Phytotoxicity of hydrochars obtained by hydrothermal carbonization of manure-based digestate

Characteristics and phytotoxicity of hydrochar-derived dissolved organic matter: Effects of feedstock type and hydrothermal temperature

The dissolved organic matter (DOM) with high mobility and reactivity plays a crucial role in soil. In this study, the characteristics and phytotoxicity of DOM released from the hydrochars prepared from different feedstocks (cow manure, corn stalk and Myriophyllum aquaticum) under three hydrothermal carbonization (HTC) temperatures (180, 200 and 220°C) were evaluated. The results showed that the hydrochars had high dissolved organic carbon content (20.15 to 37.65 mg/g) and its content showed a gradual reduction as HTC temperature increased. Three fluorescent components including mixed substance of fulvic acid-like and humic acid-like substances (C1, 30.92%-58.32%), UVA humic acid-like substance (C2, 25.27%-29.94%) and protein-like substance (C3, 11.74%-41.92%) were identified in hydrochar DOM by excitation emission matrix spectra coupled with parallel factor analysis. High HTC temperature increased the relative proportion of aromatic substances (C1+C2) and humification degree of hydrochar DOM from cow manure, while it presented adverse effects on the hydrochar DOM from corn stalk and Myriophyllum. aquaticum. The principal component analysis suggested that feedstock type and HTC temperature posed significant effects on the characteristics of hydrochar DOM. Additionally, seed germination test of all hydrochar DOM demonstrated that the root length was reduced by 8.88%-26.43% in contrast with control, and the germination index values were 73.57%-91.12%. These findings provided new insights into the potential environmental effects for hydrochar application in soil.

The effect of an acidic environment during the hydrothermal carbonization of sewage sludge on solid and liquid products: The fate of heavy metals, phosphorus and other compounds

The pH of sewage sludge is a crucial factor during the hydrothermal carbonization process that influences the characteristics of the resulting products and migration of certain compounds from the solid to liquid phase. Accordingly, this work is focused on examining the pH impact during the HTC process, in particular, pH equals 2, 3, 4, 5 and 6 on the individual hydrothermally carbonized products generated at 200 °C and 2 h residence time. For this reason, the chemical and physical indicators describing the post-processing liquid and hydrochar were determined. For instance, it was observed that the phosphorus content detected in the liquid, derived at pH2, rose significantly by 80%. Furthermore, decreasing the pH of sewage sludge had a significant impact on the ash content and the calorific value of the hydrochar. Additionally, changes in the specific surface area of hydrochar were noticed: pH = 5 and pH = 6 showed an increase of 20-30%, while for lower pH values a decrease of c.a. 26% was achieved. The distribution of heavy metals between the obtained fractions in the HTC process (solid and liquid) indicated that 92 to almost 100% of the tested heavy metals were transferred to the hydrochar. A significant effect of pH on the distribution between these fractions was observed only for Zn and Ni. For instance, for pH = 2, Zn and Ni in post-processing liquid were 34% and 29%, respectively. In addition, the sequential extraction of heavy metals from hydrochar was also performed in order to identify mobile and non-mobile phases. It was noticed that the acidic environment favours a higher amount of mobile heavy metals in hydrochar. The largest effect was observed for Cd, Pb, Cr and Cu, for which, at pH = 2, their respective amounts in the mobile fraction were 2.7; 3.6; 1.8; 6.2 times higher, compared to the hydrochar without pH correction.

Effects of biogas slurry on hydrothermal carbonization of digestate: Synergistic valorization of hydrochars and aqueous phase

In this study, livestock manure digestate (LMD) was used as feedstock for hydrothermal carbonization (HTC) at different temperature (180-260 °C) and residence time (0-4 h). Nutrient flow and distribution during the HTC process were evaluated by comparing the effects of livestock manure biogas slurry (LBS) and ultrapure water (UW) to determine the optimal reaction conditions for the synergistic production and application of hydrochars (HC) and aqueous phases (AP). Compared with UW, the HC yields derived from LBS as solvent were increased by 27.05-38.24% under the same conditions. The C content, high heating value (HHV), and energy densification of HC obtained from LMD and UW were higher than those obtained from LMD and LBS, and the ash content was lower. While, LBS circumstance improved the porosity, N content and some trace elements e.g. Ca, Fe and Mg in HC that showed excellent fertility potential. In addition, the recovery rate of K, TOC, NH4+-N, and TN concentrations in AP were significantly higher in the LBS circumstance than in UW. The results show that the addition of UW is more favorable for fuel generation, and the HC obtained from LMD and UW at 220 °C has the potential to be used as a fuel. Whereas, the addition of LBS enhanced the potential of HC and AP for agricultural applications simultaneously. It is recommended to use HC and AP obtained from LMD and LBS at 240 °C for using as fertilizer.

Biocrude extraction from human-excreta-derived hydrochar for sustainable energy and agricultural applications

Hydrothermal carbonization may be a sustainable sanitary treatment for wet organic waste including human excreta. Human-excreta-derived hydrochar properties differ from those of typical wet biomass due to the formation of a biocrude-like phase at low reaction temperatures. This study characterized the importance of this phase in terms of hydrochar combustion properties and potential agricultural use. Hydrothermal carbonization of raw human excreta was undertaken at 180, 210, and 240 °C, after which the biocrude phase was extracted with dichloromethane. Physicochemical properties, surface-area parameters, combustion profiles, and gas emissions of non-extracted hydrochar, biocrude, and extracted hydrochar were compared. The potential agricultural use of extracted hydrochar was assessed in germination experiments. Biocrude comprised up to 49.5% of hydrochar mass with a calorific value of >60% that of extracted hydrochar. Biocrude combustion properties were better than those of hydrochar, before and after extraction as demonstrated by higher combustion index value (Si). The extracted hydrochar surface area (34.7 m2 g-1) was greater than that of non-extracted hydrochar (<2 m2 g-1), and seeds germinated more readily due to the lower phytotoxin content. Most macro and micronutrients required for plant growth were retained in the extracted hydrochar. The extraction of biocrude from human-excreta-derived hydrochar not only provided a higher-quality fuel with enhanced combustion properties but also improved hydrochar characteristics, suggesting its potential as a soil additive for enhanced plant growth.

Hydrochar reduces oxytetracycline in soil and Chinese cabbage by altering soil properties, shifting microbial community structure and promoting microbial metabolism

The efficient remediation of antibiotic-contaminated soil is critical for agroecosystem and human health. Using the cost-effective and feedstock-independent hydrochar with rich oxygen-containing functional groups as a soil remediation material has become a hot concern nowadays. However, the feasibility and effectiveness of hydrochar amendment in antibiotic-contaminated soil still remain unknown. Therefore, this study investigated the remediation effect and potential mechanisms of different hydrochars from cow manure (H-CM), corn stalk (H-CS) and Myriophyllum aquaticum (H-MA) at two levels (0.5% and 1.0%) in oxytetracycline (OTC)-contaminated soil using a pot experiment. Results showed that compared with CK, OTC content in the soils amended with H-CM and H-MA was decreased by 14.02-15.43% and 9.23-24.98%, respectively, whereas it was increased by 37.03-42.64% in the soils amended with H-CS. Additionally, all hydrochar amendments effectively reduced the OTC uptake in root and shoot of Chinese cabbage by 10.41-57.99% and 31.92-65.99%, respectively. The response of soil microbial community to hydrochar amendment heavily depended on feedstock type rather than hydrochar level. The soil microbial metabolism (e.g., carbohydrate metabolism, amino acid metabolism) was enhanced by hydrochar amendment. The redundancy analysis suggested that TCA cycle was positively related to the abundances of OTC-degrading bacteria (Proteobacteria, Arthrobacter and Sphingomonas) in all hydrochar-amended soils. The hydrochar amendment accelerated the soil OTC removal and reduced plant uptake in soil-Chinese cabbage system by altering soil properties, enhancing OTC-degrading bacteria and promoting microbial metabolism. These findings demonstrated that the cost-effective and sustainable hydrochar was a promising remediation material for antibiotic-contaminated soil.

Effect of Biochar Type, Concentration and Washing Conditions on the Germination Parameters of Three Model Crops

Biochar has been recognized as a promising and efficient material for soil amendment. However, its effects on seed germination are variable due to its alkaline pH and/or the presence of phytotoxic substances. In this study, two types of biochar (B1 and B2) were mixed with soil at different concentrations (0%, 5%, 10%, 25%, 50% and 100%, w:w), and both the solid and liquid fractions of these mixtures were tested on the germination of basil, lettuce and tomato seeds. Furthermore, solid fractions subjected to a pre-washing treatment (B1_W and B2_W) were also investigated for their effects on seed germination. Three germination parameters were then measured: seed germination number (GN), radicle length (RL) and germination index (GI). Biochar B2_W at 10% increased both RL and GI in basil by 50% and 70%, respectively, while B1 at 25% increased these parameters in tomato by 25%. No effects or negative effects were recorded for lettuce. Liquid fractions (L1 and L2) generally hampered seed germination, suggesting the presence of potentially water-soluble phytotoxic compounds in biochar. These results point to biochar as a suitable component for germination substrates and highlight that germination tests are critical to select the best performing biochar according to the target crop.

Nitrated hydrochar reduce the Cd accumulation in rice and shift the microbial community in Cd contaminated soil

Rice grown on Cd-contaminated soil may accumulate Cd in grain, which is extremely harmful to human health. Several managements are developed to reduce the Cd load in rice, while in-situ immobilization by soil amendments has been attractive for its feasibility. Waste-derived hydrochar (HC) has been shown effective at immobilizing Cd in soil. However, potential plant negative effects and huge application amount are crucial to resolving in extensive application of HC. Nitric acid ageing may be an effective method to deal with these problems. In this paper, HC and nitrated hydrochar (NHC) were added to the Cd-contaminated soil at rates of 1% and 2% in a rice-soil column experiment. Results showed that NHC markedly promoted root biomass of rice by 58.70-72.78%, whereas HC had effects of 35.86-47.57%. Notably, NHC at 1% reduced the accumulation of Cd in rice grain, root and straw by 28.04%, 15.08% and 11.07%, respectively. A consistent decrease of 36.30% in soil EXC-Cd concentration was caused by NHC-1%. Following soil microbial community was shifted greatly under HC and NHC applications. The relative abundance of Acidobacteria was decreased by 62.57% in NHC-2% and by 56.89% in HC-1%. Nevertheless, Proteobacteria and Firmicutes were promoted by NHC addition. In contrast to HC, co-occurrence network of dominated bacteria was more complex and centralized generated by NHC. Key bacteria in that metabolic network of NHC such as Anaerolineae and Archangiaceae played key roles in Cd immobilization. These observations verified that NHC was more efficient to decrease Cd accumulation in rice and could alleviate the negative roles to plant by microbial changings in community composition and network. It could provide an enrichment of paddy soil microbial responds to the interaction of NHC with Cd and lay a foundation for the remediation of Cd-contaminated soil by NHC.

Reduction and valorization of dairy manure by organic chelating acid-assisted hydrothermal process: Dewatering performance, energy recovery, and effluent toxicity

Livestock manure with high moisture content is a challenge for management and further disposal. In this study, the organic chelating acid(EDTA)-assisted hydrothermal (EAHT) process was used to achieve dewatering, dry mass minimization, and volume reduction of dairy manure (DM). The hydrophobic modification of DM resulted in a 55% reduction in dry mass, and the specific resistance to filtration (SRF) showed a shift in dewatering performance from unfilterable to highly filterable. An investigation of the reaction mechanisms suggests that proteins and polysaccharides were released from the damaged extracellular polymeric substances (EPS) of the DM into effluent. The surface functional groups of the hydrochar were changed from hydrophilic to hydrophobic, which promotes the transformation of bound water to free water in the DM with enhanced dewatering performance. The obtained hydrochar at 17.5 mg/g EDTA dosage exhibited the highest calorific value (HHV_daf = 29.25 MJ/kg). The HHV_dry of samples have little difference and approach that of anthracite coal (19.2-21.1 MJ/kg)After EAHT, the combustion safety of the hydrochar was improved, which is highly significant for its use as biofuel. The by-product effluent showed lower biological toxicity after EAHT than after HT. The findings of this study demonstrated that EAHT can be efficient in achieving DM reduction and energy recovery, which provides widespread agricultural and environmental application prospects.

Digestate-derived carbonized char and activated carbon: Application perspective

The flourishment of anaerobic digestion (AD) on waste treatment emphasizes the importance of digestate valorization, which plays an essential role in determining the benefits provided by the AD process. The perception of digestate gradually shifts from waste to products to realize the concept of circular economy and maximize the benefits of digestate valorization. This review first outlined the current status of digestate valorization, focusing on thermal-chemical methods. The novel valorization methods were then summarized from the recent research, illustrating prospects for digestate valorization. Limits and perspectives are finally addressed. Methods for preparing digestate-derived activated carbon and impurity effects were elucidated. Inherent mineral content/inorganic impurity could be a niche for downstream use. High surface area and well-developed pore structure are essential for satisfying downstream use performance, but they are not the only factors. Digestate char applications other than use as an energy fuel are suggested.

Phytotoxicity and genotoxicity of agro-industrial digested sludge hydrochar: The role of heavy metals

Hydrochar is a new carbonaceous product obtained via hydrothermal carbonization of wet biomass, such as sludges or digested sludges, which often have disposal problems, also due to the presence of contaminants such as heavy metals. The properties of the hydrochar led to an interest in using it as an amendment, but the agro-environmental properties must be considered for its safe use. Raw hydrochar produced by agro-industrial digestate and relative three acidic post-treated hydrochars (for heavy metals removal) have been assessed considering their effect on phytotoxicity, soil, plant growth, mutagenicity, and genotoxicity. The chemical characterization showed the effect of post-treatment on heavy metals contents reduction, except for Cu content (hydrochar, 650 mg/kg; post-treated hydrochars, 940 mg/kg, 287 mg/kg, and 420 mg/kg). The acidic post-treatment also reduces the phytotoxicity compared to raw hydrochar (the germination index at 16 % of hydrochar concentration was: hydrochar, 61.48 %; post-treated hydrochars, 82.27 %, 58.28 %, and 82.26 %), but the low pH and the impact on N-cycle probably have caused the detrimental effect on plant growth of post-treated hydrochar. No mutagenic activity was observed in bacteria using Ames test, while all the samples induced chromosomal aberrations in plant cells (Allium cepa test). The approach adopted, which considers phytotoxicity, plant growth-soil effects, and mutagenicity/genotoxicity bioassays has been proven effective for a proper evaluation of organic products derived from waste to promote a sustainable and circular recovery of materials.

Evaluating the potential of hydrochar as a soil amendment

In this study, hydrochar (HC), a carbon-rich product originated from hydrothermal conversion treatment (HTC), was obtained from wastes of the wine and dairy industries. The effect of mixing secondary char and compost was tested, before and after the aerobic mixing of compost (COM) and HC at increasing doses (from 15 to 75 Mg ha^-1 DM), in an effort to lower the HC phytotoxicity due to potential phytotoxic compounds of secondary char. The results indicated that, after the aerobic stabilization, the mix HC/COM was able to double the plant growth in comparison to COM alone. The presence of easily degradable organic compounds probably led to poor stability of HC, increased microbial activity and, consequently, root anoxia when used at high doses. Chemical, spectroscopic and thermal investigation confirmed this hypothesis. In particular, HC shows a high content of dissolved organic matter, characterized by the presence of small molecules, which is negatively correlated with the growth index of lettuce. Furthermore, thermal characterization suggests a higher proportion of less complex and thermally stable molecular compounds in HC in comparison to COM. Therefore, co-composting of HC allows obtaining a useful amendment to support soil organic matter and fertility.

Biochar improves the performance of Avena sativa L. grown in gasoline-polluted soils

This study investigated the effect of soil contamination by different concentrations of gasoline on oat (Avena sativa L.) and tested the effect of biochar supply to the polluted soils on the performance of oat plants. Oat seeds were sowed in contaminated soils with different concentrations of gasoline: 0% (control), 1%, 2%, 6%, and 10% (v/w), and grown for 2 weeks. Germination, fresh weight, root and stem length, photosynthetic parameters (i.e., chlorophyll content, PI_ABS, F_V/F_M, and NDVI), and total antioxidant power were analyzed. The results showed a remarkable negative effect on almost all the investigated parameters starting from the gasoline concentration of 6%. Based on these results, a new experiment was run by adding 5% (w/w) biochar (a carbon-rich byproduct of wood biomass pyrolysis) to the 6% and 10% polluted soils to test whether adding biochar had a beneficial effect on oat performance. The results showed that biochar supply greatly reduced the negative effects caused by gasoline on all the investigated parameters.

A review on hydrothermal carbonization of potential biomass wastes, characterization and environmental applications of hydrochar, and biorefinery perspectives of the process

It is imperative to search for appropriate processes to convert wastes into energy, chemicals, and materials to establish a circular bio-economy toward sustainable development. Concerning waste biomass valorization, hydrothermal carbonization (HTC) is a promising route given its advantages over other thermochemical processes. From that perspective, this article reviewed the HTC of potential biomass wastes, the characterization and environmental utilization of hydrochar, and the biorefinery potential of this process. Crop and forestry residues and sewage sludge are two categories of biomass wastes (lignocellulosic and non-lignocellulosic, respectively) readily available for HTC or even co-hydrothermal carbonization (Co-HTC). The temperature, reaction time, and solid-to-liquid ratio utilized in HTC/Co-HTC of those biomass wastes were reported to range from 140 to 370 °C, 0.05 to 48 h, and 1/47 to 1/1, respectively, providing hydrochar yields of up to 94 % according to the process conditions. Hydrochar characterization by different techniques to determine its physicochemical properties is crucial to defining the best applications for this material. In the environmental field, hydrochar might be suitable for removing pollutants from aqueous systems, ameliorating soils, adsorbing atmospheric pollutants, working as an energy carrier, and performing carbon sequestration. But this material could also be employed in other areas (e.g., catalysis). Regarding the effluent from HTC/Co-HTC, this byproduct has the potential for serving as feedstock in other processes, such as anaerobic digestion and microalgae cultivation. These opportunities have aroused the industry interest in HTC since 2010, and the number of industrial-scale HTC plants and patent document applications has increased. The hydrochar patents are concentrated in China (77.6 %), the United States (10.6 %), the Republic of Korea (3.5 %), and Germany (3.5 %). Therefore, considering the possibilities of converting their product (hydrochar) and byproduct (effluent) into energy, chemicals, and materials, HTC or Co-HTC could work as the first step of a biorefinery. And this approach would completely agree with circular bioeconomy principles.

Impact of starch-based bioplastic on growth and biochemical parameters of basil plants

The recent use of bioplastics in agriculture is considered an ecological choice, aimed at limiting the environmental impact of plastics, in line with the Sustainable Development Goals of the United Nations. However, the impact of bioplastic residues on the environment is unclear as knowledge is lacking. This is the first study investigating the effect of a starch-based bioplastic on the growth and biochemical parameters of basil. Bioplastic was experimentally prepared and added to the soil at 2.5 % (w/w), corresponding to twice the concentration of plastic mulch film residues currently found in cultivated soils, in view of the increasing agricultural use of bioplastics. Basil plants were grown without (controls) and with bioplastic addition for 35 days, under controlled experimental conditions. Compared to the control, plants exposed to bioplastic showed stunted growth (in terms of shoot fresh weight, height, and number of leaves). Significant reductions in the content of chlorophyll, protein, ascorbic acid, and glucose were also observed. Finally, the treatment caused oxidative stress, as evidenced by the increased content of malondialdehyde in the shoots. The addition of bioplastic increased the electrical conductivity and reduced the cation exchange capacity of the cultivation soil. These results suggest that bioplastic in soil may promote the onset of stressful conditions for plant growth in a similar manner to plastic. They will be complemented by further investigations to unravel the mechanisms underlying these responses, involving different doses and types of bioplastics and other crop species.

Hydrochar and hydrochar co-compost from OFMSW digestate for soil application: 3. Toxicological evaluation

Modern societies produce ever-increasing amounts of waste, e.g. organic fraction of municipal solid waste (OFMSW). According to the best available techniques, OFMSW should be treated through anaerobic digestion to recover biogas and subsequently composted. An innovative scheme is under investigation, where anaerobic digestion is combined with hydrothermal carbonization (HTC) and composting. The final product, referred to as hydrochar co-compost (HCO), is under study to be used as an unconventional soil improver/fertilizer. Recent studies showed that HCO is not phytotoxic. However, nothing is known about the toxicity of HCO on cells as part and organisms as a whole. This study aims to investigate in vitro genotoxicity and cytotoxicity of the HCO and its precursors in the production process. In particular, we tested water and methanolic extracts of HCO (WEHCO and MEHCO) from one side and methanolic extracts of hydrochar (MEH) and OFMSW digestate (MED) as well as liquor produced downstream HTC (HTCL) from the other side. Genotoxicity was investigated using cytokinesis-block micronucleus assay in Chinese Hamster Ovarian K1 (CHO-K1) cells. Cytotoxicity was tested in vitro against a panel of human cells line. Zebrafish embryo toxicity upon MEH treatment was also investigated. Results show that incubation of CHO-K1 cells with all the tested samples at different concentrations did not cause any induction of micronucleus formation compared to the vehicle-treated control. Treatment of cells with MEH, MED, HTCL and MEHCO, but not WEHCO, induced some degree of cytotoxicity and MEH showed to be more cytotoxic against tested cells compared to the MEHCO. Toxicity effect at the highest tested concentrations of MEH on zebrafish embryos resulted in coagulation, induction of pericardial edema and death. In conclusion, the hydrochar co-compost cytotoxicity is similar to standard compost cytotoxicity. Hence composting the hydrochar from OFMSW digestate is a good step to eliminate the cytotoxicity of hydrochar.

Recent progress on the phytotoxic effects of hydrochars and toxicity reduction approaches

Hydrothermal carbonization of wet biomasses has been known to produce added-value materials for a wide range of applications. From catalyst substrates, to biofuels and soil amendments, hydrochars have distinct advantages to offer compared to conventional materials. With respect to the agricultural application of hydrochars, both positive and negative results have been reported. The presence of N, P and K in certain hydrochars is appealing and may contribute to the reduction of chemical fertilizer application. However, regardless of biomass, hydrothermal carbonization results in the production of phytotoxic organic compounds. Additionally, hydrochars from sewage sludge often contain heavy metal concentrations which exceed the regulatory limits set for agricultural use. This review critically discusses the phytotoxic aspects of hydrochar and provides an account of the substances commonly responsible for these. Furthermore, phytotoxicity reduction approaches are proposed and compared with each other, in view of field-scale applications.

Hydrochar and hydrochar co-compost from OFMSW digestate for soil application: 2. agro-environmental properties

The work concerns the study of the hydrochar from digestate and hydrochar co-compost characterization as amendments. The processes for hydrochar and co-compost production were described in Part 1 of this work (Scrinzi et al., 2022). The amendment properties of hydrochar (produced at 180-200-220 °C for 3 h) and co-composts (25%, 50%, and 75% hydrochar percentage of digestate substitution) were assessed by phytotoxicity, plant growth bioassay, and soil effect. Different seeds species (Lepidium sativum, Cucumis sativus, and Sorghum bicolor sp.) were dosed at increased concentrations using both wet raw amendments and their water extracts. The chemical characterization showed phytotoxic compounds content depending on both the initial feedstock (digestate) and the HTC process; at the same time, the analysis highlighted the reduction of these compounds by composting (organic acid, polyphenols, salt concentration). The dose-response was analyzed by the Cedergreen-Streibig-Ritz model and the half-maximal effective concentration (EC50) was calculated based on this equation. The soil properties and GHG emissions measurements (CH₄, CO₂, N₂O, and NH₃) highlighted the effect on N dynamics and on soil respiration induced by substrates. The HC200 soil application determined a significant impact on CO₂ and N₂O emission and NH₃ volatilization (10.82 mol CO₂/m²; 51.45 mmol N₂O/m²; 112 mol NH₃/m²) and a significant reduction of total N and TOC (46% of TKN and 49% of TOC). The co-compost (75%) showed specific effects after soil application compared to other samples an increase of available P (48%), a greater content of nitrogen (1626 mg/kg dry basis), and a reduction of organic carbon (17%). Our results demonstrate the good quality of co-compost and at the same time the validity of this post-treatment for addressing many issues related to hydrochar use in the soil as an amendment, confirming the suitability of HTC process integration for digestate treatment in anaerobic digestion plants.

Hydrothermal carbonization as an alternative sanitation technology: process optimization and development of low-cost reactor

Background: The provision of safe sanitation services is essential for human well-being and environmental integrity, but it is often lacking in less developed communities with insufficient financial and technical resources. Hydrothermal carbonization (HTC) has been suggested as an alternative sanitation technology, producing value-added products from faecal waste. We evaluated the HTC technology for raw human waste treatment in terms of resource recovery. In addition, we constructed and tested a low-cost HTC reactor for its technical feasibility. Methods: Raw human faeces were hydrothermally treated in a mild severity range (≤ 200 °C and ≤ 1 hr). The total energy recovery was analysed from the energy input, higher heating value (HHV) of hydrochar and biomethane potential of process water. The nutrient contents were recovered through struvite precipitation employing process water and acid leachate from hydrochar ash. A bench-scale low-cost reactor (BLR) was developed using widely available materials and tested for human faeces treatment. Results: The hydrochar had HHVs (23.2 - 25.2 MJ/kg) comparable to bituminous coal. The calorific value of hydrochar accounted for more than 90% of the total energy recovery. Around 78% of phosphorus in feedstock was retained in hydrochar ash, while 15% was in process water. 72% of the initial phosphorus can be recovered as struvite when deficient Mg and NH 4 are supplemented. The experiments with BLR showed stable operation for faecal waste treatment with an energy efficiency comparable to a commercial reactor system. Conclusions: This research presents a proof of concept for the hydrothermal treatment of faecal waste as an alternative sanitation technology, by providing a quantitative evaluation of the resource recovery of energy and nutrients. The experiments with the BLR demonstrate the technical feasibility of the low-cost reactor and support its further development on a larger scale to reach practical implementation.

Hydrothermal carbonization as an alternative sanitation technology: process optimization and development of low-cost reactor

Background: The provision of safe sanitation services is essential for human well-being and environmental integrity, but it is often lacking in less developed communities with insufficient financial and technical resources. Hydrothermal carbonization (HTC) has been suggested as an alternative sanitation technology, producing value-added products from faecal waste. We evaluated the HTC technology for raw human waste treatment in terms of resource recovery. In addition, we constructed and tested a low-cost HTC reactor for its technical feasibility. Methods: Raw human faeces were hydrothermally treated in a mild severity range (≤ 200 °C and ≤ 1 hr). The total energy recovery was analysed from the energy input, higher heating value (HHV) of hydrochar and biomethane potential of process water. The nutrient contents were recovered through struvite precipitation employing process water and acid leachate from hydrochar ash. A bench-scale low-cost reactor (BLR) was developed using widely available materials and tested for human faeces treatment. Results: The hydrochar had HHVs (23.2 - 25.2 MJ/kg) comparable to bituminous coal. The calorific value of hydrochar accounted for more than 90% of the total energy recovery. Around 78% of phosphorus in feedstock was retained in hydrochar ash, while 15% was in process water. 72% of the initial phosphorus can be recovered as struvite when deficient Mg and NH 4 are supplemented. The experiments with BLR showed stable operation for faecal waste treatment with an energy efficiency comparable to a commercial reactor system. Conclusions: This research presents a proof of concept for the hydrothermal treatment of faecal waste as an alternative sanitation technology, by providing a quantitative evaluation of the resource recovery of energy and nutrients. The experiments with the BLR demonstrate the technical feasibility of the low-cost reactor and support its further development on a larger scale to reach practical implementation.

Hydrochar and hydrochar co-compost from OFMSW digestate for soil application: 1. production and chemical characterization

The best available technique (BAT) for managing the organic fraction of municipal solid waste (OFMSW) is represented by anaerobic digestion (AD) and subsequent composting. This research explored a new industrial model in the framework of the C2Land international project, with the insertion of hydrothermal carbonization (HTC) as a post-treatment for OFMSW digestate. The reaction was set for 3 h at three different temperatures (180 ÷ 220 °C); the wet solid hydrochar obtained after filtration was then co-composted with greenery waste as a bulking agent and untreated OFMSW digestate in four different proportions in bench-scale bioreactors. The hydrochars and the hydrochar co-composts were suitable for agro-industrial applications, while the HTC liquors were tested in biochemical methane potential (BMP) for internal recirculation to AD. The scenarios proposed can be beneficial for plant enhancement and increased biogas production. This study reports results connected to the production phase. Mass balances confirmed that, during HTC, phosphorus precipitated into the solid products, organic nitrogen partially mineralized into ammonium, and oxidizable organic matter solubilized. The selected hydrochar obtained at 200 °C had mean (dry) solid, liquid, and gaseous yields equal to 77, 20, and 3 %_db, respectively. The dynamic respirometric index (DRI) confirmed that the reproduced BAT for the composting process was effective in producing high-quality hydrochar co-composts in terms of biological stability. The BMP tests on HTC liquors showed some inhibitory effects, suggesting the need for future studies with inoculum adaptation and co-digestion, to dilute toxic compounds and enhance biogas production. Part 2 of this study describes the agro-environmental properties of hydrochars and hydrochar co-composts, including the beneficial effect of composting on hydrochars phytotoxicity.

Benefits and Limitations of Using Hydrochars from Organic Residues as Replacement for Peat on Growing Media

New technologies for the production of peat-substitutes are required to meet the rising demand for growing media in horticulture and the need to preserve natural peatlands. Hydrothermal conversion of organic residues into char materials, hydrochars, with peat-like properties may produce such substitutes, reducing environmental impacts and CO2 emissions from improper management. To assess their potential as a component in growing media, cress seed germination tests are used to assess hydrochars from digestate (D), spent coffee grounds (SCG), and grape marc (GM). Pre- and post-treatments (extraction, washing, and drying) are applied to remove phytotoxic compounds associated with process waters retained on the hydrochars, and a nitrification bioassay with process water is used to predict their toxicity. All hydrochars achieve similar or better germination results compared to their feedstock, showing a potential to replace at least 5% of peat in growing media. SCG and GM hydrochars show inhibition above 5%, while all post-treated D-hydrochar mixtures produce >3 times longer roots than the control. The nitrification test shows a high sensitivity and good agreement with the high inhibition trends found in the germination tests with process water. Such tests can be a good way to optimize process combinations for the hydrothermal production of peat replacements.

Hydrothermal conversion of Chinese cabbage residue for sustainable agriculture: Influence of process parameters on hydrochar and hydrolysate

The demands on novel and sustainable techniques for vegetable waste (VW) valorization continues to increase during the past few decades due to the growing waste production under the flourishing vegetable industries. In this study, Chinese cabbage residues were hydrothermal carbonization (HTC) at 180, 200, 220 and 240 °C for 2 to 6 h to explore the impacts of process parameters on the characteristics of hydrochars and hydrolysates and their feasibility in sustainable agriculture. Results indicated that hydrothermal temperature had a greater impact on cabbage residue hydrolysis than the residence time. With the rising reaction severity, hydrochars became more alkaline with higher amount of ash and carbon (C), while the pH and dissolved organic nitrogen (DON) and NH₄⁺-N in the hydrolysate were gradually reduced. The thermogravimetric analysis (TG-DTG) indicated that organic constitutions in the feedstock went through incomplete decomposition. Although the recalcitrance index (R50) steadily increased through HTC (0.37-0.46), hydrochars were unstable and would not applicable for carbon sequestration. Furthermore, hydrochars and hydrolysate would be optimal media for plants seedling and growth for the abundant nutrients and dissolved organic compounds but reduced phytotoxicity. In conclusion, these results showed that HTC is highly applicable for vegetable waste management for sustainable agriculture.

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.