Abstract
Hydrothermal carbonization (HTC) is an energy-efficient thermochemical process for converting wet waste products into value added materials for water treatment. Understanding how HTC influences the physicochemical properties of the resultant materials is critical in optimizing the process for water treatment, where surface functionality and surface area play a major role. In this study, we have examined the HTC of four wet waste streams, sewage sludge, biosludge, fiber sludge, and horse manure at three different temperatures (180 °C, 220 °C, and 260 °C). The physicochemical properties of these materials were examined via FTIR, SEM and BET with their adsorption capacity were assessed using methylene blue. The yield of solid material after hydrothermal carbonization (hydrochar) decreased with increasing temperature for all samples, with the largest impact on horse manure and fiber sludge. These materials also lost the highest degree of oxygen, while HTC had minimal impact on biosludge and sewage sludge. The differences here were due to the varying compositions of each waste material, FTIR identified resonances related to cellulose in horse manure and fiber sludge, which were not detected in biosludge and sewage sludge. Adsorption capacities varied between 9.0 and 68 mg g−1 with biosludge HTC at 220 °C adsorbing the highest amount. Adsorption also dropped drastically at the highest temperature (260 °C), indicating a correlation between adsorption capacity and HTC conditions. This was attributed to the loss of oxygen functional groups, which can contribute to adsorption. These results suggest that adsorption properties can be tailored both by selection of HTC temperature and feedstock.
Highlights
Benign disposal of wet waste materials, such as sewage sludge and manures, is an expanding global problem
The highest yields were observed for the lowest temperature, an expected result as higher temperatures in Hydrothermal carbonization (HTC) typically lead to lower yields (Rodriguez Correa et al 2017; Yahav Spitzer et al 2018; Zhou et al 2019)
Fiber sludge and horse manure are rich in lignocellulosic materials (Dong et al 2016; Muhammad Nasir and Mohd Ghazi 2015), whose components decompose at different temperatures (Atta-Obeng et al 2017; Bobleter 1994; Kang et al 2012)
Summary
Benign disposal of wet waste materials, such as sewage sludge and manures, is an expanding global problem. In China, 3.8 billion tons of manure is produced annually and sewage sludge production expected to rise to 52.92 million tons by 2020 (Gao et al 2019). These amounts are expected to further increase with global population and living standards over the century. These wet waste materials are disposed in landfill, incinerated, or utilized as fertilizers; this is problematic because of contaminates (e.g., heavy metals, ash, pharmaceuticals, personal care products (PPCPs), and pathogens), bans on landfilling combustible materials, and eutrophication/acidification from nitrogen and phosphorus leeching. The high moisture content, up to 97% w/v (Tasca et al 2019), makes transportation costly and limits the disposal of these materials to facilities in proximity to the waste source
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