To investigate the influence of carbonization process parameters on the characteristics of municipal sludge carbonization products, this study selected carbonization temperatures of 300–700 °C and carbonization times of 0.5–1.5 h to carbonize municipal sludge. The results showed that with an increase in temperature and carbonization time, the sludge was carbonized more completely, and the structure and performance characteristics of the sludge changed significantly. Organic matter was continuously cracked, the amorphous nature of the material was reduced, its morphology was transformed into an increasing number of regular crystalline structures, and the content of carbon continued to decrease, from the initial 52.85 to 38.77%, while the content of inorganic species consisting continued to increase. The conductivity was reduced by 87.8%, and the degree of conversion of salt ions into their residual and insoluble states was significant. Natural water absorption in the sludge decreased from 8.13 to 1.29%, and hydrophobicity increased. The dry-basis higher calorific value decreased from 8,703 to 3,574 kJ/kg. Heavy metals were concentrated by a factor of 2–3, but the content of the available state was very low. The results of this study provide important technological support for the selection of suitable carbonization process conditions and for resource utilization.

  • Temperature and time are used to investigate the properties of carbonized sludge.

  • The structure and performance characteristics of the sludge changed significantly.

  • The EC, water absorption, particle size and calorific value tends to decrease.

  • Carbon content continues to decrease with the rising of temperature and time.

  • Heavy metals were concentrated, but the content of the soluble state was very low.

Sludge is a direct product of wastewater treatment and a carrier for pollutants accumulated in wastewater. In 2021, the volume of wastewater treated in China was 6.12 × 1010 m3, and accordingly, the amount of sludge generated (with 80% water content) exceeded 3 × 107 tons (Ministry of Housing & Urban-Rural Development of the People's Republic of China 2022). This sludge often contains large amounts of pollutants such as heavy metals (Zhang et al. 2021; Tasca et al. 2022), microorganisms (Cieślik et al. 2015), and pathogens (Anjum et al. 2016). If improperly disposed of, sludge presents significant pollution risks to the environment (Ji et al. 2022; Li et al. 2022a). On the other hand, sludge contains large quantities of organic matter (Ke et al. 2018) and is rich in nitrogen, phosphorus, and potassium (Zhang et al. 2017); therefore, it is a good resource utilization target (Raheem et al. 2018). To effectively solve the sludge disposal problem, China has been exploring sludge treatment and disposal technologies, policies and management strategies (Zhou et al. 2020; Chen et al. 2022). Generally, sludge disposal methods include land application, landfilling, and comprehensive utilization in construction materials (Raheem et al. 2018) to diversify the disposal of sludge. However, greenhouse gas mitigation in the wastewater treatment sector is indispensable in China's carbon neutral target. The use of sludge in gardens, for land improvement, and in construction materials is encouraged to achieve sustainable sludge disposal measures (Oreshkin et al. 2015; Taki et al. 2020).

Composting is an important treatment route for converting sludge into garden fertilizer (et al. 2021), but a long treatment time is needed for composting, usually 1–3 months, the longest treatment time can be 150 days (Haouas et al. 2021), and the composting footprint is large. In the context of increasing urban land use constraints, the composting process has gradually been modified to reduce or even eliminate such constraints. Therefore, more efficient pretreatment processes are needed to address sludge disposal challenges. To better follow the principles of reduction, remediation, stabilization and resourcefulness in the sludge disposal process (Chu et al. 2023), the sludge carbonization process has gradually received attention and is being applied (Tang et al. 2018; Fan et al. 2023), especially under China's carbon peaking and carbon neutrality goals.

Sludge carbonization methods can be divided into wet carbonization, i.e., hydrothermal carbonization, and dry carbonization. There are more research reports on the hydrothermal carbonization of sludge than other methods (Chen et al. 2020; Malhotra & Garg 2023; Wilk et al. 2023). Wang et al. (2021) applied a hydrothermal carbonization method to municipal sludge using various reaction temperatures (180–300 °C) and reaction time (2–15 h) to obtain sludge hydrates with high yield, carbon recovery, and polarity. Feng et al. (2023) used a combination of hydrothermal carbonization temperatures of 200–260 °C and supercritical temperatures of 400–450 °C to find a method and obtained greater H2 ratios and carbon conversions. Huezo et al. (2021) utilized hydrothermal carbonization to treat anaerobically digested sludge into hydrates and liquids and evaluated the effect of temperature, time and initial pH of hydrothermal carbonization on hydrate and liquid yields and properties. These studies showed that hydrothermal carbonization of sludge can effectively reduce colloidal structures, enhance dewatering performance, disrupt microbial structures, reduce sludge volume, and convert sludge with high moisture content and toxic pollutants into low-energy value-added products for nutrient preservation and heavy metal immobilization (Wang et al. 2019; Xu et al. 2022; Leghari et al. 2023). However, sludge carbonization products from hydrothermal carbonization have the disadvantages of low pH, low specific surface area and high toxic content (Zhi et al. 2024).

In contrast, high-temperature pyrolysis (400–800 °C) significantly reduces the oxygen content, favoring higher carbon content, specific surface area, and pH. This makes products possess higher aromaticity and carbonation (Zhi et al. 2024). Dry carbonization of sludge is advantageous compared to hydrothermal carbonization, a green technology that is considered promising (Barry et al. 2019; Dai et al. 2019). Zhou et al. (2023a) synergistically and anaerobically carbonized dewatered sludge with corn stover to obtain sludge carbonization products. Their method effectively improved sludge dewatering efficiency, with an allowable ratio of 1.5 tons of sludge per 1.2 tons of corn stover (absolutely dry). In addition, 26.7% carbon was immobilized in the sludge, the volume of each ton of sludge was reduced by 88% (with 80% water content), greenhouse gas emission was reduced by 13.2% (approximately 25.4 kg), and disposal costs were reduced to less than US$20. Ao Zhou et al. (2023b) recommended the following parameters for sludge pyrolysis: a 30% moisture content in the dried sludge, a dry-basis higher calorific value not less than 10 MJ/kg and a pyrolysis temperature of 400 °C; under these conditions thermal self-balancing of the process could be achieved. Alipour et al. (2021) used a pyrolysis process to treat municipal sludge, which altered the porosity and surface morphology of the biochar, and the biochar produced at 600 °C had high porosity. These studies suggested that dry carbonization of sludge will produce effective capacity reduction, organic pollutant decomposition and resource utilization; therefore, it is a promising method for sludge recycling (Xiao et al. 2022). When sludge is treated by the carbonization process, the treatment time is greatly shortened, the treatment footprint is greatly reduced, and the product is stable and does not have any odors; therefore, the product can be applied in the production of garden amendments, artificial stone product additives, building material product additives, fuel doping, etc. (Leghari et al. 2023) as well as high-phosphorus soil conditioners (Frišták et al. 2018), which provides diverse disposal routes for the utilization of sludge.

However, the application of the dry carbonization process is limited by energy costs, which are directly dependent on the required temperature and carbonization time (Ren et al. 2012). The higher the temperature and the longer the treatment time are, the greater the treatment cost of the carbonization process. On the other hand, different temperatures and carbonization process time result in different properties in the carbonized sludge products, affecting the subsequent disposal and utilization of carbonized sludge and product quality. For this reason, in this study, the effects of temperature and time on the performance of sludge dry carbonization products were investigated, the cost of power consumption in the dry carbonization process was assessed to optimize the cost of disposal, and important basic data were provided for diversifying the use of carbonized sludge products.

Test materials

Municipal sludge samples were taken from the Daguan Water Purification Plant of Guangzhou Sewage Purification Co., Ltd. These were irregularly shaped and granular and were brown in color, with diameters of approximately 1–3 cm. The plant's wastewater treatment adopted the modified Anaerobic–Anoxic–Oxic process with a capacity of 200,000 tons of municipal wastewater per day, and the residual sludge was co-conditioned with polymerized aluminum chloride (dose approximately 0.16% of the dry sludge) and polyacrylamide (dose approximately 0.05% of the dry sludge) and then synchronized with a hot-drying by 85–90 °C hot water and dewatering process through plate and frame press filtration. The basic properties of the sludge are shown in Table 1.

Table 1

Basic properties of municipal sludge samples

ItemsValues
pH 7.25 
EC (μS/cm) 2,120 
Water content (%) 36.4 
Organic matter (%) 56.8 
Total zinc (mg/kg) 374 
Total cadmium (mg/kg) 1.02 
Total lead (mg/kg) 34.5 
Total chromium (mg/kg) 133 
Total nickel (mg/kg) 39.8 
Total copper (mg/kg) 74.4 
ItemsValues
pH 7.25 
EC (μS/cm) 2,120 
Water content (%) 36.4 
Organic matter (%) 56.8 
Total zinc (mg/kg) 374 
Total cadmium (mg/kg) 1.02 
Total lead (mg/kg) 34.5 
Total chromium (mg/kg) 133 
Total nickel (mg/kg) 39.8 
Total copper (mg/kg) 74.4 

The municipal sludge samples were dried at 105 °C for 3 h to completely remove water and then pulverized by a pulverizer (model WF-40B, provided by Jiangyin Lingling Machinery Manufacturing Co., Ltd, China) with a pore size of 2 mm to obtain dried pulverized sludge samples, which were used for dry carbonization.

Sample preparation

This test was designed to optimize both the temperature and treatment time for dry carbonization. Approximately 1 kg of crushed and dried sludge samples was weighed, placed in a stainless-steel box, compacted and covered to reduce air entry into the sludge. The aim was to prevent the organic matter from burning and oxidizing to generate carbon dioxide. Then, the stainless-steel box filled with sludge was placed in a muffle furnace at a preset temperature of 300–700 °C, and the carbonization treatment time was set to 0.5–1.5 h. A micronegative-pressure environment was maintained in the furnace to ensure that the flue gases were discharged in a timely manner. The furnace chamber was cooled naturally, and a total of 15 carbonized sludge samples were obtained, which were used for performance testing and structural characterization.

Sample analysis

Structural characterization

Fourier Transform Infrared Spectroscopy analysis

Sludge sample powders were mixed with solid KBr at a ratio of 1:100, the powders were ground and pressed together, and infrared spectra were collected by a Necolet-670 Fourier Transform Infrared Spectroscopy (FTIR) spectrometer with a wavenumber range of 400–4,000 cm−1.

X-ray diffraction analysis

Using a 10 × 10 mm metal paper press, sludge sample powders were placed in an ESCALAB250 X-ray spectrometer (XPS/ESCA, Thermo Fisher Scientific), and the vacuum applied to the analytical room was less than 2 × 10−9 mbar.

Scanning electron microscope–energy dispersive spectrometer analysis

Sludge sample powders were placed on a conductive adhesive sheet, were sprayed with gold to make the samples conductive, and were analyzed by scanning electron microscope (SEM) using a Hitachi S-3700 N instrument. The surface structure of the carbonized sludge was observed and recorded under 4,000× magnifications, SEM images were obtained and the contents of main elements were analyzed by energy dispersive spectrometer (EDS) analysis accessories.

Sample performance test

Weight loss rate
Approximately 200 g of sludge was weighed. During the carbonization process, the sludge carbonization temperature was maintained at 600 °C, the carbonization time was set to 3.0 h, the sludge was weighed after it naturally cooled to room temperature, and then the weight loss rate was calculated:
formula
(1)
where L is the rate of weight loss, %; l0 is the weight of sludge before carbonization, g; l1 is the weight of sludge after carbonization, g.
Residual amount and volatile fraction
Approximately 1 g of the dried carbonized sludge sample was weighed, placed in a porcelain crucible, covered, placed in a muffle furnace and burned at 600 °C for 3 h. After removal, it was cooled in air for 5 min, transferred to a desiccator to cool to room temperature and weighed. Each treatment was repeated 3 times:
formula
(2)
where W is the residual amount of carbonized sludge, %; w0 is the total weight of the crucible and dried sample after calcination, g; w1 is the weight of the crucible, g; w2 is the weight of the dried sample, g.
formula
(3)
where X is the percentage of volatile fraction of carbonized sludge, %; x0 is the total weight of the crucible and dried sample, g; x1 is the total weight of the crucible and dried sample after calcination, g; x2 is the weight of the dried sample, g.
Thermogravimetric analysis

Approximately 10–30 mg of sludge sample powders were weighed, placed in the sample box of a thermogravimetric analyzer (provided by Nanjing Dazhan Detection Instrument Co., Ltd, model number DZ-TGA101), and warmed to 900 °C at a heating rate of 20 °C/min to analyze the relationship between the weight loss of samples over time. When the weight of the sample no longer changed with time, the temperature recorded was considered the temperature at which the sludge was completely carbonized, and the corresponding time was the time at which the sludge was completely carbonized.

pH and electrical conductivity

Ten grams of carbonized sludge samples were placed in a 25-mL beaker, mixed with distilled water (10 mL), and incubated for 30 min using a calibrated pH meter (model HQ1110, HACH) to determine the suspension pH. A conductivity meter (model HQ1140, HACH) was used to determine the electrical conductivity (EC) value.

Natural water absorption rate
The carbonized sludge (approximately 50 g) obtained was placed in an open glass petri dish and placed in an incubator with a humidity of 85% and a temperature of 25 °C for 72 h. Then, the carbonized sludge samples weighing 1–2 g were removed and dried to a constant weight at 105 °C (approximately 3 h), the weights of the carbonized sludge were determined before and after drying, and the water content was calculated:
formula
(4)
where Y is the natural water absorption rate of carbonized sludge, %; y0 is the of weight of carbonized sludge after moisture absorption, g; y1 is the of weight of carbonized sludge after drying, g.
Particle size

The particle size of carbonized sludge samples was determined using a Malern MS3000 laser particle sizer. During the determination, the stirring rate of the solution was 1,500 rpm, and the results of the particle size were expressed as the cut diameter Dx10, Dx50, and Dx90, where Dx90 means that 90% of the particle size was less than or equal to Dx90.

Calorific value

Approximately 1 g of carbonized sludge samples was placed in a calorimeter (provided by China Hebi Xintianhe Company, model ZDHW-600). During analysis, pure oxygen was supplied at a pressure of 3 MPa to obtain the high-level calorific value on a dry basis.

Determination of heavy metals

Samples were accurately weighed to 0.3000 g and placed in polytetrafluoroethylene digestion tubes, 1 mL of HCl, 4 mL of HF, and 5 mL of HNO3 were added, and the samples were left undisturbed for 20 min to fully oxidize the organics. Then, the samples were covered tightly with an inner lid and outer lid, and placed on a turntable. A gradient temperature program was used, and the samples were digested at 120 °C for 5 min, 180 °C for 30 min, and then 120 °C for 5 min. After the digestion program, the samples were removed, cooled to room temperature, centrifuged at 4,000 rpm for 30 min, and then filtered through quantitative filter paper. The filtrate was transferred to a volumetric flask, the volume was adjusted with HNO3 (2%) to 50 mL, and then the solution was analyzed. Total zinc, total cadmium, total lead, total chromium, total nickel and total copper were determined on an inductively coupled plasma mass spectrometry (ICP-MS) instrument (Thermo Fisher ICAPRQ01304).

Calculation of energy consumption

The theoretical total energy required to carbonize municipal sludge is the sum of the energy required to heat water, generate steam, heat sludge solids, and maintain constant equipment temperatures, and these were calculated as follows:
formula
(5)
formula
(6)
formula
(7)
formula
(8)
formula
(9)

where Etotal is the total energy of theoretical demand, kJ; Wh is the energy needed to increase the temperature of water from room temperature to 100 °C, kJ; Wv is the heat of vaporization of water, kJ; Eh is the energy needed to heat sludge solids from room temperature to the carbonization temperature, kJ; El is the energy needed to keep the carbonization temperature constant, assuming 90% insulation efficiency of the equipment, kJ; cw is the specific heat capacity of water, 4.2 kJ/kg·°C; mw is the mass of water in 1 ton of municipal sludge, 362 kg; t1 is the temperature when water is heated to boiling, 100 °C; t0 is the room temperature, 20 °C; cs is the specific heat capacity of sludge, 3.62 kJ/kg·°C; ms is the mass of dry sludge in 1 ton of municipal sludge, 638 kg; t2 is the sludge carbonization temperature, °C.

Structural characterization of carbonized sludge

To explore the influence of sludge carbonization conditions on sludge structural characteristics, five carbonized sludge samples obtained under a carbonization treatment time of 1 h and carbonization temperatures of 300–700 °C were selected as the research objects in this study, and structural changes in the carbonized sludge were analyzed by using characterization methods such as FTIR, X-ray diffraction (XRD), and SEM.

FTIR analysis

Figure 1 shows the FTIR spectra of carbonized sludge under different temperature conditions. As shown in Figure 1, the absorption peaks at 3,696–3,697 and 3,620–3,621 cm−1 show sharp absorption bands, which correspond to the stretching vibration of O–H (υO–H); the absorption peaks at 3,400–3,413 cm−1 show broad absorption bands, which are the stretching vibrations of O–H in multimolecular conjugation and in the carboxyl group. These functional group structures reflect the molecule structure of water and organic matter with carboxyl groups in the carbonized sludge.
Figure 1

FTIR spectra of carbonized sludge under different temperature conditions.

Figure 1

FTIR spectra of carbonized sludge under different temperature conditions.

Close modal

The absorption peaks at 2,914–2,927 and 2,853 cm−1 are formed due to C–H stretching vibrations in CH3– and –CH2–. The peak at 1,640–1,655 cm−1 corresponds to carbonyl (–C = O) stretching vibrations in the amide group (–CONH2). The 1,612 cm−1 peak is the double-bond stretching vibration of N = O in the nitro group (–NO2). The peak at 1,450–1,460 cm−1 is related to the in-plane bending vibrations of alkyl saturated hydrocarbon groups. The peak at 1,412–1,413 cm−1 is due to the C–N stretching vibrations of the amide group. The 1,033–1,034 cm−1 peak is attributed to the C–N stretching vibrations of the amide group with the C–O stretching vibration in alcohols. The 1,079 cm−1 peak corresponds to C–O stretching vibrations. The peaks at 913, 797 and 693 cm−1 are due to C–H out-of-plane bending vibrations in unsaturated hydrocarbons. These functional group assignments indicate that the organic matter in the carbonized sludge mainly consists of saturated groups such as CH3–, –CH2–, C–O–, C–N–, and other structures but also includes amide groups, alcohols and ethers. This is consistent with previous research findings. Frišták et al. (2018) used a temperature of 430 °C to carbonize municipal sewage sludge, and the carbonized sludge samples were analyzed by FTIR, which also yielded similar functional group compositions. Sun et al. (2022) combined synergistic pyrolysis of sewage sludge and wet waste, and these mixtures were heated from 30 to 900 °C. The change of gas functional groups with different blends (–OH, –CH, CO2, CC, phenol, CO, and NH3) was detected by FTIR.

With increasing carbonization temperature, the number of multimolecular conjugated O–H structures and O–H structures in the carboxyl groups of organic matter in carbonized sludge gradually decreased, which manifested as a gradual decrease in peak intensity. The saturated bonds in CH3– and –CH2– were also gradually cleaved with increasing carbonization temperature to form single carbon products. The peak intensities of CH3– and –CH2– groups were obviously weakened when the carbonization temperature was above 500 °C, and they disappeared at 700 °C. In addition, the peak intensities of amides, alcohols, and ethers also appeared to weaken or even disappear with increasing carbonization temperature (Zhu et al. 2023). These results indicated that with increasing carbonization temperature, the organic matter in the municipal sludge was continuously cracked, which changed the structural composition of the organic matter, indicating the cracking of carbonized sludge was more thorough and complete.

XRD analysis

Figure 2 shows the XRD spectra of carbonized sludge under different temperature conditions. As shown in Figure 2, the XRD pattern of carbonized sludge showed obvious peak shape changes due to sludge breakdown induced by thermal energy. In the 2θ = 6.7−19.1° interval, there is only one crystalline peak (2θ = 12.5°) for the uncarbonized municipal sludge. When carbonization was carried out at 300 °C for 1 h, a clear broad peak appeared, which was a result of the amorphous and irregular structure of carbonized sludge (Wang et al. 2020). When the temperature was increased to 400 °C, this broad peak appeared only at 2θ = 6.7−13.6°, the intensity was obviously weakened, and the number of amorphous structures decreased. When the temperature was further increased to 500, 600, and 700 °C, the amorphous structure represented by this broad peak began to disappear, and the amorphous structure was transformed to a regular crystal structure. At 2θ = 19.1−26.2°, all the carbonized sludges showed amorphous structures with broad peaks, but with increasing temperature, the intensity of these broad peaks gradually weakened until they disappeared. When the diffraction angle was increased above 30°, crystalline peaks (2θ = 60.0°, 77.5°) were enhanced, and the crystals were better structured. These results indicate that the higher the temperature is, and the less amorphous the structure of the carbonized sludge is. More amorphous structures were transformed into regular crystalline structure. This may be caused by the loss of amorphous organic volatile matter in the sludge and the conversion of inorganic matter to regular crystal structures. This can be corroborated by the studies of Chen et al. (2019). They co-pyrolyzed municipal sewage sludge and wood waste at 800 °C and analyzed it using XRD, and found that the regular crystal structure of biochar was effectively constructed.
Figure 2

XRD spectra of carbonized sludge under different temperature conditions.

Figure 2

XRD spectra of carbonized sludge under different temperature conditions.

Close modal

SEM–EDS analysis

Figure 3 shows the SEM images of carbonized sludge at 4,000× magnification under different temperature conditions. As shown in Figure 3, the powdered sludge particles showed irregular and amorphous microstructures both before and after carbonization. Although carbonization did not change the particle morphology of the sludge, it changed its surface microstructure. The surface of the uncarbonized sludge showed a rough structure, which was the result of the fusion of disordered organic flocs and amorphous inorganic matter in the sludge. However, after the sludge was carbonized for 1 h, the surface of the carbonized sludge began to show a smooth morphology as the temperature was increased from 300 to 700 °C, and the higher the temperature was, the smoother the sludge surface, which indicated that most of the roughness of the sludge surface was due to organic flocs, which were volatilized or transformed into inorganic carbon under high temperature conditions; as a result, the rough morphology of the surface was altered (Frišták et al. 2018). Therefore, a high-temperature carbonization process can easily carbonize the organic matter present on the surface of sludge particles and change surface microstructures.
Figure 3

SEM images of carbonized sludge under different temperature conditions.

Figure 3

SEM images of carbonized sludge under different temperature conditions.

Close modal
Figure 4 shows the EDS spectra of sludge carbonized at different temperatures. Based on elemental content analysis, the carbonized sludge from all treatments mainly consisted of nine elements, namely, C, O, Al, Si, P, Fe, Ca, K, and Cl, which accounted for a total of more than 99%. Among these, carbon and oxygen, important substances for the formation of organic matter, were absolutely dominant, and their smallest total contents were more than 69%.
Figure 4

EDS spectra of carbonized sludge under different temperature conditions.

Figure 4

EDS spectra of carbonized sludge under different temperature conditions.

Close modal

However, the elemental content of each treatment group changed with a change in temperature, and the largest change was observed in carbon. The carbon content of uncarbonized municipal sludge was 52.8%. As the carbonization temperature increased from 300 to 700 °C, the elemental carbon content decreased gradually from 49.2 to 47.9%, 45.3, 41.5, and 38.8%, respectively, which was caused by the higher temperature and the more thorough cleavage of the carbon chains in organic matter. Long carbon chains were broken to form volatile substances such as carbon dioxide or other small molecular organic volatiles, resulting in a gradual decline in the content of carbon in the sludge.

The oxygen content was not affected by the increase in carbonization temperature and showed no obvious changes, and its content essentially remained between 29 and 35%, indicating that oxygen was not only present in combination with carbon but also with other inorganic substances, such as aluminum, silicon, phosphorus, iron, calcium, and potassium (Petrovič et al. 2023). As shown in Figure 4, the content of these inorganic substances increased as the temperature was increased from 300 to 700 °C, the aluminum content increased by 5.13%, the silicon content increased by 2.57%, the phosphorus content increased by 2.18%, the iron content increased by 1.44%, and the calcium content increased by 0.84%. The greatest increase was observed in aluminum, followed by silicon, phosphorus, iron and calcium. A decrease in the content of carbon occurred because some of the carbon was lost in the form of volatile organic matter due to the cracking of organic matter. An increase in inorganic compounds occurred due to a relative increase in their concentration upon the loss of volatile organic compounds, concentrating the inorganic compounds.

Effect of temperature and time on the mass change in carbonized sludge

Mass loss of carbonized sludge

Under anaerobic micronegative-pressure environment, municipal sludge was carbonized at 600 °C for 3 h to ensure that the sludge was fully carbonized, i.e., the organic matter was fully converted into carbon monomers. The municipal sludge generated from the wastewater treatment plant was directly carbonized, and the mass loss amounted to 63.80%. These lost components included both organic matter and water, which accounted for 42.9 and 57.1%, respectively. The moisture content of the municipal sludge used for carbonization was 36.4%, and moisture loss was the main weight loss factor in the carbonization process. Sludge contains a large amount of organic matter, including proteinaceous compounds, carbohydrates, humic substances, lipids, lignin, organic acids, organic micropollutants and other biologically derived substances produced during wastewater treatment (Xiao et al. 2020). Among lost organic fractions, these substances were cracked to produce small molecules of carbon oxides, hydrocarbons, and some inorganic small molecules, such as NH3, H2S, SO2, NO2, and NO, which resulted in the generation of unpleasant and irritating odors. Tang et al. (2018) utilized a temperature of 300–500 °C for sludge carbonization, and the resulting products were mainly composed of 3- and 4-atom hydrocarbons, accompanied by a large number of cracking products such as NH3, NO2, H2S, CO2, CH3SH, SO2, and CS2, which were attributed to the compounds and minerals present in the sludge (Shahbeik et al. 2022). Therefore, sludge carbonization must be accompanied by flue gas purification and treatment. The mass loss that occurs during sludge carbonization is an inevitable result of the cracking of organic matter at high temperatures.

Changes in the amount of residue and volatile fraction in carbonized sludge

The amount of residue obtained from the carbonized sludge samples using the scorching and weighing method reflects the amount of inorganic components in the carbonized sludge, while the volatile fraction is an important indicator for assessing the organic components present in the carbonized sludge. Figure 5 reflects the variation in the amount of residue and the volatile fraction in carbonized sludge under different carbonization conditions. The amount of residue of municipal sludge was 431 mg/g, which would be considered inorganic matter. Theoretically, the amount of residue in carbonized sludge should be the same for all carbonization treatments, i.e., the same amount of residual municipal sludge is produced. However, as shown in Figure 5(a), the residual amount of carbonized sludge for all treatments was higher than that of municipal sludge, and the residual amount of carbonized sludge under treatment conditions of 300 °C and 0.5 h was 1.09% higher than that of municipal sludge. When the time was kept constant at 0.5 h, the residual amount of carbonized sludge increased by 13.2, 64.3, 55.7 and 54.6% compared with that of municipal sludge when the temperature was increased to 400, 500, 600 and 700 °C, respectively. When the treatment time was extended to 1.5 h, the residual amount of carbonized sludge obtained at 700 °C even increased by 75.4% compared with that of municipal sludge. These increases in the amount of residue were the result of the conversion of cracked organic matter into carbon monomers during the sludge carbonization process. This also indicates that the higher the carbonization temperature and the longer the carbonization time are, the more fully the municipal sludge is carbonized, and the more effectively it is converted into carbon monomers. This result is corroborated by the EDS results for carbon content (Figure 4).
Figure 5

Changes in residual amount and volatile fraction in carbonized sludge under different carbonization conditions. (a) Residual amount of scorched carbonized sludge and (b) volatile fraction percentages of scorched carbonized sludge.

Figure 5

Changes in residual amount and volatile fraction in carbonized sludge under different carbonization conditions. (a) Residual amount of scorched carbonized sludge and (b) volatile fraction percentages of scorched carbonized sludge.

Close modal

When the temperature is above 500 °C for 1.5 h, the increase in residue formation from organic matter conversion into carbon monomers gradually decreases, and the carbonization process becomes stable. Below 500 °C, significantly less conversion of organic matter into carbon monomer residues occurs in the sludge, and the amount of residue in the carbonized sludge relative to municipal sludge is no more than 20%. Pauline & Joseph (2021) analyzed the number of carbon atoms present in the carbon chains of organic matter in sludge (between C6 and C56), and after hydrothermal carbonization at different temperatures, the number of carbons in dominant chains varied, and the compounds degraded into smaller acids, alcohols and ketones in the range of C7–C8. C12, C8, C8 (with decreasing number of carbons) and C7 species were the dominant organics formed with carbonized at 150, 175, 200, and 250 °C, respectively. It can be shown that the higher the temperature is, the more the long carbon chains in sludge organic matter break and form small molecules. At lower temperatures, the small molecules in sludge are easily dispersed by carbonization. Therefore, the organic matter in municipal sludge is mainly composed of small molecular carbon chains, showing that most of the small molecules are cracked and dispersed at 300–500 °C, while the increase in residue gradually decreases beyond 500 °C, and the carbonization process tends to stabilize.

However, the municipal sludge showed insufficient carbonization, as evidenced by the fact that 24.2% of the volatiles were emitted from the carbonized sludge samples even after 1.5 h of carbonization at 700 °C under scorching conditions (see Figure 5(b)). From Figure 5(b), it can be seen that when the treatment time was kept constant at 0.5 h, the percentage of volatile matter emitted under scorching conditions was 56.4, 51.1, 29.1, 32.8, and 33.3% when the treatment temperature was set to 300–700 °C, respectively. When the treatment time was extended to 1.5 h, the volatile fraction of carbonized sludge further decreased, and its percentage decreased to 49.7, 42.9, 26.8, 25.8 and 24.3%, respectively. These results indicated that the higher the temperature and the longer the carbonization time are, the lower the amount of volatile components that remain in the carbonized sludge, which also indirectly indicates more thorough sludge carbonization. These volatiles are the cleavage products of large molecules and long-chain organic matter in the carbonized sludge. Petrovič et al. (2023) increased the nitrogen and volatile fatty acid content in process liquor by increasing the operating temperature and time of hydrothermal carbonization. Li et al. (2023) used microwave pyrolysis of sewage sludge, the elevated temperature they employed resulted in the increases of the gas production, and H2 and CO fractions present in the gas. Lin et al. (2022) investigated biogas production from microwave pyrolysis of sewage sludge and found that the H2 content gradually increased to a peak value at elevated pyrolysis temperatures while the CO content continued to increase and that high temperatures promoted the secondary pyrolysis of bio-oils for syngas generation. Increasing the temperature and extending carbonization time essentially results in more energy inputs to the sludge, which facilitates further cleavage of the sludge carbon chain.

Thermogravimetric analysis of carbonized sludge

Thermogravimetric analysis is an important technique to test the degree of sludge carbonization. The organic matter in sludge is volatilized and lost with increasing temperature, which manifests as a decrease in sample weight. When the organic matter is completely volatilized and released, the material reaches a constant weight, and at this point, only inorganic matter and simple carbons remain. The greater the weight loss in carbonized sludge is, the greater the amount of organic matter contained, which indicates that the carbonization process is incomplete.

Figure 6 shows the thermogravimetric analysis curves of municipal sludge before and after carbonization under different temperature and carbonization time. From Figure 6, it can be seen that when the municipal sludge and carbonized sludge were heated to 600 °C, the sample weights essentially remained stable and no further decreases in weight were observed with increasing temperature, indicating that the organic matter in the sludge had completely broken down at temperatures close to 600 °C. When the temperature was increased to 600 °C, weight loss reached 64.3%, which was consistent with the 63.8% weight loss observed using the direct weighing for municipal sludge carbonized at 600 °C for 3 h.
Figure 6

Thermogravimetric analysis curves of municipal sludge before and after carbonization under different carbonization temperatures and time.

Figure 6

Thermogravimetric analysis curves of municipal sludge before and after carbonization under different carbonization temperatures and time.

Close modal

When the municipal sludge was carbonized, the weight loss rates of the carbonized sludge were all lower than that of the municipal sludge, indicating that different degrees of organic matter loss had occurred during the carbonization process of the sludge. When municipal sludge was carbonized for 0.5 h, the weight loss in carbonized sludge decreased during carbonization at 300–700 °C. The 700 °C carbonization treatment reduced the weight loss in sludge compared with that at 300 °C by 31.1%. When the carbonization temperature reached a value above 500 °C, the decrease in weight loss in carbonized sludge was reduced, indicating that the material was sufficiently carbonized above 500 °C, and the higher the temperature was, the more thorough the carbonization. The weight loss in carbonized sludge decreased when the carbonization treatment time was extended to 1.0 h. The weight loss in sludge decreased by 19.4% for carbonization at 700 °C compared with that at 300 °C. When the carbonization time was further extended to 1.5 h, the weight loss in carbonized sludge decreased. These results indicated that the longer the carbonization time and the higher the carbonization temperature of the sludge are, the lower the weight loss rate of the carbonized sludge and the more thorough and complete the carbonization process of the sludge. Therefore, longer carbonization time and higher carbonization temperatures are needed to obtain fully carbonized sludge.

On the other hand, with increasing carbonization time and temperature, the temperature at which a sharp decline was observed in the thermogravimetric curve of the carbonized sludge gradually increased. When the municipal sludge was carbonized for 0.5 h, the inflection point at which a sharp decrease was observed in the thermogravimetric curves of carbonized sludge occurred at 85, 220, 260, 275, and 340 °C when the carbonization temperature was raised from 300 to 700 °C, respectively. When the carbonization time of municipal sludge was increased to 1.0 h, the inflection point in the thermogravimetric curves at which a sharp decrease was observed occurred at temperatures of 270, 280, 295, 325, and 390 °C as the carbonization temperature was increased from 300 to 700 °C, respectively. When the carbonization time of municipal sludge was increased to 1.5 h, the inflection point temperature at which a sharp decline was observed still increased. The longer the carbonization time and the higher the temperature of the municipal sludge was, the more thorough the carbonization process, which led to an increase in the temperature at which a sharp decline was observed in the thermogravimetric curve of the carbonized sludge. Consequently, the more structurally stable the carbonized sludge is, and the lower the organic matter content of the carbonized sludge is. This is because small molecules of organic matter are volatile at lower temperatures and shorter carbonization time, while large molecules of organic matter require higher temperatures and longer carbonization time, i.e., more energy is needed cleave them to produce small molecules and produce volatiles.

Effect of temperature and time on the physicochemical properties of carbonized sludge

The process of sludge carbonization changes sludge composition and structure, which manifests as changes in the various physicochemical indicators of carbonized sludge. For this reason, the effects of carbonization temperature and time on the physicochemical properties of sludge were assessed in this study.

pH

pH measurements can reflect changes in the acidity and alkalinity of substances during sludge carbonization. Figure 7 shows the pH change in carbonized sludge under different carbonization conditions. As seen from Figure 7, the pH of carbonized sludge shows a decreasing trend as carbonization temperature increased from 300 to 500 °C, indicating that the organic matter in the sludge in this stage was decomposing, forming small molecules of acidic substances such as carboxylic acids. When the temperature increased to 600 and 700 °C, the pH value of carbonized sludge slightly rebounded, which was the result of further carbonization and cleavage of weakly acidic small molecule organic matter in sludge. It can be assumed that when the temperature reached 600 and 700 °C, the degree of sludge carbonization was relatively high, and the organic matter was more thoroughly carbonized. The carbonized sludge eventually showed neutral values because the high temperatures induced the acidic groups in the sludge organic matter to cleave and further carbonize to generate carbon monomers until the organic matter was completely carbonized. In this process, the pH value of carbonized sludge shows a trend in which it first decreases, then increases, and finally becomes neutral. This is consistent with the results of past studies. Czerwińska et al. (2022) found that natural carbonization process of sludge results in neutral and weak acidity. The hydrothermal carbonation process resulted in a shift in the pH of hydrocarbon products and treated water toward neutral values (Reza et al. 2015; Zhai et al. 2016). Pauline & Joseph (2021) used a hydrothermal carbonation technique to carbonize petroleum sludge and found that the pH of hydrothermal carbonate was acidic. Due to the large amount of acid produced, the pH initially decreased significantly to 3.32 at 175 °C and then started to recover.
Figure 7

pH change in carbonized sludge under different carbonization conditions.

Figure 7

pH change in carbonized sludge under different carbonization conditions.

Close modal

In terms of carbonization time, the carbonized sludge with a treatment time of 0.5 h needed to reach a temperature greater than 600 °C to further carbonize the already acidified small molecular organic matter and complete the neutralization trend in pH. Sludge carbonized for 1.0 and 1.5 h had to reach temperatures higher than 500 °C to show a small recovery in pH. When the carbonization time was extended to 1.5 h, the pH increase in carbonized sludge was more obvious. These results indicated that the shorter the time and the lower the temperature are, the less thorough and more incomplete the carbonization process; under the opposite conditions, the carbonization process was more thorough and complete. A treatment temperature of 600 °C and time of 0.5 h and a treatment temperature of 500 °C and time of 1.5 h were the inflection points that marked the end of the acidification process in sludge organic matter decomposition. Past the inflection point, these acidified small molecular organic substances are carbonized to produce carbon monomers. Therefore, the degree of carbonization of sludge can be effectively identified from the pH of sludge.

EC

EC indicates dissolved salt content. Figure 8 shows the variation in the EC of carbonized sludge under different carbonization conditions. From Figure 8, it can be seen that when the treatment duration was 1.0 h, the conductivity decreased from 1,577 to 192 μS/cm as the treatment temperature increased from 300 to 700 °C, with a decrease of 87.8%, indicating that the higher the temperature is, the lower the dissolved salt content. This occurred because salt ions, including heavy metal ions, cured at high temperatures and transformed into insoluble compounds or precipitates.
Figure 8

Variation in the conductivity of carbonized sludge under different carbonization conditions.

Figure 8

Variation in the conductivity of carbonized sludge under different carbonization conditions.

Close modal

When the temperature was 300 °C and the carbonization time was 0.5 h, the conductivity was 1,869 us/cm, while when the carbonization time was extended to 1.5 h, the conductivity decreased to 1,200 us/cm, which was a decrease of 35.8%. When the temperature was 600 °C and the carbonization time was 0.5 h, the conductivity was 679 us/cm, while when the carbonization time was extended to 1.5 h, the conductivity decreased to 489 us/cm, showing a decrease of 28.0%. Therefore, the longer the carbonization time is, the lower the conductivity and the higher the salt immobilization in the sludge. From the collective trends in carbonization temperature and time, it can be seen that the higher the temperature and the longer the carbonization time are, the more salt ions, including metal ions, are converted to the residual state and insoluble state in the carbonized sludge particles. If carbonized sludge is applied to agricultural and forestry crops as a nutrient substrate, these immobilization processes will effectively reduce the soluble forms of heavy metal ions in the sludge and reduce biotoxicity, improving the bioeffectiveness of the carbonized sludge, which in turn will be beneficial for plant growth.

Natural water absorption rate

The natural water absorption rate is an index that reflects the hydrophilicity of carbonized sludge, which to a certain extent reflects the amount of residual organic matter in carbonized sludge. When sludge contains more organic matter and more hydrophilic functional groups such as amino, nitro and carboxyl groups, and it can more easily absorb water. Figure 9 shows the variation in the natural water absorption properties of carbonized sludge under different carbonization conditions. The carbonized sludge obtained at a carbonization temperature of 300 °C and a carbonization time of 0.5 h was the most water-absorbent, and its natural water absorption rate reached 12.7%. With an increase in carbonization time to 1–1.5 h, the water absorption rate decreased to 8.13 and 4.51%, respectively. Obviously, the longer the carbonization time is, the more complete the carbonization process and the lower the amount of organic matter in the sludge, which leads to a reduction in water absorption in carbonized sludge. As the carbonization temperature increased from 300 to 700 °C, the natural water absorption of carbonized sludge decreased to 3.64, 1.81, 1.17, and 1.29%, respectively, when the carbonization time was 1.0 h. Therefore, the higher the treatment temperature is, the poorer the water absorption of the carbonized sludge, which is the result of the increasingly complete carbonization of sludge organic matter.
Figure 9

Variation in the natural water absorption properties of carbonized sludge under different carbonization conditions.

Figure 9

Variation in the natural water absorption properties of carbonized sludge under different carbonization conditions.

Close modal

In terms of the water absorption of carbonized sludge, when the treatment temperature was above 500 °C, the water absorption of carbonized sludge was no more than 3.0%, which indicated that the municipal sludge was transformed from a strongly hydrophilic material to a strong hydrophobic material after carbonization, which occurred because hydrophilic groups were lost from the sludge and organic matter was converted into carbon monomers. These results coincided with the results of the FTIR analysis (see Figure 1).

Particle size

A change in the particle size of carbonized sludge reflects the extent to which the sludge particles shrink in volume due to the loss of water and organic volatile matter during the sludge carbonization process. Figure 10 shows the particle size distribution changes in the carbonized sludge under different carbonization conditions. The particles of size Dx90 in uncarbonized municipal sludge was 984 μm, and after carbonization treatment at different temperatures and time, the sludge particle sizes all showed decreasing trends. When the treatment temperature was increased from 300 to 700 °C and the treatment time was 0.5 h, the particles of size Dx90 in carbonized sludge decreased by 0.41, 6.50, 12.3, 10.1, and 14.6%, respectively. When the treatment time was increased to 1.0 h, the particles of size Dx90 decreased by 5.49, 7.72, 13.6, 12.6, and 19.8%, respectively. When the treatment time was further increased to 1.5 h, the particles of size Dx90 further decreased by 6.61, 23.8, 26.7, 32.3, and 61.3%, respectively. It is obvious that the longer the treatment time and the higher the treatment temperature are, the more obvious the reduction in carbonized sludge particle size. Particularly when the treatment time reached 1.5 h, the reduction in carbonized sludge particle size was very significant, which was the result of the continuous loss of water and volatile organic matter from the sludge particles with increasing energy input, so the particles were increasingly concentrated and their size decreased. At the same time, as the volume decreased, some pore structures may have formed inside the particles due to the loss of components, which provided the potential for the carbonized sludge to serve as a gas adsorption material.
Figure 10

Changes in the particle size of carbonized sludge under different carbonization conditions.

Figure 10

Changes in the particle size of carbonized sludge under different carbonization conditions.

Close modal

Calorific value

Calorific value is an important measure of whether carbonized sludge can be used as fuel for resource reuse. Figure 11 shows the variation of dry-base high-level heat calorific value (DHCV) of carbonized sludge under different carbonization conditions. The DHCV of municipal sludge without carbonization treatment was 10,774 kJ/kg, and when carbonized, the DHCV of sludge showed a decreasing trend. When the treatment temperature was increased from 300 to 700 °C and the treatment time was 0.5 h, the DHCV of carbonized sludge was 8,886, 8,489, 6,555, 5,964, and 3,997 kJ/kg, decreasing by 17.53, 21.22, 39.16, 44.65, and 62.90%, respectively, compared with municipal sludge. The results showed that the higher the temperature for the same treatment time is, the lower the DHCV of the carbonized sludge, indicating a lower content of combustible carbon, which is corroborated by the elemental content results presented in Figure 4. When the treatment time was increased to 1.5 h, a further decrease occurred in the DHCV of carbonized sludge. Compared with municipal sludge, the DHCV decreased by 41.2, 51.5, 51.7, 62.4 and 77.4%, respectively. The longer the treatment time and the higher the temperature are, the lower the DHCV of carbonized sludge, and this occurs because prolonged high-temperature exposure accelerates organic matter cracking and promotes the partial volatilization and loss of the carbon as small molecules.
Figure 11

Variation in the high-level calorific value on a dry basis for carbonized sludge under different carbonization conditions.

Figure 11

Variation in the high-level calorific value on a dry basis for carbonized sludge under different carbonization conditions.

Close modal

As shown in Figure 11, the longer the carbonization treatment time and the higher the temperature are, the more the DHCV of carbonized sludge shows an obvious decreasing trend, which is not conducive to its use as fuel. When the temperature was 600 °C for 1 h, the DHCV of carbonized sludge decreased to 6,323 kJ/kg, which was 21.6% of the calorific value of standard coal (29,307 kJ/kg). It is generally believed that substances with a calorific value of more than 5,000 kJ/kg can be burned without the addition of auxiliary fuels such as coal, oil or natural gas during stable combustion processes, and the combustion performance is better. Therefore, if the carbonized sludge is burned as a fuel, the carbonization process should be conducted at a temperature of no more than 600 °C and a time of no more than 1 h.

Effect of temperature and time on the heavy metal content of carbonized sludge

The heavy metal content of municipal sludge has consistently been an environmental safety concern (Li et al. 2022a). When sludge is treated by a carbonization process, most of the heavy metals present in the sludge are immobilized in the carbonized sludge (Barry et al. 2019; Gong et al. 2020). The heavy metal content of carbonized sludge inevitably determines its potential to be used as a component of garden amendments, coal combustion blends, and construction materials. In this study, we analyzed changes in the total quantities of six common heavy metals, including Cu, Cr, Cd, Pb, Zn, and Ni, in carbonized sludge (see Figure 12). The contents of Cu, Cr, Cd, Pb, Zn, and Ni in municipal sludge before carbonization were 74.4, 133, 1.03, 34.5, 374, and 39.8 mg/kg, respectively. The contents of the six heavy metals were higher in sludge after the carbonization process, and the higher the temperature and the longer the treatment time were, the higher the content of heavy metals and the more obvious the concentration effect. When the carbonization treatment time was 0.5 h and the temperatures were 300–700 °C, the total Cu content increased by 0.74, 38.9, 64.7, 64.2, and 122%, respectively, compared with the uncarbonized sludge; the total Cr content increased by −5.08% (no enrichment), 20.7, 42.5, 55.8, and 55.7%, respectively; the total Cd content increased by 4.16, 192, 364, 387, and 423%; the total Pb content increased by −7.81% (not enriched), 28.3, 46.7, 63.4, and 69.9%; and the total Zn content increased by 4.63, 15.0, 53.2, 85.1, and 75.2%; and the total Ni content increased by −6.33% (not enriched), −7.12% (not enriched), 47.1, 53.6, and 57.9%, respectively.
Figure 12

Variation in total heavy metals in carbonized sludge under different carbonization conditions.

Figure 12

Variation in total heavy metals in carbonized sludge under different carbonization conditions.

Close modal

When the carbonization time was extended to 1.5 h, the carbonization temperatures of 300–700 °C increased the total Cu content of the carbonized sludge by 45.7, 68.2, 98.1, 178, and 194%, respectively; the total Cr content by 3.39, 25.0, 113, 144, and 151%; total Cd content by 224, 358, 736, 692, and 801%; the total Pb content by 19.1, 52.2, 61.8, 181, and 203%; the total Zn content by 11.2, 52.1, 90.5, 190, and 208%; and the total Ni content by −6.72% (no enrichment), 45.8, 74.5, 198, and 202%, respectively. The total quantity of the six heavy metals, including Cu, Cr, Cd, Pb, Zn, and Ni, was further significantly increased from the total amount at a carbonization time of 0.5 h, and at their highest levels, they were enriched by factors of 2.93, 2.51, 9.01, 2.04, 3.08, and 3.02, respectively. These results indicated that the longer the carbonization time of municipal sludge is, the more complete and thorough the degree of carbonization and the more obvious the concentration of heavy metals.

These six heavy metals were not volatile under 700 °C condition, and their concentration was attributed to organic volatile matter and water loss from the municipal sludge. Although the heavy metal content in the carbonized sludge was concentrated exponentially, all the heavy metal contents were still far below the standard limits of China's ‘Urban Sewage Treatment Plant Sludge Disposal of Mud for Gardening and Greening’ (GB/T 23486–2009) (General Administration of Quality Supervision 2009) when the carbonized sludge is used to make gardening amendment products (see Table 2). If the carbonized sludge is blended into cement raw material at a dose of 5% and used to make cement clinkers by an incineration process, based on the Chinese standard Technical Specification for Cooperative Disposal of Solid Wastes in Cement Kilns (GB 30760-2014) (General Administration of Quality Supervision 2014), the content of all heavy metals in the carbonized sludge would still meet the standard limit. This is in line with the results of past studies. Nuagah et al. (2020) found that the heavy metal concentrations of biochar produced at temperatures of 300, 450, and 600 °C were high but were still within the acceptable range for agricultural applications. Barry et al. (2019) prepared biochar from sewage sludge for use as an alternative fuel in cement kilns. Li et al. (2022b) utilized alkaline condition-assisted pyrolysis to immobilize Zn and Ni residues by 100 and 60%, respectively, and the ecological risk of heavy metals was significantly reduced from 57.3 to 30.1, resulting in a low potential ecological risk related to sludge biochar. The final disposal approaches for municipal sludge after carbonization are diverse.

Table 2

Heavy metal limits for the use of carbonized sludge

ItemsHeavy metal contents (mg/kg)
CdZnPbCrNiCu
Disposal of sludge from municipal wastewater treatment plant – Quality of sludge used in gardens or parks(GB/T 23486-2009) (General Administration of Quality Supervision 2009pH ≥ 6.5 <20 <4,000 <1,000 <1,000 <200 <1,500 
pH < 6.5 <5 2,000 300 600 100 800 
Technical specification for coprocessing of solid waste in cement kiln (GB 30760-2014) (General Administration of Quality Supervision 2014Cement raw material 1.00 361 67 98 66 65 
Cement clinker 1.50 500 100 150 100 100 
Cement raw material mixed with 5% carbonated sludge treated by 700 °C and 1.5 h 1.41 400 69 110 68.7 72.7 
ItemsHeavy metal contents (mg/kg)
CdZnPbCrNiCu
Disposal of sludge from municipal wastewater treatment plant – Quality of sludge used in gardens or parks(GB/T 23486-2009) (General Administration of Quality Supervision 2009pH ≥ 6.5 <20 <4,000 <1,000 <1,000 <200 <1,500 
pH < 6.5 <5 2,000 300 600 100 800 
Technical specification for coprocessing of solid waste in cement kiln (GB 30760-2014) (General Administration of Quality Supervision 2014Cement raw material 1.00 361 67 98 66 65 
Cement clinker 1.50 500 100 150 100 100 
Cement raw material mixed with 5% carbonated sludge treated by 700 °C and 1.5 h 1.41 400 69 110 68.7 72.7 

Energy consumption assessment and process analysis of sludge carbonization

The sludge carbonization process is difficult to apply in industries, and the reason lies in the high cost associated with energy consumption. If energy recovery can be performed well, production costs will be greatly reduced. Studies have shown that the thermal energy recovery efficiency of solid sludge powder is more than 80% (Cheng et al. 2019), which makes the industrialization of the sludge carbonization process economically and technically feasible, allowing the sludge carbonization process to be promoted and applied.

The heat demand of the sludge carbonization process is divided into two parts: the heat demand for water evaporation and the heat demand for sludge heating and carbonization. Taking the carbonization of 1,000 kg sludge (moisture content of 36.2%) as the basis for calculation, according to the calculated Equation (5) based on the principle of thermodynamics, when the temperature is increased from 20 °C (room temperature) to 300–700 °C and the temperature is maintained for 1 h, the theoretical energy needed is 1,955, 2,361, 2,767, 3,173, and 3,579 MJ, respectively. To effectively carry out residual heat recovery from carbonized sludge, a heat recovery rate of 70% (observed in China and other countries) for the bulk solid material was used as the basis for calculations, and the theoretical energy needed for the completion of sludge carbonization was calculated to be 586 MJ, 708 MJ, 830 MJ, 952 MJ, and 1,073 MJ, respectively. If the electrothermal conversion efficiency of carbonization equipment is 75%, the corresponding cost of electricity consumption is 18.8 USD/t MS, 22.7 USD/t MS, 26.6 USD/t MS, 30.5 USD/t MS, and 34.4 USD/t MS, respectively (Table 3), which is essentially similar to the results of other studies. Barry et al. (2019) utilized carbonation pyrolysis to treat 2.1 t/h sewage sludge dry solids; the annual expenditure was $976,800, and the treatment cost was approximately 55.4 USD/t DS. Therefore, with good heat recovery, the cost of carbonization of municipal sludge is in an acceptable range even if the carbonization temperature reaches 700 °C.

Table 3

Energy and cost analysis of sludge carbonization

Processing number
T2T5T8T11T14
Carbonization process parametersTemperature (°C)300400500600700
Time (h)1.01.01.01.01.0
Sludge weight (kg) 1,000 1,000 1,000 1,000 1,000 
Sludge water content (%) 36.2 36.2 36.2 36.2 36.2 
Initial temperature (°C) 20 20 20 20 20 
Total energy of theoretical demand (MJ) 1,955 2,361 2,767 3,173 3,579 
Waste heat recovery rate for bulk materials (%) 70 70 70 70 70 
Theoretical demand energy (MJ) 587 708 830 952 1,074 
Equipment electric heat conversion efficiency (%) 75 75 75 75 75 
Equipment insulation efficiency (%) 90 90 90 90 90 
Actual power consumption (kWh) 217 262 307 353 398 
Guangzhou Comprehensive Electricity Tariff, China (USD$/kW·h) 0.0865 
Carbonized electricity costs (USD$/ton) 18.8 22.7 26.6 30.5 34.4 
Processing number
T2T5T8T11T14
Carbonization process parametersTemperature (°C)300400500600700
Time (h)1.01.01.01.01.0
Sludge weight (kg) 1,000 1,000 1,000 1,000 1,000 
Sludge water content (%) 36.2 36.2 36.2 36.2 36.2 
Initial temperature (°C) 20 20 20 20 20 
Total energy of theoretical demand (MJ) 1,955 2,361 2,767 3,173 3,579 
Waste heat recovery rate for bulk materials (%) 70 70 70 70 70 
Theoretical demand energy (MJ) 587 708 830 952 1,074 
Equipment electric heat conversion efficiency (%) 75 75 75 75 75 
Equipment insulation efficiency (%) 90 90 90 90 90 
Actual power consumption (kWh) 217 262 307 353 398 
Guangzhou Comprehensive Electricity Tariff, China (USD$/kW·h) 0.0865 
Carbonized electricity costs (USD$/ton) 18.8 22.7 26.6 30.5 34.4 

To promote the industrial application of the municipal sludge carbonization process, the authors of this study propose a set of carbonization process flows with energy recovery. The system mainly consists of a sludge drying and dewatering system, a sludge preheating system, a continuous carbonization furnace system and a carbonized sludge cooling furnace system to achieve the greatest extent of energy recovery and reuse possible to ensure the normal operation of the carbonization process, reducing production costs. Figure 13 shows the recommended sludge carbonization process flow. Municipal sludge with a water content of 30–40% and a particle size of less than 2 cm is first crushed by a pulverizer to particles of sizes of less than 2 mm and then sent to a powder flow drier to remove water; then, it is sent into a powder flow heater, which preheats and warms the sludge to 150 °C. Then, the preheated sludge is sent to a continuous carbonization furnace, and the residence time is 1 h. Finally, the carbonized sludge is cooled to room temperature in a high-temperature powder flow cooler to achieve energy recovery and reuse. In the energy recovery process, heat transfer oil is the main heat transfer medium. The drying, preheating and cooling processes produce some flue gas; therefore, a flue gas treatment system is needed to realize environmentally friendly production and meet emissions standards.
Figure 13

Schematic of the recommended sludge carbonization route.

Figure 13

Schematic of the recommended sludge carbonization route.

Close modal
  • (1)

    Characterization techniques such as FTIR, XRD, and SEM were used to analyze structural changes in carbonized sludge. The results show that with increasing carbonization temperature, the organic matter in municipal sludge is continuously cracked, changing the structural composition of the organic matter. The amorphous nature of the material decreases, large numbers of particles with regular crystal structures are formed, the surface becomes smooth, the carbon content decreases, and the inorganic species content increases.

  • (2)

    The higher the temperature and the longer the carbonization time are, the more thorough and complete the degree of carbonization of municipal sludge, which manifests as a trend in which pH first decreases and then increases and finally returns to a neutral value. Additionally, a decrease in EC occurs, a high degree of conversion is observed in salt ions to the residual and insoluble states, natural water absorption properties decrease, the material becomes more hydrophobic, an obvious reduction occurs in particle size, the dry-basis high-level calorific value of the material decreases, and a significant concentration of heavy metals occurs.

  • (3)

    The performance of carbonized sludge products directly depends on the temperature and time of carbonization, and different utilization pathways require different product properties. This provides an important reference for carbonized sludge to carry out a more suitable and economical way of resource utilization. Although the heavy metal content concentrates exponentially during carbonization, the reactive metal content is extremely low, and therefore the biotoxicity is significantly reduced. As the heavy metal contents are far below the corresponding standards in China, these carbonized sludge products will be suitable for different directions of horticultural substrate, building material product additives, fuel doping, etc., according to their properties. The carbonized sludge is to be used for plant seedling breeding, and the carbonized products with very full carbonization degree are required. Insufficiently carbonized sludge would be considered if it was to be used for afforestation or for fuel.

  • (4)

    Good heat recovery is the key to the sludge carbonization process. Even if a carbonization temperature of 700 °C is employed, and the cost is acceptable. The establishment of a set of carbonization processes with energy recovery is important to promote the industrial application of municipal sludge carbonization processes.

This study was supported by Jiangmen Science and Technology Specialist Research Program (2023760300420008577), Science and Technology Program of Guangzhou (2023A04J1636), Science and Technology Program of Guangzhou (202201011306), Science and Technology Innovation Project of Guangzhou Sewage Purification Co., Ltd (HX2022-016).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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