Carbonization Of Sewage Sludge Research Articles

Hydrothermal cocarbonization of cellulose and organic matter of municipal sewage sludge for the preparation of supercapacitor carbon materials

Two kinds of aqueous phases (AQ230 and AQAm230) prepared from hydrothermal carbonization of sewage sludge in pure water and aqueous ammonia, respectively, were separately hydrothermally cocarbonized with cellulose at 260, 280 and 300 °C. The N-heterocyclic compounds in the aqueous phase promoted the N content of the hydrochars while reducing their O content through a possible conversion of –CO into –CN-/C–N. With both AQ230 or AQAm230, increasing the temperature improved the incorporation of N into the aromatic rings, while the fluctuation in total N content in the hydrochars from each aqueous phase was not as remarkable. After KOH-activation of the hydrochars, the average pore diameter of the derived N-doped carbons (NCs) exhibited an increasing trend with increasing O/N ratio, especially at lower N-doping. A three-dimensional approximation confirmed that the specific surface area decreased when the C, N, and O content were high. In a three-electrode system, NC230280 exhibited the best performance, with a specific capacitance of 219.91 F g−1 at 0.25 A g−1 (63.64% retention at 20 A g−1), the lowest internal resistance (0.51 Ω), and a small time constant (0.49 s). In a two-electrode system, NC230280 exhibited 176.72 F g−1 at 0.25 A g−1 (59.08% retention at 20 A g−1), and a maximum energy density of 6.14 Wh⋅kg−1 was reached at a power density of 64 W kg−1 (5.86 Wh⋅kg−1 was still maintained at 518.07 W kg−1). The results reveal that, in addition to the surface properties, the type of precursor and the elemental composition of the NCs also influence the capacitance performance.

Energy yield optimization of co-hydrothermal carbonization of sewage sludge and pinewood sawdust coupled with anaerobic digestion of the wastewater byproduct

High energy recovery efficiency is expected to be obtained from biomass waste by coupling hydrothermal carbonization (HTC) with anaerobic digestion (AD). This study focused on optimizing the total energy yield (TEY) from sewage sludge (SS) and pinewood sawdust (PS) in a HTC-AD coupling process, which has attracted little attention. Response surface methodology (RSM) was used to gain the optimal operation parameters including PS proportion, liquid–solid mass ratio (L/S) of raw feedstock (SS-PS mixture slurry), HTC temperature, and residence time and discuss how the parameters influenced the TEY. The results showed that the PS proportion, HTC temperature, and residence time had significant effects on the TEY (p < 0.05), while the influence of the L/S of raw feedstock on TEY was limited. The hydrochar mass yield (HCY), higher heating value (HHV) of hydrochar, and the cumulative methane production from the AD of HTC wastewater (HTCWW) increased with PS proportion increasing. By contrast, when HTC temperature increased, HHV increased while both HCY and cumulative methane production decreased. Prolonging the HTC residence time decreased the TEY of hydrochar and methane, because the HCY decreased considerably although the carbonization degree of the fed organics increased, and the AD of HTCWW was inhibited due to the increasingly generated toxic substances in a long-term HTC process. The optimal TEY, with a value of 89.61%, was obtained at the following conditions: PS proportion of 75%, original L/S of feedstock of 7:1, HTC temperature of 190 °C, and residence time of 3 h. Among which, the energy yields of hydrochar and methane were 86.46% and 3.15%, respectively. HTCWW showed good AD performance with a cumulative methane yield of 255.64 mL-CH4/g-COD in a 16-day AD process.

Environmental Life Cycle Assessment of Hydrothermal Carbonization of Sewage Sludge and Its Products Valorization Pathways.

This study is aimed at evaluating through Life Cycle Assessment (LCA) the environmental performances of an integrated system of an existing Water Resources Recovery Facility (WRRF) and a hypothetical hydrothermal carbonization (HTC) plant applied to the generated sewage sludge (SS). Beside the valorisation of the solid product (hydrochar, HC) as a fuel substituting lignite, the possibility to valorize also the liquid fraction (process water, PW) derived by the HTC, by anaerobic digestion to produce biogas, is here proposed and analysed. Additionally, phosphorus recovery from HC, prior its use, by acid leaching with nitric acid is also suggested and evaluated. Thus, four integrated scenarios, based on SS carbonization, are proposed and compared with the current SS treatment, based on composting outside of the WRRF (Benchmark scenario). The proposed scenarios, based on HTC, show improved performances with respect to the benchmark one, for thirteen of sixteen considered impact indicators. For the Climate Change (CC) indicator, the two HTC scenarios are able to reduce the impacts up to – 98%, with respect to the Benchmark. Further, the introduction of anaerobic digestion of PW proves to reduce impacts more than other configurations in eleven on sixteen impact categories. On the contrary, the introduction of phosphorus recovery process negatively affects the values for most of indicators. Thus, possible solutions to improve the integration of this process are outlined (e.g., the use of sulfuric acid instead of nitric one, or the application of a different ratio between solid and acidified solution during acid leaching of HC to recover phosphorus).Graphical abstract Supplementary InformationThe online version contains supplementary material available at 10.1007/s12649-022-01821-x.

Co-hydrothermal carbonization of sewage sludge and coal slime with sulfuric acid for N, S doped hydrochar

In this study, co-hydrothermal carbonization (co-HTC) of sewage sludge(SS) and coal slime(CS) was studied for preparation of N, S doped hydrochar. The effect of blending ratios and temperature was also investigated. The results showed that there existed a synergistic effect during co-HTC, especially at the ratio of 7:3. Compared with the calculated values, the experimental values of the fixed carbon content increased by 51.00%, the carbon content increased by 21.73%, the mass yield increased by 13.52% and the carbon retention increased by 42.34%; while the H/C and O/C decreased by 32.20% and 31.97%, respectively. Furthermore, Raman spectrum and XPS analysis indicated that adding CS could promote the aromatization degree. XPS and FTIR analysis showed that the sulfuric acid could affect the occurrence mode of sulfur and nitrogen and led to abundant oxygen-containing functional groups such as –SO, O–CO and –OH on the hydrochar surface during the HTC process. This study could provide a theoretical basis for the utilization of SS and CS, and also provide a novel method for the preparation of N, S doped hydrochar.

Effects on the Physicochemical Properties of Hydrochar Originating from Deep Eutectic Solvent (Urea and ZnCl2)-Assisted Hydrothermal Carbonization of Sewage Sludge

Effects on the Physicochemical Properties of Hydrochar Originating from Deep Eutectic Solvent (Urea and ZnCl₂)-Assisted Hydrothermal Carbonization of Sewage Sludge

Theoretical modeling of hydrochar precursor formation during the hydrothermal carbonization of sewage sludge

Hydrothermal carbonization (HTC) is a promising technique to process biomass resources such as sewage sludge, which is rich in carbohydrates and easily forms hydrochar. However, as research on the HTC of sewage sludge is largely experimental, the mechanism behind the related transformations of small-molecule carbohydrates remains poorly understood. In this study, glucose was employed as a model of carbohydrate hydrolysates in sludge, and its conversion mechanism into hydrochar precursors was investigated using density functional theory. The obtained results suggested that glucose was first isomerized into fructose, which subsequently underwent three dehydration reactions with different energy barriers to produce hydroxymethylfurfural (HMF). The following hydration and dehydration of HMF involved small molecules, such as formic acid, levulinic acid, 2,5-dioxo-6-hexanal (DHH), and 1,2,4-benzenetriol, some of which (including HMF and DHH) underwent polycondensation to form soluble polymers as hydrochar precursors. Thus, the present study sheds light on the disputed conversion mechanism of sludge-to-hydrochar conversion upon HTC and guides further related experimental studies.

Preparation of Citric Acid-Sewage Sludge Hydrochar and Its Adsorption Performance for Pb(II) in Aqueous Solution.

In order to seek the value-added utilization method of sewage sludge and develop low-cost and high-efficient adsorbents, a hydrochar was prepared by the co-hydrothermal carbonization of sewage sludge and citric acid and then characterized. The differences in Pb(II) adsorption performance between the citric acid–sewage sludge hydrochars (AHC) and the hydrochar prepared solely from sewage sludge (SSHC) were also investigated. When citric acid dose ratio (mass ratio of citric acid to dry sewage sludge) is 0.1, the obtained hydrohcar (AHC0.1) has the highest specific surface area (59.95 m2·g−1), the most abundant oxygen-containing functional groups, the lowest pHpzc (5.43), and the highest equilibrium adsorption capacity for Pb(II). The maximum adsorption capacity of AHC0.1 for Pb(II) is 60.88 mg·g−1 (298 K), which is approximately 1.3 times that of SSHC. The potential mechanisms can be electrostatic attraction, co-precipitation, complexation, and cation-π interaction. It was demonstrated that by incorporating citric acid into the hydrothermal carbonization, resource utilization of sewage sludge can be accomplished effectively.

Waste-based hydrothermal carbonization aqueous phase substitutes urea for rice paddy return: Improved soil fertility and grain yield

Hydrothermal carbonization is an emerging and efficient technology for utilizing wet waste. However, the utilization of hydrothermal carbonization aqueous phase (HAP), a carbon-rich liquid product containing massive quantities of nutrient elements, has been largely unexplored. In this study, HAP produced by hydrothermal carbonization of sewage sludge (SHAP) and pig manure (PHAP) at 180 °C and 260 °C was applied as a substitute for partial (9.1–55.9%) urea in rice paddy field. Results showed that HAP increased the abundance of humic acid-like substances and microbial metabolic by-products in soil dissolved organic matter (DOM) by 3.3–7.2 times and 42.1–101.6%, respectively, compared to the control treatment without HAP addition (CK). Meanwhile, HAP significantly increased soil fluorescence and humification index of soil DOM by 28.5–41.9% and 12.3–26.8%, respectively, in comparison with CK. Soil fertilized with SHAP260 had greater mineral nitrogen, total organic carbon and Olsen-P compared to CK. HAP increased the grain N uptake by 34.0–58.8% and rice yield by 6.3–34.1% with essential amino acids in grains improved by 8.6–20.9% compared to CK. This study stated that HAP as a substitute of partial urea for rice paddy return has positive effects on soil fertility and grain yields.

Random forest-based modeling for insights on phosphorus content in hydrochar produced from hydrothermal carbonization of sewage sludge

The hydrochar produced from hydrothermal carbonization(HTC) of sewage sludge (SS) usually has a high phosphorous (P) content, and that would result in fouling and energy efficiency reduction. Therefore, it is important to monitor the P content during the hydrochar production process. This work suggests a data-driven Random Forest-based model to predict the total P content in the hydrochar (TP-hc) from the HTC of SS. Various configurations of inputs features were examined, including the data of proximate analysis, ultimate analysis, ultimate and proximate analyses, and for each configuration, either if the total P in the SS (TP-ss) was known or not. Overall, the models including TP-ss as input have accurately predicted the TP-hc with an R2 located in [92–95%]. Features’ importance approach and partial dependence analysis pointed out that the TP-ss, ash content, reaction temperature (T), reaction time (t), and initial pH of feedwater exhibit positive effect on the TP-hc. In contrast, contribution of the volatile matter (VM) of SS was mostly negative. Dry matter loading exhibits no obvious monotonicity with TP-hc. This work could guide the production of SS-hydrochar with the desired P content, and thus avoid time and resources consuming for many trials.

Influence of Post- and Pre-Acid Treatment during Hydrothermal Carbonization of Sewage Sludge on P-Transformation and the Characteristics of Hydrochar

Phosphorus (P) recovery from alternative P-rich residues is essential to meet the growing demands of food production globally. Despite sewage sludge being a potential source for P, its direct application on agricultural land is controversial because of the obvious concerns related to heavy metals and organic pollutants. Further, most of the available P recovery and sludge management technologies are cost-intensive as they require mandatory dewatering of sewage sludge. In this regard, hydrothermal carbonization (HTC) has gained great attention as a promising process to effectively treat the wet sewage sludge without it having to be dewatered, and it simultaneously enables the recovery of P. This study was conducted to analyse and compare the influence of acid (H2SO4) addition during and after HTC of sewage sludge on P leaching and the characteristics of hydrochar. The obtained results suggested that despite using the same amount of H2SO4, P leaching from solid to liquid phase was significantly higher when acid was used after the HTC of sewage sludge in comparison with acid utilization during the HTC process. After HTC, the reduction in acid-buffering capacity of sewage sludge and increase in solubility of phosphate precipitating metal ions had a greater influence on the mobilization of P from solid to liquid phase. In contrast, utilization of H2SO4 in different process conditions did not have a great influence on proximate analysis results and calorific value of consequently produced hydrochar.

Integrated Study of Organic Components for Hydrochar's Properties and the Nitrogen Transformation Mechanisms During Hydrothermal Carbonization of Sewage Sludge

Integrated Study of Organic Components for Hydrochar's Properties and the Nitrogen Transformation Mechanisms During Hydrothermal Carbonization of Sewage Sludge

Hydrothermal Carbonization of Sewage Sludge: Optimization of Operating Conditions Using Design of Experiment Approach and Evaluation of Resource Recovery Potential

Hydrothermal Carbonization of Sewage Sludge: Optimization of Operating Conditions Using Design of Experiment Approach and Evaluation of Resource Recovery Potential

Energy Yield Optimization for Co-Hydrothermal Carbonization of Sewage Sludge and Pinewood Sawdust Coupling with Anaerobic Digestion of the Produced Wastewater

Energy Yield Optimization for Co-Hydrothermal Carbonization of Sewage Sludge and Pinewood Sawdust Coupling with Anaerobic Digestion of the Produced Wastewater

Investigation of aqueous phase recirculation on co-hydrothermal carbonization of sewage sludge and lignite: Hydrochar properties and heavy metal chemical speciation

Excessive process water remained an important challenge in hydrothermal carbonization (HTC). In this work, co-HTC of sewage sludge (SS) and lignite (LIG) along with the aqueous phase recirculation (APC) was employed to explore the feasibility of producing clean solid fuels at 230 °C. Chemical speciation and concentration of heavy metals (HMs) in hydrochars were evaluated, as well as fuel properties. The results showed that the co-HTC with LIG was beneficial for the decline of total concentrations of Cu, Cr, Ni, Mn, and Zn. Furthermore, APC further decreased the risk of Ni without expanding the pollution of HMs. Compared with the hydrochar produced from SS, the coalification degree, higher heating values (5.61─6.19 MJ/kg), and combustion behaviour of the hydrochar derived from co-HTC were upgraded and improved. The yield of hydrochar was enhanced via APC but several fuel properties changed slightly. These findings offered a viable technology that could lower the output of wastewater and realize the energy utilization of SS.

Results of Chemical Activation of Biochar Obtained by the Method of Accelerated Hydrothermal Carbonization of Sewage Sludge

Results of Chemical Activation of Biochar Obtained by the Method of Accelerated Hydrothermal Carbonization of Sewage Sludge

Co-hydrothermal carbonization of sewage sludge and model compounds of food waste: Influence of mutual interaction on nitrogen transformation

This study reports the transformation behavior of nitrogen during the co-hydrothermal carbonization of sewage sludge and model compounds (microcrystalline cellulose, starch, lignin, and xylan) of food waste at 220 °C, with a focus on the reaction routes between starch/xylan and NH4+. Most of the nitrogen in the raw sludge was transformed into organic-N (44.6%) and NH4+ (23.3%) in the aqueous product, and only 20.3% of nitrogen was retained in the hydrochar. The added model compounds could react with organic-N (i.e., amino acids and amines) and NH4+ in aqueous products through Maillard and Mannich reactions, generating heterocyclic-N (especially pyrrole-N) which further polymerizes to form nitrogen-containing polyaromatic hydrochar. This leads to an increase in the retention rate of nitrogen to 36.8–50.9%, especially upon the addition of starch and xylan. During the hydrothermal carbonization of starch/xylan in the NH4+ solution, the polymers are first hydrolyzed into monomers, followed by their further reaction with NH4+ to generate pyrrole-N and pyridine-N in aqueous products (especially xylan), and the pyrrole-N can then polymerize with aromatic clusters to form hydrochar-N. The results show that the model compounds of food waste substantially affect the nitrogen transformation pathways during hydrothermal carbonization, mainly because of the structures of their monomers. These findings can guide the production of sludge-based hydrochar with the targeted regulation of nitrogen content and species.

Correlating phosphorus transformation with process water during hydrothermal carbonization of sewage sludge via experimental study and mathematical modelling

Recently, hydrothermal carbonization (HTC) based phosphorus (P) recovery from sewage sludge (SS) has attracted considerable interests worldwide. However, they concentrated on P transformation in the hydrochars, while ignored that the variations of process water (PW) might influence P transformation, since it exposed to water thoroughly during HTC. In this study, correlation of P transformation with PW were examined via experimental study and mathematical modelling. The results showed that statistical significance (p < 0.05) of HTC temperature and feedwater pH on NH4+-N concentration in the PW was observed due to deamination and ring opening reactions of amino acids, confirming by their excellent correlation with R2 = 0.988. NH4+-N concentration dominated increasing PW pH, which stimulated the transformation of NAIP to AP. Associated model was developed with satisfactory R2 = 0.938. Although P transformation during HTC was significantly influenced by HTC temperature and feedwater pH, supporting by their strong correlation with R2 = 0.956, its transformation was PW pH dependent. Ultimately, detailed P transformation pathways during HTC was proposed with incorporation into the impact of PW. This work can provide new insights into HTC-based P transformation in the pristine SS.

Effects of baking soda on Co-hydrothermal carbonization of sewage sludge and Chlorella vulgaris: Improved the environmental friendliness of hydrochar incineration process

Incineration is one of the prevailing methods of sewage sludge management worldwide. However, the pollutants of NOX, SO2, and heavy metals emitted severely limit this disposal method during the incineration process. By adding baking soda to the co-hydrothermal carbonization process of sewage sludge and Chlorella vulgaris, this study improved the cleanliness of hydrochar. The characteristics of hydrochar produced with 0, 4, 8, and 12 g baking soda/15 g sample were evaluated. When added 8 g baking soda/15 g sample, the content of N and S in hydrochar dropped to the lowest, 0.98% and 0.16%, respectively. Compared with the hydrochar produced with no baking soda, the contents of N and S of hydrochar with 8 g baking soda/15 g sample decreased by 66.78% and 80.72%, respectively. TG-FTIR analysis shown that NOX and SOX emissions from the hydrochar combustion process were reduced. As for heavy metals, they can be divided into two categories. One is easy to combine with amino acids hydrolyzed from protein to dissolve in the liquid phase; the other is easy to combine with amino acids and sedimented as a part of hydrochar. The fixation ratios of Zn, Cr, Cu, and Mn of the hydrochar produced with 12 g baking soda/15 g sample reached 74.34%, 93.53%, 79.47%, and 81.74%, respectively. The comprehensive assessment using multi-dimensional decision-making methods showed that the hydrochar produced with 12 g baking soda/15 g sample had the best environmental friendliness.

Co-Hydrothermal Carbonization of Sewage Sludge with Wood Chip: Fuel Properties and Heavy Metal Transformation Behavior of Hydrochars

Co-Hydrothermal Carbonization of Sewage Sludge with Wood Chip: Fuel Properties and Heavy Metal Transformation Behavior of Hydrochars

Co-hydrothermal carbonization of sewage sludge and polyvinyl chloride for the production of high-quality solid fuel with low nitrogen content

Sewage sludge (SS) and polyvinyl chloride (PVC) are typical solid wastes. Their co-hydrothermal carbonization behavior was investigated in this study. The low-nitrogen solid fuel (0.94 wt%) with high heating value (9.84 MJ·Kg−1) was prepared through parameter optimization at 240 °C for 1.5 h under water loading amount of 0.84 g·cm−3. In an acidic environment, the stubborn protein in SS could be converted into free amino acids, which were generated by the decomposition of PVC under hydrothermal conditions. The stubborn N could be translated into easy-to-remove N, such as nitrile–N and inorganic N, and the dehydration reaction was evidently promoted. The acidic environment at high temperatures caused the dissolution of ash in SS and improved the combustion performance of hydrochar. FT-IR results showed that, with increased PVC loading proportion, -C=N- was converted into -C=O-. Co-hydrothermal carbonization could effectively improve the combustion performance of hydrochar. The addition of PVC could lead to the generation of increased volatile matter, fixed carbon, and unsaturated CC, and the combustion temperature range shifted to a high range. However, the generation of graphite-like carbon was caused by further increasing the PVC loading proportion, which hindered the improvement of its combustion performance. In the parameter optimization study, the increased water loading amount (from 0.54 g·cm−3 to 0.84 g·cm−3) had the most evident effect on the N content in the hydrochar (from 1.50 wt% to 0.94 wt%), which promoted the denitrification efficiency (from 60.11% to 75.00%) and the conversion of -C=N- components, and prevented further polymerization of solid products.