Mild Thermal Pretreatment Research Articles

Tannery sludge valorization through zeolite-assisted anaerobic process for short-chain fatty acids (SCFAs) production

Tannery sludge, a challenging waste, was utilized as a substrate for the production of Short-Chain Fatty Acids (SCFAs) through a series of six thermophilic Continuous Stirred-Tank Reactor runs. The sludge was subjected to a mild thermal pre-treatment and incorporated zeolites (chabazite in run II, and clinoptilolite in run III) in the acidification process. Results highlighted zeolites' impact on chromium concentration and the SCFAs/CODSOL ratio. Ammonia release remained consistent at around 47 % and 51 % for run I and II, respectively, but surpassed 60% in run III, suggesting limited zeolite effectiveness in NH4 absorption. Chromium release in the liquid fraction, due to thermal pretreatment, reached 335 mg/L. While in tests without zeolite, complete removal proved challenging, in zeolite-amended runs, complete removal was achieved, showcasing the materials' heavy metal absorption capacity. SCFA concentrations reached 20260 mgCOD/L, with acidification efficiency varying; runs I and III had ratios around 0.70 COD/COD, while run II showed substantial improvement (0.92) with chabazite. Anaerobic fermentation-digestion mass balance indicated a 41% reduction in landfill sludge mass, reducing its environmental footprint while yielding valuable byproducts like biogas and SCFAs. These findings underscore zeolites' potential in heavy metal absorption and acidification process enhancement, paving the way for applications with tannery sludge.

Viability and removal assessment of Escherichia coli and Salmonella spp. by real-time PCR with propidium monoazide in the hygienization of sewage sludge using three anaerobic processes

Sewage sludge should be stabilized for its beneficial use and pathogens, among other factors, should comply with environmental regulations. Three sludge stabilization process were compared to assess their suitability for producing Class A biosolids: MAD-AT (mesophilic (37 °C) anaerobic digestion (MAD) followed by an alkaline treatment (AT)); TAD (thermophilic (55 °C) anaerobic digester); and TP-TAD (mild thermal (80 °C, 1 h) pretreatment (TP) followed by a TAD). E. coli and Salmonella spp. were determined, differentiating three possible states: total cells (qPCR), viable cells using the propidium monoazide method (PMA-qPCR), and culturable cells (MPN).Culture techniques followed by the confirmative biochemical tests identified the presence of Salmonella spp. in PS and MAD samples, while the molecular methods (qPCR and PMA-qPCR) showed negative results in all samples. The TP + TAD arrangement reduced the concentration of total and viable E. coli cells in a greater extent than the TAD process. However, an increase of culturable E. coli was observed in the corresponding TAD step, indicating that the mild thermal pretreatment induced the viable but non-culturable state in E. coli. In addition, the PMA technique did not discriminate viable from non-viable bacteria in complex matrices. The three processes produced Class A biosolids (fecal coliforms < 1000 MPN/gTS and Salmonella spp, < 3 MPN/gTS) maintaining compliance after a 72 h storage period. It appears that the TP step favors the viable but not culturable state in E. coli cells, a finding that should be considered when adopting mild thermal treatment in sludge stabilization process arrangements.

Impact of thermal pretreatment on anaerobic digestion of dewatered sludge from municipal and industrial wastewaters and its economic feasibility

This study aimed to evaluate the effect of thermal pretreatment on anaerobic digestion (AD) of dewatered sludge. Thermal pretreatment was conducted at varying temperatures (40–220 °C) and durations (1–3 h), prior to batch biomethane production. The highest biomethane yield of 195 mL CH4/g VSfeed was achieved at 140 °C for 3 h, which was 81% higher than that of the control. However, the net cash flow of 76,942 USD/yr was achieved at 60 °C for 3 h. The solubilization of chemical oxygen demand and cell lysis were accompanied by a relative abundance of Firmicutes in the bacterial population during batch digestion, implying that thermal pretreatment promoted the growth of hydrolytic bacteria. In the methanogenic archaeal population, the proportion of hydrogenotrophs and electrotrophs increased, indicating that pretreatment enhanced the interspecies transfer of H2 and electrons. Economic analysis revealed that pretreatment at 40 °C for 1 h, 60 °C for 1–3 h and 100 °C for 3 h could be beneficial to AD of dewatered sludge. However, a sensitivity analysis confirmed that 60 °C for 3 h is the most economical pretreatment condition. AD accompanied by mild thermal pretreatment is a viable treatment method for dewatered sludge discharged from small wastewater treatment facilities.

Effects of mild thermal pre-treatment combined with H2O2 addition on waste activated sludge digestibility

The pre-treatment of waste activated sludge (WAS) has become more common since it often results in improved bioconversion to methane, in both rate and extent. However, thorough insights on the possible effects and mechanisms of mild pre-treatment techniques, such as temperatures <100 °C combined with the addition of H2O2, are still limited. This study reports the effects of the addition of 5–30 mgH2O2/g TS and its interaction with thermal pre-treatment at 70 °C on methane production, using WAS as the substrate. It was found that the addition of H2O2 increased the methane production rate, coinciding with a decrease in apparent viscosity of WAS, which probably improved mass transfer under non-ideal mixing conditions. While H2O2 solubilized proteins and carbohydrates and mineralized a small fraction of the humic substances in WAS, these biochemical transformations did not suffice to explain the observed extent and rate of methane production. A decreased particle size, the presence of Fenton’s reagent, and the presence of cationic polymers in the WAS were discarded as the reasons for the observed decrease in apparent viscosity. It was concluded that the pre-treatment conditions applied in the present study might be a strategy to enhance mixing conditions in full-scale anaerobic digesters.

Influence of torrefaction on yields and characteristics of densified solid biofuel

Torrefaction is a mild thermal pretreatment for enhancing the combustion property of biomass to be properly applied to biomass or co-firing power plants. The present research investigated mass yields, fuel characteristics and hydrophobicity of rubber wood pellet torrefied at different temperatures and residence times. Proximate and ultimate analyses, the enhancement factor of higher heating value, energy yield, lignocellulosic components and Fourier transform infrared spectroscopy (FT-IR) of torrefied samples were performed to evaluate the influence of the process parameters and compared to those of the untreated ones. The non-condensable and condensable products were analyzed by GC and GC-MS. The results showed that the torrefaction severity index (TSI) correlated sufficiently with the properties of torrefied biomass and was more sensitive to torrefaction temperature than residence time. The increase in the TSI value resulted in an increase in heating value but a decrease in energy yield. The energy density increased by 14% at 260 °C and reached up to 35% at torrefaction temperature of 300 °C. FT-IR analysis revealed that the contents of hydroxyl and carbonyl groups appeared in torrefied wood pellets were decreased and increased, respectively, due to the torrefaction process. Furthermore, the hydrophobicity of torrefied pellets can be significantly improved as torrefaction temperature increases. The non-condensable gaseous products were mostly composed of CO2, CO, and traces of CH4. While the organic compounds found in condensable liquid products were acids, ketones, furans and phenols.

Effect of pulsed electric field, mild thermal pretreatment and calcium on texture changes of potato (Solanum tuberosum L.) during subsequent cooking

The effect of pulsed electric field (PEF) and/or mild thermal processing at 60 °C (T60) on the cooking behaviour of potato tissue in media with different Ca2+ concentrations (0–0.5%) was investigated. The rate constant for texture degradation during cooking seemed to decrease after the different pretreatments in the order Untreated > T60 > PEF > PEF-T60, but only a significant effect (α = 0.05) could be found in the case of the combination pretreatment in 0.5% Ca2+ medium. These texture changes were linked with changes in pectin degree of methylesterification (DM) and Ca2+ crosslinking obtained after the pretreatments. The mild thermal pretreatment has little effect on the pectin DM but promotes ionic crosslinking. Both the PEF and combination pretreatment reduce the pectin DM and increase ionic crosslinking significantly. An exponential correlation was found between the texture degradation rate constant and the pectin properties studied.

The role of growth media on composition, bioconversion and susceptibility for mild thermal pre-treatment of waste activated sludge

The highly variable characteristics of waste activated sludge (WAS) hinder the comparison of experimental results on WAS bioconversion between the different studies that use excess sludge from different origin. Sludge grown under laboratory conditions with synthetic wastewater as feed showed high resistance to commonly applied pre-treatment techniques, such as thermal pre-treatment. However, a distinctly higher bioconversion of this sludge was recorded compared to WAS from a full-scale wastewater treatment plant (WWTP). The observed results casted concern on the suitability of the experimental laboratory-based data for practice. The physicochemical and biochemical characteristics of both WAS and lab-grown sludge are dependent on the wastewater characteristics or growth media on which the sludges were grown. The objective of this study was to formulate a growth medium that results into a lab-grown sludge which shows high similarity to the WAS coming from a specific full-scale WWTP in response to a pre-treatment technique. More specifically, in this study we targeted the formation of slowly-biodegradable lab-grown sludge that is similarly responsive to mild thermal pre-treatment with H2O2 addition. By comparing real and synthetic wastewaters, we discussed the various wastewater constituents that may lead to a higher degree of recalcitrance of the produced sludge. We then formulated a growth medium, which was fed to a lab-scale activated sludge reactor and evaluated the nutrient removal capacity, as well as the characteristics of the cultivated sludge before and after pre-treatment. Finally, the growth medium was modified to provoke a change in both the bioconversion and in the response to mild thermal pre-treatment. The growth medium proposed in this study resulted in a slowly-biodegradable sludge (195 ± 3.7 NLCH4/kgVSadded) that after thermal pre-treatment resulted in an increase in methane production of 9 %, which was similar to the WAS coming from the full-scale WWTP. It was concluded that not only the bioconversion but also the response to mild thermal pre-treatment of lab-grown sludge was determined by the composition of the growth media.

3-ROUTES PLATFORM FOR RECOVERY OF HIGH VALUE PRODUCTS, ENERGY AND BIO-FERTILIZER FROM URBAN BIOWASTE: THE REVENUE PROJECT

This innovative Biorefinery platform is based on the integration of a mild thermal pre-treatment and a solid/liquid separation unit to parallel-integrated bioprocesses specifically selected on food waste distinctive chemical composition: a liquid fraction, rich in readily fermentable sugars, to be transformed into valuable biobased products, and a solid organic residue to enhance biomethane production generating a fully hygienized digestate to be recycled. The preliminary results in terms of VFAs yields and composition from the acidogenic stage, and the methane conversion rate from the anaerobic digestion of the solid residue, are here presented

Comparison of Microwave Short Time and Oven Heating Pretreatment on Crystallization of Raisins.

Crystalline material can develop on the surface of raisins during storage and transport, affecting the final acceptability of the product. In this work, a mild thermal pretreatment was applied to raisins to melt the pre-existing crystals and the effect of such thermal treatments on the development of crystals over a storage period was investigated. The raisins selected for this study were of the Thompson seedless variety from one Chilean company. The thermal pretreatment of raisins at 50 °C and 70 °C for 20 min in an oven and microwave (800 W) irradiation for 15 s resulted in a reduction in the percentage of crystallized raisins (w/w) from more than 50% in the control samples to less than 10% after 35 days of storage at 15 and 25 °C in a 57% relative humidity environment. The results showed that some textural parameters, such as cohesiveness and chewiness, were not affected by thermal treatment and were independent of storage temperature.

Mild Thermal Pre-Treatment of Waste Activated Sludge to Increase Loading Capacity, Biogas Production, and Solids’ Degradation: A Pilot-Scale Study

Sludge pre-treatments are emerging as part of the disposal process of solid by-products of wastewater purification. One of their benefits is the increase in methane production rate and/or yield, along with higher loading capacities of existing digesters. In this study, we report the performance of a pilot-scale compartmentalized digester (volume of 18.6 m3) that utilized a mild thermal pre-treatment at 70 °C coupled with hydrogen peroxide dosing. Compared with a reference conventional anaerobic digester, this technique allowed an increased organic loading rate from 1.4 to 4.2 kg volatile solids (VS)/(m3d) and an increment in the solids degradation from 40 to 44%. To some extent, these improvements were promoted by the solubilization of the tightly-bound fraction of the extracellular polymeric substances to looser and more accessible fractions without the formation of refractory compounds. In sum, our results suggest that this pre-treatment method could increase the treatment capacity of existing digesters without significant retrofitting.

Pulsed electric field and mild thermal processing affect the cooking behaviour of carrot tissues (Daucus carota) and the degree of methylesterification of carrot pectin

For the first time, the effect of pulsed electric field (PEF) and mild thermal processing on the texture of cortex and vascular carrot tissue during subsequent thermal processing (i.e. cooking behaviour) was compared and the degree of methylesterification (DM) of pectin from the pretreated tissues was investigated. The PEF and mild thermal pretreatment slowed down the cooking behaviour of the carrot tissues, especially when the pretreatments were combined. The DM of pectin from vascular tissue was lowered after both types of pretreatments, the effect being most pronounced in the case of the combination of the PEF and mild thermal pretreatment. In contrast, the DM of cortex pectin only decreased after the mild thermal pretreatment and after the combination pretreatment. This study demonstrates that besides mild thermal pretreatments also PEF pretreatments can be considered in the context of texture preservation of thermally processed fruits and vegetables.

Mild Thermal Pre-treatment as a Method for Increasing the Methane Potential of Food Waste

Mild Thermal Pre-treatment as a Method for Increasing the Methane Potential of Food Waste

Impact of thermal pretreatment on crystallization of Thompson raisins

After prolonged storage or during the shipment of raisins, a crystalline material may be developed on their surface, affecting the final acceptability of the product. The mild thermal pretreatment (50 °C for a period of 12 h and 48 h) was applied to dissolve any potential tiny crystals and nuclei already present in the raisins. The thermal pretreatment of raisins resulted in a reduction on the percentage of crystallized raisins (w/w) from more than 46% in control samples to less than 10% after 30 days of storage at 57% and 66% of relative humidity and 15 °C and 25 °C. X-ray diffraction analysis identified d-Glucose monohydrate as the main crystalline component. All the thermal treatments increased the hardness of the raisins, when they were incubated at 57% relative humidity. To mitigate the problems of crystallization of raisins during shipping, it is recommended that the raisins are thermally pretreated and the shipping conditions are controlled.

Kitchen waste valorization through a mild-temperature pretreatment to enhance biogas production and fermentability: Kinetics study in mesophilic and thermophilic regimen

Biowaste valorization through anaerobic digestion is an attractive option to achieve both climate protection goals and renewable energy production. In this paper, a complete set of batch trials was carried out on kitchen waste to investigate the effects of mild thermal pretreatment, temperature regimen and substrate/inoculum ratio. Thermal pretreatment was effective in the solubilisation of macromolecular fractions, particularly carbohydrates. The ability of the theoretical methodologies in estimating hydrogen and methane yields of complex substrates was evaluated by comparing the experimental results with the theoretical values. Despite the single batch configuration, a significant initial hydrogen production was observed, prior to methane yield. Main pretreatment effect was the gain in hydrogen production; the extent was highly variable according to the other parameters values. High hydrogen yields, up to 113 mL H2/g VSfed, were related to the prompt transformation of soluble sugars. Thermophilic regimen resulted, as expected, in faster digestions (up to 78 mL CH4/gVS/day) and sorted out pH inhibition. The relatively low methane yields (342–398 mL CH4/g VSfed) were the result of the consistent lignocellulosic content and low lipid content. Thermal pretreatment proved to be a promising option for the enhancement of hydrogen production in food waste dark fermentation.

A Mild Thermal Pre-treatment of the Organic Fraction of Municipal Wastes Allows High Ethanol Production by Direct Solid-state Fermentation

A solid standard mixture (SSM) representing the annual composition of fresh fruits and vegetables residues generated at the Supply Center in Mexico City was used for bioethanol production. This type of residues allows bioethanol production with a single thermal pre-treatment instead of hard thermochemical or enzymatic treatments. The release of fermentable carbohydrates from the SSM by a mild thermal pretreatment was firstly optimized. After that, mixed and single cultures of Saccharomyces cerevisiae, Scheffersomyces stipitis, and Schwanniomyces occidentalis were evaluated for bioethanol production. The maximum ethanol production, 282.61 ± 13.09 L ethanol per ton of dry matter (DM), was reached using a severity factor (SF) of 2.35 and a mixed culture composed of Saccharomyces cerevisiae, Scheffersomyces stipitis, and Schwanniomyces occidentalis. The improved lab scale conditions were evaluated in a pilot scale (18 Kg) stirred bioreactor with an SF of 2.35 and the mixed culture, obtaining 245.72 ± 17.76 L ethanol per ton DM. The obtained results demonstrate for the first time the use of fresh fruits and vegetables residues for bioethanol production under solid-state culture conditions without any thermochemical or enzymatic pre-treatment.

Variability of food waste chemical composition: Impact of thermal pre-treatment on lignocellulosic matrix and anaerobic biodegradability

A comprehensive sustainable Food Waste (FW) management is globally needed in order to reduce the environmental pollution and the financial costs due to FW disposal; anaerobic digestion is considered as one of the best environmental-friendly alternatives to this aim. A deep investigation of the chemical composition of different Food waste types (cooked kitchen waste (CKW), fruit and vegetable scraps (FVS) and organic fraction of municipal solid waste (OFMSW)) is here reported, in order to evaluate their relevant substance-specific properties and their impact on anaerobic biodegradability by means of a sophisticated automatic batch test system. Suitability for a mild thermal pre-treatment (T = 134 °C and p = 3.2 bar) to enhance the biological degradation of hardly accessible compounds was investigated. The pre-treatment affected significantly the carbohydrates solubilisation, and was able in reducing part of the lignocellulosic matrix. Moreover, in mesophilic conditions, the high solubilized sugars content favoured the initial recovery of hydrogen (not consumed by hydrogenotrophic methanogenesis), allowing to newly assess the extent of prompt fermentability. Pre-treatment enhanced hydrogen yields of FVS and OFMSW, with gains up to +50%, while the successive methane production, occurring in the same reactor, resulted affected by the lack of the soluble part of carbohydrates, “subtracted” for H2 production. Only in thermophilic conditions, when no hydrogen in the biogas was detected, pre-treatment of OFMSW significantly increased methane yield (from 0.343 to 0.389 L CH4 g−1 VSfed). A thermal pre-treatment seems the recommended solution in order to reduce part of the recalcitrant lignocellulosic matrix of food waste, to improve energy recovery and to eliminate the extra cost needed for pasteurization.

Two-Step Thermal Cracking of an Extra-Heavy Fuel Oil: Experimental Evaluation, Characterization, and Kinetics

This work deals with an efficient two-step thermal upgrading process for converting extra-heavy fuel oil to light olefins (ethylene, propylene, and butenes) and fuels (gasoline and diesel fuel). In the first step, mild thermal pretreatment was implemented at different temperatures (360–440 °C) in the liquid phase to obtain a more suitable feedstock for an olefin production unit. Thanks to this cost-effective pretreatment, the upgraded feedstock demonstrated considerable flowability and crackability compared to the initial fuel oil, making the subsequent vapor-phase operation easier to handle at temperatures as high as 800 °C with no severe operational impediments. The quantitative 1H and 13C NMR studies shed light on the enhanced features of the thermally treated feedstock toward lighter and more valuable products. As a result, remarkable olefin production (74.7 or 55.1 wt % light olefins based on the upgraded or the original feedstock) was accomplished in this two-step process. The process could be alter...

Effect of thermal pretreatment at 70 °C for one hour (EU hygienization conditions) of various organic wastes on methane production under mesophilic anaerobic digestion

The impact of hygienization as mild thermal pretreatment on the methane production of various organic wastes was investigated, including digestate issued from hydrolysis tank, thickened sludge from a municipal wastewater treatment plant (MWWTP sludge) and from a mixed domestic-industrial wastewater treatment plant (D-I WWTP sludge), sludge from a meat-processing plant (MP sludge), sieving rejection from a pork slaughterhouse, pork liver, cattle slurry, cattle scraping slurry and date seeds. They were thermally pretreated at 70 °C for one hour and subsequently put into AD digesters incubated at 37 °C for individual methane potential test. The modified Gompertz model was employed to evaluate the kinetic parameters of methane production curves (R2 = 0.944–0.999). The results were compared with the untreated samples. Significant enhancement of methane potentials induced by thermal treatment (p < 0.05) was observed when it comes to the pork liver (+8.6%), the slaughterhouse sieving rejection (+11.1%), the thickened MWWTP sludge (+12.5%) and the digestate issued from hydrolysis tank (+18.0%). The maximum methane production rates of the 4 substrates mentioned above were increased by thermal pretreatment as well (from 13.5% to 64%, p < 0.05). The lag time of the methane production was shortened for the digestate from hydrolysis tank and the MWWTP sludge (by 48.6% and 62.2% respectively, p < 0.05). No significant enhancement was obtained for the cattle slurry, the cattle scraping slurry and the D-I WWTP sludge. Additionally, the maximum methane production rate and the methane potential were reduced by thermal pretreatment for the MP sludge and the date seeds respectively (p < 0.05). In this paper, possible mechanisms were discussed to explain the different methane production behaviors of substrates after the mild thermal pretreatment.

Using a novel spiral-filter press technology to biorefine horticultural by-products: The case of tomato. Part I: Process optimization and evaluation of the process impact on the antioxidative capacity

Abstract With tomato as a model crop, the use of a novel, low-oxygen spiral-filter press technology for juice production was demonstrated on pilot-scale. Our results show that a robust process could be developed with a juice yield of 82.5% which could be increased to 97.0% with an additional mild thermal pretreatment (40 °C for 3 min). A comprehensive insight was gained in the underlying mechanisms through which process parameters can affect juice yield and juice quality parameters such as turbidity and precipitate weight ratio. Additionally, the antioxidative capacity (AOC) was investigated, showing a preservation of antioxidants during pressing (102 ± 12%) which may be attributed to the low-oxygen processing. Finally, also an insight was gained in the antioxidative distribution of the resulting fractions, demonstrating the potential of the press residue and confirming the relevance of designing a biorefinery system where all fractions are valorized. Industrial relevance This pilot-scale study illustrates the potential of a novel spiral-filter press technology in refining biomass. Besides a high juice yield and retention of the antioxidative potential, it proves flexible towards characteristics of the input biomass and can customize end-products in function of the aimed application. This study contributes to the essential technical knowledge necessary for processing other matrices with this novel technology.

Biomethane potential of food waste: modeling the effects of mild thermal pretreatment and digestion temperature

ABSTRACTIn order to enhance anaerobic biodegradability of food waste (FW), thermal pretreatment was applied. The effectiveness in terms of biodegradability extent and process rate improvement was investigated. To this aim, Biomethane Potential tests were carried out under mesophilic and thermophilic conditions. The IWA anaerobic digestion Model 1 (ADM1), a powerful tool for modeling the anaerobic digestion (AD) of different substrates, was implemented to predict the methane production. Disintegration constant (k_dis) and maximum acetate uptake rate (km_ac) were identified as the most sensitive parameters and were calibrated over the observed methane production. Pretreatment improvement was more evident in enhancing parameters related to the process rate, such as solubilization extent (+153%) and disintegration constant (+18%), rather than increasing substrate biodegradability. Thermophilic conditions proved to be effective in speeding up the whole AD process, since all the kinetics were significantly improved (disintegration rate increased up to fivefold). Furthermore, it was demonstrated that, after k_dis and km_ac calibration, default thermophilic ADM1 parameters can be suitable to model FW digestion.