Improving nutrients removal and energy recovery from wastes using hydrochar

Machine learning prediction of dye adsorption by hydrochar: Parameter optimization and experimental validation

In response to escalating global wastewater issues, particularly from dye contaminants, many studies have begun using hydrochar to adsorb dye from wastewater. However, the relationship between the preparation conditions of hydrochar, the properties of hydrochar, experimental conditions, types of dyes, and equilibrium adsorption capacity (Q) has not yet been fully explored. This study conducted a comprehensive assessment using twelve distinct ML models. The Gradient Boosting Regressor (GBR) model exhibited superior performance with R² (0.9629) and RMSE (0.1166) in the test dataset, marking it as the most effective among the evaluated models. Moreover, this study also proved the feasibility of the GBR model through stability testing and residual analysis. A feature importance analysis prioritized the variables as follows: experimental conditions (41.5 %), properties of hydrochar (26.0 %), preparation conditions (18.1 %), and type of dye (14.4 %). Meanwhile, experimental conditions (C0 > 30 mmol/g, pH > 8, and higher solvent temperatures) and hydrochar properties (the BET surface area > 2000 m²/g, an (O+N)/C molar ratio < 0.6, and an H/C molar ratio of approximately 0.06) show higher Q for dyes. Experimental validation of the GBR model confirmed its practical utility with a suitable predictive accuracy (R² = 0.8704). Moreover, the study developed a Python-based GUI that has integrated the best GBR models to facilitate researchers' ongoing application and improvement of this predictive model. This study not only underscores the efficacy of ML in enhancing the understanding of dye adsorption by hydrochar but also sets a precedent for future research on sustainable contaminants removal through bio-based adsorbents.

Soil application of activated hydrochar derived from sewage sludge enhances plant growth and reduces nitrogen loss

Sewage sludge treatment and disposal is a considerable environmental and economic burden, and is considered a major global challenge. Here, sewage sludge treatment and disposal were studied with a focus on hydrothermal carbonization and the use of hydrochar (HC) as a soil amendment after Fenton-reaction activation. The underlying hypothesis was that enhanced adsorption of nutrients (e.g., ammonium) by activated HC (AHC) increases their availability, thus enhancing plant growth and reducing environmental impacts such as greenhouse gas emission and N leaching relative to conventional soil-amendment techniques. The impact of AHC on lettuce plant growth, N leaching, ammonia volatilization, soil trace-gas emissions, and respiration was studied in a net-house planting experiment. Four treatments were tested in quadruplicate using sandy loam soil with addition of either AHC, urea fertilizer, or AHC plus urea, and a control with no amendment. Activation-induced changes in AHC surface properties (indicated by SEM and XPS analyses) resulted in an NH4+ adsorption capacity 60 % higher than that of untreated HC. The AHC + urea soil treatment yielded the most enhanced plant growth, followed by urea and AHC treatments with comparable growth rates. Least growth occurred in the control with no amendment. Nitrogen loss through gas emissions, per kg of lettuce, was lowest with AHC + urea treatment, although its mean N emission as nitrous oxide (N2O) was notably higher at 2.3 mg N2O-N kg-1 than for other treatments (∼0.4 mg N2O-N kg-1). Dissolved-N leaching was reduced by up to four times with AHC treatment due to its higher NH4+ adsorption capacity, indicating reduced environmental impact of the AHC amendment. AHC application is therefore considered a sustainable soil amendment, enhancing plant growth and reducing N loss and sewage environmental impact.

Enhancing sustainable waste management: Hydrothermal carbonization of polyethylene terephthalate and polystyrene plastics for energy recovery

Hydrothermal carbonization (HTC) of single plastic polymers such as polyethylene terephthalate (PET) and polystyrene (PS) has not yet been explored on a large scale, particularly their thermal behavior, chemical transformations under subcritical conditions, and the energy properties of the resultant hydrochar. This study investigated these aspects by employing techniques, such as thermogravimetric analysis (TGA), Fourier transformed infrared spectroscopy (FTIR), elemental and calorific analysis. The results show that PET hydrochar has a superior energy densification (1.37) and energy yield (89 %) compared to PS hydrochar (1.13, 54 %). Hydrothermal carbonization modifies the chemical structure of the polymers by increasing the number of carbonyl groups (CO) in PET and forming new ones in PS, and by enhancing hydroxyl groups (OH) in PET while retaining them in PS. Both materials preserve their aromatic and aliphatic structures, with the introduction of alkenes groups (CC) in the PET hydrochar. PET hydrochar begins to decompose at lower temperatures (150-270 °C) than PS hydrochar (242-283 °C) but reaches higher peak temperatures (420-585 °C vs. 390-470 °C), with both types achieving similar burnout temperatures (650-800 °C). PET hydrochar recorded a higher activation energy (121-126 kJ/mol) than PS hydrochar (67-74 kJ/mol) with the Mampel first-order reaction model as the best fit.

Hydrothermal carbonization of food waste: Process parameters optimization and biomethane potential evaluation of process water

Food waste (FW) is one of the major biomasses produced in large quantities in urban areas, which contributes to more than one-third of global greenhouse gas emissions. FW must be properly managed to minimize its environmental consequences. Hydrothermal carbonization (HTC) of FW is a promising technology compared to conventional methods. The objective of the present study is to maximize the mass yield (MY), higher heating value (HHV) and energy yield (EY) of FW by optimizing the operational variables of HTC process. Additionally, process water generated during HTC of FW under optimal conditions was evaluated for methane yield using anaerobic digestion. To optimize the HTC process, three operational variables, including solid-to-liquid (S/L) ratio, temperature, and reaction time, were manipulated using response surface methodology (RSM). According to RSM studies, the optimum operating conditions are 198.5 °C for 150 min with a 0.2 S/L ratio, resulting in MY, HHV and EY as 62.5%, 21.24 MJ/kg and 81.71%, respectively. Proximate and elemental analysis for the hydrochars synthesized at various operating conditions reveals that the temperature and reaction time have a significant impact on fixed carbon and carbon percentage. The anaerobic digestion results showed that the combination of process water and hydrochar, yielded a maximum cumulative methane production of 298.5 ± 16.34 mL/g COD. To mimic methane production, the modified Gompertz model was utilized. Thus, this finding contributes towards the commercialization of the HTC process to produce solid fuel (hydrochar) and provides a way to find an alternative energy source that enhances the HTC process and tackles the problem of process water disposal.

Influence of hydrothermal carbonization conditions on the porosity, functionality, and sorption properties of microalgae hydrochars

Green microalgae is a possible feedstock for the production of biofuels, chemicals, food/feed, and medical products. Large-scale microalgae production requires large quantities of water and nutrients, directing the attention to wastewater as a cultivation medium. Wastewater-cultivated microalgae could via wet thermochemical conversion be valorised into products for e.g., water treatment. In this study, hydrothermal carbonization was used to process microalgae polycultures grown in municipal wastewater. The objective was to perform a systematic examination of how carbonization temperature, residence time, and initial pH affected solid yield, composition, and properties. Carbonization temperature, time and initial pH all had statistically significant effects on hydrochar properties, with temperature having the most pronounced effect; the surface area increased from 8.5 to 43.6 m2 g-1 as temperature was increased from 180 to 260 °C. However, hydrochars produced at low temperature and initially neutral pH generally had the highest capacity for methylene blue adsorption. DRIFTS analysis of the hydrochar revealed that the pH conditions changed the functional group composition, implying that adsorption was electrostatic interactions driven. This study concludes that un-activated hydrochars from wastewater grown microalgae produced at relatively low hydrothermal carbonization temperatures adsorb methylene blue, despite having low surface area.

Hydrochar mediated anaerobic digestion of bio-wastes: Advances, mechanisms and perspectives

Bio-wastes treatment and disposal has become a challenge because of their increasing output. Given the abundant organic matter in bio-wastes, its related resource treatment methods have received more and more attention. As a promising strategy, anaerobic digestion (AD) has been widely used in the treatment of bio-wastes, during which not only methane as energy can be recovered but also their reduction can be achieved. However, AD process is generally disturbed by some internal factors (e.g., low hydrolysis efficiency and accumulated ammonia) and external factors (e.g., input pollutants), resulting in unstable AD operation performance. Recently, hydrochar was wildly found to improve AD performance when added to AD systems. This review comprehensively summarizes the research progress on the performance of hydrochar-mediated AD, such as increased methane yield, improved operation efficiency and digestate dewatering, and reduced heavy metals in digestate. Subsequently, the underlying mechanisms of hydrochar promoting AD were systematically elucidated and discussed, including regulation of electron transfer (ET) mode, microbial community structure, bio-processes involved in AD, and reaction conditions. Moreover, the effects of properties of hydrochar (e.g., feedstock, hydrothermal carbonization (HTC) temperature, HTC time, modification and dosage) on the improvement of AD performance are systematically concluded. Finally, the relevant knowledge gaps and opportunities to be studied are presented to improve the progress and application of the hydrochar-mediated AD technology. This review aims to offer some references and directions for the hydrochar-mediated AD technology in improving bio-wastes resource recovery.

Combination of biochar and AMF promotes phosphorus utilization by stimulating rhizosphere microbial co-occurrence networks and lipid metabolites of Phragmites

Agricultural biochar and arbuscular mycorrhizal fungi were used to promote the growth of Phragmites in the structural damaged and nutritional imbalanced littoral zone soils. Wheat straw biochar played a significant role in improving soil porosity and supplementing available phosphorus to 79.20 ± 3.20 mg/kg, compared with CK at 17.50 ± 0.88 mg/kg. The addition of Diversispora versiformis improved the plant net photosynthetic rate reaching up to 25.66 ± 0.65 μmol·m^-2·s^-1, which was 36.60 % higher than CK. The combination of biochar and fungi contributed to the whole plant dry weight biomass of 32.30 % and 234.00 % higher than the single biochar or AMF amendment groups, respectively. Meanwhile, the analysis of microbial co-occurrence networks showed the most relevant networks node species were mainly Talaromyces, Chaetomiacea and Gemmatimonadetes etc. Root lipid metabolite of Glycerophospholines further proved that phosphorus utilization was also enhanced endogenously in the rhizosphere soil. These results indicate that the combination of biochar and arbuscular mycorrhizal fungi play synergic role in enhancing phosphorus utilization endogenously and exogenously.

Hydrochar prepared from digestate improves anaerobic co-digestion of food waste and sewage sludge: Performance, mechanisms, and implication

This work reported a new waste functionalization and utilization method, which use digestate to prepare hydrochar to improve methane production from food waste (FW) and sewage sludge (SS). Experimental results presented that 10 g/L hydrochar obtained the cumulative methane production of 133.11 ± 1.18 mL/g volatile solids added, 26.99 % higher than that without hydrochar addition. By monitoring the conversion of model metabolic intermediates, 10 g/L hydrochar was determined to favor hydrolysis, acidogenesis and methonogenesis bio-processes involved in methane production, thus improving the degradation of solubilized organics and consumption of short-chain fatty acids (SCFAs) during the co-digestion. Microbial investigation revealed that 10 g/L hydrochar enriched the microbes relevant to methane production (e.g., Methanosaeta and Syntrophomonas), but reduced the abundances of hydrolysis- and acidogenesis-related microbes (e.g., Acinetobacter). This hydrochar-based preparation and utilization strategy might offer a novel paradigm for waste-control-waste, bringing economic and environmental benefits.

Phosphorus release from hydrothermally carbonized digested sewage sludge using organic acids

Hydrothermal carbonization (HTC) is a treatment technique with great potential for sanitizing digested sewage sludge (SS) and converting it into valuable products. In particular, phosphorus (P) recovery from hydrothermally carbonized SS has attracted special attention in recent years. This work aims to examine the leaching efficiency of P and the consequent release of metals and heavy metals from SS hydrochars (at 180, 215 and 250 °C) using organic acids (oxalate and citrate) over a range of pH values (0-4) and extraction times (5 min-24 h). Both organic acids triggered P extraction efficiencies exceeding 75 % at the lowest pH, but only oxalate reached a nearly complete P release from hydrochars at pH > 0 and for all carbonization temperatures. Low HTC temperature (180 °C) and short extraction time (5 min) were the optimal conditions treatment for P recovery when reacted in oxalate solutions of maximal pH buffering capacity (pH = 1.4). However, oxalate leaching also transferred metals/heavy metals into the P-leachate, with the exception of Ca being retained in the solid residue from HTC as Ca-oxalate precipitate. Different characterization methods confirmed the presence of this precipitate, and provided information about the surface and morphological changes of the SS hydrochars following acid treatment. The results suggest that HTC not only a promising technique to sanitize and reduce the volume of SS, but also an efficient means for P recovery using oxalic acid, thus contributing to the circular economy of P.

Multiple microplastics induced stress on anaerobic granular sludge and an effectively overcoming strategy using hydrochar

Previous studies mostly focused on the responses of anaerobic granular sludge (AGS) to one kind of microplastics during wastewater treatment. However, a wide variety of microplastics has been detected in wastewater. The multiple microplastics induced stress on AGS and the effectively mitigating strategy still remain unavailable. Herein, this work comprehensively excavated the influences of multiple microplastics (i.e., polyethylene terephthalate (PET), polystyrene (PS), polyethylene (PE) and polypropylene (PP)) coexisting in the wastewater on AGS system from macroscopic to microcosmic aspects. Experimental results illustrated that microplastics decreased AGS granule size, increased cell inactivation and caused deteriorative methane recovery from wastewater. As such, this study then put great emphasis on proposing a mitigating strategy using hydrochar and disclosing the role of hydrochar in overcoming the stress induced by coexisting microplastics to AGS system. Physiological characterization and microbial community analysis demonstrated that hydrochar effectively mitigated the reductions in methane production by 50.6% and cell viability by 68.8% of microplastics-bearing AGS and reduced the toxicity of microplastics to microbial community in the AGS. Mechanisms investigation by fluorescence tagging and excitation emission matrix fluorescence spectroscopy with fluorescence regional integration (EEM-FRI) analysis revealed that hydrochar adsorbed/accumulated microplastics and enhanced microplastics-bearing AGS to secrete extracellular polymeric substance (EPS) with more humic acid generation, thus reducing the direct contact between microplastics and AGS. In addition, hydrochar weakened the AGS intracellular oxidative stress induced by microplastics, thereby completely eliminating the inhibition of microplastics on acidification efficiency of AGS, and partially mitigating the suppression on methanation.

Hydrochar as an environment-friendly additive to improve the performance of biodegradable plastics

Plastic additives affect the properties of plastics, which further determine the application range of plastics. However, most plastic additives have environmental friendliness or performance issues limiting their application. Hydrochar (HC) from waste biomass by hydrothermal carbonization has been proved to contain organic matter as function substances, like a binder, and is environment-friendly material. Currently, hydrochar as a plastic additive has not been previously reported. In this study, the HC/PBAT composites were produced by hydrochar blending with poly (butylene adipate-co-terephthalate) (PBAT) which is a biodegradable polymer. The hydrochar produced at different hydrothermal carbonization temperatures (180 °C, 210 °C, 240 °C, 270 °C, and 300 °C) and the addition of hydrochar (10 wt%, 20 wt%) were investigated. The results showed that the elastic modulus of the composites was increased by 27.4 MPa and 32.5 MPa compared with virgin PBAT while adding 10 wt% and 20 wt% hydrochar, respectively. Moreover, the stiffness of the composite was improved, and the balance of stiffness and toughness of the composites was effectively maintained when adding 10 wt% hydrochar treated at 300 °C. The elongation at break, tensile strength, and the elastic modulus of its composites were 630.8 ± 13.7%, 23.0 ± 0.4 MPa, and 100.5 ± 2.7 MPa, respectively. Furthermore, the crystallization temperature of the composites was increased after hydrochar was added into PBAT, and the maximum was 87.9 °C. It also means that hydrochar has a great nucleation effect during plastic processing. Therefore, hydrochar can be used as an environment-friendly additive to promote the performance of biodegradable plastic and promise to be applied in the field of biodegradable plastics.

Synthesis of magnetic hydrochar from Fenton sludge and sewage sludge for enhanced anaerobic decolorization of azo dye AO7

A novel magnetic hydrochar synthesized from Fenton sludge (FS) and sewage sludge (SS) was employed in the anaerobic decolorization of acid orange 7 (AO7). The stable presence of Fe₃O₄ in magnetic hydrochar was confirmed by physicochemical characterization. The degradation efficiency of AO7 in the anaerobic system with the addition of hydrochar prepared in an optimal proportion (SS:FS=1:3, named as HC-1:3) could reach 98.55%, which was 1.91 times higher than the control system. Particularly, superior electrical conductivity, electron transport system activity and azo reductase activity of the sludge in anaerobic system with HC-1:3 were achieved. The redox of Fe(Ⅲ)/Fe(Ⅱ) in anaerobic system was realized by dissimilatory iron-reducing bacteria enriched with HC-1:3. According to the six-cycle batch experiments and 120-day continuous-flow UASB experiments, the addition of HC-1:3 into the anaerobic system facilitated the diversity of microbiological community and increased the ecological stability of anaerobic system. The possible electron transfer mechanism involving in the magnetic hydrochar-based anaerobic system for AO7 removal was speculated preliminarily. The as-prepared magnetic hydrochar not only showed a promising future in anaerobic system for recalcitrant contaminants degradation, but also provided a new approach for the resource utilization of FS and SS.

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.

Estimation of methane production through the anaerobic digestion of greenhouse horticultural waste: A real case study for the Almeria region

The methane production of greenhouse horticultural waste (GHW) from Almeria (Spain), from where fruits and vegetables are exported to all parts of Europe, was calculated in this work through a combination of experimental and theoretical methods. To this end, eight samples of GHW were collected and characterized in a waste treatment plant. The collection of samples was fairly distributed throughout the year to ensure a representative characterization. The amount of methane produced in a hypothetical anaerobic digestion process was predicted through empirical models fed by experimental data. The experimental characterization revealed that GHW contained an adequate content of volatile matter (65.72% TS), but a high value for total dry matter (53.46%) and lignin content (9.36%), as well as a low moisture content (46.54%) and C/N ratio (17.46). Inhibiting compounds were also observed in the characterization, such a S (0.43%) and Cl (1.41%). The methane production predicted was 0.229 Nm³ CH₄/kg volatile matter, which may seem low in comparison to other waste potentially usable for anaerobic digestion. Nonetheless, the co-digestion of GHW with other waste could be an interesting alternative to enhance methane production and solve seasonality issues. Suitable pre-treatment can be also explored to increase the usability of GHW in anaerobic digestion. All in all, this work establishes a theoretical basis for potential solutions to manage the GHW produced in Almeria.

Valorization of oat husk by hydrothermal carbonization: Optimization of process parameters and anaerobic digestion of spent liquors

The hydrothermal carbonization (HTC) optimization of oat husk was performed using a response surface methodology. Furthermore, anaerobic digestion (AD) of spent liquor and hydrochar addition were evaluated in the biomethane potential (BMP) test. Results found that temperature influences the most in the studied responses (i.e., mass yield (MY) and higher heating value (HHV)). Optimal hydrochar MY (53.8%) and HHV (21.5 MJ/kg) were obtained for 219.2 °C, 30 min, and 0.08 of biomass/water ratio. A successful prediction capability of the optimization approach was observed, archiving an error < 1% between predicted and validated responses. The BMP experiment showed the feasibility of spent liquor as a potential substrate to be treated by AD (144 NmLCH₄/gCOD). Hydrochar boosted the methane production of spent liquor increasing up to 17% compared to digestion with no hydrochar addition. These findings provide new insights regarding oat husk valorization by integrating HTC and AD for energy production.