Hydrothermal Carbonization Of Sewage Sludge Research Articles

Hydrothermal carbonization of sewage sludge for hydrochar valorization: Role of feedwater pH on sulfur removal and transformation

Adjusting the pH value is a useful method to improve the hydrochar properties during hydrothermal carbonization (HTC). The distribution of various sulfur species in hydrochar is considered as an important parameter affecting its further utilization. Nevertheless, the detailed effects of pH on the redistribution of different sulfur forms are still unknown. This study investigated the sulfur transformation pathways during HTC of sewage sludge with different feedwater pH (0–14) at 220 °C. Both acidic and alkaline feedwater facilitate the conversion of organic sulfur from the hydrochar into the stable species in aqueous products (i.e., sulfate-S). The latter especially leads to a decreased sulfur retention ratio of 31.97 % at pH=0 and 60.13 % at pH=14. Under acidic conditions, organic sulfur species such as sulfone-S and aliphatic-S undergo enhanced cyclization to form aromatic-S/thiophene-S in aqueous and hydrochar products; additionally, the oxidation of sulfone-S predominates the generation of sulfate-S in aqueous products at pH < 1. Under alkaline conditions, the aromatic-S/thiophene-S can be further oxidized into sulfate-S in aqueous products at pH>13. Furthermore, the interaction between water-soluble sulfate-S and hydrolytic intermediates (i.e., glucose, heterocyclic compounds) is intensified at pH=14, resulting in increased formation of sulfone-S and sulfite-S in aqueous products. The insights gained from this study can offer guidance for controlling sulfur distribution during HTC by adjusting the feedwater pH.

Ionic liquid-catalyzed hydrothermal carbonization of sewage sludge: Effect of residence time and liquid phase circulation on hydrochar characteristic

The ionic liquid (IL, [DMAPA]HSO4) was used to catalyze hydrothermal carbonization (HTC) of sewage sludge (SSL), the study discussed how residence time and liquid-phase circulation influence the characteristics of the hydrochar. The decarboxylation process in IL-catalyzed HTC led to lower hydrochar yield but higher higher heating value (HHV). And the secondary hydrochar formation promoted by IL favored the system for high energy feedback (EF), but iron was involved in the secondary hydrochar formation. At the optimum hydrothermal time (90 min), the EF reached 124 % and the removal efficiencies of Fe, Mg, Mn, and Zn were 66 %, 73 %, 96 %, and 90 %, respectively. Functional groups on organic acids (COO–), amides (CO), and polysaccharides (C–O–C) in the SSL were sequentially removed during the IL-catalyzed HTC process, while cross-linking and graphitization occurred, resulting in higher ignition temperatures and lower metal contents. However, multiple cycles of the liquid phase resulted in a decrease in the catalytic properties of IL, resulting in low EF hydrochar with high metal content and functional group intensity. Catalytic graphitization is the key to obtaining hydrochar with high HHV and low metal content. Therefore, future development of catalysts is needed to facilitate the dehydration, decarboxylation and graphitization process of HTC.

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.

Hydrothermal carbonization of sewage sludge for hydrochar production: optimization of operating conditions using Box-Behnken design coupled with response surface methodology

Hydrothermal carbonization of sewage sludge for hydrochar production: optimization of operating conditions using Box-Behnken design coupled with response surface methodology

Coupling of struvite crystallization and aqueous phase recirculation for hydrochar upgrading and nitrogen recovery during hydrothermal carbonization of sewage sludge

Recycling of aqueous phase (AP) as a by-product after hydrothermal carbonization (HTC) of sewage sludge (SS) has been of interest. The combination of magnesium ammonium phosphate (MAP) or the so-called struvite crystallization and aqueous phase (AP) recirculation has great potential for resource recovery and hydrochar enhancement. In this study, both the aqueous phase of HTC after MAP recovery of NH4+-N (AP-MAP) and the untreated aqueous phase of HTC (AP-HTC) were reused for HTC of fresh SS, and both aqueous phases were recycled four times. The effects of the two AP cycles on the properties of AP and hydrochar at 200, 230, and 260 °C were studied, and the effect of temperature on the two AP cycles was similar. The hydrochar produced by the AP-MAP cycle had lower nitrogen content than that of the AP-HTC cycle due to the low ammonia nitrogen (NH4+-N) content, and the combustion performance was improved. MAP recovery reduces the accumulation of NH4+-N in the AP cycle and MAP is also a high-quality fertilizer. Therefore, the combination of MAP recovery and AP recycling provides a feasible technical approach for resource utilization, eutrophic AP treatment, and production of high-quality hydrochar in the HTC process of SS.

Effect of aqueous phase from hydrothermal carbonization of sewage sludge on heavy metals and heavy metal resistance genes during chicken manure composting

Livestock manure is often contaminated with heavy metals (HMs) and HM resistance genes (HMRGs), which pollute the environment. In this study, we aimed to investigate the effects of the aqueous phase (AP) produced by hydrothermal carbonization (HTC) of sewage sludge (SS) alone and the AP produced by co-HTC of rice husk (RH) and SS (RH-SS) on humification, HM bioavailability, and HMRGs during chicken manure composting. RH-SS and SS increased the humic acid content of the compost products by 18.3 % and 9.7 %, respectively, and significantly increased the humification index (P < 0.05) compared to the CK (addition of tap water). The passivation of HMs (Zn, Cu, As, Pb, and Cr) increased by 12.17–23.36 % and 9.74–15.95 % for RH-SS and SS, respectively, compared with that for CK. RH-SS and SS reduced the HMRG abundance in composted products by 22.29 % and 15.07 %, respectively. The partial least squares path modeling results showed that SS and RH-SS promoted compost humification while simultaneously altering the bacterial community and reducing the bioavailability of metals and host abundance of HMRGs, which has a direct inhibitory effect on the production and distribution of HMRGs. These findings support a new strategy to reduce the environmental risk of HMs and HMRGs in livestock manure utilization.

Improvements in dewaterability and fuel properties of hydrochars derived from hydrothermal co-carbonization of sewage sludge and organic waste

The hydrothermal co-carbonization of sewage sludge and organic additives, namely 10 and 20 % of charcoal, fir, grass, and an undersieved fraction of municipal solid waste, was studied. The benefits of this combined process included the spectacular dewaterability performance of slurry, proved by positive filtration tests and shorter capillary suction times. For instance, a 20 % fir addition decreased c.a. 60 % of pressure filtration time when compared to the hydrothermal carbonization of sewage sludge. A 10 % undersieved fraction of municipal solid waste resulted in 15.72 s of capillary suction time. Moreover, hydrothermal co-carbonization produced effective solid energy sources. The addition of organic origin waste to sewage sludge prior to the process caused higher heating values, carbon and fixed carbon contents of hydrochars (e.g. a 20 % charcoal addition generated 21 % higher heating value, 30 % carbon and 2.8 times higher fixed carbon), which corresponded with easier and more stable combustion processes compared to hydrochar from sewage sludge determined by thermal analysis. Possible exploitation problems during combustion have been assessed by determining the tendency risks of slagging and fouling based on oxides identified in ash by XRF analysis. Furthermore, changes in the structural and morphological properties of hydrochars were identified by SEM and FTIR analyses.

The role of chemical functional groups in dewaterability of hydrochar: A molecular simulation study

Hydrothermal carbonization (HTC) is a promising technique for conversion of wet biomass materials to hydrochar, a carbon-rich material with a wide range of applications. A crucial property of hydrochar, particularly that obtained from HTC of sewage sludge, is the dewaterability, which may be controlled by certain chemical functional groups of the hydrochar itself. This paper presents results from our recent study on the role of chemical functional groups in the dewaterability of hydrochar, employing a novel approach that integrates molecular dynamics simulations with extended semi-empirical tight-binding calculations. By examining various hydrochar models with distinct chemical functional groups such as hydroxyl, carboxyl, and ketone, multiple factors affecting the dewaterability were investigated. The results show that removal of specific functional groups led to substantial reductions in hydration energy of hydrochar molecules, from -340 kJ/mol to -61 kJ/mol. Findings from this work not only contribute to better understanding the dewaterability of hydrochar in connection to the functional groups, but also provide valuable knowledge for customizing the design and production of hydrochar with specific chemical functional groups to suit diverse applications.

Conversion of Sewage Sludge into Biofuels via Different Pathways and Their Use in Agriculture: A Comprehensive Review

The valorisation of sewage sludge for sustainable agricultural use and biofuel production proposes an effective and beneficial management of sewage sludge in a closed-loop cycle. The management of sewage sludge biowaste is a rising problem due to increasing waste storage expenses. In this sense, the use of circular economy principles in sewage sludge management creates opportunities to develop new technologies for processing. The biorefinery model allows the application of wasteless technologies via sewage sludge valorisation in terms of agricultural use and biofuel production, especially with the hydrothermal carbonisation method. Applying hydrothermal carbonisation in the treatment of biosolid sewage sludge has numerous benefits due to processing highly hydrated organic waste into carbon hydro char, a high-quality solid biofuel. The direct use of sewage sludge in the soil does not allow for full use of its functional properties. However, the hydrothermal carbonisation of sewage sludge results in biocarbon pellets, making it a viable approach. This work also discusses the barriers (legal, chemical, biological, and technical) and possibilities related to sewage sludge biorefining processes.

Hydrothermal Carbonization of Sewage Sludge: New Improvements in Phosphatic Fertilizer Production and Process Water Treatment Using Freeze Concentration

In recent years, promising developments in the hydrothermal carbonization (HTC) of sewage sludge, as well as the potential to reclaim phosphorus and nitrogen, have emerged. In this study, the HTC of digested sewage sludge (DSS) was investigated for the downstream production of heavy metal (HM)-free fertilizer and the use of freeze concentration (FC) as a novel technology for process water treatment. To obtain clean fertilizer, phosphatic acid extracts were first treated with ion-exchange resins to remove dissolved HM, as well as phosphorus precipitating agents (i.e., aluminum and iron). Over 98% of the aluminum (Al) and 97% of the iron (Fe) could be removed in a single treatment step. The purified extract was then used for the precipitation of HM-free struvite crystals, with P-recovery rates exceeding 89%. Process water (PW) makes up the largest share of the two main HTC-products (i.e., hydrochar and PW) and is very rich in organic compounds. Compared to evaporation or membrane separation, FC is a promising technology for concentrating solutes from PW. Separation experiments resulted in the recovery of over 90% of the dissolved compounds in the concentrate. In our study, the concentrate was later utilized as an ammonium source for struvite precipitation, and the subsequent aerobic digestion of the remaining ice water resulted in an 85% reduction in chemical oxygen demand (COD) in 15 days.

The effect of an acid catalyst on the hydrothermal carbonization of sewage sludge

This research is focused on the addition of a catalyst, sulphuric acid (VI), to sewage sludge, and its effect on solid and liquid products resulting from the hydrothermal carbonization process. Consequently, for hydrochars, proximate and ultimate analyses, higher heating value and specific surface area were determined. Additionally, Fourier-transform infrared spectrophotometric and thermogravimetric analyses were conducted. The heavy metal contents in the ash composition of hydrochars were identified by X-ray fluorescence spectrometry. It was confirmed that the catalyst addition changed the structure as well as the physical and chemical properties of hydrochars and their ashes. Regarding post-processing water, both pH value and conductivity were determined and the element composition, including heavy metals, was conducted by the inductively coupled plasma optical emission and mass spectrometry analyses. It was found that the addition of the catalyst caused a decrease in heavy metal contents and an increase of phosphorus compound in filtrates.

Hydrothermal carbonization of sewage sludge coupled with Fenton oxidation pretreatment: Moderate oxidation to enhance hydrochar yield and properties

Hydrothermal carbonization (HTC) is a promising technology for sewage sludge (SSL) utilization. However, the yield and performance of hydrochar need to be further improved produced when SSL is used as a feedstock must be improved further. Herein, a process combining physicochemical pretreatment, namely, Fenton oxidation (FO) and HTC was developed for SSL utilization. Moderate oxidation was found to improve the yield and performance of the produced hydrochar. The product distribution, yield, and properties of the hydrochar produced using the proposed combined process were compared with those of the hydrochar produced via direct HTC treatment. The pretreatment affected the surface structure and organic composition of SSL and promoted the carbonization of the intermediate products. Thus, compared with the yield of hydrochar (50.7 %) obtained via the direct HTC treatment, the yield of the hydrochar obtained using the combined process increased to 55.0–65.2 % (depending on the pretreatment condition). Hydrochar properties were enhanced using the combined process, and the energy density of hydrochar slightly decreased after pretreatment (1.2–13.1 %); however, the energy yields increased by 0.6–30.1 % due to the enhanced hydrochar yield. The carbonization degree of hydrochar improved, the carbon in the feedstock distributed to the hydrochar increased from 33.40 % to 46.09 %. Moreover, the hydrochar showed more distinct pore channels, slightly improving its combustion activity. This study shows the efficacy of the combined pretreatment–HTC process and provides an experimental basis for the combined hydrothermal treatment of SSL.

Insights into acid-base properties of hydrotalcite materials during hydrothermal carbonization of sewage sludge

Based on proteins hydrolysis requiring alkaline conditions while polysaccharide hydrolysis requiring acidic conditions during hydrothermal carbonization (HTC) of sewage sludge (SS), different types of layered double oxides (LDOs) were introduced as catalysts to understand their role and structural alterations due to its containing acidic and basic sites. Results indicated that the structure of LDOs changes during HTC of SS with structural reconstruction influenced by properties of the aqueous phase, including anions and organic matter. As a result, the properties of the resulting hydrochar are also altered. Structural reconstruction of LDOs is a key factor that influences the HTC of SS. Basic sites of Mg/Al/Zn LDOs are slightly reduced, while acidic sites are remarkably enhanced, leading to an improvement in nitrogen removal and the recovery of organic matter from the HTC aqueous phase.

Hydrothermal carbonisation of sewage sludge and resulting biofuels as a sustainable energy source

Hydrothermal carbonisation of sewage sludge and resulting biofuels as a sustainable energy source

Effect of the aqueous phase from the hydrothermal carbonization of sewage sludge as a moisture regulator on nitrogen retention and humification during chicken manure composting

The hydrothermal carbonation (HTC) of sewage sludge (SS) produces a large amount of an aqueous phase (AP), which has become a bottleneck for the development of HTC. Herein, the effectiveness of the AP from HTC as a moisture regulator for composting was investigated in terms of nitrogen (N) retention and humification. The AP from the HTC of SS (AP-SS) and that from the co-HTC of rice husks and SS (AP-RH–SS) were considered. Experimental results showed that AP-RH–SS and AP-SS reduced the cumulative ammonia emissions from compost by 41.74% and 9.98%, respectively; reduced the N losses by 36.68% and 14.91%, respectively; and increased the total N concentrations by 28.17% and 21.08%, respectively, compared with those from control treatment. AP-RH–SS and AP-SS promoted the humification process and considerably increased the humic content, humification index, and germination index of the compost products. Moreover, AP-SS and AP-RH–SS promoted the degradation of aliphatic proteins and increased the amount of unsaturated and aromatic components during composting while decreasing the average molecular weight of the dissolved organic matter in the compost products. These findings provide new insights into the design and implementation of joint management strategies for the AP and composting.

Hydrothermal co-carbonization of sewage sludge and whey: Enhancement of product properties and potential application in agriculture

The impacts of operating conditions on hydrothermal carbonization of sewage sludge (SS) in water media were investigated in this study. Hydrothermal co-carbonization (co-HTC) of SS and cheese whey was also carried out under selected conditions (T = 250 °C, t = 5 h), to investigate the effects of replacing water with whey on product properties. Besides, the potential application was also tested of HTC products for agricultural use.The HTC treatment increased the content of nutrients (P, Ca, and Mg) in hydrochars compared to untreated SS, but fuel properties were not improved. Increasing the operating temperature and time increased the contents of nitrogen and volatile fatty acids in the process liquids. The replacement of water with whey resulted in remarkable improvement of the fuel properties of hydrochar, as the hydrochar yield increased from 57.10 to 64.14 wt%, the calorific value from 14.75 to 18.29 MJ/kg, and carbon content from 36.89 to 45.24 wt%. The ash content decreased from 46.55 to 40.06 wt%. The content of N, P, Ca, Mg and K increased significantly. SEM-EDS, FTIR and thermogravimetric analysis revealed the strong influence of the co-carbonization on the thermogravimetric behaviour of the hydrochars and their chemical properties. The HTC products proved efficient as fertiliser in a germination test with plants. Process liquids can be used as an N fertiliser, while hydrochars as a P fertiliser. Undiluted process liquids were toxic to the freshwater organisms Daphnia magna.

The treatment of post-processing liquid from the hydrothermal carbonization of sewage sludge

The liquid phase, being a major product of the hydrothermal carbonization of sewage sludge, is highly problematic due to numerous toxic compounds that make it impossible to dispose of without adequate purification. Therefore, this study is focused on the two groups of selected advanced methods of post-processing water derived from the hydrothermal carbonization process of sewage sludge. The first group included membrane processes, namely ultrafiltration, nanofiltration and double nanofiltration. The second included coagulation, ultrasonication and chlorination. Chemical and physical indicators were determined to confirm the validity of these treatment methods. The highest reduction values were found for double nanofiltration, which showed a spectacular decrease in the Chemical Oxygen Demand by 84.9 %, specific conductivity by 71.3 %, nitrate nitrogen by 92.4 %, phosphate phosphorus by 97.1 %, total organic carbon by 83.3 %, total carbon by 83.6 %, and inorganic carbon by 88.5 %, as compared to the liquid phase after hydrothermal carbonization. In the second group, for the largest number of parameters, the greatest reduction was achieved when 10 cm3/L of iron coagulant was added to the permeate from ultrafiltration. Additionally, COD decreased by 41 %, P-PO43− content by 78 %, phenol content by 34 %, TOC content by 97 %, TC content by 95 % and IC content by 40 %.

Hydrothermal carbonization of sewage sludge: Hydrochar properties and processing water treatment by distillation and wet oxidation

Hydrothermal carbonization is an innovative method of sewage sludge disposal. The solid product hydrochar (c.a.10%), may be converted to a fuel or fertilizer. Gaseous products (less than 10%) are mainly CO2 and nitrogen, but c.a. 80% of post-processing liquid presents as a challenging issue due to its high toxicity. Therefore, the main aim of this study was to determine the potentiality of hydrochars derived from sewage sludge as a fuel, at two temperatures: 200 and 220 °C, and 2 h of residence time, and successfully utilize the filtrates after the implementation of an adequate purification process. The results of hydrochars depicted better energy properties and dewaterability than in the case of raw sewage sludge, including improved chemical composition, a higher fixed carbon content and fuel ratio, and it also gained more effective filtration. The comprehensive analysis of the Thermal Analysis data proved that hydrochar can be combusted, but based on X-ray Fluorescence data a high tendency to slagging and contamination is expected. The highly toxic processing liquids indicated a great need for an appropriate treatment. Accordingly, distillation and the preliminary study of wet oxidation processes were applied. In both methods the chemical oxygen demand, an indicator of organic pollution, was significantly reduced.

Hydrothermal Carbonization of Sewage Sludge into Solid Biofuel: Influences of Process Conditions on the Energetic Properties of Hydrochar

Hydrothermal carbonization (HTC) is an attractive, green technology for the management of sewage sludge. In this study, low-value secondary sewage sludge was subjected to an HTC treatment in a 1 L batch hydrothermal reactor and transformed into a high-energy-density hydrochar under varying HTC conditions (temperature of 150–300 °C, carbonization time of 30–150 min and a solid loading of 10–30%). The resulting hydrochar fuel characteristics were analyzed for ultimate and proximate analyses, functional group composition and energetic parameters. It was found that the hydrochar yield decreased with the increasing HTC temperature and reaction time, primarily due to the loss of organic volatile matter and functional groups. Under the optimum conditions of 150 °C, 30 min of carbonization time and 30% solid loading, 80.56% of the hydrochar was recovered, providing a maximum energy yield of 90.32% and a high heating value of 18.49 MJ/kg. Compared to the raw sewage sludge (H/C ratio of 2.67 and O/C ratio of 0.51), the hydrochar also had lower H/C and O/C atomic ratios of 1.42 and 0.18, respectively. The results suggest that significant dehydration and decarboxylation during the HTC treatment of sewage sludge have resulted in the formation of carbonaceous hydrochar with energetic properties close to the sub-bituminous coals.

Sulfite enhancing nitrogen removal from sewage sludge during hydrothermal carbonization

In order to prepare low nitrogen content hydrochar, Na2SO3, and NaHSO3 have been utilized respectively as an additive to remove nitrogen from sewage sludge (SS) during hydrothermal carbonization (HTC). Compared to without additives, the maximum removal efficiency of nitrogen in hydrochar can increase as high as 68.68% at 210 °C when 10% of Na2SO3 was added. Results demonstrate that Na2SO3 has a greater capability than NaHSO3 to remove nitrogen. The key role of NS during HTC of SS was the degradation of extracellular polymeric substances (EPS) to release polysaccharides to the aqueous phase and destroying proteins structure, thus inhibiting Maillard reaction to reduce nitrogen content in hydrochar. This gave a new insight to remove nitrogen during HTC via the release of polysaccharides to the aqueous phase to avoid interaction and subsequent reaction between polysaccharides and proteins. In addition, the possible removal pathways are also analyzed and proposed in this investigation.