Co-pyrolysis Of Sewage Sludge Research Articles

Co-Pyrolysis of Sewage Sludge, Two-Component Special Municipal Waste and Plastic Waste

Co-Pyrolysis of Sewage Sludge, Two-Component Special Municipal Waste and Plastic Waste

Co-pyrolysis of Sewage Sludge with Paint Sludge: Kinetics and Thermodynamic Analysis via Iso-conversional Methods

Co-pyrolysis of Sewage Sludge with Paint Sludge: Kinetics and Thermodynamic Analysis via Iso-conversional Methods

Co-pyrolysis of sewage sludge with hydrogen-rich polythene: Effects on synergistic promotion and bio-oil quality

Co-pyrolysis of biomass with hydrogen-rich feedstock is a promising technology to improve bio-oil quality, in which understanding the synergistic effects and product of sewage sludge (SS) with two types of plastic waste (HDPE, LDPE) is essential to develop co-pyrolysis technology. This work used in situ tracking to detect co-pyrolysis promotion/inhibition effects with different mixed feedstocks and mixing ratios, and analyzed volatile and bio-oils in detail. The results show that co-pyrolysis synergistic promotion was mainly created by the release of a large amounts of free radicals, and the largest promotion occurred near the maximum mass loss rate (DTGmax). The synergistic promotion effect was 190 % higher for SS mixed with HDPE (S–H) than for SS mixed with LDPE (S-L). Co-pyrolysis reduced polluting gases (e.g., NO2, and SO2), enhanced the degradation of oxygenated functional groups, contains 2.7 times of hydrocarbons in bio-oil than SS mono-pyrolysis. S-L promoted short-chain generation and produced more naphthalene compounds (e.g., methylnaphthalene and 2-methylnaphthalene) in hydrocarbons. However, S–H tends to produce longer chains and more monocyclic aromatic hydrocarbons (e.g., alkylbenzenes). The gasoline fraction in bio-oil could reach 89.7 % (S7L3) and 55.8 % (S5H5). This information helps to promote clean disposal and resource utilization of sewage sludge and plastic waste through co-pyrolysis.

Energy recovery from sewage sludge waste blends: Detailed characteristics of pyrolytic oil and gas

Using waste as an alternative energy source may partially replace fossil fuels. In this paper, energy recovery from waste blends (sewage sludge (SS), polyethylene (LDPE) and polypropylene (PP), paper rejects (PR), and waste tyres) was tested through laboratory-scale co-pyrolysis. For each co-pyrolysis test, 7 l of waste blend in different ratios were used. The results of pyrolytic oil and gas analyses show valuable compounds and chemical materials recovered. The detailed analyses show the effects of co-pyrolysis parameters, especially the ratios of waste blends with respect to the process temperature of 600°C, on the pyrolytic gas and oil. The highest volumes of gas (0.66 m3) and oil (699 ml) were obtained from sample 5_8 (PES:SS:PR). Waste blend 15_1 (LDPE:SS) had the highest gross calorific value (GCV) of 43.82 MJ/m3 in gas, and had the highest proportion of flammable components (46.31% of methane). Sample 5_8 had the highest calorific value in pyrolytic oil (45 MJ/kg) and sample 5_3 had the lowest (28.113 MJ/kg), which is comparable to coal. High-quality pyrolytic gas and oil were obtained with average GCV of 44 MJ/m3 and 40 MJ/kg respectively, where the average GCV of the most calorific fossil fuels is 43.6 MJ/kg in crude oil and 34 MJ/m3 in natural gas. We conclude that co-pyrolysis of sewage sludge and waste is a convenient and simple method of waste disposal under a synergetic effect of efficient waste management, energy production, and reduction of dependence on fossil fuels.

Co-pyrolysis of sewage sludge and phosphate tailings: Synergistically enhancing heavy metal immobilization and phosphorus availability

Phosphate tailings (PT) was used to reduce the release of heavy metals (HMs) during pyrolysis and the leachable rate of residual HMs, and simultaneously improve the bioavailability of phosphorus in the sludge-based biochar. The concentration of heavy metals and the fractions determined by BCR method was used to investigate the release and the transformation of Zn, Pb, Mn, Ni and Cu during pyrolysis involved with the effects of temperature and the addition of PT. The respective pyrolysis experiments shows that the release of Zn and Pb increases with temperature for both sewage sludge (SS) and PT, and the bioavailable fractions (F1 + F2) of Mn, Ni, and Cu increases with temperature for PT. During co-pyrolysis, blended samples released lower quantities of Zn and Pb and presented lower bioavailability of HMs than the individual SS or PT. A synergistic effect of co-pyrolysis was evident for volatile Zn and Pb. The decomposition of CaMg (CO3)2 from PT produced CaO, by which the volatile ZnCl2 and PbCl2 were transformed into ZnO and PbO with less volatility and higher reactivity with SiO2 and Al2O3 than the chlorides. Then SiO2 and Al2O3 from SS acted as the final stabilizer to immobilize the oxides. The final product combined with SiO2 and Al2O3, such as ZnSiO4 and ZnAl2O4, were detected. The addition of PT also introduced more Ca and P into sludge to produce biochar with higher concentration of apatite phosphorus with higher bioavailability.

Enhanced phosphorus and potassium recovery through co-pyrolysis of nutrient-rich biomass feedstocks for engineered biochar production

Engineered biochars were produced via slow pyrolysis at 600 °C for 1 h from selected carbon-rich (diyar wood, sawdust, cotton stalk, sunflower straw, and banana stem) and nutrient-rich (eggshell, chicken bone, chicken manure, and sewage sludge) biomass residues with enhanced P and K recovery. Separate pyrolysis of both category feedstocks and co-pyrolysis of selected carbon-rich and nutrient-rich feedstocks mixed at 3:1 ratio was conducted. Higher biochar yield (41 %) was achieved in the co-pyrolysis of sewage sludge and sunflower stalk compared to average (38.5 %) of their separate pyrolysis. Moreover, increased K recovery (5.3 % to 19.5 %) and increased P recovery (1.8 % to 25.5 %) was achieved in biochar through co-pyrolysis of nutrient-rich and selected carbon-rich feed stocks compared with their separate pyrolysis. The chicken bone and sewage sludge can be ideal feedstocks for enhanced P and K recovery through their co-pyrolysis with carbon rich biomass such as banana stem or sunflower stalk.

Co-pyrolysis of sewage sludge and sawdust: Synergistic effects of product yield and synthetic gas calorific value

Co-pyrolysis of sludge and sawdust, which are two renewable and abundant biomass resources, offers a promising environmental-friendly way to produce valuable energy products. In this study, the synergistic effect between sludge and sawdust was quantified by fixed bed reactor and thermogravimetric experiments. The product yield and product quality were also studied. The results showed that both temperature and sawdust addition significantly affected the distribution of co-pyrolysis products. Biochar yield decreased with increasing temperature, with the highest yield at 700 °C. With the addition of sawdust, the yield of biochar first increased and then decreased. The highest biochar yield was 58.1 wt% after adding 10 wt% sawdust. The yield of bio-oil initially decreases and then increases with temperature. With the addition of sawdust, the yield of bio-oil first decreased and then increased. The process condition of a temperature of 700 °C–800 °C and a sawdust addition ratio of less than 30 wt% is more favorable for the production of bio-oil. Increasing temperature and the addition of sawdust boosteded syngas production and calorific value. When the sawdust ratio was 10 wt%, it exhibited the most pronounced influence on gas yield and demonstrated less susceptibility to temperature variations. The most obvious increase in the calorific value of syngas was observed at 700 °C and 10 wt% sawdust ratio. Taking into account the product yield and product quality, the optimum conditions for co-pyrolysis were determined to be 750 °C and a sawdust ratio of 10 wt%.

The development of plastic waste and sewage sludge co-pyrolyzed biochar composites with improved interfacial characteristics for the effective removal of ciprofloxacin

The growing global proliferation of plastic waste and sewage sludge (SS) has emerged as a prominent environmental dilemma. This study assessed the efficacy of highly efficient carbon-based adsorbent materials derived from co-pyrolysis of SS and plastic waste in mitigating the concentration of ciprofloxacin (CPX) in an aqueous solution. Each formulated material included varying proportions—20% and 50%—of either polyethylene (PE) or polyethylene terephthalate (PET). Notably, composite with PET—50% exhibited a significant increase in specific surface area to 194.7 m2/g. Efficient adsorption of CPX up to 113.97 mg/g (qms) was reported at pH 5. Pollutant removal was recorded in 12 hours of retention time due to induced π–π interactions, electrostatic and hydrophobic surface interactions highlighting chemosorption. The alkaline pH impacted the adsorption capacity, causing a prominent decline. The SS-PET formulation exhibited a substantial increase in the number of active sites, thereby showing a strong interaction with ionized CPX molecules and yielding superior sorption efficiency by utilizing the novel combination of the material. The regenerative investigations also confirmed the high adsorption for four consistent cycles. Overall, acquiring comprehensive knowledge and practical information from this study will contribute to the effective management of CPX-contaminated wastewater and the mitigation of plastic pollution.

Sewage sludge co-pyrolysis with agricultural/forest residues: A comparative life-cycle assessment

This study aims to determine the sustainability and energy efficiency of co-pyrolysis scenarios as treatment processes for municipal sewage sludge through a life cycle assessment (LCA). In addition, sensitivity and energy recovery analyses are conducted to determine the possible methods for optimizing the co-pyrolysis process from a circular bioeconomy perspective. Corncob and wood residue have been selected as potential co-feed materials for co-pyrolysis with sewage sludge at three mixing ratios (25, 50, and 75 wt%). The functional unit (FU) for this study is 1000 kg of dried single or mixed feedstock. LCA results indicate that sewage sludge, in a singular pyrolysis scenario, demonstrated the most unfavorable outcome by causing a rise in all negative environmental indicators. In contrast, the overall environmental impacts are reduced by up to 48 %, when the sewage sludge is mixed with co-feed biomass (wood or corncobs), with corncob co-pyrolysis performing better than wood residue in most impact indicators. Energy recovery from a gas turbine provides significant benefits, generating about nine times of the required energy for gas turbine operation and supplying sufficient energy to sustain the whole process. This is notably evident for corncob co-pyrolysis, where the energy produced from gas recovery is equivalent to 59–181 % of energy requirement of the whole process and achieved the highest net positive energy balance (+1368 kWh/FU). Sensitivity analysis indicates that co-pyrolysis is more sensitive to bio-oil yield fluctuations and feedstock transportation. In conclusion, this study establishes that sewage sludge co-pyrolysis is a more environmentally friendly treatment approach when compared to single pyrolysis.

Co-pyrolysis of sewage sludge and biomass waste into biofuels and biochar: A comprehensive feasibility study using a circular economy approach

Enormous annual sewage sludge (SS) volumes pose global environmental challenges owing to contamination and significant greenhouse gas emissions. Here, we investigated the economic viability of co-pyrolyzing SS and biomass waste to produce biofuels (bio-oil and gas) and biochar. Net present worth (NPW) analysis, the sale product break-even price, and sludge handling price (SHP) were used to determine the profitability of co-pyrolysis compared with SS pyrolysis alone and conventional treatment methods. In this study, the sale prices of biochar based on quality (i.e., stability, carbon sequestration effectiveness, and heavy metal content) were estimated to be 2.24, 1.44, and 0.98 CAD/kg for high-, medium-, and low-grade biochar. The bio-oil prices, estimated based on the higher heating values of bio-oil and diesel, ranged from 0.80 to 1.22 CAD/kg. Sawdust (SD) and wheat straw (WS) were the chosen co-pyrolysis feedstocks, with four mixing ratios (20, 40, 60, and 80 wt%). Economically, SD (40 wt% mixing ratio) co-pyrolysis achieved the best performance, with a maximum NPW of 8.71 million CAD. SD single and co-pyrolysis were the only profitable scenarios. Moreover, SS single pyrolysis and WS co-pyrolysis exhibited higher profitability than conventional SS treatment methods, with SHPs of 65 and 40 CAD/1000 kg dry sludge, respectively. Sensitivity analysis highlighted the dependence of economic performance on biochar and bio-oil market value. This study offers the first economic analysis of this approach and enhances our understanding of the potential of co-pyrolysis for biofuel and biochar production, providing innovative solutions for the environmental challenges of SS disposal.

CO2 assisted Ca-based additives on pyrolytic characteristics and products from the co-pyrolysis of sewage sludge and biomass

In this study, the CO2-looping system was firstly attempted to build for exploring the effect of calcium-based (Ca-based) additives on pyrolytic characteristics and products in co-pyrolysis of sewage sludge (SS) and biomass. Then, an innovation method for the consumption of CO2 (CPCO2) evaluation was proposed and carried out in the CO2-looping system, in which it was evaluated based on the syngas volume and CO2 proportion. The results revealed that the addition of CaO and Ca(OH)2 could significantly improve CPCO2 by 21.2% in the co-pyrolysis of 500 ℃, while the CaCO3 addition decreased the CPCO2 by 1.0%. Besides, the co-pyrolysis temperature more than 700 ℃ would play more significant role on the CPCO2 than the addition of Ca-based additives. Then the effect of Ca-based types on biochar characteristics was also identified accompanied by the bio-oil composition. Results showed that the CaO and Ca(OH)2 additives not only increased biochar mass and pH, but also enhanced the dissolved organic carbon content and the variety of dissolved organic matter, and reduced the total HMs content. Meanwhile, it was 15 types of components that was identified in bio-oil by NIST mass spectral library, found that the intensity of heptadecanenitrile and estra-1,3,5(10)-trien-17 beta-ol was influenced by Ca-based additives. Moreover, the CaO and Ca(OH)2 played important roles for improving the CPCO2 in the CO2-looping system, which was further verified by Fact Sage. Finally, the mechanism for the Ca-based additives on CPCO2 was proposed at different co-pyrolysis temperatures. These findings can widen paths for biochar application and development of CO2 capture materials.

Comprehensive insight into co-pyrolysis of sewage sludge and tobacco stalk: Effects of mixing ratio and pyrolysis conditions on product distribution and characteristics

Co-pyrolysis of sewage sludge (SS) and tobacco stalk (TS) was conducted, where TS are rich in inorganic elements. The effects of the mixing ratio and pyrolysis temperature on pyrolysis products were investigated through the tube furnace system. The components of gas and tar were determined. In addition, the N/S ratio, pore distribution characteristics, surface functional group characteristics, aromatisation characteristics and inorganic element content of char were studied. Results show that the addition of TS to SS has a significant synergistic effect. Particularly, when the ratio of SS to TS is 1:3, the pyrolysis gas yield is the largest. The inorganic elements with a catalytic effect can promote the conversion of char and tar into pyrolysis gas. However, with the increase in pyrolysis temperature, the synergistic effect will gradually weaken. The increase in pyrolysis temperature will promote the development of mesopores in char but will reduce the order degree of the char structure. The addition of TS in SS will further increase the number of mesopores in char, and the obtained char carbon structure is more ordered. When the pyrolysis temperature is 600 °C, the tar yield is the highest, and the higher the mixing ratio of SS, the better the formation of polyaromatic compounds and aliphatic hydrocarbons.

Co-pyrolysis of sewage sludge with lignocellulosic and algal biomass for sustainable liquid and gaseous fuel production: A life cycle assessment and techno-economic analysis

Biowastes are marked sources of global greenhouse gas emissions, however, locally accessible biowaste could be a sustainable source for biofuel production to generate clean energy. This study examined the environmental sustainability and economic feasibility of using co-pyrolysed sewage sludge (SS) with lignocellulosic and algal biomass for liquid and gaseous fuel production from the perspective of life cycle assessment and techno-economic analysis. Wastewater-grown microalgae, sawdust, and wheat straw were selected as co-feed materials, and three mixing ratios (25, 50, and 75 wt%) of each feedstock with SS were examined. Wastewater-grown microalgae and SS were obtained from wastewater treatment plants (WWTPs), which were assumed to be near the pyrolysis plant facility, obviating the need for transportation. An Aspen simulation was used to estimate the amount of energy recovered from the pyrolysis as gaseous fuel for electricity and heat generation via gas turbines. A sensitivity analysis was conducted to identify future considerations for SS co-pyrolysis to make it a more environmentally and economically viable industrial process for producing liquid and gaseous fuels. The co-pyrolysis of SS with biomass reduced the overall environmental burden, with sawdust co-pyrolysis outperforming wheat straw and microalgal co-pyrolysis. All scenarios achieved a high net positive energy balance (from 2,595 to 6,323 kWh/1,000 kg bio-oil) and reduced global warming potential (GWP) (from −793 to −1,970 kg CO2 eq/1,000 kg bio-oil). Sawdust co-pyrolysis resulted in the highest GWP reduction (from −1,510 to −1,970 kg CO2 eq/1,000 kg bio-oil) and energy recovery (from 5,551 to 6,323 kWh/1,000 kg bio-oil). Energy recovery from gaseous fuel through gas turbine operation generated a process energy demand of 23 %–224 %. Sawdust co-pyrolysis exhibited the highest net present worth (NPW) of 6.16 million CAD. Sensitivity analysis demonstrated that the GWP and terrestrial ecotoxicity potential were affected by the transportation of sawdust and wheat straw. Overall, the co-pyrolysis of SS with both lignocellulosic and algal biomass is sustainable and economically feasible for producing liquid and gaseous fuels and treating hazardous waste originating from WWTPs, thereby facilitating environmental sustainability and a circular economy from the sale of pyrolysis products. Broader implication of this study is using effective catalysts to further strengthen the sustainability and increase the profitability of the co-pyrolysis process, which has not been yet investigated.

Effect of biochar derived from co-pyrolysis of sewage sludge and rice straw on cadmium immobilization in paddy soil.

The remediation of cadmium (Cd) contaminated cropland has been related to food safety and public health. While biochar derived from sewage sludge (SS) has been widely used for soil remediation due to its high efficiency of Cd immobilization, it has a low specific surface area and the ecological risk of heavy metals. Co-pyrolysis of straws and SS could resolve these issues. To date, little is known about the effect of biochar from SS/rice straw (RS) on Cd immobilization in soils. Here, we explored the soil remediation efficiency and mechanism of biochar derived from different mixing ratios (1:0, 3:1, 2:1, 1:1, 1:2, 1:3 and 0:1) of RS and SS named as RBC, R3S1, R2S1, R1S1, R1S2, R1S3 and SBC. It was shown that R1S2 amendment had the most efficiency of Cd immobilization among all amendments, which decreased the bioavailable Cd by 85.61% and 66.89% compared with RBC and SBC amendments. Results of biochar after soil remediation revealed that cation-π interaction, complexation, ion exchange and precipitation were the key mechanisms of Cd immobilization by biochar. All biochar amendments indirectly promoted Cd immobilization through increasing soil pH values, cation exchange capacity (CEC), soil organic carbon (SOC) and available phosphorous (AP). Compared with RBC, R1S2 reduced bioavailable Cd mainly through the increased soil pH, CEC and AP. However, the enhanced efficiency of Cd immobilization in R1S2 amendment than that in SBC amendment because of the more developed pore structure, functional groups and larger specific surface area of R1S2. Overall, our study showed a new type of biochar for the effective remediation of Cd-contaminated soil.

Synergistic interactions between sewage sludge, polypropylene, and high-density polyethylene during co-pyrolysis: An investigation based on iso-conversional model-free methods and master plot analysis

Sewage sludge (SS) is a hazardous by-product of wastewater treatment processes that requires careful management for minimal environmental impacts and effective resource recovery. Through thermochemical processes such as pyrolysis, clean energy is recovered from SS in the form of bio-oil, biogas, and biochar. To improve the yield and quality of products, the co-pyrolysis of more than two materials is increasingly gaining interest. Here, the thermal behaviour, kinetics, and synergistic interactions during the co-pyrolysis of SS with polypropylene (PP) and high-density polyethylene (HDPE) were comparatively evaluated with thermogravimetric analysis at different mixing ratios and heat rates. Activation energies and reaction mechanisms were determined through iso-conversional model-free methods and master plot analysis. Evolved gases were monitored with thermogravimetric-mass spectrometry. Increased volatile conversion and degradation rates, and reduced activation energies during co-pyrolysis were mediated by synergistic interactions between H-radicals of PP/HDPE and oxygenated intermediates of SS. Contrary to the pyrolysis of SS, PP and HDPE, the co-pyrolysis processes are predominantly diffusion-controlled. Insights into the co-pyrolysis processes of SS/PP and SS/HDPE gained from this work provide the theoretical support for subsequent investigation, facilitate design of waste-to-energy reactor, and aid the adoption of the technology to harness the bioenergy potential of the feedstocks.

Study on characteristics, hydrogen-rich gas, bio-oil production, and process optimization of co-pyrolysis of sewage sludge and wheat straw

Sewage sludge (SS) and wheat straw (WS) are wastes that need to be treated urgently. In this research, the co-pyrolysis of SS and WS was investigated utilizing a thermogravimetric analyzer (TG) and a fixed bed. Using the Coats-Redfern approach, the effects of the mix ratio and the interactions of the SS and WS were investigated, and the effects of mix ratio, N2 flow rate, and pyrolysis temperature were examined for fixed bed pyrolysis. The response surface method (RSM) was applied in fixed bed pyrolysis to optimize the process. Increasing WS ratio can reduce pyrolysis residual rate to 23.8% for TG analysis. The reaction activation energy (E) increased with WS ratio. For the co-pyrolysis, there were substantial interactions between 200 and 400°. For fixed bed pyrolysis, syngas and bio-oil yields were mainly affected by N2 flow rate and pyrolysis temperature. The components with the highest amount in bio-oil gradually changed from aliphatic to aromatic hydrocarbons as the WS proportion grew, demonstrating that WS pyrolysis produced mostly aromatic compounds whereas SS pyrolysis produced more aliphatic compounds. Temperature 600 °C, sludge mixing ratio 32%, flow rate 2.5L/min, anticipated maximum H2 0.0421 m3/kg, and maximum bio-oil of 50.7114 g/kg are the best circumstances according to RSM.

Optimal Co-pyrolysis of municipal sewage sludge and microalgae Chlorella Vulgaris: Products characterization, synergistic effects, mechanism, and reaction pathways

Co-pyrolysis of municipal sewage sludge (MSS) and microalgae Chlorella Vulgaris (MCV) was carried out in a fixed-bed reactor to investigate the effect of feedstocks interaction on pyrolysis products and synergistic effects. Based on previous authors' research, the optimum values of reaction parameters were T = 520 °C, mixing ratio (MCV/MSS) = 0.82, and an inert gas flow rate of 0.55 L/min. For the investigation of synergetic effects at optimum point, theoretical values anticipated from each characteristic were computed as an arithmetic sum of the single pyrolysis values obtained. For the first time, by using GC analysis, changes in gas components (H2, CO, CO2, CH4) at different temperatures (220–520 °C) and reaction times (0–30 min) have been investigated. Referring to GC-MS results the percentages of oxygen-containing species, alkanes, alkenes, aliphatic, aromatic, and total hydrocarbons were changed from the theoretical value of (41.719%), (6.004%), (11.215%), (17.219%), (4.317%), and (21.536%) to experimental value (39.292%), (3.693%), (12.103%), (15.796%), (0%), and (15.796%) respectively. Fatty acids and N&O-containing compounds (amides) were increased from (12.297%) to (26.009%) and (6.989%) to (16.217%), respectively. Co-pyrolysis bio-oil was rich in fatty acids and amides compounds. So, co-pyrolysis of sewage sludge and Chlorella Vulgaris provides a new orientation for high value-added utilization of biomass. It was observed that the nitrogen and oxygen content of biochar was reduced (0.29% and 1.01% respectively) compared to the theoretical condition. In the end, a comprehensive mechanism, synergistic, and reaction pathways of co-pyrolysis of MCV and MSS were presented.

Co-pyrolysis of sewage sludge and poplar sawdust under controlled low-oxygen conditions: Biochar properties and heavy metals behavior

Biochar production by pyrolysis of sewage sludge (SS) is one of effective ways to realize resource recovery and harmless treatment of SS. However, biochar prepared by direct pyrolysis of SS has a lower specific surface area and higher heavy metal content. This study was aimed to evaluate the effect of low-oxygen and addition of poplar sawdust (PS) on sludge-derived biochar. The properties of biochar and heavy metals behavior were systematically characterized. The presence of 10% O2 during SS pyrolysis had a significant effect on the specific surface area of the pyrolytic biochar and the chemical species of heavy metals. In comparison with the pyrolytic biochar under N2 atmosphere, the yield of co-pyrolytic biochar from 10% O2 decreased by 6.7% at 350 °C and 8.3% at 650 °C, while it had the highest fixed carbon content of 19.6%. Moreover, the addition of PS also increased the specific surface area of pyrolysis biochar to 26.8 m2 g−1 at 650 °C. With the increase of pyrolysis temperature, heavy metals including Zn, Cr, and Cu in SS were transformed from the exchangeable and reducible states to oxidizable states and residue states. Especially, the presence of 10% O2 can also promote the formation of oxidizable states of heavy metals in biochar. Based on risk assessment code analysis, the ecotoxicity of heavy metals in biochar with the presence of O2 was evidently lowered. Therefore, a low oxygen atmosphere and co-pyrolysis of sludge and biomass could be effective ways to improve the quality of sludge-based biochar.

Co-pyrolysis of sewage sludge as additive with phytoremediation residue on the fate of heavy metals and the carbon sequestration potential of derived biochar

Considering the characteristics of municipal sewage sludge (MS) and Sedum alfreddi L. (SA, a hyperaccumulator plant), we attempted to use MS to enhance the enrichment and stability of heavy metals (HMs) in pyrolysis residue during SA pyrolysis. The effects of pyrolysis temperature (400–800 °C) and co-pyrolysis on migration behavior, chemical speciation, long-term leaching toxicity of HMs, and the environmental risk and carbon sequestration potential of biochar were systematically investigated. Besides, thermodynamic equilibrium simulations were performed to study the transformation of HM compounds during pyrolysis. When the pyrolysis temperature increased from 400 °C to 800 °C, the unstable fractions (F1+F2) of Cd, Pb, Cu, and Cr in MS1SA3 800 had decreased to less than 6% and Zn to 20.4%, and long-term leachability of HMs decreased continuously. Meanwhile, biochar's ecological risk was reduced to a low level, while its carbon sequestration potential improved with little released HMs. Compared with SA pyrolysis alone, adding MS increased the relative residue content of Cd and Zn in biochar, whereas no apparent effect on Pb, Cu, and Cr, and the proportion of stable fractions (F3+F4) increased. Co-pyrolysis enhanced the carbon sequestration potential of biochar, attributed to the inherent minerals of MS. Equilibrium calculations showed that the influence of MS on the fate of HMs during SA pyrolysis is mainly attributed to its high sulfur content, while Si and Al preferentially combine with alkali metal (K)/alkaline earth metal (Ca) and then interact with Zn. The findings in this paper suggest that co-pyrolysis of MS as an additive with hyperaccumulator plants is a feasible proposal, and the co-pyrolysis biochar obtained at suitable temperatures has the potential for safe application.

Immobilization effect of heavy metals in biochar via the copyrolysis of sewage sludge and apple branches

The excess sludge produced by sewage treatment plants can be recycled into energy through pyrolysis, and the byproduct biochar can be used for soil remediation. However, the heavy metals in sludge are retained in biochar after pyrolysis and may cause secondary pollution during its soil application. Herein, a fast copyrolysis method of activated sludge (AS) and apple branches (AT) was proposed to immobilize heavy metals while improving bio-oil yield. The results showed that the heavy metal release from the copyrolyzed biochar was markedly reduced compared with that from the biochar produced through the pyrolysis of AS alone (78% for Cr and 28% for Pb). The kinetic behavior of ion release from different biochars could be described by a first-order kinetic model. The excellent fixation of heavy metals was attributed to complexation by abundant oxygen-containing surface functional groups (–O–, =O, and –CHO) that were mainly donated by AT. Furthermore, high-temperature pyrolysis was conducive to the fixation of metals, and the release of Pb2+ and Cr3+ from the biochar pyrolyzed at 600 °C was approximately 2/3 and 1/10 of that from the biochar pyrolyzed at 400 °C, respectively. A growth experiment on Staphylococcus aureus and Escherichia coli revealed that the toxicity of the copyrolyzed biochar was greatly reduced. This work can provide a method for heavy metal fixation and simultaneous resource recovery from organic wastes.