Relation between hydrophilic/hydrophobic characteristics of sludge extracellular polymeric substances and sludge moisture-holding capacity in hot-pressing drying.

Sludge poses a serious threat to the environmental health. Hot-pressing drying has been proven efficient in sludge treatment because of the reduced thermal contact resistance, rapid increase in sludge temperature, and high drying rate. Sludge extracellular polymeric substances (EPS) significantly influence moisture transfer. However, whether in hot-pressing or traditional thermal drying, the effect of EPS on sludge moisture-holding capacity is rarely reported. Thereby, this study investigated the relationship between hydrophilic/hydrophobic characteristics of EPS and sludge moisture-holding capacity at various drying time and mechanical compression using XAD resin fractionation. Thermodynamic analysis indicated that sludge moisture desorption isotherms, net isosteric heat of desorption, and differential entropy presented a downward trend with the increase in drying time and mechanical compression, suggesting reduced sludge moisture-holding capacity. EPS analysis showed that at the same drying time, applying 25 kPa mechanical compression increased sludge temperature by 16 % and protein content by 13.8 %. At the same sludge temperature, protein content rose by 7.3 % compared to the drying without mechanical compression. It was concluded that the fast rise in sludge temperature and the mechanical extrusion facilitated the destruction of sludge microbial flocs, accelerating the release of intracellular and EPS-bound moisture and contributing to the decrease in moisture-holding capacity. Besides, tryptophan protein-like substances were the major source of hydrophilic/hydrophobic organic matter, compared to polysaccharide and humic acid-like substances. The gradually reduced sludge moisture-holding capacity was divided into three stages. Below 67 °C, the moisture desorption was dominated by the release of intracellular moisture. Below 85 °C, the increase in protein and the enhanced exposure of hydrophobic functional groups in protein improved the hydrophobicity of EPS. Above 85 °C, protein consumption due to thermal decomposition and browning reaction facilitated the desorption of EPS-bound moisture. Hence, this study provided novel insights into the mechanism of sludge drying.

Experimental analysis on products distribution and characterization of medical waste pyrolysis with a focus on liquid yield quantity and quality.

The massive generation of medical waste (MW) poses a serious risk to the natural environment and human health. The pyrolysis technique is proposed as a potential treatment for MW to tackle the associated environmental issues and produce value-added products. In this work, medical waste pyrolysis has been conducted at various temperatures using a fixed bed reactor with a 20 °C·min-1 heating rate and nitrogen was used as a career gas with a flow rate of 100 ml·min-1. In addition, the effect of temperature on products yield and chemical composition of MW pyrolysis have been investigated. The maximum yield of 57.1% for liquid oil was observed from the mixed MW pyrolysis at 500 °C. The gas and char yield were found between 26.5-37.3% and 24.2-12.4%, respectively, for the pyrolysis temperature of 450 °C to 600 °C. According to GC analysis, the concentration of the main gaseous products such as CH4, H2, and C2H4 was increased with increasing temperature, while CO and CO2 experienced a decreasing trend. The results of GC-MS analysis revealed that the main components of MW pyrolysis oil were aromatic hydrocarbons, cyclic hydrocarbons, aliphatic hydrocarbons, alcohol, carboxylic acids, and their derivatives. The aromatic and cyclic hydrocarbons content increased up to 38.2% at a pyrolysis temperature of 600 °C. As pyrolysis oil tends to have more long-chain hydrocarbons therefore carbon distributions from C7 to C35 were observed. The ultimate analysis of oil and char revealed that the increased temperature enhanced the carbon content up to 78.6% and 68.0%, respectively. Furthermore, the higher heat values of 41.8, 24.4, and 52.7 MJ·kg-1 were reported for oil, char, and gas, respectively.

Enhancement of conductive drying of sewage sludge with mechanical compression: Drying kinetics, and interfacial heat transfer behavior.

Improving sludge drying efficiency is of tremendous importance for public health, subsequent treatment, and comprehensive utilization. The interfacial thermal resistance between sludge and hot wall greatly limits the conductive drying performance. This study employed mechanical compression to decrease the interfacial thermal resistance. The drying kinetics and interfacial heat transfer behavior were investigated at mechanical loads of 25 to 200 kPa, temperatures of 120 to 210 °C, and sludge thicknesses of 1.0 to 3.0 mm, and were compared to those in the conventional drying process without mechanical load. The increase of temperature and mechanical load and the decrease of thickness improved drying rates. The drying experienced one warm-up period and two falling rate periods. The breakthrough of interfacial vapor film was responsible for the rapid rise in drying rates initially. At the thickness of 3.0 mm, 210 °C, and 100 kPa, the effective moisture diffusivity was increased by 2.5 times, and the apparent activation energy was reduced by 34% compared to the traditional process in the first falling rate period, implying that mechanical compression facilitated moisture migration and bound water desorption. The effective moisture diffusivity in the first falling rate period was increased by 35% compared to the diffusivity in the second falling rate period because of the pressure-driven flow. The decrease in drying rates was due to the transformation from the pressure-driven flow to vapor diffusion-limited flow in the first falling rate period. Additionally, this study provided essential information on developing a new sludge treatment method and establishing the drying model.

Mechanical compression assisted conductive drying of thin-film dewatered sewage sludge: Process performance, heat and mass transfer behavior.

The improvement in heat transfer efficiency between the hot wall and sewage sludge was a critical issue to enhance the conductive drying performance. The drying behavior of thin-film dewatered sewage sludge was investigated based on a conductive dryer assisted with mechanical compression at hot wall temperatures of 120-210 °C. The heat and mass transfer behavior of the sludge in the conductive drying process alone was compared to those in the mechanical compression assisted conductive drying process at three external mechanical loads of 25, 100, and 200 kPa. The average drying rates with mechanical compression were higher than those without mechanical load and were enhanced with the increase of mechanical loads at 210 °C. The extrusion of interfacial vapor film and the reduction of sludge surface roughness was responsible for the enhanced interfacial heat transfer efficiency under mechanical compression. The effective moisture diffusivity, mass transfer coefficient, and effective thermal conductivity were enhanced by mechanical compression. The improved moisture transfer inside sludge and on the open surface, and the decreased heat transfer resistance of sludge was due to the generated pressure-driven flow and the reduced gas cavities in sludge, resulting in the higher drying rates. Additionally, this finding provided reference data for developing a new sludge drying method.