The migration and transformation behavior of heavy metals during the liquefaction process of sewage sludge

Hydrothermal liquefaction of municipal sludge and its products applications

Hydrothermal liquefaction (HTL) is an effective medium-temperature, high-pressure thermochemical process to dispose municipal sludge (MS), and biocrude (a crude bio-oil) is its main product. Many efforts are continued extensively to improve conversion efficiency and to promote industrial application of this technology. This work focuses on critical influencing factors (e.g., reaction temperature, residence time, atmosphere, solvent, catalyst, and pretreatment) and fundamental transformation mechanisms of main components (i.e., lipids, proteins, and carbohydrates) in MS HTL. It also analyzes migration behavior of heavy metals during MS HTL, which can provide a reference for subsequent recovery of nutrients from HTL products. Moreover, the applications of MS HTL products are systematically expounded, and potential challenges and opportunities are highlighted as well. It is necessary to develop advanced methods of catalyst recovery and innovative biocrude upgrading methods so as to reduce HTL investment and operating costs. Reusing aqueous phase and solid phase products as reaction medium and catalyst carrier separately after MS HTL is feasible to realize resource utilization of MS. This information can provide valuable guidance to promote MS HTL industrialization.

Synergistic optimization of bio-oil quality and heavy metal solidification during microwave co-pyrolysis of cow dung and red mud

To decrease the environmental risks caused by heavy metals (HMs) in red mud (RM) and improve the quality of pyrolysis oil from biomass, high-temperature pretreated RM and cow dung (CD) were microwave co-pyrolyzed. Then, the optimization potential of energy consumption was evaluated and the interaction mechanism between RM and CD was explored. The results showed that the increase in transition metal oxides and specific surface area improved the microwave-absorption and catalytic capacity of the pretreated RM. By optimizing the parameters, a pretreatment temperature of 650 °C resulted in a 21.65% reduction in acid content of bio-oil, higher HMs immobilization rates (>91%) and a 7.44% reduction in energy consumption. The synergistic optimization of bio-oil quality, HMs immobilization and energy consumption was achieved. After microwave co-pyrolysis with cow dung, the larger specific surface area (92.90 m² g^-1) and higher carbon crystallinity (I_D/I_G = 1.02) of pyrolysis residues enhanced the physical adsorption to HMs. The complexation of HMs with -OH could further enhance the solidification of HMs. This work will provide support to efficient resource utilization of solid waste, and demonstrate the great potential of microwave co-pyrolysis in HMs immobilization.

Toxic metals and the risks of sludge from the treatment of wastewater from beryllium smelting

The release of highly toxic beryllium in sludge (BCS) produced by physico-chemical treatment of beryllium-containing wastewater from Be smelting production has become a growing concern with the widespread use of Be in the defense industry. This work investigated the potential mobility of Be in BCS. The toxicity characteristic leaching procedure (TCLP) of BCS showed that the amount of leached Be was up to 202 mg L^-1, which exceeded the regulated limit by nearly 10,000 times. The chemical fractionation analysis further revealed that the excessive amount of Be leached from BCS was contributed to the high content of acid-soluble fraction and reducible fraction of Be, which accounted for over 70% of the Be content. The results obtained from mineralogical automatic analyzer (MLA) showed that gypsum (23.23%) and epidote (19.55%) were the two major mineralogical phases of BCS. Both were small and loosely structured agglomerated particles with a D₅₀ of 6.61 μm and 3.31 μm. ToF-SIMS results revealed that the Be distribution on the surface of BCS particles was relatively dispersed, with no aggregation or encapsulation. Be co-precipitated with gypsum and chlorite in the form of unstable Be(OH)₂, which attached to the surface of these small particles. The unstable state of Be and the small size, loose structure and high liberation of the host material phases are the main reasons for the high leaching mobility of Be. The results of the risk assessment indicated that BCS posed an extremely high potential ecological risk, with Be being the most significant contributor.

Research progress, trends, and future prospects on hydrothermal liquefaction of algae for biocrude production: a bibliometric analysis

The rising demand to settle a sustainable energy source is guiding researchers in the production of biofuels. The liquefaction process is an alternative to obtaining biocrude from different types of renewable biomass and can mitigate environmental impacts. All papers published since 2000, which are related to the hydrothermal liquefaction process that aims to obtain biocrude are analyzed in the present study using the bibliometric approach to provide the selected database. Furthermore, the use of algae biomass in the liquefaction was also a discussed topic considering its high relevance in the process. The focus of the present study was to evaluate the evolution of the current state of the art in these topics and also to indicate trends and courses that it might be taken in the future. The database used in the bibliometric analysis was taken from the Web of Science (WoS) and the papers were selected by two different search equations. With the selected data, the use of BibExcel, VOSviewer, and PowerBi software was useful to guide the discussion and to create graphics and visual networks. As shown in the results, it was noticeable the influence of China and the USA on the field, considering the high number of publications from these countries. Moreover, the main authors were indicated considering their citation numbers, publications, and local h-index factor. Based on the author's keywords, the most significant and recent topics on liquefaction were listed. Among them, technical-economic analysis, nutrient, and energy recovery, response surface methodology, and kinetic model are highlighted. This may indicate a new direction being taken by researchers besides the operational parameters' studies.

Graphical Abstract

Supercritical water gasification of hyperaccumulators for hydrogen production and heavy metal immobilization with alkali metal catalysts

The high moisture content and heavy metal concentration of hyperaccumulator are the main bottlenecks of resource utilization. Supercritical water gasification technology was used to convert Sedum plumbizincicola (a hyperaccumulator of Zn and Cd) into hydrogen gas and to immobilize HMs into biochar. Homogeneous alkali metal catalysts such as NaOH, Na₂CO₃ and Ca(OH)₂ were added to optimize the experimental conditions. The results showed that NaOH was effective in capturing CO₂in-situ, thereby shifting the water-gas shift reaction equilibrium in the forward direction. And the increase of NaOH concentration had a significant promotion effect on hydrogen production. In the non-catalytic gasification of Sedum plumbizincicola, the highest hydrogen (1.5 mol/kg) and H₂ selectivity (22.9%) with greater carbon gasification efficiency (19.3%) and lower H₂ gasification efficiency (8.7%) of the gas products were obtained at 400 °C with 6 wt% material concentration for 20 min. However, NaOH at 5% mass fraction maximized hydrogen and H₂ selectivity up to 7.5 and 98.2%, respectively. Alkali catalyst not only promoted the generation of hydrogen-rich bio-gas but also enhanced the immobilization efficiency of heavy metals. Compared to non-catalytic, when the addition amount of NaOH was 1 wt%, the Zn、Mn、Cd、Pb、Cr accumulated in biochar increased significantly for 76.8, 42.5, 80.8, 75.6 and 80.0%, respectively. This study highlights the remarkable ability of SCWG with alkali catalyst for hydrogen production and heavy metal stabilization.

Effect of CaO and montmorillonite additive on heavy metals behavior and environmental risk during sludge combustion

Serious pollution is caused by heavy metals (HMs) emission during sludge combustion treatment, but the addition of minerals has the ability to alleviate the migration of HMs to the gaseous state. In this study, HMs (As, Cr, Zn and Cu) behavior, speciation, and environmental risk during sludge combustion with CaO and montmorillonite (MMT) additive was investigated in the lab-scale tube furnace. The results showed that the sludge combustion was mainly determined by volatile matter. In general, CaO inhibited the volatilization of Cr, Zn, and Cu, but promoted As volatilization. MMT inhibited the volatilization of HMs, but the effect was not obvious at high temperatures. Besides, the improvement of retention effect was not found for Cr and Cu with the increase of CaO at 1000 °C, there might exist threshold value for CaO on HMs retention process. Meanwhile, CaO increased acid-soluble fraction of As significantly at high temperatures, decreased residual fraction of Cr by oxidation, converted Zn and Cu to residual fraction. MMT increased the acid-soluble fraction of As and residual fraction of Cr. In view of the HMs environmental risk in ash, the combustion temperature of sludge was necessary to control under 1000 °C and minerals additive amount was needed to manage above 1000 °C.

Thermochemical conversion of heavy metal contaminated biomass: Fate of the metals and their impact on products

With the rapid depletion of fossil energy and increasingly severe environmental pollution, the development of biomass resources for biorefineries has become a new research focus. However, heavy metals may be released during the thermochemical treatment when the biomass materials used in biomass conversion are contaminated by heavy metals. This can cause secondary environmental pollution or transference to the target products, reducing product quality. Therefore, having a systematic understanding of the fate of heavy metals in biomass conversion is necessary for alleviating potential risks. This study presents the current status of contaminated biomass and conversion products involving thermochemical processes, the migration, transformation, and impact of heavy metals in biomass conversion was investigated, and the utilization of heavy metals in contaminated biomass was briefly outlined. This review aims to link biomass conversion to the fate of heavy metals, avoid existing risks as much as possible to produce cleaner products efficiently, and promote the sustainable development of heavy metal contaminated biomass resources.

Influence of persulfate on transformation of phosphorus and heavy metals for improving sewage sludge dewaterability by hydrothermal treatment

Activated persulfate oxidation has been proven to be an efficient advanced sludge treatment technique to improve sludge dewaterability. This study investigates the influence of persulfate on the transformation of phosphorus (P) and heavy metals (HMs) during the hydrothermal treatment of sewage sludge. The hydrothermal temperature, time, and persulfate concentration are optimized by a Box-Behnken design to obtain the best sludge dewaterability, which is expressed by capillary suction time (CST). The highest CST reduction efficiency is 90.5% at the optimal hydrothermal temperature, time, and concentration of persulfate, which are 145 °C, 2 h, and 150 mg/g dry sludge (DS), respectively. The distribution and transformation of P and HMs with different persulfate concentrations (100-200 mg/g DS) during the hydrothermal process are investigated. Results show that more than 90% of the P and HMs in the sludge are retained in sludge cakes after the hydrothermal treatment. The addition of SPS can make the P in the sludge cakes transform into more stable P species according to the extraction capacity of sequential extracts. It can be found from the ecological risk indexes of the HMs that the addition of SPS during the hydrothermal treatment of sludge can reduce the environmental risk of HMs. This study provides insights into the P and HM distribution and transformation during hydrothermal treatment with persulfate, providing a reference for sludge recovery strategies.

Study on the process wastewater reuse and valorisation during hydrothermal co-carbonization of food and yard waste

The commercial success of hydrothermal carbonization (HTC) is contingent on seeking solutions for the downstream wastewater (process water) generated during the process which is still regarded largely as a nuisance. In the present study, the reusability and valorization strategy of process wastewater generated during co-HTC of organic fraction of municipal solid waste (food and yard waste) at 220 °C for 1 h was established. The process wastewater was anaerobically digested in the first part to determine its methane-generating capacity; and in the second part, it was recirculated up to five times to understand the evolution of physicochemical and fuel characteristics of hydrochar. The process water was characterized by the presence of high total organic carbon (up to 40 g/L) and chemical oxygen demand (up to 96 g/L). The decreasing trend of heavy metals with increasing recirculation suggested possible adsorption/immobilization mechanism taking place to the hydrochar surface. The process water generated from co-HTC condition has anaerobic biodegradability of 72% while experimental and theoretical methane yield observed were 224 mL/g COD and 308 mL/g COD, respectively. The presence of high organic and ionic species in recirculated process water promoted the overall carbonization process which was evident from the increased energy yield (86 to 92%), carbon content (68 to 71%) and calorific value (20 to 27 MJ/kg). The recirculation also enhanced overall combustion characteristics of hydrochar as analyzed by thermogravimetric analysis. The recirculation strategy enhanced fuel properties of hydrochar while making sure upstream and downstream water related burden is reduced (as illustrated by life cycle analysis) to create a cleaner production system for renewable solid biofuels production.

Ash-to-emission pollution controls on co-combustion of textile dyeing sludge and waste tea

Given the globally increased waste stream of textile dyeing sludge (TDS), its co-combustion with agricultural residues appears as an environmentally and economically viable solution in a circular economy. This study aimed to quantify the migrations and chemical speciations of heavy metals in the bottom ashes and gas emissions of the co-combustion of TDS and waste tea (WT). The addition of WT increased the fixation rate of As from 66.70 to 83.33% and promoted the chemical speciation of As and Cd from the acid extractable state to the residue one. With the temperature rise to 1000 °C, the fixation rates of As, Cd, and Pb in the bottom ashes fell to 27.73, 8.38, and 15.40%, respectively. The chemical speciation perniciousness of Zn, Cu, Ni, Mn, Cr, Cd, and Pb declined with the increased temperature. The ash composition changed with the new appearances of NaAlSi₃O₈, CaFe₂O₄, NaFe(SO₄)₂, and MgCrO₄ at 1000 °C. The addition of WT increased CO₂ and NO_x but decreased SO₂ emissions in the range of 680-1000 °C. ANN-based joint optimization indicated that the co-combustion emitted SO₂ slightly less than did the TDS combustion. These results contribute to a better understanding of ash-to-emission pollution control for the co-combustion of TDS and WT.

Hydrothermal liquefaction accelerates the toxicity and solubility of arsenic in biowaste

Arsenic (As) is one of notorious metalloids due to its high toxicity to human beings and ecological system. Understanding its fate and speciation transformation mechanism during hydrothermal liquefaction (HTL) of microalgae is of crucial importance for the application of its HTL products. 80.0-96.7% of As in raw microalgae was migrated into the liquid phase (aqueous phase and biocrude oil) with the increase of reaction severity from 0.108 to 0.517. HPLC-ICPMS reveals that 67% of the As in microalgae accounted for As(V) with a concentration of 68.4 mg/kg. The other fractions in microalgae were primarily As(III) with a concentration of 36.3 mg/kg. Model compounds experiments illustrate that over 30% of the As(V) in feedstocks was unexpectedly converted into more soluble and toxic As (III). Hydrochar containing O-containing groups (e.g., aliphatic C-OH) was probably contribute to the reduction transformation of As(V) to higher toxic As(III). Meantime, the aqueous phase facilitated the reduction reaction via providing a reducing environment and serving as hydrogen donator. This study firstly revealed the speciation transformation of As(V) to As(III) during HTL of wastewater cultivated microalgae.

Optimizing environmental pollution controls in response to textile dyeing sludge, incineration temperature, CaO conditioner, and ash minerals

The dynamics of heavy metal speciation and flue gas emissions during the incineration of textile dyeing sludge (TDS) were quantified as a function of four addition levels of CaO, incineration temperature, and ash minerals using thermogravimetric analysis and experimental tube furnace. The TDS incineration was most improved with the addition of 10% CaO. The increased fractions of CaO coupled with the ash minerals changed the retention behaviors of eight heavy metals. The CaO addition increased the Cu, Zn, As, and Pb retentions, did not significantly change Cr, Mn, and Cd, but decreased the Ni retention. The CaO addition enhanced the speciation stability of Cu and transferred the Cr, Cd, and As speciations to the mobile fractions. The increased temperature weakened the Zn and Pb retentions and the speciation stabilities of As and Pb and turned the Cr, Mn, Ni, Cu, Zn, and Cd speciations into the stable fractions. The CaO addition inhibited HCN, NO, NO₂, COS, SO₂, CS₂, and SO₃ emissions from the TDS incineration. Neural network-based multi-response optimization was implemented to determine the optimal operational temperature for the TDS incineration and the reduction of the 12 gas emissions. The range of 640-755 °C with(out) 5% CaO appeared to be most beneficial in terms of both environmental quality and economic efficiency.

Microwave pyrolysis of oily sludge under different control modes

The effects of temperature and power on product distribution and characteristics of oily sludge (OS) pyrolysis were investigated in a microwave reactor. The maximum oil yield was 72.55 wt% at 550 °C and 71.47 wt% at 800 W, respectively. X-ray photoelectron spectroscopy (XPS) indicated that C-C and C-O were the main forms of carbon in OS char (OC). The sulfur (S) content in OC increased as the temperature/power rose, implying that S might exist in the form of inorganics or OC had S retention ability. In temperature control mode, the changes of functional groups on OC surface were more sensitive. The maximum hydrocarbon content in oil was 14.56% at 350 °C and 13.40% at 900 W, respectively. The contents of oxygenated compounds and heterocycles in oil from temperature control mode were higher. The CO yield increased with increasing temperature/power, reaching the maximum of 9.60 wt% at 650 °C and 7.75 wt% at 900 W, respectively. Compared with power control mode, it seemed that more heavy metals (HMs) were retained in OC in temperature control mode. The E_r of HMs were at the clean level and RI indicated the HMs in OC had a low environmental risk.

Hydrochar derived from municipal sludge through hydrothermal processing: A critical review on its formation, characterization, and valorization

Additional options for the sustainable treatment of municipal sludge are required due to the significant amounts of sludge, high levels of nutrients (e.g., C, N, and P), and trace constituents it contains. Hydrothermal processing of municipal sludge has recently been recognized as a promising technology to efficiently reduce waste volume, recover bioenergy, destroy organic contaminants, and eliminate pathogens. However, a considerable amount of solid residue, called hydrochar, could remain after hydrothermal treatment. This hydrochar can contain abundant amounts of energy (with a higher heating value up to 24 MJ/kg, dry basis), nutrients, and trace elements, as well as surface functional groups. The valorization of sludge-derived hydrochar can facilitate the development and application of hydrothermal technologies. This review summarizes the formation pathways from municipal sludge to hydrochar, specifically, the impact of hydrothermal conditions on reaction mechanisms and product distribution. Moreover, this study comprehensively encapsulates the described characteristics of hydrochar produced under a wide range of conditions: Yield, energy density, physicochemical properties, elemental distribution, contaminants of concern, surface functionality, and morphology. More importantly, this review compares and evaluates the current state of applications of hydrochar: Energy production, agricultural application, adsorption, heterogeneous catalysis, and nutrient recovery. Ultimately, along with the identified challenges and prospects of valorization approaches for sludge-derived hydrochar, conceptual designs of sustainable municipal sludge management are proposed.

Effects of co-pyrolysis of rice husk and sewage sludge on the bioavailability and environmental risks of Pb and Cd

In this study, biochars were produced by co-pyrolysis of rice husk and sewage sludge, the environmental risk of heavy metal (Pd and Cd) in the biochars was assessed. Co-pyrolysis resulted in a lower yield but a higher C content compared with sewage sludge pyrolysis alone, the relative contents of Pb and Cd in biochars were declined. Co-pyrolysis process transformed the bioavailable heavy metals into stable speciation. The environmental risk assessment codes of Pb and Cd were reduced by 1-2 grades. The co-pyrolysis technology provides a feasible method for the safe disposal of heavy metal-contaminated sewage sludge.

In Situ hydrochar regulates Cu fate and speciation: Insights into transformation mechanism

Cu is one of the dominant heavy metals toxic to human health and environmental ecosystems. Understanding its fate and chemical speciation is of great importance for hydrothermal liquefaction (HTL) of Cu-rich hazardous streams. Herein, we investigated its evolution during the HTL of wastewater algae through ICP-MS, XRD, XANES, and EXAFS. Cu-cysteine complexes (51.5%) and Cu₂S (40.4%) were the main components of Cu in algae, whereas the predominant form was CuS (70.9%) in 220 °C-hydrochar. Model compound experiments indicated that Cu-cysteine could be converted into CuS, while Cu₂S was stable during HTL. However, Cu₂S was partially converted into CuS in the hydrochar. Subsequently, the positive Gibbs free energy (36.8 KJ/mol) indicates that the oxidation from Cu⁺ to Cu²⁺ can't occur spontaneously. Furthermore, cyclic voltammograms demonstrated that hydrochar facilitated the oxidation of Cu₂S due to its higher capability of electron acceptance. All these results prove that hydrochar serves as a catalyst for the conversion of Cu₂S to CuS during HTL. This study firstly elucidated that Cu₂S was oxidized into CuS in the presence of hydrochar, and Cu-cysteine was converted into CuS under HTL. This study provides a critical insight into the transformation mechanism of Cu during the HTL of hazardous streams.

Transformation behaviors and environmental risk assessment of heavy metals during resource recovery from Sedum plumbizincicola via hydrothermal liquefaction

Environmentally sound disposal of hyperaccumulator harvests is of critical importance to industrialization of phytoremediation. Herein, transformation behaviors and environmental risk of heavy metals were comprehensively examined during subcritical hydrothermal liquefaction of Sedum plumbizincicola. It is concluded that low temperature liquefaction favored resource recovery of heavy oil and hydrochars in terms of higher energy density, improved carbon sequestration and less energy consumption. Heavy metals were mainly distributed into hydrochars and water soluble phase with less than 10% in heavy oil. All metal elements except As could be accumulated in hydrochars by extending reaction time, whereas more than 96% of As was redistributed into water soluble phase. Prolonged liquefaction time facilitated immobilization of Cd, Cr and As in hydrochars, but fast liquefaction favored Pb stabilization. Liquefaction significantly reduced environmental risk level of Cd, Zn and As, but may mobilize Pb and Mn, especially for Mn to very high risk level at 240 ºC. High temperature with long reaction time tended to inhibit leaching rate of Mn, whereas low liquefaction temperature with short reaction time prevented the leaching of Zn and As from hydrochars. Overall, these findings are essential for downstream upgrading of heavy oil and metals recovery from hydrochars.

Simultaneous heavy metal removal and sludge deep dewatering with Fe(II) assisted electrooxidation technology

A hybrid sludge conditioning strategy with electrooxidation and Fe(II) addition was used for heavy metal removal from sewage sludge and industrial sludge, with simultaneous sludge dewatering and stabilization. With the addition of 82 mg/g DS Fe(II) and treatment time of 4.5 h, heavy metal removals of 72.95% and 78.49% for Cu, 66.29% and 84.26% for Zn, and 36.52% and 36.99% for Pb were achieved from sewage sludge and industrial sludge samples respectively. The system pH decreased to 2.33 and 2.98 and the oxidation-reduction potential (ORP) values increased to 435.90 mV and 480.60 mV in sewage sludge and industrial sludge samples, respectively, which was conducive to the desorption and dissolution of heavy metals from sludge structures and the degradation of the organic compounds that complexed with heavy metals. In addition, the hybrid conditioning process demonstrated excellent dewatering performance due to the efficient electrochemical disintegration of sludge flocs together with the coagulation of sludge particles by Fe(III) generated via electrooxidation. The strong acidic and oxidative environment produced by the enhanced electrooxidation process was also responsible for pathogen inactivation.

The migration, transformation, and risk assessment of heavy metals in residue and bio-oil obtained by the liquefaction of pig manure

The total contents and chemical speciation analysis of Zn, Cu, Pb, Cr, Mn, Ni, Cd, and As in pig manure (PM), liquefaction residues (LRs), and bio-oils (BOs) derived from PM by liquefaction with ethanol as a solvent at 180-300 °C were thoroughly investigated in this study. The environment risk assessment, leachability, and bioavailability of heavy metals in PM and LRs were studied. The results showed that more than 75% of heavy metals remained in LRs. The total contents of heavy metals in LRs were markedly elevated, but those in BOs gradually decreased with the increase in liquefaction temperature. Moreover, the acid soluble/exchangeable fraction and reducible fraction (F1 + F2) of heavy metals in LRs and BOs was significantly reduced, while oxidizable fraction and stable fraction (F3 + F4) desirably increased after liquefaction. Furthermore, the potential risk of heavy metals in LRs was decreased in comparison to that in PM, but the risk of Pb, Mn, and As had not been obviously reduced; therefore, the LRs from the liquefaction of PM should be pretreated before recycling. Temperatures from 220 to 260 °C were the optimum conditions for disposing of PM by liquefaction with ethanol.

Co-pyrolysis of monobasic potassium phosphate and plastic processing sludge: Characteristics and environmental risks of potentially toxic elements

A high concentration of potentially toxic elements (PTEs) can be frequently observed in the plastic processing sludge (PPS), thereby restricting its environmental applications. The main objective of this study was to investigate the effects of the co-pyrolysis of PPS and KH₂PO₄ (0, 5, 10 and 20 wt%) on the characteristics and environmental risks associated with the PTEs in PPS and derived chars. General characteristic analysis revealed that the char yield, ash content, pH, and particle size of the chars prepared with KH₂PO₄ were greater than those of the char prepared without KH₂PO₄ by 3.13-4.89 wt%, 2.95-4.4 wt%, 0.77-0.93, and 9.64-30.07 µm, respectively. The results of sequential extraction indicated that co-pyrolysis with KH₂PO₄ could considerably increase the distribution of PTEs in the F4 fraction (non-bioavailable) in PPS by 1.30-65.90% when compared with that obtained via co-pyrolysis with 5 wt% of KH₂PO₄. The toxic leaching tests indicated that the leaching concentrations of Cr, Ni, Cu, Zn, Cd, and Pb in the char prepared without KH₂PO₄ decreased to different extents when PPS was subjected to co-pyrolysis with KH₂PO₄, especially in case of co-pyrolysis with 5 wt% of KH₂PO₄. The range of decrease was 26.40-88.34%. However, in case of Cu, Zn, and Pb, the leaching concentration of the chars prepared with more than 10 wt% of KH₂PO₄ increased owing to the decomposition of (Cu Zn)PbVO₄(OH) in an acidic environment. The results obtained using Hakanson's equations revealed that the potential ecological risk associated with the PTEs in chars obtained by co-pyrolysis with KH₂PO₄ decreased, with a minimum decrease of 38.17%. In addition, the risk level associated with PPS reduced from considerable to low after co-pyrolysis with KH₂PO₄. The observations of this study imply that the co-pyrolysis of PPS with KH₂PO₄ can be a promising treatment for PTE immobilization.

Hydrogen production and heavy metal immobilization using hyperaccumulators in supercritical water gasification

The dispersion of hyperaccumulators used in the phytoremediation process has caused environmental concerns because of their heavy metal (HM) richness. It is important to reduce the environmental risks and prevent the HM to reenter the ecological cycle and thereby the human food web. In this work, supercritical water gasification (SCWG) technology was used to convert Sedum plumbizincicola into hydrogen (H₂) gas and to immobilize HMs into biochar. The H₂ production correlated with temperature ranging from 380 to 440 ℃ with the highest H₂ yield of 2.74 mol/kg at 440 ℃. The free-radical reaction and steam reforming reaction at high temperatures were likely to be the mechanism behind the H₂ production. The analyses of bio-oil by the Gas Chromatography-Mass Spectrometer (GC-MS) and Nuclear magnetic resonance spectroscopy (NMR) illustrated that the aromatic compounds, oxygenated compounds, and phenols were degraded into H₂-rich gases. The increase of temperature enhanced the HM immobilization efficiency (>99.2 % immobilization), which was probably due to the quickly formed biochar that helped adsorb HMs. Then those HMs were chemically converted into stable forms through complexation with inorganic components on biochar, e.g., silicates, SiO₂, and Al₂O₃. Consequently, the SCWG process was demonstrated as a promising approach for dispersing hyperaccumulators by immobilizing the hazardous HMs into biochar and simultaneously producing value-added H₂-rich gases.

Induced changes of pyrolysis temperature on the physicochemical traits of sewage sludge and on the potential ecological risks

Biochar from sewage sludge is a low-cost sorbent that may be used for several environmental functions. This study evaluates the induced effects of pyrolysis temperature on the physicochemical characteristics of sewage sludge (SS) biochar produced at 350 (SSB₃₅₀), 450 (SSB₄₅₀) and 600 (SSB₆₀₀), based on the metal enrichment index, metal mobility index (MMI), and potential ecological risk index (PERI) of Cd, Cu, Pb, and Zn. Increased pyrolysis temperature reduced the biochar concentration of elements that are lost as volatile compounds (C, N, H, O, and S), while the concentration of stable aromatic carbon, ash, alkalinity, some macro (Ca, Mg, P₂O₅, and K₂O) and micronutrients (Cu and Zn), and toxic elements such as Pb and Cd increased. Increasing the pyrolysis temperature is also important in the transformation of metals from toxic and available forms into more stable potentially available and non-available forms. Based on the individual potential ecological risk index, Cd in the SS and SSB₄₅₀ were in the moderate and considerable contamination ranges, respectively. For all pyrolysis temperature biochar Cd was the highest metal contributor to the PERI. Despite this, the potential ecological risk index of the SS and SSBs was graded as low.

The factors influencing sludge incineration residue (SIR)-based magnesium potassium phosphate cement and the solidification/stabilization characteristics and mechanisms of heavy metals

Magnesium potassium phosphate cement (MKPC) is prepared from MgO and KH₂PO₄ through an acid-base reaction and has been widely used in the rapid repairs of building structures and the solidification/stabilization (S/S) of heavy metals (HMs). The use of sludge incineration residue (SIR) rich in phosphorus resources to prepare SIR-based MKPC can achieve the reclamation of SIR and efficient HM S/S. Herein, based on the exploration of the optimal MKPC magnesia/phosphate ratio (M/P), the effects of SIR and HMs on the performance of the matrix and its interaction mechanism were comprehensively investigated. The results indicated that the compressive strength of the SIR-based MKPC increased first and then decreased with the gradual increase of SIR incorporation; the optimal was reached at 40.31 MPa when the SIR incorporation was 5 wt%. The peak signal and crystal lattice of Pb₂(PO₄)₃ indicated that there is a mixed effect between HMs (in SIR) and KH₂PO₄. The Visual MINTEQ analysis results also indicated that HMs are precipitated as HM phosphates. The formation of HM phosphates not only increases the M/P (with 30 wt% SIR, M/P increased by 0.019), affecting the microstructure and changing the compressive strength of the matrix, but also promotes the transformation of HMs from the bioavailable to the more stable residual forms. The residual forms of the six HMs were all above 84% after S/S. Therefore, the SIR-based MKPC preparation significantly immobilized the HMs; particularly, the leaching toxicities of Cu (96.6%) and Zn (96.3%) were alleviated.

Inorganic flocculant for sludge treatment: Characterization, sludge properties, interaction mechanisms and heavy metals variations

As an industrial waste, phosphogypsum was modified to produce flocculant for sludge dewatering. In this paper, characteristics of flocculant, properties of treated sludge, and interactions of sludge and flocculant were investigated. Results suggested that after modification, flocculant showed a positive electrical property and a porous structure. Besides, larger sludge flocs formed in treated sludge showed a higher settleability and filterability. Flocculant could narrow sludge colloid network by compressing its Electrical double-layer due to the presence of CaSO₄. With potential change, the electronegative colloidal network cracked quickly and released sludge particles, active groups, unstable heavy metals and 82.91% of bound water. Moreover, porous adsorption between sludge particles and flocculant was found under molecular electrostatic potential and Van Der Waals force caused by flocculant addition. After modification, shear modulus of CaSO₄, SiO₂ and Al₂O₃ in modified phosphogypsum increased by 21%, 23% and 17%, respectively. This provided a strong skeleton support for sludge particles, which is significant to sludge dewatering. Particularly, through chelation, adsorption and rolling-sweeping process, risk level of unstable heavy metals excepting Cu in sludge filter cake was largely weakened. Immobilized rate of risky heavy metals was 23.96% (Cd_F1/F2), 39.92% (Cr_F1), 11.11% (Pb_F1/F2), 21.21% (Zn_F1), 35.49% (Ni_F1/F2), and 78.61% (As_F1/F2), respectively. Therefore, this study provided significant insight for developing efficient method to promote bound water removal from sludge, and to stabilize risky heavy metals in sludge.

Treatment of municipal sludge by hydrothermal oxidation process with H2O2

The recent increase in municipal sludge worldwide has led to a great deal of interest in developing an efficient and environmentally friendly sludge treatment method. In the paper, the treatment of municipal sludge by hydrothermal oxidation (HTO) process with H₂O₂ as the oxidant was proposed. The impacts of HTO temperature and H₂O₂ mass fraction on the distribution of products, the moisture content, the migration behaviors of the heavy metals (HMs) of the resulted solid products, the concentration of volatile fatty acids (VFAs) and NH₃-N contained in the resulted aqueous phase products and the pH value were investigated. The results indicated that the sludge reduction was achieved by HTO treatment, the increasing H₂O₂ mass fraction and HTO temperature can significantly improve the dewatering performance of the sludge. The potential toxicity fraction of Pb and Cd contained in the resulted solid residual increased with the increasing HTO severity and the potential toxicity fraction of solid residues was still lower than that of raw material. Acetic acid was the main VFAs produced from HTO treated sludge, and its concentration reached to the maximum value of 2923.41 mg/L at 230 °C under H₂O₂ mass fraction of 15%. The change in the pH of the resulted aqueous phase products was caused by the competition between the acidic (VFAs or CO₂) or alkaline (NH₃-N) substances derived from the sludge during HTO process. The HTO process was expected to be an efficient method for municipal sludge treatment due to its mild conditions and high heavy metal safety.

Effects of rice straw/wood sawdust addition on the transport/conversion behaviors of heavy metals during the liquefaction of sewage sludge

Lignocellulosic biomass has been widely introduced into the liquefaction process of sewage sludge (SS) to improve the yield/quality of liquefaction products (bio-oil/biochar). This study explores the effect of adding rice straw (RS) and wood sawdust (WS) on the transport/conversion behaviors of heavy metals (HMs) during the liquefaction of SS. The introduction of lignocellulosic biomass, especially for RS, substantially lowers the total content of HMs in biochar. Most HMs (except Cd) still remain in biochar, although the introduction of RS/WS enhances the transport of HMs into bio-oils. The addition of RS/WS raises the percentage of HMs in active form, but the contents of bioavailable/leachable HMs are not considerably increased and even decreased in some cases, especially when RS is introduced. The overall pollution degree and environmental risk of HMs in biochars are lowered to a certain extent with the addition of RS/WS. Considering that the pollution degree and environmental risk of HMs present in biochars are still at a considerable level, appropriate pollution management measures should be undertaken when using such biochars for agricultural use.

Resource recovery and waste-to-energy from wastewater sludge via thermochemical conversion technologies in support of circular economy: a comprehensive review

With the rapid rise in global population over the past decades, there has been a corresponding surge in demand for resources such as food and energy. As a consequence, the rate of waste generation and resultant pollution levels have risen drastically. Currently, most organic solid wastes are either land applied or sent to landfills, with the remaining fraction incinerated or anaerobically digested. However, with the current emphasis on the reduction of emissions, nutrient recovery, clean energy production and circular economy, it is important to revisit some of the conventional methods of treating these wastes and tap into their largely unrealized potential in terms of environmental and economic benefits. Wastewater sludge, with its high organic content and fairly constant supply, provides a great opportunity to implement some of these strategies using thermochemical conversion technologies, which are considered as one of the alternatives for upcycling such waste streams. This paper summarizes the results of prominent studies for valorizing wastewater sludge through thermochemical conversion technologies while drawing inferences and identifying relationships between different technical and operating parameters involved. This is followed by sections emphasizing the environmental and economic implications of these technologies, and their corresponding products in context of the broader fields of waste-to-energy, nutrient recycling and the progress towards a circular economy.

Energy recovery and nutrients recycling from municipal sewage sludge

Hydrothermal Liquefaction (HTL) could be a promising and better alternative to other techniques for energy recovery from municipal sewage sludge (MSS). However, the nutrients (i.e., N, and P) recovery potential from the byproducts, generated in the HTL of MSS, needs to be studied so that a comprehensive sludge management practice could be adopted. In this study, HTL process temperature (275-400 °C), and reaction time (30-120 min) were first investigated for biocrude yield and release of the nutrients to the aqueous phase liquid (APL) and biochar. The maximum energy recovery (i.e., 59%) and maximum energy return on investment (i.e., 3.5) were obtained at 350 °C and 60 min of holding time. With the increase in HTL reaction time, the concentration of nitrogen in the APL increased (5.1 to 6.8 mg/L) while the concentration of phosphorus decreased (0.89 to 0.22 mg/L); the opposite was observed for the biochar. The nutrient recycling efficiency from the APL using microalgae was found to be strain-specific; nitrogen recycling efficiency by Picochlorum sp. and Chlorella sp. were 95.4 and 58.6%, respectively. The APL, derived from 1 kg MSS, could potentially produce 0.49 kg microalgal biomass. Since the concentrations of various metals in the biochar samples were substantially lower compared to their concentrations in raw MSS, the application of biochar as a soil conditioner could be very promising. Overall, net positive energy could be recovered from MSS using the HTL process, while the nutrients in the APL could be used to cultivate specific microalgae, and biochar could be applied to enhance the soil quality.

Stabilization of heavy metals during co-pyrolysis of sewage sludge and excavated waste

In this paper, excavated waste was added to sewage sludge for co-pyrolysis, aiming to stablize the heavy metals in sewage sludge. The effect of co-pyrolysis with various pretreatment (e.g. cooling, drying and hydrothermal pretreatment) on heavy metals stabilization was studied using orthogonal test. The results showed that the optimal conditions are 600 °C, nitrogen flow rate of 200 mL/min, mixing excavated waste with sewage sludge (25:75, wt%) and hydrothermal pretreatment. 90% of the heavy metals in the sewage sludge and excavated waste mixtures were transformed to biochars after co-pyrolysis. Moreover, the state of heavy metals changed from bio-available fractions to stable state, thereby reducing the potential ecological risk index (RI) from 116.8 to below 50, which represented a reduction in contamination levels and ecological risks from considerate to low. Finally, the study found that the synergy between hydrothermal and pyrolysis made full use of the moisture in sewage sludge and was more conducive to the solidification of heavy metals. This paper provides a good option to dispose multiple wastes and reduce their environmental risks.

Risk Evaluation of Pyrolyzed Biochar from Multiple Wastes

This paper aims at demonstrating the significance of biochar risk evaluation and reviewing risk evaluation from the aspects of pyrolysis process, feedstock, and sources of hazards in biochar and their potential effects and the methods used in risk evaluation. Feedstock properties and the resultant biochar produced at different pyrolysis process influence their chemical, physical, and structural properties, which are vital in understanding the functionality of biochar. Biochar use has been linked to some risks in soil application such as biochar being toxic, facilitating GHGs emission, suppression of the effectiveness of pesticides, and effects on soil microbes. These potential risks originate from feedstock, contaminated feedstock, and pyrolysis conditions that favor the creation of characteristics and functional groups of this nature. These toxic compounds formed pose a threat to human health through the food chain. Determination of toxicity levels is a first step in the risk management of toxic biochar. Various sorption methods of biochar utilized low-cost adsorbents, engineered surface functional groups, and nZVI modified biochars. The mechanisms of organic compound removal was through sorption, enhanced sorption, modified biochar, postpyrolysis thermal air oxidation and that of PFRs degradation was through activation, photoactive functional groups, magnetization, and hydrothermal synthesis. Emissions of GHGs in soils amended with biochar emanated through physical and biotic mediated mechanisms. BCNs have a significance in reducing the health quotient indices for PTEs risk contamination by suppressing cancer risk arising from consumption of contaminated food. The degree of environmental risk assessment of HM pollution in biomass and biochars has been determined by using potential ecological risk index and RAC while organic contaminant degradation by EPFRs was considered when assessing the environmental roles of biochar in regulating the fate of contaminants removal. The magnitude of technologies’ net benefit must be considered in relation to the associated risks.

Characterisation of ashes from waste biomass power plants and phosphorus recovery

Biowastes, such as meat and bone meal (MBM), and poultry litter (PL), are used as energy sources for industrial combustion in the UK. However, the biomass ashes remaining after combustion, which contain nutrients such as phosphorus, are landfilled rather than utilised. To promote their utilisation, biomass ashes from industries were characterised in terms of their elemental and mineral compositions, phosphorus extractability, and pH-dependent leachability. These ashes were highly alkaline (pH as high as 13), and rich in calcium and phosphorus. The P bio-availabilities in the ash evaluated by Olsen's extraction were low. Hydroxyapatite and potassium sodium calcium phosphate were identified by X-ray powder diffraction (XRD) as the major phases in the MBM and PL ashes, respectively. The leaching of P, Ca, and many other elements was pH dependent, with considerable increase in leaching below about pH 6. P recovery by acid dissolution (e.g., with H₂SO₄) seems feasible and promising; the optimized acid consumption for ~90% P recovery could be as low as 3.2-5.3 mol H⁺/mol P.

How does zero valent iron activating peroxydisulfate improve the dewatering of anaerobically digested sludge?

Zero valent iron (ZVI) activating peroxydisulfate (PDS) was demonstrated to be effective in improving the dewaterability of anaerobically digested sludge (ADS). However, details of how ZVI/PDS enhances the dewaterability remain largely unknown. This work therefore aims to reveal the facts of what happen in ZVI/PDS involved ADS systems. Experimental results showed that ZVI/PDS treatment remarkably improved the dewaterability of ADS, with the minimal normalized capillary suction time of 8.6 ± 0.5 s L/g·VSS being obtained at the dosages of 2 g/g TSS ZVI and 0.5 g/g TSS PDS, which was 42.5% of that in the control. In this case, 71.2% ± 1.8% of water content (press filtration) was measured, which was 16.9% lower than that determined in the control. The mechanism investigations showed that ZVI activating PDS produced substantially reactive species, i.e., SO₄^•- and •OH, and these strong oxidative radicals decreased surface negative charges of ADS flocs, caused disruption of extracellular polymeric substances (EPS) and release of intracellular substances, and changed the secondary structure of proteins. Additionally, the products of ZVI oxidation, i.e., Fe²⁺ and Fe³⁺, were effective flocculants, thus their generation benefited the coagulation of ADS flocs through compressing double electric layers and neutralizing negative charges of sludge colloidal particles. As a result, the flocculability, hydrophobicity, and flowability of ADS were enhanced, but the bound water content, fractal dimension, and viscosity of ADS were decreased, which were responsible for the improvement of dewaterability. Further analyses exhibited that the contributions of these major contributors were different, and their contributions to the dewaterability improvement were in the order of SO₄^·- > ·OH > Fe²⁺/Fe³⁺. It was also found that ZVI/PDS treatment enhanced the degradation of recalcitrant organics, inactivation of the fecal coliforms, and mitigation of the toxicity of heavy metals in the dewatered sludge, which were beneficial to its land application.

Eisenia fetida and biochar synergistically alleviate the heavy metals content during valorization of biosolids via enhancing vermicompost quality

Impact of different biochars supplemented (10% w/w) to promote vermicomposting of sewage sludge (SS) and kitchen waste (KW) mixture (SS + KW, 70:30) was studied on the growth, reproduction and survival of earthworms, and ultimately the quality of vermicompost. Four types of biochar used as secondary material for preincubation (16 days) and vermicomposting (30 days) were: pine tree biochar (PTB), poplar plant biochar (PPB), wetland plant biochar (WPB) and yard waste biochar (YWB). Preincubation and vermicomposting of biomass mixture were undertaken in 60 L and 2 L capacity round-shaped bioreactors, respectively. Samples of biomass undergoing degradation were drawn after every 2 days during preincubation and with 5 days interval during vermicomposting to analyze them for plant nutrients and heavy metals contents. Amendment of vermicompost substrate (SS + KW) with biochars; PTB, PPB, WPB and YWB increased the reproduction rate of earthworms (Eisenia fetida) by 44.6, 53.9, 29.3 and 38.8%, respectively as compared to control (no biochar, NB). There has been significant reduction in total content of Cd (0.2-5.1%), Cr (7.3-10.8%), Cu (3.1-7.4%), Mn (3.2-8.4%), Pb (9.0-45.9%) and Zn (1.1-5.7%) by the application of different biochars as compared to NB after vermicomposting. The SEM/EDS images also reflected reduced concentration of these heavy metals in the final vermicompost as compared to initial mixtures. Progressively, biochar amendments increased the concentration of all macronutrients, viz., TN (15.8-31.0%), TP (8.6-9.9%), TK (2.8-17.3%), Ca (4.1-9.9%) and Mg (0.8-12.2%); while, reduced the pH (1.9-2.3%), content of Na (6.6-22.3%), TOC (6.6-15.4%), OM (5.0-8.2%) and C:N ratio (2.6-18.9%). Earthworm body accumulation factor (BAF) of heavy metals was: Cd > Zn > Pb > Cu > Mn > Cr at the termination stage of experiment. In conclusion, amending the SS + KW mixture with 10% (w/w) PPB for vermicomposting rendered higher count of cocoons, growth rate and reproduction rate of earthworms, which ultimately produce nutrients-rich vermicompost lower in heavy metals.

Turning sewage sludge into sintering fuel based on the pyrolysis I: lipid content and residual metal

The use of pyrolysis to produce oil from sludge by the evaporation-condensation process is a promising technique. However, the resulting lipids are prone to be acidized under exposure to oxygen, which can affect their quality and use. To eliminate the need for this oil separation process, the present work uses blended pyrolysis to preserve the oil in the char and to prevent it from deteriorating. At the same time, metals are eliminated to a secure level of combustion emissions. The sludge was pyrolyzed into a sintering fuel through blended pyrolysis with SiO₂, Al₂O₃, and sand. These materials are the main components of the sintered ceramsite obtained. Therefore, the influence of these substances and residence time on lipid formation and metal residue in the char were investigated. Non-blended pyrolysis required a 40-min duration, whereas sand-pyrolysis required 10 min to achieve the same yield. The concentration of C_16:0 produced by blended pyrolysis with sand reached 2177 mg kg^-1, which is 57% higher than that of non-blended pyrolysis. Blended pyrolysis with SiO₂ required at least 20 min to immobilize As metal. In summary, blended pyrolysis simplifies the process, reduces time, and produces char with lipid-rich and low metal leaching, which can be used as a sintering fuel.

Dewaterability enhancement and heavy metals immobilization by pig manure biochar addition during hydrothermal treatment of sewage sludge

Hydrothermal treatment (HTT) of sewage sludge (SS) with pig manure biochar (PMB) addition at 160-200 °C was conducted in this study. The effects of PMB addition on the dewaterability of SS and the speciation evolution, leaching toxicity, and potential ecological risk of heavy metals were investigated. The results showed that the solid contents of the filter cakes after adding PMB increased from 20.24%, 24.03%, and 28.69% to 21.57%, 27.69%, and 32.91% at 160, 180, and 200 °C, respectively, compared with traditional HTT of SS. Furthermore, PMB could reduce the bioavailable fractions of Cr, Ni, As, and Cd in the filter cakes obtained at 160 and 180 °C compared with the theoretical value. The leaching toxicity of heavy metals in the filter cakes after adding PMB decreased significantly at 160 and 180 °C and the potential ecological risk index (RI) declined from 62.13 and 44.83 to 55.93 and 42.11, respectively. The obtained filter cake had low potential ecological risk when used in the environment. The mechanisms on the improvement of the dewaterability and heavy metals immobilization were related that PMB acted as the skeleton builder providing the outflow path for free water and implanting heavy metals into SS structure. And the optimal results were obtained at 180 °C during HTT of SS with PMB addition. This work provides a novel and effective method for the treatment of SS.

Research on the variations of organics and heavy metals in municipal sludge with additive acetic acid and modified phosphogypsum

Concentration and fraction distribution of organics and heavy metals in municipal sludge treated by modified phosphogypsum and acetic acid (signed as MPG/HAC) were studied. The results showed that MPG/HAC conditioning significantly produce synergistic enhancement effect to dissolution of unstable heavy metals wrapped in the stable colloid network. Simultaneously, after conditioning, about 45.16% of organics such as proteins, polysaccharides and humic acid in supernatant was degraded, thus dissociating large amount of active group which accelerated immobilization of dissolved heavy metals and weaken its toxicity. In addition, MPG with a porous structure could adsorb unstable heavy metals and transform them into residual fraction, leading to a considerable decrease in their mobility risk level. Besides, linear regression models showed that a strong oxidizability of sludge, and destruction of colloidal network could greatly promote dissolution of unstable heavy metals. Simultaneously, sludge oxidizability and organics degradation rate, and disintegration of extracellular polymeric substances (EPS) layer highly accelerate immobilization of unstable metals. Excepting Cd, environmental risk of Cr, Cu, Pb, Zn, Ni and As can be effectively weakened after conditioning. Additionally, MPG/HAC conditioning might be appropriate for stabilization of Cd, Cr and Zn in water supply sludge, especially for Zn.

Influence of pyrolysis temperature on characteristics and environmental risk of heavy metals in pyrolyzed biochar made from hydrothermally treated sewage sludge

A novel approach was used to prepare sewage sludge (SS)-derived biochar via coupling of hydrothermal pretreatment with pyrolysis (HTP) process at 300-700 °C. The influence of the pyrolysis temperature on the characteristics and environmental risk of heavy metals (HMs) in biochar derived from SS were investigated. The HTP process at higher pyrolysis temperature (≥500 °C) resulting in a higher quality of SS-derived biochar and in HMs of lower toxicity and environmental risk, compared with direct SS pyrolysis. Surface characterization and micromorphology analysis indicate that the N₂ adsorption capacity and BET surface area in biochar (SRC₂₂₀-500) obtained from hydrothermally treated SS at 220 °C (SR₂₂₀) pyrolysis at 500 °C, significantly increased the BET surface area and achieved its maximum value (47.04 m²/g). Moreover, the HTP process can promote the HMs in SS be transformed from bioavailable fractions to more stable fractions. This increases with the pyrolysis temperature, resulting in a remarkable reduction in the potential environmental risk of HMs from the biochar obtained from the HTP process.

Effect of pyrolysis temperature on characteristics, chemical speciation and risk evaluation of heavy metals in biochar derived from textile dyeing sludge

Textile dyeing sludge (TDS) was pyrolyzed at temperature ranging from 300 to 700 °C to investigate characteristics and to evaluate the risk of heavy metals (Zn, Cu, Cr, Ni, Cd, and Mn) in biochar derived from the TDS. The analyzation of characteristics and potential environmental risk evaluation of heavy metals were conducted by the BET-N₂, FTIR, and BCR sequential extraction procedure. The results showed that the pyrolysis treatment of the TDS contributed to the improvement of the pH value and specific surface areas with increasing pyrolysis temperature. Conversion of the TDS to biochar significantly decreased the H/C and O/C ratios, resulting in a far stronger carbonization and a higher aromatic condensation for the TDS derived biochar. The total contents of Zn, Cu, Cr, Ni and Mn in biochar increased with pyrolysis temperature owing to the thermal decomposition of organic matter in the TDS; but for Cd, the portion distributed in the biochars decreased significantly when the temperature increased up to 600 °C. However, using BCR sequential extraction procedure and analysis, it was found that pyrolysis process promoted changes in the chemical speciation and biochar matrix characteristics, leading to reduce bio-available fractions of heavy metals in the biochars. The potential environmental risk of heavy metals decreased from considerable risk in the TDS to low risk or no risk in biochar after pyrolysis above 400 °C. This work demonstrated that the pyrolysis process was a promising method for disposing of the TDS with acceptable environment risk.

The effect of hydrolysis on combustion characteristics of sewage sludge and leaching behavior of heavy metals

The aim of this paper is to present the effect of hydrolysis treatment on the thermal reaction characteristics of sludge and the leaching test of heavy metals contained in the sludge. Raw and hydrolysis-treated sludge (at temperature of 200°C and pressure 0.4 MPa) samples were collected from local municipal sewage sludge treatment plant. Thermogravimetric analysis was carried out between 25°C and 900°C at the heating rate of 10°C/min and 20°C/min under incineration and pyrolysis atmospheres. The leaching behavior of toxic heavy metals in the sludge was studied along with ash composition. The heating rate significantly changed the thermogravimetric and differential thermogravimetric curve profiles for studied fuels under different atmospheres. After hydrolysis treatment, the heavy metals of hydrolysis-treated sludge becomes more stable. The leaching concentration of Cr, Mn, Cu and Pb in hydrolysis-treated sludge were greatly reduced, when compared to raw sludge; however, no significant change was observed in Cd concentration. The X-ray diffraction pattern of bottom ash of hydrolysis-treated sludge was significantly reduced than raw sludge. This decrease is due to possible reduction in grain growth, which degrades the crystallinity of hydrolyzed sludge, implying that the hydrolysis treatment of sewage sludge could be a promising and beneficially safe disposal technology.

Role of phosphoric acid in the bioavailability of potentially toxic elements in hydrochars produced by hydrothermal carbonisation of sewage sludge

The effect of phosphoric acid addition to the feed-water on the speciation and transformation behaviour of potentially toxic elements (PTEs) in the hydrothermal carbonisation (HTC) of sewage sludge was explored. Over 70% of each of the PTEs (As, Cd, Cr, Cu, Mn, Ni, Pb and Zn) was in the directly bioavailable and potentially bioavailable fraction in the raw sludge, and especially Cu and Zn at 97.5 and 98.6%, respectively. Through the HTC process the directly bioavailable and potentially bioavailable fractions of PTEs in the sludge hydrochar clearly decreased, and the residual fraction in the hydrochar showed an observable increase. Further stabilisation of PTEs in hydrochar occurred during HTC with the addition of phosphoric acid solution to the feed-water. As the concentration of phosphoric acid in the feed-water increased the percentages of the residual fraction of Cd, Cr, Ni, Pb and Zn in hydrochars each exceeded 80%, but different PTEs behaved differently with increasing phosphate molar ratio in the feed-water. When the molar ratio of phosphate was 15%, the percentages of the residual fractions of Cd, Mn and Zn reached their maximum values in accordance with the changing trend in aromaticity of the hydrochar. Moreover, a large number of phosphate mineral crystals effectively occluded the PTEs in hydrochar. In conclusion, the addition of phosphoric acid to the feed-water during HTC further deactivated PTEs leading to a substantial decline in the potential environmental risk associated with the land application of the sewage sludge.

Supercritical water treatment of heavy metal and arsenic metalloid-bioaccumulating-biomass

Hyperaccumulator biomass, as a promising resource for renewable energy that can be converted into valuable fuel productions with high conversion efficiency, must be considered as hazardous materials and be carefully treated before further reuse due to the high contents of heavy metals. In this study, Pteris vittata L., an As-hyperaccumulator biomass was treated by an effective and environmental friendly method-supercritical water gasification (SCWG) using a bench-scale batch reactor. The contents of heavy metals (Cd, Pb and Zn) and arsenic metalloid in solid, liquid and gaseous products during SCWG process were thoroughly investigated. The speciation fractions including exchangeable, reducible, oxidizable and residual fractions of each heavy metal as the proportion of the total contents in solid residue were presented and the transformations trend of these heavy metals during the SCWG process was especially demonstrated. The significant operating parameters, including reaction temperature (395-445 °C), pressure (21-27 MPa) and residence time (0-40 min) were varied to explore their effects on the contents and forms. Moreover, the environmental risks of heavy metals in solid residues were evaluated based on risk assessment code, taking into consideration the speciation fractions and bioavailability. It was highlighted that although heavy metals particularly Pb and Zn tended to accumulate in solid residues with a maximum increment of about 50% in the total content, they were mostly converted to more stable oxidizable and residual fractions, and thus the ecotoxicity and bioavailability were greatly mitigated with no obvious increase in direct toxicity fractions. Each tested heavy metal presented no or low risk to the environments after SCWG treatments, meaning that the environmental pollution levels were markedly reduced with no or low risk to the environment. This study highlights the remarkable ability of SCWG for the heavy metal stabilization.

Effect of ultrasound pretreatment of municipal sewage sludge on characteristics of bio-oil from hydrothermal liquefaction process

Hydrothermal liquefaction experiments with a moist (water content 80.64 wt%) municipal sewage sludge (SS) were conducted at 280, 300, 320, 340 and 360 °C for raw and ultrasound pretreated (energy input 4500 kJ/kg of solid matter) sludge. The main aim of the experiments was to explore the effects of ultrasounds on yields and composition of bio-oil and liquefaction by-products (gas and char). The bio-oils obtained were viscous, dark brown liquids with a specific odor and heating values in the range 31.39-37.35 MJ/kg. A temperature of 320 °C was found to be the optimum for the maximum yields of the SS-derived bio-oil. During the tests a significant effect of ultrasounds on bio-oil yields was proven. A positive effect of ultrasound energy input was observed at each of the liquefaction temperatures and the maximum effect (increase of 19 wt% in bio-oil yield) was achieved at 320 °C. The GC-MS analysis revealed that the bio-oils were complex mixtures of many chemical compounds belonging to different chemical classes: aliphatic hydrocarbons, fatty alcohols, N-aliphatics, phenols, aromatic esters, aldehydes, ketones, alcohols and carboxylic acids. It was found that influence of ultrasounds on the chemical composition of the bio-oils was negligible.

The Migration and Transformation of Heavy Metals in Sewage Sludge during Hydrothermal Carbonization Combined with Combustion

The migration and transformation behaviors of heavy metals (HMs), including Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb, during the hydrothermal carbonization (HTC) of sewage sludge (SS) were investigated. The immobilization of HMs during the combustion of solid residual (SR) produced from HTC of SS was also analyzed. With increasing HTC temperature and residence time, the majority of HMs (except As) accumulated in the SR. The residual rate of As in the SR decreased from 73.95% to 56.74% when the residence time was increased from 1h to 3h and reduced significantly from 73.95% to 37.48% when the temperature increased from 220°C to 280°C, implying that numerous arsenic compounds dissolved into liquid phase products. Although the HTC process has a positive influence on the transformation of HMs from weakly bound fractions to the more stable fractions, the exchangeable and reducible fractions of Mn, Zn, As, and Cd in the SR were still high. In addition, the leached amounts of Zn and As were high (14.61 and 6.16 mg/kg, respectively) and showed a high leaching risk to the environment. An increase in HTC temperature and residence time led to an increase of the residual rate of HMs in the combustion residual of SR, implying that the HTC process promotes the stabilization of HMs in the combustion process.

The migration and transformation behavior of heavy metals during co-liquefaction of municipal sewage sludge and lignocellulosic biomass

Co-liquefaction of municipal sewage sludge (MSS) and heavy metal (HM) contaminated lignocellulosic biomass (rice straw or wood sawdust) was conducted at 300 °C with ethanol as the solvent to study the transformation behavior of HMs (e.g., Cu, Cd, Pb, Cr, Zn, and Ni). The results indicate that HMs in rice straw or wood sawdust transferred heavily to bio-oils (up to 10-25% of the total Cu, Cd, and Zn) when they were liquefied individually, compared with MSS with only ∼5% distributed to bio-oil. The bio-available fraction of HMs in bio-chars and bio-oils produced from liquefaction of individual biomass were assessed to show medium to high risk to the environment. Co-liquefaction promoted the distribution of HMs to solid bio-char. Moreover, co-liquefaction benefited the immobilization of HMs in bio-chars and bio-oils. Synergistic effects were found for HMs immobilization during co-liquefaction.

Use of microalgae to recycle nutrients in aqueous phase derived from hydrothermal liquefaction process

Hydrothermal liquefaction (HTL) of microalgae biomass generates an aqueous phase (AP) byproduct with limited energy value. Recycling the AP solution as a source of nutrients for microalgae cultivation provides an opportunity for a cost-effective production of HTL based biofuel and algal biomass feedstock for HTL, allowing a closed-loop biofuel production in microalgae HTL biofuel system. This paper aims to provide a comprehensive overview of characteristics of AP and its nutrients recycling for algae production. Inhibitory effects resulted from the toxic compounds in AP and alleviation strategies are discussed.

Speciation evolution of zinc and copper during pyrolysis and hydrothermal carbonization treatments of sewage sludges

Thermal and hydrothermal treatments are promising techniques for sewage sludge management that can potentially facilitate safe waste disposal, energy recovery, and nutrient recovery/recycling. Content and speciation of heavy metals in the treatment products affect the potential environmental risks upon sludge disposal and/or application of the treatment products. Therefore, it is important to study the speciation transformation of heavy metals and the effects of treatment conditions. By combining synchrotron X-ray spectroscopy/microscopy analysis and sequential chemical extraction, this study systematically characterized the speciation of Zn and Cu in municipal sewage sludges and their chars derived from pyrolysis (a representative thermal treatment technique) and hydrothermal carbonization (HTC; a representative hydrothermal treatment technique). Spectroscopy analysis revealed enhanced sulfidation of Zn and Cu by anaerobic digestion and HTC treatments, as compared to desulfidation by pyrolysis. Overall, changes in the chemical speciation and matrix properties led to reduced mobility of Zn and Cu in the treatment products. These results provide insights into the reaction mechanisms during pyrolysis and HTC treatments of sludges and can help evaluate the environmental/health risks associated with the metals in the treatment products.

A process combining hydrothermal pretreatment, anaerobic digestion and pyrolysis for sewage sludge dewatering and co-production of biogas and biochar: Pilot-scale verification

To fully dispose of/utilize sewage sludge, a process combing hydrothermal pretreatment (HTPT), anaerobic digestion (AD) and pyrolysis was developed and tested at the pilot scale. First, the improvement in sludge dewaterability by HTPT at 180 °C for 30 min was verified, and the water content decreased from 85 to 33 wt% after filter pressing. Then, the resulting filtrate underwent continuous mesophilic (37 ± 2 °C) AD in an up-flow anaerobic sludge bed (UASB) reactor for producing biogas to compensate for the energy required for HTPT. Meanwhile, the filter cake was pyrolyzed in a rotary furnace (600 ± 50 °C) to generate biochar, and heavy metals were well immobilized in the biochar. Finally, the material/energy balance made according to the pilot data showed that the proposed process was effective for full resource reuse of sewage sludge.

Effect of hydrothermal carbonization on migration and environmental risk of heavy metals in sewage sludge during pyrolysis

The heavy metals distribution during hydrothermal carbonization (HTC) of sewage sludge, and pyrolysis of the resultant hydrochar was investigated and compared with raw sludge pyrolysis. The results showed that HTC reduced exchangeable/acid-soluble and reducible fraction of heavy metals and lowered the potential risk of heavy metals in sewage sludge. The pyrolysis favored the transformation of extracted/mobile fraction of heavy metals to residual form especially at high temperature, immobilizing heavy metals in the chars. Compared to the chars from raw sludge pyrolysis, the chars derived from hydrochar pyrolysis was more alkaline and had lower risk and less leachable heavy metals, indicating that pyrolysis imposed more positive effect on immobilization of heavy metals for the hydrochar than for sewage sludge. The present study demonstrated that HTC is a promising pretreatment prior to pyrolysis from the perspective of immobilization of heavy metals in sewage sludge.

Changes of chromium speciation and organic matter during low-temperature pyrolysis of tannery sludge

The application or disposal of char derived from tannery sludge is directly influenced by the mobility and bioavailability of Cr during pyrolysis process. This study focused on the changes of Cr speciation and organic matter in tannery sludge during low-temperature pyrolysis (100-400 °C) to evaluate the toxicity of char in terms of the leaching possibility of Cr. The results showed that (1) lower char yield and more porous structure were observed after pyrolysis. (2) Higher pyrolysis temperature increased Cr content in the char; however, Cr in this case was converted into the residual fraction which minimized its bioavailability therefore lowers its potential risk to the environment. (3) Organic matters in the acid and alkali leachates were mainly humic acid-like substance, and condensed organic matter might appear at 200 °C and then destruct. (4) Despite the comparatively high content of Cr in the char, the leaching toxicity of char was within the security range according to the national standard of China. The Cr content in the acid and alkali leachates decreased to the range of 16.5-35.3 and 0.2-6.8 mg/L, respectively. It was suggested that the potential toxicity of tannery sludge from Cr could be reduced before utilization or disposal by pyrolysis, especially under 400 °C.