH₄⁺) adsorption and desorption potential of hydrochar activated via Fenton oxidation were systematically investigated. Hydrochar was prepared from domestic wastewater treatment plant sludge and activated by Fenton oxidation using different H₂O₂ concentrations, H₂O₂/Fe²⁺ ratios, and activation times. The activated hydrochars (AHs) were characterized by ATR-FTIR, high-resolution XPS, BET specific surface area, and SEM, and their NH₄⁺ adsorption capacity was analyzed. The NH₄⁺ adsorption isotherms and kinetics, adsorption in the presence of competing ions (with and without humic acid), and NH₄⁺ desorption were investigated. The results show that following hydrochar activation, the acidic groups' concentration and the BET surface area increased, but the morphology remained essentially unchanged. It was also found that the activation occurs within a few minutes when using a relatively low concentration of reagents, and without extensive post-treatment steps. The NH₄⁺ adsorption onto AH at equilibrium fitted the Langmuir isotherm model, with a maximum adsorption capacity of 30.77 mg g^-1, and the NH₄⁺ adsorption kinetics fitted the pseudo-second-order model. NH₄⁺ adsorption in the presence of competing ions decreased by up to 33 ± 3%. NH₄⁺ desorption experiments demonstrated that NH₄⁺ recovery can reach 33 ± 5% with ultrapure water and 67 ± 2% with 2 M KCl. The results of this study indicate that Fenton oxidation is a promising alternative for hydrochar activation, and can be used as an adsorbent for NH₄⁺ remediation in wastewater treatment processes.

A review on nitrogen transformation in hydrochar during hydrothermal carbonization of biomass containing nitrogen

Biomass is a type of renewable and sustainable resource that can be used to produce various fuels, chemicals, and materials. Nitrogen (N) in biomass such as microalgae should be reduced if it is used to produce fuels, while the retention of N is favorable if the biomass is processed to yield chemicals or materials with N-containing functional groups. The engineering of the removal and retention of N in hydrochar during hydrothermal carbonization (HTC) of biomass rich in protein is a research hot spot in the past decade. However, the N transformation during HTC has not yet been fully understood. In order to mediate the migration and transformation of N in hydrochar, the present review overviewed i) the characteristics of hydrochar and the original feedstock, ii) the possible N transformation behavior and mechanisms, and iii) the effect of factors such as feedstock and pyrolysis parameters such as temperature on hydrochar N. The high temperature and high protein content promote the dehydration, decarboxylation, and deamination of biomass to produce hydrochar solid fuel with reduced N content, while the Millard and Mannich reactions for lignocellulosic biomass rich in carbohydrate (cellulose, hemicellulose, and lignin) at medium temperatures (e.g., 180-240 °C) significantly promote the enrichment of N in hydrochar. The prediction models can be built based on properties of biomass and the processing parameters for the estimation of the yield and the content of N in hydrochar.

Organoarsenic conversion to As(III) in subcritical hydrothermal reaction of livestock manure

Liquid phase produced by the subcritical hydrothermal liquefaction (HTL) of livestock manure is extensively used in agronomic and environmental applications, but the potential risks caused by inherent pollutants (e.g., roxarsone, ROX) of the livestock manure have not been considered. This study shows that less toxic ROX is completely converted into highly toxic As(III) and As(V) in the HTL reaction with temperature more than 240 °C. Moreover, more than 81.5% of As is distributed in the liquid phase generated by the livestock manure HTL reaction. Notably, the hydrothermal products of livestock manure facilitate the conversion of As(V) to As(III). The resulting hydrochar and aldehydes act as electron donors for As(V) reduction, thus resulting in the formation of As(III). Furthermore, the dissociated As promotes the depolymerization and deoxygenation of the macromolecular compounds to produce more small oxygen-containing compounds such as aldehydes, further boosting the As(V) reduction to As(III). These results indicate that the liquid phase of the livestock manure has potential risks in applications as a fertilizer. Such findings have substantial implications in biomass utilization and redox reactions of envirotechnical and biogeochemical relevance.

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

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

Bioenergy recovery from wastewater produced by hydrothermal processing biomass: Progress, challenges, and opportunities

Hydrothermal carbonization (HTC)/liquefaction (HTL)/gasification (HTG) are promising processes for biofuel production from biomass containing high moisture. However, wastewater, the aqueous phase (AP) byproduct from these hydrothermal processes, is inevitably produced in large amounts. The AP contains >20% of the biomass carbon, and the total organic carbon in AP is as high as 10-20 g/L. The treatment and utilization of AP are becoming a bottleneck for the industrialization of hydrothermal technologies. The major challenges are the presence of various inhibitory substances and the high complexity of AP. Bioenergy recovery from AP has attracted increasing interest. In the present review, the compositions and characteristics of AP are first presented. Then, the progress in recovering bioenergy from AP by recirculation as the reaction solvent, anaerobic digestion (AD), supercritical water gasification (SCWG), microbial fuel cell (MFC), microbial electrolysis cell (MEC), and microalgae cultivation is discussed. Recirculation of AP as reaction solvent is preferable for AP from biomass with relatively low moisture; AD, MFC/MEC, and microalgae cultivation are desirable for the treatment of AP produced from processing biomass with low lignin content at relatively low temperatures; SCWG is widely applicable but is energy-intensive. Finally, challenges and corresponding strategies are proposed to promote the development of AP valorization technologies. Comprehensive analysis of AP compositions, clarification of the mechanisms of valorization processes, valorization process integration detoxification of AP, polycultures and co-processing of AP with other waste, enhancement in pollutant removal, scaling-up performance, and the techno-economic analysis and life-cycle assessment of valorization systems are promising directions in future investigations.

The effect of aqueous phase recirculation on hydrothermal liquefaction/carbonization of biomass: A review

Aqueous phase (AP) recirculation is attracting increasing interest in hydrothermal process field as it has the potential to increase the yield of bio-crude and/or hydrochar and decrease the cost of hydrothermal wastewater disposal. This work summarizes the effect of AP recirculation on hydrothermal processing biomass, including the discussions on the mechanisms account for the increased yield and the changing properties of the hydrochar and bio-crude. However, the application of AP recirculation in hydrothermal process is limited by the enrichment of nitrogen in bio-crude and the applicability of only specific biomass type. To alleviate these limitations, the feasibility of combining AP recirculation with other strategies (e.g., co-solvent and co-feed) has been discussed. The possibility of using AP as a resource (e.g., nutrient source, and material mediator) can be increased by AP recirculation due to the accumulation of substances.

Poultry litter hydrochar as an amendment for sandy soils

The conversion of poultry litter to hydrochar has been proposed for stabilization of the soils and to eliminate pathogens. Still, research on the hydrochar's effect on soil properties as a function of production temperature, and its direct use with plants is limited in general and even less so on poultry litter. We characterized poultry litter hydrochar as an amendment for sandy soils in terms of changes to the soil's bulk density, porosity, water-retention capacity, and fertility. Soil bulk density, porosity and water-retention capacity were determined in a pneumatic tension plate system for sand with hydrochar-amendment rates of 0.5, 1 and 2%, and hydrochar-production temperature of 180, 220, and 250 °C. Soil fertility was assessed by growing lettuce seedlings in a randomized block design planter experiment, consisting of 16 blocks that were sampled every 10 days. The addition of poultry litter hydrochar resulted in decreased soil bulk density. Soil porosity increased with hydrochar generated at a temperature of up to 220 °C, and decreased with hydrochar generated at 250 °C. Soil water content increased as compared to unamended sand, but decreased with increasing hydrochar-production temperature, probably due to increasing hydrophobicity of the poultry litter hydrochar. The addition of hydrochar at concentrations of 0.5 and 1% resulted in improved plant growth despite an initial delay. While increased soil moisture due to increased soil water-retention capacity was confirmed, it did not seem to be responsible for the improved plant growth. It was also demonstrated for the first time that hydrochar decreases nitrate leaching from soils. Therefore, poultry litter-derived hydrochar seems to be an adequate amendment for sandy soils.