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Abstract: The aim of the study was to analyse two small wastewater treatment plants (SWTPs) and compare their treatment performance with the aim of developing a zero-waste strategy. The analyses showed that the second wastewater treatment plant had a problem with an increased phosphorus concentration in the wastewater as well as fluctuations in nitrogen removal. One of the key aspects was the recovery of phosphorus in the form of struvite precipitated on zeolite, which can be used as a fertiliser due to its significant nutrient content. The main benefit of the same SWTP is the separate collection of cellulosic material (labelled as sample RS), which has the potential for reuse as biochar, while from the first SWTP the sludge sample (labelled as sample SS) was taken and torrefied for comparison with RS. Both samples were torrefied at 250 ◦C and 350 ◦C in nitrogen and carbon dioxide atmospheres. The RS products obtained in CO2 atmosphere at 350 ◦C showed the best biochar properties, as they had a higher heating value (HHV) and a higher C content. Elemental analysis showed that the carbon content in RS and SS increased from 43 to 65 % and from 35 to 39 %, respectively. Measurement of the HHV of the torrefied RS product showed an increase from 17 to 26 MJ/kg, while for SS it increased from 15 to 17 MJ/kg. The comprehensive combustion index and the EMCI index show higher values for the RS samples torrefied at 350 ◦C. The H/C and O/C ratios are favourable for these samples, as the high quality of torrefied biomass is associated with the highest prices on the market. According to the TGA and FTIR analysis, the type of torrefaction atmosphere, in contrast to the torrefaction temperature, has little influence on the subsequent use of the torrefied samples in combustion. The results show that the integration of torrefaction for biochar production with phosphorus recovery by struvite precipitation is an efficient solution for waste management in small wastewater treatment plants.
Keywords: wastewater treatment plant efficiency, torrefication, N2 atmosphere, CO2 atmosphere, solid biofuels, proximate and ultimate analyses, thermogravimetry
Published in DKUM: 29.10.2025; Views: 0; Downloads: 7
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Abstract: Four biomass residues–rosemary pomace, rosemary cake, grape seed and apple pomace– were torrefied at 250, 350 and 450 ◦C, and the physical, chemical and structural changes were characterized. The mass and energy yield decreased with increasing torrefaction temperature; the lowest mass (~10.4%) and energy yield (~10.6%) were observed for rosemary cake torrefied at 450 ◦C. The HHV increased the most for all feedstocks at 350 ◦C, with rosemary cake reaching a peak value of 36.4 MJ/kg at 350 ◦C. Ash content increased with temperature due to organic mass loss, while volatiles decreased and fixed carbon increased in most samples. The FTIR spectra showed the progressive loss of hydroxyl, carbonyl and C–O functionalities and the appearance of aromatic C=C bonds, indicating the formation of the biochar. TGA and DTG analyses revealed that the torrefied samples exhibited higher initial and maximum temperatures for decomposition, confirming improved thermal stability. The TGA-FTIR analyses of gas emissions during pyrolysis and combustion showed that the emissions of CO2, CH4, NOx and SO2 decreased with increasing degree of torrefaction. Overall, 350 ◦C was optimal to maximize energy density. The results show that agro-industrial residues can be effectively converted into sustainable biofuels, which offer the dual benefit of reducing waste disposal problems and providing a renewable alternative. In practice, such residues could be used for decentralized power generation in rural areas, co-combustion in existing power plants, or as feedstock for advanced bioenergy systems.
Keywords: torrefication, fuel, thermogravimetric analysis, biowaste, mass yield, energy yield
Published in DKUM: 04.09.2025; Views: 0; Downloads: 11
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Abstract: Particulate matter (PM10 and PM2.5) is a key contributor to urban air pollution and poses significant health risks, particularly in densely populated areas. While conventional air quality monitoring focuses on particle size and concentration, this study emphasizes the importance of understanding chemical composition and emission sources for effective air pollution management. PM samples were collected between 2019 and 2022 at two locations in the Republic of Slovenia: a traffic-dominated urban site and an industrial area. Annual average PM10 concentrations ranged from 14 to 34 μg/m3, and those of PM2.5 ranged from 9 to 22 μg/m3. In addition to decreasing annual concentrations, a notable reduction in exceedance days was observed between 2019 and 2022, indicating the effectiveness of recent air quality improvement measures. Meteorological data and statistical models were used to assess environmental influences on PM variability. Advanced SEM-EDS analysis revealed substantial seasonal and spatial differences in particle composition, with key elements such as silicon (4.3–28.4%), carbon (13.1–61.7%), and trace amounts of lead and zinc varying across sites and particle types. Mineral dust (Si, Al, Ca, Fe, Mg), originating from soil resuspension, construction, and Saharan dust, was dominant. Combustion-related particles containing C, Pb, Zn, and Fe oxides were associated with vehicle emissions, industrial processes, and biomass burning. Secondary aerosols, such as sulphates and nitrates, showed seasonal trends, with higher concentrations in summer and winter, respectively. The results confirm that PM levels are driven by complex interactions between local emissions, weather conditions, and seasonal dynamics. The study supports targeted policy measures, particularly regarding residential heating and traffic emissions, to improve air quality.
Keywords: air pollution, air quality, PM particles, SEM-EDS, Slovenia
Published in DKUM: 30.05.2025; Views: 0; Downloads: 5
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Abstract: In this study, three locally available biomasses, namely miscanthus, hops, sewage sludge, and additionally, their mixtures, were subjected to the torrefaction process to improve their fuel properties. The torrefaction process was conducted at 250–350 ◦C and 10–60 min in a nitrogen (N2) environment. The torrefaction temperature and time were studied to evaluate the selected biomass materials; furthermore, heating values, mass and energy yields, enhancement factors, torrefaction severity indexes (TSI), and energy-mass co-benefit indexes (EMCI) were calculated. In addition, thermogravimetric (TGA) and Fourier transform infrared analyses (FTIR) were performed to characterize raw and torrefied biomass under the most stringent conditions (350 ◦C and 60 min). The results showed that with increasing torrefaction temperature and duration, mass and energy yields decreased, and heating values (HHVs) increased for all studied biomasses. The results of the TSI and EMCI indexes showed that the optimum torrefaction conditions were as follows: 260 ◦C and 10 min for pure miscanthus and hops, whilst this could not be confirmed for the sewage sludge. Furthermore, the combination of sewage sludge and the above-mentioned types of lignocellulosic biomass exhibited better fuel properties than sewage sludge alone.
Keywords: lignocellulosic biomass, sewage sludge, torrefication, thermogravimetry, TSI
Published in DKUM: 10.04.2025; Views: 0; Downloads: 16
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Abstract: The aim of the research was to study the torrefaction processes of wood biomass, compare the product characteristics at different torrefaction temperatures, and assess both moisture adsorption on raw and torrefied samples, as well as metal (Cu(II) and Ni(II)) adsorption on torrefied biomass. The novelty of the research was to investigate whether the presence of adsorbed metals in torrefied biomass significantly affects the energetic properties of the torrefied biomass, compared to torrefied biomass without metals. First, wood samples were torrefied at temperatures of 250 °C, 350 °C, and 400 °C. Following torrefaction, thermogravimetric analysis (TGA) was performed to evaluate mass loss and thermal stability. Next, changes in surface functional groups were examined, and higher heating values (HHV) were measured to assess the energy content. The results showed that torrefaction significantly increased the hydrophobicity of the biomass, leading to reduced moisture adsorption and enhanced material properties. Additionally, the adsorption of Cu(II) and Ni(II) ions on torrefied biomass was investigated. The results showed that the adsorption efficiency for Cu(II) was higher, reaching 62.4%, compared to Ni(II) at 21.2%. The adsorption process followed a pseudo-second-order kinetic model, which indicated that chemisorption was the dominant mechanism.
Keywords: adsorption, torrefication, nitrogen atmosphere, metals
Published in DKUM: 12.03.2025; Views: 0; Downloads: 6
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