Changes in Selected Organic and Inorganic Compounds in the Hydrothermal Carbonization Process Liquid While in Storage

Anaerobic valorization of sewage sludge pretreated through hydrothermal carbonization: Volatile fatty acids and biomethane production

Hydrothermal carbonization (HTC) emerged as an effective technology for the treatment of various types of wet biomass and organic residues, including sewage sludge, offering the potential for sludge reduction and resource recovery. HTC pretreatment impact on downstream sludge fermentation is investigated. Results obtained at optimal conditions for HTC pretreatment (170 °C for 30 min) indicated that soluble carbon was significantly increased in the liquid fraction, enhancing feedstock availability for fermentation. Semi-continuous fermentation of HTC-treated sludge resulted in a stable process in which a mixed microbial community produced volatile fatty acids (VFAs) with longer chain acids content, acidification yield of 0.59 ± 0.05 g COD-VFA g-1 CODin and volumetric productivity of 1.6 ± 0.5 g COD-VFA L-1 d-1. Biomethane Potential tests evidenced high values for hydrochar. Overall, the HTC pretreatment enables improved conversion efficiencies, in the view of valorizing the liquid for VFA synthesis and the hydrochar for biomethane production.

The effect of an acidic environment during the hydrothermal carbonization of sewage sludge on solid and liquid products: The fate of heavy metals, phosphorus and other compounds

The pH of sewage sludge is a crucial factor during the hydrothermal carbonization process that influences the characteristics of the resulting products and migration of certain compounds from the solid to liquid phase. Accordingly, this work is focused on examining the pH impact during the HTC process, in particular, pH equals 2, 3, 4, 5 and 6 on the individual hydrothermally carbonized products generated at 200 °C and 2 h residence time. For this reason, the chemical and physical indicators describing the post-processing liquid and hydrochar were determined. For instance, it was observed that the phosphorus content detected in the liquid, derived at pH2, rose significantly by 80%. Furthermore, decreasing the pH of sewage sludge had a significant impact on the ash content and the calorific value of the hydrochar. Additionally, changes in the specific surface area of hydrochar were noticed: pH = 5 and pH = 6 showed an increase of 20-30%, while for lower pH values a decrease of c.a. 26% was achieved. The distribution of heavy metals between the obtained fractions in the HTC process (solid and liquid) indicated that 92 to almost 100% of the tested heavy metals were transferred to the hydrochar. A significant effect of pH on the distribution between these fractions was observed only for Zn and Ni. For instance, for pH = 2, Zn and Ni in post-processing liquid were 34% and 29%, respectively. In addition, the sequential extraction of heavy metals from hydrochar was also performed in order to identify mobile and non-mobile phases. It was noticed that the acidic environment favours a higher amount of mobile heavy metals in hydrochar. The largest effect was observed for Cd, Pb, Cr and Cu, for which, at pH = 2, their respective amounts in the mobile fraction were 2.7; 3.6; 1.8; 6.2 times higher, compared to the hydrochar without pH correction.

Machine learning-based modeling of malachite green adsorption on hydrochar derived from hydrothermal fulvification of wheat straw

This study investigated the efficiency of hydrochar derived from hydrothermal fulvification of wheat straw in adsorbing malachite green (MG) dye. The characterizations of the hydrochar samples were determined using various analytical techniques like SEM, EDX, FTIR, X-ray spectroscopy, BET surface area analysis, ICP-OES for the determination of inorganic elements, elemental analysis through ultimate analysis, and HPLC for the content of sugars, organic acids, and aromatics. Adsorption experiments demonstrated that hydrochar exhibited superior removal efficiency compared to feedstock. The removal efficiency of 91 % was obtained when a hydrochar dosage of 2 g L-1 was used for 20 mg L-1 of dye concentration in a period of 90 min. The results showed that the study data followed the Freundlich isotherms as well as the pseudo-second order kinetic model. Moreover, the determined activation energy of 7.9 kJ mol-1 indicated that the MG adsorption was a physical and endothermic process that increased at elevated temperatures. The study also employed an artificial neural network (ANN), a machine learning approach that achieved remarkable R2 (0.98 and 0.99) for training and validation dataset, indicating high accuracy in simulating MG adsorption by hydrochar. The model's sensitivity analysis demonstrated that the adsorbent dosage exerted the most substantial influence on the adsorption process, with MG concentration, pH, and time following in decreasing order of impact.

FTIR spectrum analysis to predict the crystalline and amorphous phases of hydroxyapatite: a comparison of vibrational motion to reflection

Hydroxyapatites were synthesized from calcium carbonate and ortho-phosphoric acid in amorphous and crystalline phases by varying sintering temperature from 300 to 1100 °C maintaining an increment of 200 °C. The asymmetric and symmetric stretching, and bending vibrations of phosphate and hydroxyl groups were explored in Fourier transformation infrared (FTIR) spectra. Although the FTIR spectra revealed identical peaks in the full range (400-4000 cm^-1 wavenumber), the narrow spectra exerted variations by splitting peaks and intensity. The intensities of peaks at 563, 599, 630, 962, 1026, and 1087 cm^-1 wavenumbers were intensified gradually with the augmentation of sintering temperature, and the relation between the relative peak intensity and sintering temperature was correlated with the aid of the good linear regression coefficient. Peak separations were also found in the case of 962 and 1087 cm^-1 wavenumbers when the sintering temperature was equal to or exceeded 700 °C. The conventional X-ray diffraction (XRD) technique was also employed to explore the crystalline and amorphous phases of synthesized hydroxyapatites.