Multiscale experimental insights into vacuum drying of sludge for enhanced energy efficiency and emission control.

This study provides a comprehensive analysis of the vacuum drying process for sludge drying, with a focus on optimizing energy efficiency and emission control. The study used both lab-scale static and pilot-scale vacuum drying systems to test various parameters like vacuum levels, heat source temperatures, and sludge thicknesses. The results indicated that optimal drying conditions were achieved at a vacuum level of -0.06 MPa, a heat temperature of 140 °C, and a sludge thickness of 3.4 mm, where the drying rate reaches 0.13278 g·g-1·min-1. The study underscores the significant influence of vacuum level, temperature, and sludge thickness on drying rates. The Page model was used to analyze drying kinetics, elucidating how changes in these parameters affect drying characteristics. Furthermore, the study also examined the pollutant emissions and energy efficiency at the pilot scale. It found that high vacuum environments could efficiently dry sludge using low-temperature heat source, leading to average energy consumption per unit evaporation of 3020.29 kJ/kg, which is lower compared to traditional methods. By harnessing low-grade industrial waste heat, this can be further reduced to 875.76 kJ/kg. This study offers valuable insights for sustainable sludge management systems, highlighting the environmental and economic benefits of vacuum drying technology. The detailed experimental approach and thorough analysis make a significant contribution to the field of the sludge drying.

Impacts of poly(lactic acid) microplastics on organic compound leaching and heavy metal distribution during hydrothermal treatment of sludge.

This study provides an in-depth examination of the role of poly(lactic acid) microplastics (PLA-MPs) during sludge treatment, particularly in relation to organic compound leaching and heavy metal distribution. Through the application of advanced analytical techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermal analysis, and gas chromatography-mass spectrometry (GC-MS), the release of degradation byproducts was quantified, and the effects on organic compound leaching and heavy metal distribution were assessed. Specifically, the results demonstrated that PLA-MPs significantly impacted the hydrolysis reaction, with the pH value descending in pure water as the hydrothermal temperature escalated. At 140 °C, the hydrolysate contained 20.66 % propylene ester and 16.57 % lactic acid. Furthermore, an increase in total organic carbon (TOC) was observed with increasing temperature, with TOC content at 140 °C in water almost doubling from that at 120 °C and 130 °C. With respect to heavy metals, the presence of PLA-MPs influenced the migration of Cr(VI) between solid and liquid phases in sludge. Notably, after 180 °C hydrothermal treatment, the content of Cr(VI) in the liquid phase of sludge with PLA-MPs was 9.72 %, which is higher than that of sludge without PLA-MPs at 5.80 %. These findings underline the need to consider PLA-MPs' influence on organic compound leaching and heavy metal distribution during sludge treatment.

Aging of polylactic acid microplastics during hydrothermal treatment of sewage sludge and its effects on heavy metals adsorption.

Microplastics' (MPs) aging process and environmental behavior have attracted extensive attention due to the potential long-term ecological impact. MPs enriched in sludge may accelerate aging during sludge treatment and the affecting environmental behavior, i.e., adsorption performance for pollutants. However, the related studies have not been well researched, especially for the biodegradable MPs. This study revealed the influences of hydrothermal treatment on the characteristics of polylactic acid microplastics (PLA-MPs) and the consequences on heavy metals adsorption. The changes in PLA-MPs' physiochemical properties were characterized and compared. PLA-MPs' surface became irregular, and the oxygen-containing functional groups increased through FTIR and XPS analysis. Meanwhile, the molecular weight and crystallinity of PLA-MPs decreased significantly with the rising in hydrothermal temperature. Accordingly, the adsorption capacity of PLA-MPs for Pb2+ increased from 93.97 µg g-1 for the raw PLA-MPs to 1058.03 µg g-1 for the aged PLA-MPs. Multiple adsorption kinetics and isotherms were discussed for the Pb2+ adsorption onto PLA-MPs with different aging of the PLA-MPs. The adsorption mechanisms of Pb2+ relate to electrostatic interaction and complexation. The main difference is that the adsorption for raw PLA-MPs is dominated by physical and chemical adsorption, whereas the adsorption for the aged PLA-MPs prefers chemical adsorption. In addition, we carefully evaluated the influences of pH, dissolved organic matter, and ionic strength on the PLA-MPs adsorption. The present study highlighted the significance of hydrothermal treatment on the MPs aging and the adsorption performance.

Insights into the characteristics and mechanism of vacuum drying technology for municipal sludge processing.

Vacuum drying is an effective approach for sludge treatment and valorization. However, the vacuum drying of sludge has not been industrialized at present. The objective of this study was to elucidate the vacuum drying characteristics of static sludge and crack initiation mechanism. Our results indicate that crusting on the sludge surface under a high vacuum inhibited drying by reducing major cracks at sludge thicknesses of 13.6 and 10.2 mm. The inhibition effect weakened with decreasing sludge thickness. At 6.8 mm, the mean drying rate (VM) was the lowest at 0.08 MPa, while VM decreased with increasing vacuum degree at thicknesses of 13.6 and 10.2 mm. The decrease in drying rate could be attributed to rapid evaporation on the sludge surface under a high vacuum, leading to crusting, which inhibited crack initiation. VM was raised by 67.9-162.2% from 10.2 to 6.8 mm because the suction force of vacuum on water was much higher than the resistance to water diffusion of small isolation piles at 6.8 mm. Additionally, this study provided essential information to improve existing sludge treatment methods.

The application of sulfate radical-based advanced oxidation processes in hydrothermal treatment of activated sludge at different stages: A comparative study.

Integrating hydrothermal treatment (HT) and advanced oxidation processes (AOP) was proved to be a promising approach for improving sludge dewaterability. In this study, the EPS valorization under elevated temperature and sulfate radical-based AOP were investigated to clarify the valorization of organic matter in different EPS layers and its effects on the sludge dewaterability. Results indicated that the organic matters in the inner layer of EPS decreased sharply with the elevated temperature, and released into the soluble EPS. Sulfate radical-based AOP significantly accelerated the degradation of organics and microbial cells lysis, especially in the presence of ZVI. The protein with the higher hydrophobicity was detected under the AOP enhanced HT. A better synergistic effect on sludge dewaterability was obtained by integrated the AOP at the initial hydrothermal stage. 3D-EEM and parallel factor analysis indicated that the protein and microbial by-product like substances in tightly bound EPS significantly affected the dewaterability.

The sorption and short-term immobilization of lead and cadmium by nano-hydroxyapatite/biochar in aqueous solution and soil.

In this study, the composite materials using different ratios of biochar (BC) to nano-hydroxyapatite (nHAP) were prepared for the remediation of lead (Pb) and cadmium (Cd) contaminated water and soil. The sorption and the immobilization experiments indicated a higher sorption capacity and immobilization efficiency of Pb compared to those of Cd. The characteristics of XRD, FTIR, SEM, and XPS manifested that dissolution-precipitation, cation exchange, complexation, and cation-π interaction were the main four mechanisms for the sorption of Pb2+ and Cd2+ using composite material PC1 (nHAP/BC = 1/1). From semi-quantitative analysis, the mineral effect accounted for the majority of the immobilization of Pb and Cd. Due to obvious Pb-precipitates in the sorbed material, dissolution-precipitation primarily affected the sorption of Pb using PC1, while the immobilization of Cd was mainly attributable to cation exchange. Such results corresponded to the stable Pb-precipitates and unstable Cd-compounds in soil, among which the latter was prone to be released into the environment. The sorption capacity in aqueous solutions and the immobilization efficiencies in the soil for both Pb and Cd increased with the addition of nHAP, which were linearly correlated to the nHAP proportion in the composite materials. In future practical applications, the percentages of composite materials can be designed according to the specific pollutant concentration. This study sheds light on the explicit immobilization mechanisms for Pb and Cd in aqueous solutions to better understand their behaviors in the soil remediated by relevant materials.

Insights into dynamic adsorption of lead by nano-hydroxyapatite prepared with two-stage ultrasound.

Nano-hydroxyapatite (nHAP) has an excellent effect on the remediation of Pb contaminated water and soil. In this study, an efficient modified nHAP was prepared assisted with two-stage ultrasonic irradiation. The effects of ultrasound modification on the nHAP were tested using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform InfraRed spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) and combined batch Pb uptake experiments. The nHAP with ultrasound has a fine structure with the width and length of around 9 nm and 40 nm respectively. The ultrasound parameter of 1s/36s in stage 1 and 16s/18s in stage 2 was verified as the optimum under which the nHAP prepared performed the best with the maximum adsorption capacity of 1300.93 mg/g. The results of XRD and SEM indicated that the sorbent after uptake of Pb2+ was mainly Pb10(PO4)6OH2 (HPY) with insignificant Ca10Pb10-x(PO4)6OH2. Compared the results of Pb/Ca, pH and XRD with the metal fraction of Pb in adsorbents during the dynamic sorption process, this research proved that the effects of complexation, cation exchange and dissolution and precipitation coexisted in the initial stage, while the dissolution and precipitation gradually dominated the adsorption mechanism with contact time. The processes of Pb2+ uptake by nHAP sorbents prepared under different ultrasound parameter presented almost the same dynamic mechanism with a little difference in time node. The research of dynamic mechanism of Pb2+ uptake by a superior nHAP is essential for both contaminated water and soil remediation.