Elucidating pyrolysis behaviour of activated sludge in granular and flocculent form: Reaction kinetics and mechanism

Kinetic analysis of slow pyrolysis of oily sludge at medium temperature (350 ℃-650 ℃) and the effects of heating rate on pyrolysis

ABSTRACTPyrolysis is an effective way for the harmless treatment of oily sludge. The composition, physicochemical properties, and pyrolysis of oily sludge were experimentally studied in the present study. The Starink and Coats-Redfern methods were used to analyze the pyrolysis kinetics of oily sludge. Pyrolysis of oily sludge is divided into four stages: water evaporation stage, light component evaporation stage, heavy component pyrolysis stage, and final pyrolysis stage. The light component evaporation and heavy component pyrolysis stages are the main stages of medium-temperature pyrolysis. The pyrolysis characteristic parameters under heating rates of 10, 20, 30, and 40 K/min were obtained, and the effects of heating rates on the pyrolysis characteristics of oily sludge were discussed. The results show that with the increase in heating rate, the temperature range of each stage expands, and the temperature of the pyrolysis peaks also increases, with an average increase of 14.88%. The activation energies of the main pyrolysis stages obtained by the Starink method and Coats-Redfern method are consistent. In the light component evaporation stage, the activation energies obtained by the two methods are 61.93kJ/mol and 68.6kJ/mol, while the activation energies are 294.88kJ/mol and 367kJ/mol in the heavy component pyrolysis stage. The pyrolysis mechanism functions are obtained, and the pyrolysis kinetic equations under 10, 20, 30, and 40 K/min were constructed and validated by comparison with the results of the calculated properties and experimental measurement. This study can provide a better insight into the heat and mass transfer processes of oily sludge in pyrolysis reactors for further development and optimization.

Oat straw pyrolysis with ammonium chloride doping: Analysis of evolved gases, kinetic triplet, and thermodynamic parameters

The purpose of this work was to determine the effect of the addition of NH4Cl to oat straw on the evolved gases, kinetic triplet, and thermodynamic parameters of the pyrolysis process at 873 K. A complementary approach allowed to assess the effects of the pyrolysis of chlorine- and nitrogen-enriched biomass. The thermal analysis of biomass was performed for four heating rates (5, 10, 20, and 30 K/min). The doping of NH4Cl in the straw favoured i) carbonisation of the chars, ii) formation of C-N bonds, iii) reduction of evolved CH4 and CO2, and iv) an increase in the mean values of the effective activation energy and all thermodynamic parameters. A group of reactions that best fit the experimental data of the pyrolysis process was selected. It was necessary to use unspecified mechanisms to describe the reaction model, particularly for samples enriched with NH4Cl.

Recovery of structural extracellular polymeric substances (sEPS) from aerobic granular sludge: Insights on biopolymers characterization and hydrogel properties for potential applications

Nowadays, wastewater treatment plants (WWTPs) are transforming into water resource recovery facilities (WRRFs) where the resource recovery from waste streams is pivotal. Aerobic granular sludge (AGS) is a novel technology applied for wastewater treatment. Extracellular polymeric substances (EPS) secreted by microorganisms promote the aggregation of bacterial cells into AGS and the structural fraction of EPS (sEPS) is responsible for the mechanical properties of AGS. sEPS can be extracted and recovered from waste AGS by physico-chemical methods and its characterization is to date of relevant concern to understand the properties in the perspective of potential applications. This study reports on: characterization of sEPS extracted and recovered from AGS; - formation and characterization of sEPS-based hydrogels. Briefly, sEPS were extracted by a thermo-alkaline process followed by an acidic precipitation. sEPS-based hydrogels were formed by a cross-linking process with a 2.5% w/w CaCl₂ solution. The following key-findings can be drawn: i) hydrogels can be formed starting from 1% w/w sEPS on, by diffusion of Ca²⁺ into sEPS network; ii) the Ca/C molar ratio of hydrogels decreased with increasing concentration of sEPS from 1 to 10% w/w; iii) the thermogravimetric and spectroscopic behaviours of sEPS show that the cross-linking reaction mainly involves the polysaccharidic fraction of biopolymers; iv) water-holding capacity up to 99 gH₂O/g_sEPS was registered for 1% w/w sEPS-based hydrogels, suggesting applications in several industrial sectors (i.e. chemical, paper, textile, agronomic, etc.); v) rheological results highlighted a solid-like behaviour (G'≫G") of sEPS-based hydrogels. The power-law fitting of G' vs. sEPS concentration suggests that the expansion of the sEPS network during cross-linking occurs through a percolative mechanism involving the initial formation of sEPS oligomers clusters followed by their interconnection towards the formation of 3D network. These findings provide additional information about the mechanisms of sEPS-based hydrogel formation and reveal the peculiar physico-chemical characteristics of sEPS which nowadays are increasingly gaining interest in the context of resource recovery.

Systematic understanding of char-volatile evolution and interaction mechanism during sewage sludge pyrolysis through in-situ tracking solid-state reaction and products fate

The complexity of sludge components and the heterogeneity of pyrolysis products make it challenging to trace char-volatile evolutions and interaction mechanisms during pyrolysis. Herein, we systematically dissected the solid-state reactions and volatile dynamic variations via in-situ infrared/mass spectral probes coupled signal amplification techniques. The identification of hidden reactions was further enhanced by comparing the discrepancies in the pyrolysis of three systems: raw sludge, sludge-extracted organics, and pseudo-components of organics. A three-stage sludge pyrolysis of bond cleavage (α = 0.2-0.5), intermediates diffusion (α = 0.5-0.7), and interface interaction (α = 0.7-0.8) was proposed through solid-state reaction tracing, and the pyrolysis reaction was found to be dominated by the first two stages. The generation of reactive intermediates accelerated the collision frequency between reactants, which increased the order of solid-state reactions and raised the energy barrier from 148 to 180-261-297 kJ/mol. The temperature-response sequence of the major pyrolysis volatiles was H₂O/CO₂/furans/alcohols (<250 °C), amine-N/acids/ketones/esters (250-350 °C), heterocyclic-N/phenols/C_2-3 (300-400 °C), CH₄/aromatics/nitrile-N (350-450 °C), and CO/HCN (>450 °C). The temperature-dependent evolution of these volatiles was consistent with the variations of chars in terms of pyrolysis behaviors, reaction models, and surface characteristics. The comprehensive understanding of the staged pyrolysis pathways and the char-volatile interaction mechanisms may provide critical information for pyrolysis procedure design and product targeted regulation.

Co-pyrolysis performances, synergistic mechanisms, and products of textile dyeing sludge and medical plastic wastes

This study aimed to quantify the co-pyrolysis of textile dyeing sludge (TDS) and the two medical plastic wastes of syringes (SY) and medical bottles (MB) in terms of their performances, synergistic mechanisms, and products. The pyrolysis of polyolefin plastics with its high calorific value and low ash content can offset the poor mono-pyrolytic performance of TDS. The synergistic mechanisms occurred mainly in the range of 400-550 °C. The addition of 10% SY or MB achieved the best co-pyrolysis performance with the lowest activation energy. The co-pyrolysis increased the contents of CH₄ and CH but reduced CO₂ emission. The co-pyrolysis released more fatty hydrocarbons, alcohols, and cyclic hydrocarbon during but reduced the yields of ethers and furans, through the synergistic mechanisms. The addition of the polyolefin plastics made the micro surface particles of chars smaller and looser. Our results can benefit energy utilization, pollution control, and optimal operational conditions for the industrial thermochemical conversions of hazardous wastes.

Coupled mechanisms of reaction kinetics, gas emissions, and ash mineral transformations during combustion of AlCl3-conditioned textile dyeing sludge

Though commonly used in the dewatering of textile dyeing sludge (TDS) before its incineration, chemical conditioning has yet to be evaluated in terms of its impact on the reaction mechanisms, emissions, and ash minerals. This study combined experiments and equilibrium simulations to disentangle the interaction mechanism among the combustion behaviors, gas emissions, ash minerals of TDS conditioned with(out) three blend ratios of the AlCl₃ conditioner. The use of the AlCl₃ conditioner slightly improved the performance of the combustion stage of volatiles and chars. No significant effect of AlCl₃ conditioner was detected on the kinetic mechanism of its main combustion stage best elucidated by the nth-order and diffusion models. SO₂ was the main evolved gas whose reduction between 600 and 800 °C was attributed to its increased retention rate by CaO from the decomposition of CaCO₃. Aluminum compounds acted as a stimulator in SO₂ emission between 800 and 1000 °C since the formation of calcium aluminosilicates. At above 1060 °C, CaSO₄ decomposed rapidly, thus almost completely releasing inorganic S. This study supplies new insights into pollution `controls on the combustion of TDS conditioned with Al salt coagulant.

Comparative (co-)pyrolytic performances and by-products of textile dyeing sludge and cattle manure: Deeper insights from Py-GC/MS, TG-FTIR, 2D-COS and PCA analyses

Not only does pyrolysis recover energy and value-added by-products but also reduces waste stream volume. The low volatiles and high ash contents of textile dyeing sludge (TDS) limit its mono-pyrolysis performance. This study aimed to conduct an in-depth analysis of its co-pyrolytic performance with cattle manure (CM). The co-pyrolysis enhanced the volatiles emission from the early devolatilization stage whose reaction mechanism shifted from a diffusion model to a reaction-order model. The further cracking of macromolecular materials was mainly elucidated by the reaction-order model. The temperature dependency of the co-pyrolytic gases was of the following order: aliphatic hydrocarbons > CO₂ > alcohols, phenols, ethers, aldehydes, ketones, and carboxylic acids. The main co-pyrolytic volatile products were coumaran and 4-vinylguaiacol. The relative content of guaiacol-type components could be enhanced by co-pyrolysis and lowering the operational temperature to 450 °C. The interaction of co-pyrolysis enriched the char aromaticity. Our findings provide practical insights into the control and application opportunities and limitations on the high value-added energy and products from the co-pyrolysis of TDS and CM.

Synergistic effects, gaseous products, and evolutions of NOx precursors during (co-)pyrolysis of textile dyeing sludge and bamboo residues

This study aimed to investigate the synergistic influences of the textile dyeing sludge (TDS) and bamboo residues (BR) co-pyrolysis, and its effects on the formation mechanisms of NH₃ and HCN. The mass loss rate was lower for TDS than BR, with the co-pyrolysis with 50% BR exerting the strongest synergistic effect. The pyrolysis stages 1 (< 400 °C) and 2 (400-800 °C) were best described using the diffusion and third-order reaction mechanisms, respectively. Activation energy and frequency factor were lower for the pyrolysis of TDS than BR. The addition of no less than 50% BR significantly increased the emissions of CO₂, CO, CH₄, CO, and CO and reduced the aromatic compounds. The thermal stability of N-A structure was lower in TDS than BR. The co-pyrolysis with 50% BR significantly inhibited the formations of NH₃ and HCN and improved the aromaticity of biochar. This may due to the weakened hydrogenation reaction at N sites, the enhanced conversion of NH₃, the inhibition of the ring cleavage in the char-secondary cracking, and the formation of more quaternary-N. Our results provide insights into the co-treatment of TDS and BR, and controls over NO_x precursors for a cleaner energy production.

CO2-assisted co-pyrolysis of textile dyeing sludge and hyperaccumulator biomass: Dynamic and comparative analyses of evolved gases, bio-oils, biochars, and reaction mechanisms

The CO₂-activated co-pyrolysis technology presents promising potential to mitigate the environmental pollution and climate change. The dynamic analyses of evolved syngas, bio-oils, biochars, interaction effects, and reaction mechanisms of the co-pyrolysis of textile dyeing sludge (TDS) and Pteris vittata (PV) (hyperaccumulator biomass) were characterized and quantified comparatively in the three atmospheres. In the CO₂-assisted atmosphere, the gasification of PV began to prevail between 600 and 900 °C, while in the N₂ atmosphere, PV and TDS were stable at 750 °C. The CO₂-assisted co-pyrolysis reduced the apparent activation energy. The higher CO₂ concentration during gasification led to the higher activation energy. The CO emission level of the CO₂ and mixed atmospheres was almost 20 and 14 times that of the N₂ atmosphere, respectively. The CO release from the CO₂ atmosphere was 1.4 times that from the mixed atmosphere. CO₂ significantly changed the production pathway of biochar in the N₂ atmosphere, as was evidenced by the enhanced temperature sensitivity of O-C = O/hydroxy (-OH) in ester. Our findings research can provide new insights into the effectiveness of the CO₂-assisted co-pyrolysis associated with reduced costs and hazardous wastes.

Persulfate oxidation for alternative sludge treatment and nutrient recovery: An assessment of technical and economic feasibility

The introduce of tighter waste disposal regulations and increasing resource scarcity make the re-utilization of waste activated sludge a hot and crucial research topic. Compared with traditional sludge disposal technologies (e.g. landfill and incineration), advanced oxidation processes have been proven to be an environmentally friendly method for sludge stabilization and disintegration. However, the effectiveness of persulfate oxidation for sludge degradation, and the re-utilization of its embedded nutrients have been rarely reported. Therefore, this work is to investigate the technical and economic feasibility of using persulfate oxidation and struvite precipitation for sludge degradation and nutrient recovery. The results show that with the assistance of ultraviolet radiation, released phosphate and ammonia nitrogen from sludge could reach 233.4 and 265.6 mg/L. Besides, 92.8% phosphate and 32.6% ammonia-nitrogen could be recovered by struvite precipitation at a pH of 9.5, with an Mg: P molar ratio of 1.1:1. The economic analysis shows that the operational cost of the proposed process was 25% higher than traditional sludge disposal (267.5 $/ton), but its capital investment is much lower. Investigations on chemical dosage minimization, energy reclamation and process optimization are suggested to reduce the process's operating cost in the future.

Activated sludge and other aerobic suspended culture processes

This paper provides an overview of activated sludge related to suspended growth processes for the year 2019. The review encompasses process modeling of activated sludge, microbiology of activated sludge, process kinetics and mechanism, nitrogen and phosphorus control, design, and operation in the activated sludge field. The fate and effect of xenobiotics in activated sludge, including trace organic contaminant and heavy metal xenobiotics, which had influence on the growth of suspended sludge, are covered in this review. Compared to past reviews, many topics show increase in activity in 2019. These include, biokinetics process of aerobic granular sludge formation, pyrolysis kinetic mechanism of granular sludge. These topics are referred to formation and disintegration of granular sludge. Other sections include activated sludge settling model, toxicity resistant microbial community, nitritation-anammox processes for nitrogen removal, and respirometry used in the operation of real wastewater treatment plant are especially highlighted in this review. PRACTITIONER POINTS: Biokinetics process of aerobic granular sludge formation Toxicity resistant microbial community in activated sludge Nitritation-anammox processes for nitrogen removal in activated sludge.