Novel insights into the biopolymers transformation under wastewater sludge drying process at different temperatures in relation to drying behavior

The biopolymers are regarded as the leading role on the water evaporation in thermal drying process for sludges. Herein, the relationships between molecular transformation and water-holding capacities of biopolymers in sludge drying was investigated by multispectral methods coupled with FT-ICR MS technique. Our results showed that the exposure of hydrophobic groups in biopolymers was affected by drying temperature, causing significant differences upon water molecules binding and evaporation. In particular, below 120 C-degrees, there was no obvious changes on the structural compositions of hydrophilia and hydrophobic compounds in sludge. Between 120 C-degrees and 150 C-degrees, the macromolecules were broken into smaller molecules with abundant hydrophobic groups exposing, resulted in efficient water evaporation with lower moisture in sludge. However, organics containing -NH and -C=O groups were aggregated via Maillard reaction at higher temperature (>150 C-degrees), the water molecules were trapped inside the Maillard products, and more energy was required to destroy the structure of organics for water evaporation, ended with higher moisture of dried sludges. In addition, a strong correlation between the hydrophilic index and CHON/CHONS distributions of extracted proteins in dried-sludge (DS) indicated that the water holding capacity and evaporation relied more on the spatial exposure of hydrophilic groups rather than their content. These implied that preventing the aggregation of hydrophilic functional groups (-NH and -C=O) in proteins was the key to eliminating the inhibitory of hydrophilic compounds in the biopolymers-bound water evaporation, and drying temperature below 150 C-degrees was recommended. This study provides a mechanistic basis for better understanding of the biopolymer transformation (especially hydrophilic compounds) in drying process, and enable development of a more efficient and energy-saving drying procedure for sludges.