Hydrothermal Carbonization Of Sewage Sludge Research Articles

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

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.

Fe(NO3)3 assisted hydrothermal carbonization of sewage sludge: Focusing on characteristics of hydrochar and aqueous phase

• Enhancing hydrolysis was a key step to improving dehydration, decarboxylation, deamination and oxidation reaction for SS during HTC. • FN can notably decrease TOC, NH 3 -N and TN and decrease elemental content in hydrochar especially at high temperatures. • Formation Fe 2 O 3 also could catalyze hydrochar combustion. • The role of oxidation also can be limited by the amount of FN. Fe(NO 3 ) 3 was selected as a typical nitrate (5, 10 wt.%) to analyze its effects on the hydrothermal carbonization (HTC) of sewage sludge (SS). Enhancing hydrolysis was a key step to improving dehydration and decarboxylation of SS HTC. Fe(NO 3 ) 3 could notably decrease the total organic carbon, ammonia nitrogen, and total nitrogen of the aqueous phase and elemental content of the hydrochar especially produced at high temperatures. Formation Fe 2 O 3 could also catalyze hydrochar combustion. At different HTC temperatures, the difference between dehydration and decarboxylation could also be observed. The results would give more help to the treatment of protein-rich organic waste, particular using HTC method.

Hydrothermal carbonization of sewage sludge: Catalytic effect of Cl− on hydrochars physicochemical properties

Abstract In order to analyze the influence of disruption extracellular polymeric substances (EPS) and cell wall on the hydrothermal carbonization (HTC) of sewage sludge (SS), ultrasonic pretreatment was selected to dispose. According to solid phase and aqueous phase analysis results, it indicated that ultrasonic pretreatment can effectively break the EPS and cell wall structure, resulting in hydrolysis, dehydration and decarboxylation enhancing during HTC. The role of free radical increased elemental content in hydrochar. NaCl, which promoted hydrolysis, dehydration and decarboxylation, affected on the “real” hydrochar properties for it can be adsorbed in the surface of hydrochar. Disruption EPS structure and enhancing hydrolysis were key steps to HTC of SS, which would influence solid and liquid phase properties. The results would give more help to understand HTC process and dispose to other organic waste.

A novel machine learning-based approach for prediction of nitrogen content in hydrochar from hydrothermal carbonization of sewage sludge

In this work, 138 datapoints, including elemental composition and ultimate analysis of various types of sewage sludge, and the hydrothermal carbonization reaction conditions, are used to develop a prediction model for the nitrogen content of the hydrochar. The results suggested that a two-layer feedforward neural network with five (05) neurons in the hidden layer can accurately predict the nitrogen content of the hydrochar based on the reaction temperature and the contents of nitrogen, carbon, volatiles and fixed carbon in the feedstock. Over 100 runs, the R2 and RMSE are in [87.547–99.097%] and [0.243–1.431] wt.% (db), respectively. Moreover, a statistical and regression analysis revealed that the sewage sludge-N is the main contributor to the hydrochar-N. Mostly, 40–70% of sewage sludge-N goes to hydrochar-N. The results are consistent with previous experimental reports, and this model can be used to predict the sewage sludge-derived hydrochar-N.

Towards Engineered Hydrochars: Application of Artificial Neural Networks in the Hydrothermal Carbonization of Sewage Sludge

Sewage sludge hydrochars (SSHs), which are produced by hydrothermal carbonization (HTC), offer a high calorific value to be applied as a biofuel. However, HTC is a complex processand the properties of the resulting product depend heavily on the process conditions and feedstock composition. In this work, we have applied artificial neural networks (ANNs) to contribute to the production of tailored SSHs for a specific application and with optimum properties. We collected data from the published literature covering the years 2014–2021, which was then fed into different ANN models where the input data (HTC temperature, process time, and the elemental content of hydrochars) were used to predict output parameters (higher heating value, (HHV) and solid yield (%)). The proposed ANN models were successful in accurately predicting both HHV and contents of C and H. While the model NN1 (based on C, H, O content) exhibited HHV predicting performance with R2 = 0.974, another model, NN2, was also able to predict HHV with R2 = 0.936 using only C and H as input. Moreover, the inverse model of NN3 (based on H, O content, and HHV) could predict C content with an R2 of 0.939.

Distribution and toxicity of polycyclic aromatic hydrocarbons during CaO-assisted hydrothermal carbonization of sewage sludge

Hydrothermal carbonization (HTC) of sewage sludge (SS) with and without calcium oxide (CaO) introduction was conducted at 160–240 °C, and the yield and distribution of polycyclic aromatic hydrocarbons (PAHs) were evaluated for the first time. PAHs (2972.99 μg/kg) and toxic equivalent quantity (TEQ) (373.09 μg/kg) yields in SS decreased by 13.61% and 14.65%, respectively, after treatment at 160 °C and substantially increased as temperatures increased. More PAHs were distributed in the hydrochar than in the aqueous products. Hydrochar yields decreased linearly with temperature, thus increasing PAH concentration in hydrochar; 6221.98 μg/kg of PAHs in hydrocar at 240 °C exceeded agricultural use standard limits. PAH and TEQ yields at 200 °C decreased by 5.55–15.98% and 2.88–3.54%, respectively, when 3–9% CaO was added, which could be ascribed to CaO inhibition in the free radical reaction for PAH generation. Additionally, 6% CaO addition substantially weakened the acceleration effect of high temperatures on PAH formation; the decrease of PAH yield at 240 °C was 22.14%, which is higher than that at other temperatures. Consequently, the PAH concentration in hydrochar declined by 2.33–22.37%. PAH content in hydrochar obtained from CaO-assisted HTC of SS fell within agriculture use standard limit and exhibits potential for use as a soil conditioner. However, condition with a CaO amount of 15% would significantly increase TEQ yields. Considering both PAH and TEQ yields and the ecological risks of PAHs in hydrochar derived from HTC of SS, the appropriate reaction conditions were found to be 200 °C with 3–6% added CaO.

Hydrothermal Carbonization of Sewage Sludge: Analysis of Process Severity and Solid Content

AbstractHydrothermal carbonization of sewage sludge was carried out with the aim to evaluate the influence of process severity and initial solid content. Response surface methodology was applied to model yield and C yield responses. Enhanced dewaterability performance was recorded under mild processing conditions. The treatment promoted concentration and immobilization of Pb, Cd, Ni, Zn, and Cu. Variation of the solid content showed a stronger influence than severity on average yield and C yield. Higher heating values (HHVs) and energy retention efficiencies (EREs) of hydrochars obtained at the lowest solid content displayed the lowest values. Hence, the energy requirements of a first dewatering step should be compared with the related improvement in terms of HHV and ERE when sludge is used as feedstock.

Hydrothermal carbonization of sewage sludge coupled with anaerobic digestion: Integrated approach for sludge management and energy recycling

Sewage sludge management is a major operational task that challenges sustainability in municipal wastewater treatment plants. On the other hand, the high level of organic carbon in sewage sludge suggests its utilization potential for renewable energy applications. This study investigates hydrothermal carbonization (HTC) of anaerobic sludge at increasing temperatures (200, 250 and 300 °C) and retention times (30, 60 and 120 min), providing a wide spectrum of reaction severities (logR0 = 4.4–8.0). The goal of the study was to quantify the energy recovery from hydrothermal co-products (i.e., hydrochar and process water) for their use as a solid fuel and a feedstock for anaerobic digestion (AD), respectively. The hydrochar yield was quantified and its quality was evaluated for elemental composition, higher heating value and combustion behavior using thermogravimetric analysis. The hydrothermal process water was analyzed for its chemical oxygen demand and biochemical methane potential. The hydrochar yield was negatively affected by reaction severity with the highest value of 73% at logR0 of 4.4 and the lowest value of 49% at logR0 of 8.0. On the other hand, the hydrochar quality was found to be positively influenced by reaction severity, which enhanced the reduction of oxygen by decarboxylation processes. The biomethane yields were also affected by the reaction severity, with the highest potential value of 227 mL CH4 g COD−1 at the lowest severity and the lowest potential value of 28 mL CH4 g COD−1 at the highest severity. An overall energy balance suggests that mild reaction severities can serve as a suitable platform returning more than 70% of the energy from anaerobic sludge using the HTC-AD process integration.

Transformation of nitrogen during hydrothermal carbonization of sewage sludge: Effects of temperature and Na/Ca acetates addition

This study reports the effects of temperature and Ca/Na acetates addition on the transformation of nitrogen during hydrothermal carbonization of sewage sludge at 160–250 °C. The nitrogen species in the hydrochar, aqueous, oil and gas products from sludge hydrothermal carbonization at different temperatures are well characterized, with a focus on the amino acid species in various products. Temperature is found to greatly affect the nitrogen transformation during sludge hydrothermal carbonization. At 160 °C, 47.3% of nitrogen is transformed into the aqueous product. When the temperature increases to 250 °C, only 27.1% of nitrogen is retained in the hydrochar, while 69.2 and 6.7% of nitrogen is present in the aqueous and oil products, respectively. During hydrothermal carbonization, the protein-N is first converted into the polypeptide-N in the aqueous product, followed by its further decomposition into the NH+ 4-N. This leads to a high content of the NH+ 4-N in the aqueous product, especially at increased temperatures. The labile protein-N is also transformed into the heterocylic-N (especially the pyrrole-N) in the hydrochar as the temperature increases. Among all nitrogen species in the aqueous product, the polypeptide-N consisting of amino acids with the alkyl group is the most stable. Moreover, the addition of NaAc and CaAc2 reduces the nitrogen retention in the hydrochar, mainly due to enhanced hydrolysis of the protein-N. While for CaAc2 addition, the deamination of the polypeptide-N is also enhanced, leading to a higher NH+ 4-N in the aqueous product. Our results show that the type of amino acid in protein is important to determine the nitrogen transformation pathways, and acetate addition is an important strategy for enhancing nitrogen removal in the hydrochar during hydrothermal carbonization.

Migration and Transformation of Phosphorus during Hydrothermal Carbonization of Sewage Sludge: Focusing on the Role of pH and Calcium Additive and the Transformation Mechanism

Recovering phosphorus (P) from sewage sludge has been proposed as a promising substitute for phosphate ores, while hydrothermal carbonization (HTC) has emerged as an energy-efficient method for sew...

Hydrothermal carbonization of sewage sludge and in-situ preparation of hydrochar/MgAl-layered double hydroxides composites for adsorption of Pb(II)

Hydrothermal carbonization is a novel technology for dewatering of sewage sludge. Layered double hydroxides (LDHs) are a widely used functional material, which can be prepared by hydrothermal method. In this study, we proposed a new process combining hydrothermal carbonization of sludge and in-situ preparation of LDHs. The addition of Mg and Al salts (as LDHs precursor) to the slurry before hydrothermal treatment can act as a sludge conditioner to effectively accelerate the sludge sedimentation. The obtained products hydrochar/LDHs (HL) composites with various mass ratios were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that the well-crystallized LDHs slice is thinner after combining with hydrochar. The HL products can be used as a highly effective adsorbent for removal of Pb2+ from water. The adsorption experiment results showed that the adsorption behavior of Pb2+ onto HL composites can be well expressed by a pseudo-second-order kinetic model and Langmuir isotherm with the maximum adsorption capacity of 62.441 mg/g. Ionic strength had no significant effect on the adsorption capacity. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectra indicated that the adsorption mechanisms of Pb2+ by HL composites mainly included electrostatic interaction and surface coprecipitation. Therefore, this is a cost-effective method to obtain a high efficiency adsorbent.

Distribution and transformation behaviors of heavy metals and phosphorus during hydrothermal carbonization of sewage sludge.

This study investigated the distribution and transformation behaviors of heavy metals (HMs) and phosphorus (P) during hydrothermal carbonization (HTC) of sewage sludge. In addition to a large reduction in sludge volume, HTC significantly decreased the exchangeable fraction of HMs (Zn, Cu, Cr, Ni, and Mn) and increased their residual fraction, which resulted in immobilization of HMs accumulated in hydrochar. The ecological toxicity of HMs was greatly reduced, and all HMs exhibited their lowest risk levels after HTC at 220°C for 1h in 2% H2SO4 solution. Most of the P (~ 97%) in the input sludge remained in the hydrochar after HTC. HTC facilitated transformation of organic P to inorganic P and promoted conversion of apatite P to non-apatite inorganic P under acidic conditions. The feasibility of recovering P from sludge by HTC was verified by an acid extraction experiment utilizing hydrochar, which recovered more than 90% P. Graphical abstract.

Effect of temperature on the sulfur fate during hydrothermal carbonization of sewage sludge

To understand the effect of reaction temperature on sulfur during hydrothermal carbonization (HTC) of sewage sludge (SS), seven group of temperature (180–300 °C) were chosen to investigate the distributions and evolution of sulfur-containing compounds in hydrochar and the liquid products. Elemental analysis, X-ray photoelectron spectroscopy (XPS), and X-Ray powder diffraction (XRD) were used to characterize the distribution of sulfur in hydrochar. The concentrations of sulfate ions and sulfide were determined in the liquid sample. The experimental results showed that as the temperature increased, the O/C ratio decreased because of the improved carbonization degree of SS. After hydrothermal carbonization, 90% of the sulfur in SS remained in hydrochar. As the temperature increased, the amount of sulfur in the liquid, mainly in the form of sulfate ions, tended to decrease. However, the experimental results for the gas phase were the opposite of the liquid phase.

Benign-by-design N-doped carbonaceous materials obtained from the hydrothermal carbonization of sewage sludge for supercapacitor applications

This contribution discloses a new methodology for the preparation of N-doped carbon via the hydrothermal carbonization (HTC) of sewage sludge (SS).

Hydrothermal carbonization of sewage sludge: effect of inorganic salts on hydrochar's physicochemical properties

NaCl and (NH4)2SO4 were selected as typical inorganic salts (10 wt% addition) in order to analyze their effects on the hydrothermal carbonization (HTC) of sewage sludge (SS).

Assessing the effect on the generation of environmentally persistent free radicals in hydrothermal carbonization of sewage sludge

Environmentally persistent free radicals (EPFRs) have attracted increasing research interest in recent years. Herein, the generation of EPFRs during the hydrothermal carbonization of sewage sludge (SS) was studied. First, the surface morphology, functional groups, constituent elements and free radicals were characterized for a holistic description of the raw SS and the selected hydrochar obtained from hydrothermal carbonization of SS (SHC). Then, the impact of hydrothermal temperature, residence time and initial pH on the formation of EPFRs was explored in detail through the investigation of g-factors and intensities of EPFRs identified in SHC. The results have shown that the formation of EPFRs was affected by the factors mentioned above, in which the impact of temperature is the greatest. Two types of EPFRs were spotted in the hydrochar, oxygen-centered (O-centered) and carbon-centered (C-centered) EPFRs, which were caught in 120–150 °C and 260–280 °C, respectively. Moreover, the intensities of Electron Paramagnetic Resonance (EPR) signals enhanced with increasing hydrothermal temperature. Whereas, residence time and initial pH only affected the amount of EPFRs in a manner. Additionally, the half-life of the O-centered EPFRs and the C-centered EPFRs was determined as long as 160.45 days and 401.10 days, respectively, indicating that EPFRs are stable in a long time.

Hydrothermal carbonization of sewage sludge: Effect of aqueous phase recycling

In this study, the aqueous phase produced from the hydrothermal carbonization (HTC) of sewage sludge (SS) with pure water was reused for the HTC of fresh SS, aiming to maximize energy recovery from the aqueous phase. The aqueous phase was recycled four times. The effects of aqueous phase recycling on the properties of the aqueous phase and hydrochar produced at temperatures of 200, 230, and 260 °C were studied. The hydrochar yield always decreased with increasing temperature regardless of whether the aqueous phase was recycled. The C and N contents and higher heating values of the hydrochars produced with aqueous phase recycling were all higher than that of the hydrochar produced with pure water and slightly increased as the number of recycling times increased. The O/C and H/C atomic ratios of the hydrochars revealed that dehydration was the main reaction pathway during HTC of SS. The pH value of the aqueous phase increased as the number of recycling times increased, indicating that the concentration of NH4+-N in the aqueous phase increased. The process of aqueous phase recycling affected the carbonization of the hydrochar. These results suggest that aqueous phase recycling was favorable for energy recovery from the aqueous phase produced from the HTC of SS and for improving the combustion properties of the hydrochar. Thus, we believe that aqueous phase recycling was a promising strategy for energy recovery, aqueous phase disposal, and the production of high-quality hydrochar with respect to the HTC of SS.

Experimental and thermodynamic studies of phosphate behavior during the hydrothermal carbonization of sewage sludge.

Recovered phosphate from sewage sludge is becoming a key product in the fertilizer market. This study investigates the fate of phosphate during the hydrothermal carbonization of digested sewage sludge to support the development of an economic and sustainable solution for dealing with sewage sludge for phosphate recovery. The solid products from the hydrothermal carbonization of digested sewage sludge in a batch reactor (180, 220, and 260 °C; 1, 2, and 4 h; digested sewage sludge-to-water ratios of 0.2 and 0.1 w/w) were analyzed using a sequential chemical extraction procedure to understand and predict the formation of phosphate species and the related extraction behavior of phosphate. The obtained results were compared with the thermochemical equilibrium composition of hydrothermal carbonization products, calculated using the software FactSage 7.2. The majority of phosphate was retained as Al, Ca, and Fe salts in hydrochar. The decomposition of organic phosphates was observed by processing at lower temperatures. Hydrothermal carbonization at temperatures higher than 180 °C resulted in the transformation of the Al-bound phosphate into Ca-bound phosphate. Hydroxyapatite (Ca5(PO4)3(OH)) and Fe7(PO4)6 were calculated as stable phosphate-containing minerals at equilibrium. This study suggests that kinetic constraints inhibit the formation of these minerals in the batch reactor and presents a mechanism of phosphate transformation using the obtained data. The results allow for targeted optimization of phosphate recovery strategy.

Upgrading Hydrothermal Carbonization (HTC) Hydrochar from Sewage Sludge

As a treatment method of sewage sludge, the hydrothermal carbonization (HTC) process was adopted in this work. HTC has a great advantage considering the economic efficiency of its process operation due to its reduced energy consumption and production of solid fuel upgraded through the increased fixed carbon and heating value. The ash of sewage sludge, however, contains up to 52.55% phosphate, which degrades the efficiency of the thermochemical conversion process such as pyrolysis, combustion, and gasification by causing slagging. In this study, three kinds of organic acids, i.e., oxalic, tartaric, and citric acid, were selected to eliminate phosphorus from hydrochars produced through the HTC of sewage sludge. The efficiency of the phosphorus removal and the properties of the corresponding HTC hydrochars were analyzed by adding 20 mmoles of organic acids per 1 g of phosphorus in the HTC sample. In addition, the phosphorus reduction effect and the applicability to an upgrading process were verified. Oxalic acid was selected as the most appropriate organic acid considering the economic efficiency of its process operation. Furthermore, the optimal conditions were selected by analyzing the efficiency of the phosphorus elimination and the characteristic property of the HTC hydrochars with the weight fraction of oxalic acid.