Structural characteristics of organics released from sludge pretreatment and their performance in the synthesis of biomass plastics

Abstract

The conversion of organics from sludge into high-value added products is essential for achieving circular economy and sustainable development in wastewater treatment processes. Currently, research focuses on promoting the release of organics from sludge, but there is a lack of in-depth understanding of the properties and structures of the released organics. To find out a suitable sludge pretreatment method (ultrasound, alkaline, thermal, ultrasound-alkaline, alkaline-thermal hydrolysis) for biodegradable plastic synthesis, the composition, properties and structures of the released organics from sludge were studied. Alkaline treatment (pH 12) resulted in the highest surface charge with Zeta potential of –23.53 mV. The release concentration and the radius of rotation (17.79 nm) of proteins were maximized after pH 12 + 150 °C pretreatment. Ultrasound treatment disrupted the extracellular polymeric substances of sludge, releasing organics with the highest thermal stability and hydrophobicity indices (20,313). Simultaneously, β-sheets was the main secondary structure of proteins. Random coils were the main proteins secondary structure after the other pretreatments instead. Meanwhile, small-angle scattering calculations showed that the released proteins tended to be discoidal for all pretreatment methods. In addition to proteins, polysaccharides, and humic acids, small organic compounds were also produced, such as amino acids (D-alanine), aromatic compounds (pyridine), alcohols, sugars (pyranose), fatty acids (oleic and palmitic), small molecule acids (acetic), etc. Furthermore, pretreatment with pH 12 + 150 °C was more efficient for the whole process of sludge synthesis into plastics. Correlation analysis shows that the radius of rotation of proteins is the main factor affecting the tensile strength and elongation at break of synthetic plastics. This study conducted an in-depth analysis of the structure and properties of organics after different pretreatments and provided a new pathway for the resource utilization of sludge through the synthesis of plastics using sludge organics.