Unlocking the potential flame-retardant mechanisms of extracellular polymeric substances recovered from municipal sludge

Limited understanding of the thermal properties and flammability of extracellular polymeric substances (EPS) from municipal sludge has constrained their use in fire safety applications. This study investigates the thermal stability and flame retardancy of EPS derived from secondary sludge, waste sludge, and digested sludges. Comprehensive analyses of chemical composition, activation energy, char formation, and heat and gas release during pyrolysis and combustion reveal potential flame-retardant mechanisms. All EPS from different types of sludge can release both free-radical scavenging compounds and non-flammable gases during combustion to reduce fire propagation in the gas phase. In the condensed phase, EPS demonstrate their char-forming capability as a major flame-retardant mechanism to diminish heat release rate due to the presence of nitrogen, phosphorus, and sulfur elements. While nitrogen-based compounds in EPS contribute to flame-retardant mechanisms in both phases, phosphorus and sulfur mainly act in the condensed phase. Additionally, EPS from secondary sludge show the highest activation energy (372.3 kJ/mol) and ignite 24 s later than other EPS. Cone calorimeter tests confirm that EPS from secondary sludge have superior fire-resistant properties, with increased char contents (31.2 %) and enhanced thermal stability compared to other EPS. This study provides the first comprehensive assessment of EPS as eco-friendly flame retardants, advancing wastewater sludge valorisation and fire safety applications.