Nitrogen conversion in relation to NH3 and HCN during microwave pyrolysis of sewage sludge

Fe(II) activated persulfate assisted hydrothermal conversion of sewage sludge: Focusing on nitrogen transformation mechanism and removal effectiveness

In this study, Fe(II)-activated persulfate-assisted hydrothermal treatment (Fe(II)-PS-HT) was used to improve the efficiency of removing nitrogen (N) from the sewage sludge (SS) under relatively mild conditions (i.e., at 150 °C, for 20min), and the N transformation mechanism was investigated. The total N content in the solid residue was used to evaluate the N removal efficiency. Further, the redistribution of N in the solid and liquid products was characterized and quantified to obtain a N transformation mechanism during sequential persulfate oxidation (Fe(II) and persulfate) assisted hydrothermal treatment (HT). The experimental results denote that the N removal efficiency obtained from the Fe(II)-PS-HT (persulfate/C = 0.085 and Fe(II)/persulfate = 0.5) treated SS was increased by 35.0% at a relatively mild temperature (i.e., 150 °C) when compared with that obtained by treating SS using normal HT. Elevating Fe(II)/persulfate ratio to 1.25 promoting the N removal efficiency by 59.9%-65.9%. Furthermore, the electron paramagnetic resonance (EPR) and scanning electron microscopy (SEM) results clearly denote a N removal mechanism where the sulfate radicals (SO₄^∙-) produced by Fe(II)-PS destroy the sludge structure and destructed extracellular polymers (EPS). In the absence of EPS protection, proteins were directly exposed to extreme hydrothermal circumstances, and were rapidly transformed from the SS into the liquid residue. The free radicals also provided energy for the denitrification of Heterocycle-N. Consequently, a high N removal efficiency was obtained by Fe(II)-PS-HT with persulfate/C = 0.085 and Fe(II)/persulfate = 1.25 at 150 °C for 20 min.

Nitrogen containing functional groups of biochar: An overview

Biochar is a carbonaceous material produced by thermal treatment, e.g., pyrolysis, of biomass in oxygen-deficient or oxygen-free environment. Nitrogen containing functional groups of biochar have a wide range of applications, such as adsorption of pollutants, catalysis, and energy storage. To date, many methods have been developed and used to strengthen the function of N-containing biochar to promote its application and commercialization. However, there is no review available specifically on the development of biochar technologies concerning nitrogen-containing functional groups. This paper aims to present a review on fractionation, analysis, formation, engineering, and application of N-functional groups of biochar. The effect of influencing factors on biochar N-functional groups, including biomass feedstock, pyrolysis parameters (e.g., temperature), and additional treatment (e.g., N-doping) were discussed in detail to reveal the formation mechanisms and performance of the N-functional groups. Future prospective investigation directions on the analysis and engineering of biochar N-functional groups were also proposed.

The Preparation of Biochar Particles from Sludge and Corncobs and Its Pb2+ Adsorption Properties

In the present study, biochar particles (BPs) produced by the co-pyrolysis of sewage sludge and corncobs at temperatures of 300, 500, and 700°C were characterized. The Pb²⁺ adsorption properties and the heavy metal leaching toxicity rates of the BPs were investigated. It was found that the adsorption kinetics of the Pb²⁺ can be accurately described by a pseudo-second-order model, and the equilibrium adsorption data were well represented by both the Langmuir and the Freundilich Equations. The toxicity characteristic leaching procedure (TCLP) results indicated that the leaching concentrations of all the heavy metals were below the set limit of China's national standard (Identification Standard for Hazardous Waste Extraction Toxicity Identification, China National Standard, GB 5085.3-2007). The results of this study can successfully provide scientific support for future corncob treatment and sludge pollution control.

Investigation on emission control of NOx precursors and phosphorus reclamation during pyrolysis of ferric sludge

In this study, a method to reduce the emission of NO_x precursors (e.g., hydrogen cyanide (HCN) and ammonia (NH₃)) while simultaneously reclaim more plant-available P was proposed through pyrolyzing ferric sludge (sludge conditioned by Fenton's reagents) rather than raw sludge. The nitrogen and phosphorus transformation at different pyrolysis temperatures was investigated. The results indicated that in comparison with the pyrolysis of raw sludge, the remaining iron compounds in ferric sludge can fix char-N in more stable forms (e.g., appearance of pyrrole-N at 900 °C). The secondary cracking of amine-N compounds in tar-N (e.g., 81.67% increase of amine-N at 900 °C) can be inhibited. Hence, more amine-N was remained and less heterocyclic-N and nitrile-N compounds were generated in tarN. Less generation of NH₃-N and HCN-N was also observed in NO_x precursors (e.g., 5.46% decrease of NH₃-N and 6.91% decrease of HCN-N at 900 °C). Moreover, the results of X-ray diffractometry, liquid ³¹P nuclear magnetic resonance spectroscopic, X-ray photoelectron spectroscopic, and chemical analyses collectively indicated that iron present in ferric sludge also favored reclamation of more plant-available P. In comparison with the pyrolysis of raw sludge, an increase in the total phosphorus pool was noted (18.06-36.26 versus 15.54-30.59 mg g^-1 dry solids). A decrease in mobility with the predominant P as sodium hydroxide (NaOH)-P, and an increase in plant-available P can be also obtained. This study indicated that pyrolysis of ferric sludge was a feasible way to simultaneously reduce emission of NO_x precursors, reclaim plant-available P, and reuse ferric sludge.

Thermal Transformation of Carbon and Oxygen-Containing Organic Compounds in Sewage Sludge During Pyrolysis Treatment

Organic carbon (C) and oxygen (O) contained in sewage sludge strongly impact its thermal behavior during pyrolysis treatment. This study was aimed at getting an insight into the decomposition mechanism of organic compounds containing C and O during sludge pyrolysis using thermo-gravimetric Fourier transform infrared spectroscopy (TG-FTIR) and pyrolysis-gas-chromatography/mass spectrometry (Py-GC/MS) and helpfully improving energy conversion of sewage sludge. The temperature domains of transformation were determined and indications of the main compounds produced during each stage were obtained. Results showed that the volatile compounds that evolved after sludge pyrolysis were mainly distributed into six groups: alkenes, aromatic hydrocarbons, alcohols, aldehydes, phenols and carboxylic acids. Comparison in thermal behavior and composition of the evolved volatile compounds were observed. In the low temperature stage (<350 °C), compounds containing O–C=O accounted for the highest proportion in the evolved gas (55%). Over 350 °C, the production of C=C, –OH, and –C6H5 compounds gradually increased; but little was found of compounds containing O–C=O. Above 550 °C, as thermal chemical reaction involving oxygen-containing groups enhanced, compounds containing O–C=O and –OH tended to disappear, and an increasing amount of macromolecular polycyclic aromatic hydrocarbon was formed. Finally, the thermal transformation pathways of the oxygen and carbon-containing compounds were proposed.

The Influence of Biochar and Solid Digestate on Rose-Scented Geranium (Pelargonium graveolens L’Hér.) Productivity and Essential Oil Quality

In recent years, biochar has generated global interest in the areas of sustainable agriculture and climate adaptation. The main positive effects of biochar were observed to be the most remarkable when nutrient-rich feedstock was used as the initial pyrolysis material (i.e., anaerobic digestate). In this study, the influence of solid anaerobic digestate and biochar that was produced by the slow pyrolysis of solid digestate was evaluated by comparing the differences in the crop growth performances of Pelargonium graveolens. The experiment was conducted in a greenhouse while using three different growth media (i.e., solid digestate, biochar, and vermiculite). The results indicated that: (i) the pyrolysis of solid digestate caused a reduction in the bulk density (−52%) and an increase in the pH (+16%) and electrical conductivity (+9.5%) in the derived biochar; (ii) the best crop performances (number of leaves, number of total branches, and plant dry weight) were found using biochar, particularly for plant dry weight (+11.4%) and essential oil content (+9.4%); (iii) the essential oil quality was slightly affected by the growth media; however, the main chemical components were found within the acceptable range that was set by international standard trade; and, iv) biochar induced the presence of leaf chlorosis in Pelargonium graveolens.

Kinetics, thermodynamics, gas evolution and empirical optimization of (co-)combustion performances of spent mushroom substrate and textile dyeing sludge

Spent mushroom substrate (SMS) and textile dyeing sludge (TDS) were (co-)combusted in changing heating rates, blend ratios and temperature. The increased blend ratio improved the ignition, burnout and comprehensive combustion indices. A comparison of theoretical and experimental thermogravimetric curves pointed to significant interactions between 350 and 600 °C. High content of Fe₂O₃ in TDS ash may act as catalysis at a high temperature. Ignition activation energy was lower for TDS than SMS due to its low thermal stability. 40% SMS appeared to be the optimal blend ratio that significantly decreased the activation energy, as was verified by the response surface methodology. D3 model best described the (co-)combustions. SMS led to more NO and NO₂ emissions at about 300 °C and less HCN emission than did TDS. The addition of 40% SMS to TDS lowered SO₂ emission. The co-combustion of TDS and SMS appeared to enhance energy generation and emission reduction.

The Sewage Sludge Biochar at Low Pyrolysis Temperature Had Better Improvement in Urban Soil and Turf Grass

In recent years, continuous efforts have been made to understand the impact of biochar on arable soil fertility. Little is known about whether the biochar derived from municipal sewage sludge has positive impacts on urban soil. In this study, we pyrolyzed spray-dried municipal sewage sludge at 200 °C, 300 °C, 500 °C, and 700 °C for 2 h in a muffle furnace and then amended it into an urban soil to grow turf grass in pots. The outcomes demonstrated that biochar incorporation caused remarkable increases in soil organic C, black C, total N, available P, and K by 3–8, 7–25, 2–9, 10–19, and 1.4–2 times, respectively. The dry matter of turf grass increased by 43–147%, probably due to the nutritional improvement after biochar addition. The turf grass grown in biochar-added soil had 4–70% lower heavy metals than that in the control, although the soils had much higher total heavy metals, which might imply that biochar amendment reduced the bioavailability of heavy metals. Considering the cost of biochar production and its impacts on both urban soil and grass, it would be alternative to convert the spray-dried municipal sewage sludge into biochar at 200 °C for 2 h and then used as an urban soil amendment.

Nitrogen transformation during pyrolysis of oilfield sludge with high polymer content

Oilfield sludge is recognized as a hazardous solid waste in China because of its considerable hydrocarbon content and other toxic components. In this study, the nitrogen transformation during pyrolysis of typical oil sludge samples produced in the Daqing oilfield (the largest oilfield in China), was experimentally investigated. X-ray photoelectron spectroscopy (XPS) and thermos gravimetric (TG) were used in laboratory-scale experiments and batch pyrolysis experiments were conducted at 200-800 °C in a tubular reactor to obtain quantitative information on the change of nitrogen conversion in the char, tar, and gas fractions. The thermal decomposition of amine-N compounds contributed to NH₃ release below 250 °C. However, the amine-N compounds in the char were converted to amine-N and imine-N compounds in the tar, and no obvious nitrogenous gases were found when the temperature increased from 250 to 500 °C. Between 500 and 800 °C, a large amount of NH₃ and a small portion of HCN were found owing to the secondary cracking of tar and ring opening of heterocyclic-N compounds in the char. Therefore, NH₃, HCN, and NO emissions can be reduced by accurate control of the pyrolysis temperature. A temperature of 400 °C was determined to be the optimal temperature for oilfield sludge pyrolysis, with 24.5% residual char and 43.0% tar produced. The results indicated that a suitable oilfield sludge pyrolysis temperature is helpful for reducing nitrogenous pollutant emissions, volume reduction, and energy recovery, which are beneficial for environmentally friendly production practices.

Effects of red mud on emission control of NOx precursors during sludge pyrolysis: A protein model compound study

The nitrogen-containing gases pyrolyzed from sewage sludge can be converted into NO_x compounds, which would cause severe environmental pollution. This study developed a new strategy to reduce the emission of NO_x precursors such as ammonia (NH₃) and hydrogen cyanide (HCN) using red mud. The highest reduction efficiencies (15.10% for NH₃ and 24.72% for HCN) were achieved at 900 °C while compared with those pyrolyzed from raw sludge without the addition of red mud. The transformation and distribution of nitrogenous compounds in three-phase pyrolysates were studied at 400-800 °C for pyrolysis process of a model soybean protein compound. The nitrogenous compounds, i.e., amine-N, heterocyclic-N, and nitrile-N, were identified as the three main intermediates related with the production of NO_x precursors. Ferric oxide (Fe₂O₃) and calcium oxide (CaO) presented in red mud were identified as the driving force which facilitated nitrogen stabilization in char (e.g., at 800 °C, 21.63% increase of char-N after addition of Fe₂O₃, and 41.54% increase of char-N after addition of CaO). These metal oxides possibly reacted with protein-N to form Fe_xN and CaC_xN_y, inhibited the secondary cracking of amine-N compounds in tar (e.g., at 800 °C, 2.33% increase of amine-N after addition of Fe₂O₃, and 0.38% increase of amine-N after addition of CaO), and reduced the production of nitrile-N (e.g., at 800 °C, 30.41% reduction of nitrile-N after addition of Fe₂O₃, and 27.40% reduction of nitrile-N after addition of CaO) and heterocyclic-N compounds (e.g., at 800 °C, 21.60% reduction of heterocyclic-N after addition of Fe₂O₃, and 13.98% reduction of heterocyclic-N after addition of CaO). Hence, the emission of NH₃ and HCN in gas phase can be controlled. Moreover, Fe₂O₃ showed better capability in controlling the emission of NO_x precursors than CaO (higher reduction of NH₃-N and higher reduction of HCN-N). These results indicate that red mud is an efficient catalyst to reduce emission of NO_x precursors through controlling intermediates at 400-800 °C.

Heavy metal leaching and distribution in glass products from the co-melting treatment of electroplating sludge and MSWI fly ash

Melting is a common solidification treatment that concentrates and encapsulates heavy metals into a glass matrix for waste containing heavy metals (Chae et al., 2016). To control the risk of heavy metal leaching into the glass product, a reduction in the amount of heavy metal was achieved in a pilot-scale furnace by co-melting electroplating sludge (EPS) and municipal solid waste incineration fly ash (MSWI FA). Through the melting process, the chloride from MSWI FA led to heavy metals volatilization in the form of chlorine salts. The fly ash additionally increased heavy metals volatilization by 4%-91%. The highest volatilization ratios of Zn, Pb, Cu, Cd, Cr and Ni were 33%, 96%, 33%, 79%, 81% and 31%, respectively. The concentrations of Pb and Zn in the secondary fly ash were close to the Pb and Zn concentrations in lead-zinc ore that are required in smelting industry. Moreover, glass sand was produced after the melting treatment. With an increase in the fly ash mixing ratio, the leaching concentration of Zn and Cu decreased to 3.8 mg/L and 2.1 mg/L. The leaching concentrations of other heavy metals stayed below 1 mg/L in all cases. When the ratio of MSWI FA reached 10%, the glass sand contained the least amount of impurities and a large amount of phosphate and silicate, which were probably responsible for the stability of the heavy metals. Therefore, our results provided a promising approach to the stability of the waste by the recovery of heavy metals in the co-treatment of heavy metal-bearing wastes.

Co-pyrolysis of microalgae and plastic: Characteristics and interaction effects

To improve the quality of the oil produced from microalgae, the co-pyrolysis of low-density polyethylene (LDPE) and Nannochloropsis sp. (NS) in a fixed bed reactor was investigated at different mixing ratios. Co-pyrolysis improved the gas yield, and the lower heating value of the gas products increased obviously with an increase in the LDPE amount. Furthermore, co-pyrolysis promoted the generation of CH₄ and C₂₊, especially C₂H₄, with the maximum C₂₊ yield (84.86 mL/g) obtained with 75% LDPE. Meanwhile, the amounts of oxygenous and nitrogenous compounds in the liquid products decreased rapidly with LDPE addition. The aliphatic hydrocarbon content of the liquid products increased from 22.63% for NS pyrolysis to 77.4% with 25% LDPE. During co-pyrolysis with LDPE, O tended to evolve as H₂O and CO (rather than as CO₂ for NS pyrolysis) and N was more likely to be released into gas products, which enhanced the quality of the pyrolysis oil.

Mechanistic study on NO reduction by sludge reburning in a pilot scale cement precalciner with different CO2 concentrations

The reducing gases of CO and NH₃ produced by sludge reburning make a major contribution to NO reduction.

Nitrogenous compounds produced by catalytic pyrolysis of cyanobacteria over metal loaded MCM-41 with vaporized methanol

The synergistic effect of methanol and an acidic catalyst promotes the production of nitriles and the enrichment of pentadecanenitrile.

A review on the production of nitrogen-containing compounds from microalgal biomass via pyrolysis

Nitrogen-containing compounds (NCCs) which may be produced from nitrogen-rich biomass such as microalgae, may find important biochemical and biomedical applications. This review summarizes the recent knowledge about the formation mechanism of NCCs during pyrolysis of microalgae. The key technical and biological aspects of microalgae and pyrolysis process parameters, which influence the formation of NCCs, have been analyzed. The mechanism of formation of NCCs such as indole, pyridine, amides, and nitriles during primary and secondary pyrolysis reactions are elaborated. It has been emphasized that the pyrolysis conditions and the use of catalysts had significant impacts on the yields and compositions of NCCs. The available information shows that the transformation of nitrogen and nitrogen functionalities during pyrolysis are strongly associated with the formation process of NCCs. The challenges in the development of pyrolysis technologies for the production of NCCs from microalgae are identified with future research needs identified.

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.

Influence of Biochar Addition on Nitrogen Transformation during Copyrolysis of Algae and Lignocellulosic Biomass

Algae are extremely promising sustainable feedstock for fuels and chemicals, while they contain high nitrogen content, which may cause some serious nitrogen emission during algae pyrolysis utilization. In this study, we proposed a feasible method to control the nitrogen emission during algae pyrolysis by introducing lignocellulosic biomass and biochar addition. Nitrogen transformation mechanism was investigated through quantitative analysis of N-species in the pyrolysis products. Results showed that copyrolysis of algae and lignocellulosic biomass greatly increased nitrogen in solid char with large amount of NH₃ and HCN releasing (∼20 wt %), while nitrogen in bio-oil decreased largely. With biochar addition, NH₃, HCN, and N-containing intermediates (amines/amides and nitriles) reacted with higher active O-species (O-C═O, -OH, and -COOH) in biochar addition, and formed large amounts of amine/amide-N, pyridinic-N, pyrrolic-N, and quaternary-N on the surface of biochar addition, which led to most nitrogen being enriched in char product and biochar addition (over 75 wt %) at the expense of amines/amides, nitriles, and N-containing gas (only 3 wt % NH₃ and HCN emission; decrease of 85%). These results demonstrated that biochar addition showed a positive influence on fixation of N-species during thermochemical conversion of algae and could convert nitrogen to valuable N-doped biochar materials.

The transformation of nitrogen during pressurized entrained-flow pyrolysis of Chlorella vulgaris

The transformation of nitrogen in microalgae during entrained-flow pyrolysis of Chlorella vulgaris was systematically investigated at the temperatures of 600-900 °C and pressures of 0.1-4.0 MPa. It was found that pressure had a profound impact on the transformation of nitrogen during pyrolysis. The nitrogen retention in bio-char and its content in bio-oil reached a maximum value at 1.0 MPa. The highest conversion of nitrogen (50.25 wt%) into bio-oil was achieved at 1.0 MPa and 800 °C, which was about 7 wt% higher than that at atmospheric pressure. Higher pressures promoted the formation of pyrrolic-N (N-5) and quaternary-N (N-Q) compounds in bio-oil at the expense of nitrile-N and pyridinic-N (N-6) compounds. The X-Ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) results on bio-chars clearly evidenced the transformation of N-5 structures into N-6 and N-Q structures at elevated pressures. The nitrogen transformation pathways during pyrolysis of microalgae were proposed and discussed.

Denitrification and desulphurization of industrial biowastes via hydrothermal modification

In attempt to decrease NO_X and SO₂ emission from thermochemical utilization, three industrial biowastes (penicillin mycelia waste, sewage sludge and peat waste) contained high nitrogen (N) and sulfur (S) were chosen to investigate the denitrification and desulphurization of hydrothermal modification. The results demonstrated that hydrothermal modification destroyed the structure of N- and S-containing components, thereby altering their existed conformations. Inorganic-N (N-IN) and most of amino-N/polyamide-N (N-A) were enriched by liquid phase in the forms of NH₄⁺-N and soluble organic-N (Org-N), respectively; subsequently, Org-N could further decompose to NH₄⁺-N at higher temperature. Residual N in hydrochars was converted from N-A to heterocyclic-N (pyrrolic-N, pyridinic-N and quaternary-N) via hydrolysis and cyclization. Similarly, over 60% of S was remove form biowastes at 240 °C. In solid phase, part of organic-S was altered to thiophenes-S after modified, while the remainder was transformed to inorganic-S; but the variation of inorganic-S in hydrochars strongly affected by its specific species.

Co-pyrolysis characteristics and kinetic analysis of organic food waste and plastic

In this work, typical organic food waste (soybean protein (SP)) and typical chlorine enriched plastic waste (polyvinyl chloride (PVC)) were chosen as principal MSW components and their interaction during co-pyrolysis was investigated. Results indicate that the interaction accelerated the reaction during co-pyrolysis. The activation energies needed were 2-13% lower for the decomposition of mixture compared with linear calculation while the maximum reaction rates were 12-16% higher than calculation. In the fixed-bed experiments, interaction was observed to reduce the yield of tar by 2-69% and promote the yield of char by 13-39% compared with linear calculation. In addition, 2-6 times more heavy components and 61-93% less nitrogen-containing components were formed for tar derived from mixtures.

Influence of temperature on nitrogen fate during hydrothermal carbonization of food waste

The influence of temperature (180-260°C) on the fate of nitrogen during hydrothermal carbonization (HTC) of food waste (FW) was assessed. The distribution and evolution of nitrogen in aqueous products and bio-oil, as well as hydrochar, were conducted. Results suggested that elevated temperature enhanced the deamination and the highest ammonium concentration (929.75mg/L) was acquired at 260°C. At temperatures above 220°C, the total N in the hydrochar became stable, whereas the mass percentage of N increased. Amines and heterocyclic-N compounds from protein cracking and Maillard reactions were identified as the main nitrogen-containing compounds in the bio-oil. As to the hydrochar, increasing temperature resulted in condensed nitrogen-containing aromatic heterocycles (e.g. pyridine-N and quaternary-N). In particular, remarkable Maillard reactions at 180°C and the highest temperature at 260°C enhanced nitrogen incorporation (i.e. quaternary-N) into hydrochar.

The thermochemical conversion of non-lignocellulosic biomass to form biochar: A review on characterizations and mechanism elucidation

Biochar obtained from non-lignocellulosic biomass (NLBM) has attracted wide interests in various fields like pollutants removal, catalysis, and energy storage. However, the thermochemical conversion processes from NLBM to non-lignocellulosic biochar (NLBC) have not been well summarized until now. To fill the knowledge gap, this review presents a systematical summary of NLBM characteristics, thermochemical behaviors of main components (e.g., C, O, N, P and metals), characterization methods for NLBC and conversion process, and the main applications of NLBC. Moreover, the vacancy and limitations of the current researches are pointed out to provide some guidance for future study. This review would contribute to deepen the understanding of NLBC, meanwhile optimize the efficient disposal and value-added utilization of NLBM wastes via thermochemical conversion.

The transformation pathways of nitrogen in sewage sludge during hydrothermal treatment

Hydrothermal treatment (HT) has been proved as a significant pretreatment in decreasing emissions of NO_X pollutants from thermochemical utilization of sewage sludge (SS) derived solid fuel. This study aims to investigate the denitrification of HT and the redistribution of nitrogen (N) in different products so as to speculate the comprehensive pathway of N transformation during hydrothermal process. Results found that only 20% of N remained in hydrochar, whereas the rest of N (nearly 80%) was transformed into other phase. A majority of amino-N in SS was enriched in liquid phase in the form of Org-N at first, then further decomposed to NH₄⁺-N. The remaining amino-N converted to pyrrole-N, pyridine-N and quaternary-N as temperature progresses. Meanwhile, amine-N derived from protein-N formed heterocyclic-N in oil phase via Diels-Alder reaction. NH₃, the major nitrogenous gas, was dissolved in liquid as NH₄⁺-N immediately after producing, but increased with prolonged reaction time.

Activation of peroxymonosulfate by nitrogen-functionalized sludge carbon for efficient degradation of organic pollutants in water

Nitrogen-functionalized sludge carbon (NSC) was prepared by urea-mediated pyrolysis of sewage sludge (SS) and was introduced, for the first time, as a potential metal-free catalyst to activate peroxymonosulfate (PMS) for oxidative removal of organic pollutants in water. The nitrogen functionalization of NSC catalysts significantly affected the chemical micro-environments as well as microstructures (morphology and porosity), improving the PMS activation activity towards removing various pollutants, e.g., acid orange 7, phenol and rhodamine B. On the basis of quenching studies and electron paramagnetic resonance, the formed dominant reactive oxidative species (ROS) in the NSC/PMS system was clarified to be nonradical singlet oxygen, in addition to the typical radical ROSs, sulfate and hydroxyl radicals. The incorporated pyridine N, graphite N and pristine CO in the NSC framework promoted the generation of ROS. This study provided new insights into environmentally friendly resourcing SS and exploiting novel cost-effective metal-free catalyst for PMS activation.

Demethanation Trend of Hydrochar Induced by Organic Solvent Washing and Its Influence on Hydrochar Activation

Hydrochar derived from hydrothermal carbonization (HTC) has been recognized as a promising carbonaceous material for environmental remediation. Organic solvents are widely used to extract bio-oil from hydrochar after HTC, but the effects of solvent extraction on hydrochar characteristics have not been investigated. This study comprehensively analyzed the effects of different washing times and solvent types on the hydrochar properties. The results indicate that the mass loss of hydrochar by tetrahydrofuran washing occurred mainly in the first 90 min, and the loss ratios of elements followed a descending order of H > C > O, resulting in a decrease in the H/C atomic ratio and an increase in the O/C atomic ratio. The use of various solvents for washing brought about hydrochar loss ratios in a descending order of petroleum ether < dichloromethane < acetone < tetrahydrofuran. The results from the Van Krevelen diagram and Fourier transform infrared, ¹³C nuclear magnetic resonance, and X-ray photoelectron spectroscopies further confirmed that demethanation controlled this washing process. Most importantly, the surface area of hydrochar increased after bio-oil removal via washing, which promoted the surface area development for hydrochar-derived magnetic carbon composites, but to some extent decreased the microporosity. Additionally, hydrochar washing by organic solvent has important implications for the global carbon cycle and its remediation application.

Autocatalytic Pyrolysis of Wastewater Biosolids for Product Upgrading

The main goals for sustainable water resource recovery include maximizing energy generation, minimizing adverse environmental impacts, and recovering beneficial resources. Wastewater biosolids pyrolysis is a promising technology that could help facilities reach these goals because it produces biochar that is a valuable soil amendment as well as bio-oil and pyrolysis gas (py-gas) that can be used for energy. The raw bio-oil, however, is corrosive; therefore, employing it as fuel is challenging using standard equipment. A novel pyrolysis process using wastewater biosolids-derived biochar (WB-biochar) as a catalyst was investigated to decrease bio-oil and increase py-gas yield for easier energy recovery. WB-biochar catalyst increased the py-gas yield nearly 2-fold, while decreasing bio-oil production. The catalyzed bio-oil also contained fewer constituents based on GC-MS and GC-FID analyses. The energy shifted from bio-oil to py-gas, indicating the potential for easier on-site energy recovery using the relatively clean py-gas. The metals contained in wastewater biosolids played an important role in upgrading pyrolysis products. The Ca and Fe in WB-biochar reduced bio-oil yield and increased py-gas yield. The py-gas energy increase may be especially useful at water resource recovery facilities that already combust anaerobic digester biogas for energy since it may be possible to blend biogas and py-gas for combined use.

Removal behaviors and mechanisms of hexavalent chromium from aqueous solution by cephalosporin residue and derived chars

Cephalosporin residue (CR) was used to produce biochar (BC) and activated carbon (AC) at 600°C (BC600 and AC600). To compare the removal behaviors and mechanisms of Cr(VI) by CR and derived chars, batch adsorption tests were performed in Cr(VI) microenvironment like pH, Cr(VI) concentration, adsorbent dosage, combing with the characterization of adsorbents before and after adsorption. Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and Brunauer-Emmett-Teller (BET) techniques were used. Results showed that the Cr(VI) removals by CR and CR-chars fitted Freundlich and Langmuir models. Based on the Langmuir model, the maximum adsorption capacities of CR, BC600 and AC600 towards Cr(VI) were 107.41, 88.19 and 74.07mgg^-1, respectively. The CR rich in dissolved carbon (DOC), -NH₂ and -COOH, chiefly acted as chelating and reducing agents, while the AC600 with high surface area mainly supported Cr(VI) adsorption during Cr(VI) removal process.

Investigation on the removal of H2S from microwave pyrolysis of sewage sludge by an integrated two-stage system

In this study, an integrated two-stage system, including the in-situ catalytic microwave pyrolysis (ICMP) and subsequent catalytic wet oxidation (CWO) processes, was proposed to remove H₂S released from microwave-induced pyrolysis of sewage sludge. The emission profile and H₂S removal from the pyrolysis of raw sewage sludge (SS) and sewage sludge spiked with conditioner CaO (SS-CaO) were investigated. The results showed that CaO played a positive role on sulfur fixation during the pyrolysis process. It was found that SS-CaO (10 wt.%) contributed to about 35% of H₂S removal at the first stage (ICMP process). Additionally, the CWO process was demonstrated to have promising potential for posttreatment of remaining H₂S gas. At the Fe³⁺ concentration of 30 g/L, the maximum H₂S removal efficiency of 94.8% was obtained for a single Fe³⁺/Cu²⁺ solution. Finally, at the pyrolysis temperature of 800 °C, 99.7% of H₂S was eliminated by the integrated two-stage system meeting the discharge standard of China. Therefore, the integrated two-stage system of ICMP + CWO may provide a promising strategy to remove H₂S dramatically for the biomass pyrolysis industry.

Heteroatoms doped porous carbon derived from hydrothermally treated sewage sludge: Structural characterization and environmental application

The heteroatoms (N and S) doped porous carbons (HAPCs) were prepared from sewage sludge by hydrothermal carbonization and chemical activation for the first time. The porous structures and surface properties of HAPCs were characterized by multiple techniques including SEM-EDS, TEM, BET, XRD, Raman spectroscopy and Boehm's titration. The resultant materials were showed to be naturally N and S dual-doped porous carbons (HAPCs), especially for HAPC_HCl+HF obtained by HCl-HF-washing, which was typical 3D hierarchically porous structure with abundant mesopores as well as big pore diameter. Then the performance of HAPC_HCl+HF on AO7 removal was determined through Response surface methodology. The results showed the adsorption behavior obeyed Langmuir isotherm model and the maximum adsorption capacity was up to 440.53 mg g^-1 at 25 °C. Kinetics study revealed that the adsorption followed pseudo second-order kinetic and intra-particle diffusion was the main control step. The high removal rate of AO7 was ascribed to the unique properties of HAPC_HCl+HF. The great V_mes and big pore diameter facilitated the diffusion of AO7 into the intra surface of particle. Meanwhile, the basic groups and doping of N and S made HAPC_HCl+HF surface had positive charges, then strong π-π stacking interaction and electrostatic attraction contributed to the highly effective adsorption. This study indicated hydrothermal carbonization coupled with chemical activation was a cost-effective approach to prepare efficient heteroatoms doped porous carbon from sewage sludge towards azo dye contaminated wastewater treatment.

Transformation of Nitrogen and Evolution of N-Containing Species during Algae Pyrolysis

Transformation and evolution mechanisms of nitrogen during algae pyrolysis were investigated in depth with exploration of N-containing products under variant temperature. Results indicated nitrogen in algae is mainly in the form of protein-N (∼90%) with some inorganic-N. At 400-600 °C, protein-N in algae cracked first with algae pyrolysis and formed pyridinic-N, pyrrolic-N, and quaternary-N in char. The content of protein-N decreased significantly, while that of pyrrolic-N and quaternary-N increased gradually with temperature increasing. Pyridinic-N and pyrrolic-N formation was due to deamination or dehydrogenation of amino acids; subsequently, some pyridinic-N converted to quaternary-N. Increasing temperature decreased amides content greatly while increased that of nitriles and N-heterocyclic compounds (pyridines, pyrroles, and indoles) in bio-oil. Amides were formed through NH₃ reacting with fatty acids, that underwent dehydration to form nitriles. Besides, NH₃ and HCN yields increased gradually. NH₃ resulted from ammonia-N, labile amino acids and amides decomposition, while HCN came from nitrile decomposition. At 700-800 °C, evolution trend of N-containing products was similar to that at 400-600 °C. While N-heterocyclic compounds in bio-oil mainly came from pyrifinic-N, pyrrolic-N, and quaternary-N decomposition. Moreover, cracking of pyridinic-N and pyrrolic-N produced HCN and NH₃. A mechanism of nitrogen transformation during algae pyrolysis is proposed based on amino acids decomposition.

Release of hydrogen sulfide during microwave pyrolysis of sewage sludge: Effect of operating parameters and mechanism

The effects of sludge characteristics, pyrolysis temperature, heating rate and catalysts on the release of H₂S and mechanism of H₂S formation during sludge pyrolysis were investigated in a microwave heating reactor (MHR). The evolution of sulfur-containing compounds in the pyrolysis chars obtained at temperature range of 400-800°C was characterized by XPS. For a given temperature, the maximum concentration of H₂S appeared at moisture content of 80%. Compared to the influence of heating rate on the H₂S yields, pyrolysis temperature and catalyst played a more significant role on the release of H₂S during microwave pyrolysis process. The H₂S concentration increased with increasing temperature from 400°C to 800°C while decreased with increasing heating rate. Both the Nickel-based catalyst and Dolomite displayed significant desulfurization effect and Ni-based catalyst exhibited the larger desulfurization capability than that of Dolomite. The organic sulfur compounds accounted for about 60% of the total sulfur in the sludge which was the main reason for the formation of H₂S. The mechanism analysis indicated that the cleavage reactions of mercaptan and aromatic-S compounds at temperatures below 600°C and the cracking reaction of sulfate above 700°C respectively were responsible for the H₂S release during sludge pyrolysis.

Microwaving human faecal sludge as a viable sanitation technology option for treatment and value recovery - A critical review

The prolonged challenges and terrible consequences of poor sanitation, especially in developing economies, call for the exploration of new sustainable sanitation technologies. Such technologies must be: capable of effectively treating human faecal wastes without any health or environmental impacts; scalable to address rapid increases in population and urbanization; capable of meeting environmental regulations and standards for faecal management; and competitive with existing strategies. Further and importantly, despite its noxiousness and pathogenic load, the chemical composition of human faecal sludge indicates that it could be considered a potentially valuable, nutrient-rich renewable resource, rather than a problematic waste product. New approaches to faecal sludge management must consequently seek to incorporate a 'valuable resource recovery' approach, compatible with stringent treatment requirements. This review intends to advance the understanding of human faecal sludge as a sustainable organic-rich resource that is typically high in moisture (up to 97 per cent), making it a suitable candidate for dielectric heating, i.e. microwave irradiation, to promote faecal treatment, while also recovering value-added products such as ammonia liquor concentrate (suitable for fertilizers) and chars (suitable for fuel) - which can provide an economic base to sustain the technology. Additionally, microwaving human faecal sludge represents a thermally effective approach that can destroy pathogens, eradicate the foul odour associated human faecal sludge, while also preventing hazardous product formations and/or emissions, aside from other benefits such as improved dewaterability and heavy metals recovery. Key technological parameters crucial for scaling the technology as a complementary solution to the challenges of onsite sanitation are also discussed.

Pollutant emissions from the pyrolysis and combustion of viscoelastic memory foam

Thermal degradation of viscoelastic memory foam (VMF) in a horizontal laboratory scale reactor has been studied. Pyrolysis and combustion experiments under sub-stoichiometric conditions were performed at four different temperatures (550°C, 650°C, 750°C and 850°C) for the determination of pollutants. Analyses of gas and semivolatile compounds, including polychlorodibenzo-p-dioxins and furans (PCDD/Fs) and dioxin-like polychlorobiphenyls (dl-PCBs) are shown. From the results, it was deduced that pyrolytic conditions favor the formation of PAHs, methane, ethylene, NH₃ and dl-PCBs, whereas the presence of oxygen involves a higher emission of PCDD/Fs and simple N-containing compounds such as NO and HCN. The toxic levels calculated for PAHs, PCDD/Fs and dl-PCBs in all cases were low confirming that the incineration of VMF mattress waste could be a good option for waste management. Nevertheless, relatively high emissions of NO, NH₃ and HCN were obtained and their reduction must be considered.

Sulfur Transformation during Microwave and Conventional Pyrolysis of Sewage Sludge

The sulfur distributions and evolution of sulfur-containing compounds in the char, tar and gas fractions were investigated during the microwave and conventional pyrolysis of sewage sludge. Increased accumulation of sulfur in the char and less production of H₂S were obtained from microwave pyrolysis at higher temperatures (500-800 °C). Three similar conversion pathways were identified for the formation of H₂S during microwave and conventional pyrolysis. The cracking of unstable mercaptan structure in the sludge contributed to the release of H₂S below 300 °C. The decomposition of aliphatic-S compounds in the tars led to the formation of H₂S (300-500 °C). The thermal decomposition of aromatic-S compounds in the tars generated H₂S from 500 to 800 °C. However, the secondary decomposition of thiophene-S compounds took place only in conventional pyrolysis above 700 °C. Comparing the H₂S contributions from microwave and conventional pyrolysis, the significant increase of H₂S yields in conventional pyrolysis was mainly attributed to the decomposition of aromatic-S (increasing by 10.4%) and thiophene-S compounds (11.3%). Further investigation on the inhibition mechanism of H₂S formation during microwave pyrolysis confirmed that, with the special heating characteristics and relative shorter residence time, microwave pyrolysis promoted the retention of H₂S on CaO and inhibited the secondary cracking of thiophene-S compounds at higher temperatures.

Recovery of agricultural nutrients from biorefineries

This review lays the foundation for why nutrient recovery must be a key consideration in design and operation of biorefineries and comprehensively reviews technologies that can be used to recover an array of nitrogen, phosphorus, and/or potassium-rich products of relevance to agricultural applications. Recovery of these products using combinations of physical, chemical, and biological operations will promote sustainability at biorefineries by converting low-value biomass (particularly waste material) into a portfolio of higher-value products. These products can include a natural partnering of traditional biorefinery outputs such as biofuels and chemicals together with nutrient-rich fertilizers. Nutrient recovery not only adds an additional marketable biorefinery product, but also avoids the negative consequences of eutrophication, and helps to close anthropogenic nutrient cycles, thereby providing an alternative to current unsustainable approaches to fertilizer production, which are energy-intensive and reliant on nonrenewable natural resource extraction.

Disintegration of Waste Activated Sludge by Thermally-Activated Persulfates for Enhanced Dewaterability

Oxidation by persulfates at elevated temperatures (thermally activated persulfates) disintegrates bacterial cells and extracellular polymeric substances (EPS) composing waste-activated sludge (WAS), facilitating the subsequent sludge dewatering. The WAS disintegration process by thermally activated persulfates exhibited different behaviors depending on the types of persulfates employed, that is, peroxymonosulfate (PMS) versus peroxydisulfate (PDS). The decomposition of PMS in WAS proceeded via a two-phase reaction, an instantaneous decomposition by the direct reaction with the WAS components followed by a gradual thermal decay. During the PMS treatment, the WAS filterability (measured by capillary suction time) increased in the initial stage but rapidly stagnated and even decreased as the reaction proceeded. In contrast, the decomposition of PDS exhibited pseudo first-order decay during the entire reaction, resulting in the greater and steadier increase in the WAS filterability compared to the case of PMS. The treatment by PMS produced a high portion of true colloidal solids (<1 μm) and eluted soluble and bound EPS, which is detrimental to the WAS filterability. However, the observations regarding the dissolved organic carbon, ammonium ions, and volatile suspended solids collectively indicated that the treatment by PMS more effectively disintegrated WAS compared to PDS, leading to higher weight (or volume) reduction by postcentrifugation.

Tracking the conversion of nitrogen during pyrolysis of antibiotic mycelial fermentation residues using XPS and TG-FTIR-MS technology

Antibiotic mycelial fermentation residues (AMFRs), which are emerging solid pollutants, have been recognized as hazardous waste in China since 2008. Nitrogen (N), which is an environmental sensitivity element, is largely retained in AMFR samples derived from fermentation substrates. Pyrolysis is a promising technology for the treatment of solid waste. However, the outcomes of N element during the pyrolysis of AMFRs are still unknown. In this study, the conversion of N element during the pyrolysis of AMFRs was tracked using XPS (X-ray photoelectron spectroscopy) and online TG-FTIR-MS (Thermogravimetry-Fourier transform infrared-Mass spectrometry) technology. In the AMFR sample, organic amine-N, pyrrolic-N, protein-N, pyridinic-N, was the main N-containing species. XPS results indicated that pyrrolic-N and pyridinic-N were retained in the AMFR-derived pyrolysis char. More stable species, such as N-oxide and quaternary-N, were also produced in the char. TG-FTIR-MS results indicated that NH3 and HCN were the main gaseous species, and their contents were closely related to the contents of amine-N and protein-N, and pyrrolic-N and pyridinic-N of AMFRs, respectively. Increases in heating rate enhanced the amounts of NH3 and HCN, but had less of an effect on the degradation degree of AMFRs. N-containing organic compounds, including amine-N, nitrile-N and heterocyclic-N, were discerned from the AMFR pyrolysis process. Their release range was extended with increasing of heating rate and carbon content of AMFR sample. This work will help to take appropriate measure to reduce secondary pollution from the treatment of AMFRs.

Release and control of hydrogen sulfide during sludge thermal drying

The release of hydrogen sulfide (H2S) during sludge drying is a major environmental problem because of its toxicity to human health. A series of experiments were performed to investigate the mechanisms and factors controlling the H2S release. Results of this study show that: (1) the biomass and activity of sulfate-reducing bacteria (SRB) in sludge were the major factors controlling the amount of H2S release, (2) the sludge drying temperature had an important effect on both the extent and the timing of H2S release from the sludge, and (3) decreasing sludge pH increased the H2S release. Based on the findings from this study, a new system that integrates sludge drying and H2S gas treatment was developed, by which 97.5% of H2S and 99.7% of smoke released from sludge treatments was eliminated.

Efficient selective catalytic reduction of NO by novel carbon-doped metal catalysts made from electroplating sludge

Electroplating sludges, once regarded as industrial wastes, are precious resources of various transition metals. This research has thus investigated the recycling of an electroplating sludge as a novel carbon-doped metal (Fe, Ni, Mg, Cu, and Zn) catalyst, which was different from a traditional carbon-supported metal catalyst, for effective NO selective catalytic reduction (SCR). This catalyst removed >99.7% NO at a temperature as low as 300 °C. It also removed NO steadily (>99%) with a maximum specific accumulative reduced amount (MSARA) of 3.4 mmol/g. Gas species analyses showed that NO removal was accompanied by evolving N2 and CO2. Moreover, in a wide temperature window, the sludge catalyst showed a higher CO2 selectivity (>99%) than an activated carbon-supported metal catalyst. Structure characterizations revealed that carbon-doped metal was transformed to metal oxide in the sludge catalyst after the catalytic test, with most carbon (2.33 wt %) being consumed. These observations suggest that NO removal over the sludge catalyst is a typical SCR where metals/metal oxides act as the catalytic center and carbon as the reducing reagent. Therefore, our report probably provides an opportunity for high value-added utilizations of heavy-metal wastes in mitigating atmospheric pollutions.

Pyrolysis characteristics and kinetic analysis of different dewatered sludge

Pyrolysis behavior and kinetic properties of four different sludge, including raw sludge and three sludge respectively dewatered with FeCl3/CaO, FeCl3/CaO/coal and Fenton's reagent (Fe(2+)+H2O2)/CaO, were analyzed by using thermogravimetry coupled with Fourier transform infrared spectrometry (TG-FTIR). The results show that organics of raw sludge mainly decomposed at 378-676K, and the decomposition temperature of conditioned sludge was prolonged to 823K. Addition of coal and catalysis of CaO/ferric salt both promoted sludge pyrolysis, leading to more NH3, CH4 and CO productions. Compared with dry sludge, wet sludge pyrolysis was hard to finish completely, and the first peak of organics' decomposition appeared at higher temperature (about 573K). Additionally, the global reaction model was suited to determine kinetic parameters, which showed that dry sludge conditioned with more CaO addition had higher E values than those of dry raw sludge. Opposite results were obtained when sludge samples were wet.

Investigation on the evolution of N-containing organic compounds during pyrolysis of sewage sludge

Pyrolysis is an emerging technology for the disposal of huge amounts of sewage sludge. However, the thermochemical decomposition mechanism of organic compounds in sludge is still unclear. We adopt a novel online TG-FTIR-MS technology to investigate the pyrolysis of sludge. The sludge samples were pyrolyzed from 150 to 800 °C with heating rates of 10, 50, and 200 K min(-1). We found for the first time that the heating rate of pyrolysis can significantly change the species of liquid organic compounds produced, but cannot change the gaseous species produced under the same conditions. The contents of produced gas and liquid compounds, most of which were produced at 293-383 °C, are influenced by both the heating rate and temperature of pyrolysis. The results also showed that heterocyclic-N, amine-N, and nitrile-N compounds are obtained from the decomposition of N-compounds in sludge, such as pyrrolic-N, protein-N, amine-N, and pyridinic-N. Heterocyclic-N compounds are the dominant N-containing products, which can be due to the thermochemical decomposition of pyridine-N and pyrrole-N, whereas fewer amine-N compounds are produced during the pyrolysis. A mechanism for the decomposition of N-containing compounds in sludge is proposed based on the obtained data.

Thermochemical behavior of tris(2-butoxyethyl) phosphate (TBEP) during co-pyrolysis with biomass

Co-pyrolysis of plastic waste and wood biomass to recover valuable chemicals is a cost-effective waste-recycling technology. However, widely used organophosphate ester additives in plastic, such as tris(2-butoxyethyl) phosphate (TBEP), can form diverse phosphorus (P)-containing species. These P-containing compounds can pose new environmental challenges when the biochar is reused. In this study, a mixture of TBEP and lignin was used to simulate the feedstock of plastic waste and wood biomass, and the thermochemical behavior of TBEP in slow pyrolysis (20 K min(-1)) and fast pyrolysis at 400-600 °C was investigated. The results show that low temperature in fast pyrolysis favors the enrichment of P in char. Up to 76.6% of initial P in the feedstock is retained in the char resulting from 400 °C, while only 51% is retained in the char from 600 °C. Slow pyrolysis favors the formation of stable P species regardless of the temperature; only 7% of the P retained in the char is extractable from char from slow pyrolysis, while 20-40% of P can be extracted from char resulting from fast pyrolysis. The addition of CaCl2 and MgCl2 can significantly increase the fraction of P retained in the char by the formation of Ca, Mg-P compounds. Online TG-FTIR-MS analysis suggests that TBEP undergoes decomposition through different temperature-dependent pathways. The P-containing radicals react with the aromatic rings produced by the pyrolysis of lignin to form Ar-P species, which is an important factor influencing the distribution and stabilization of P in char.