Comparison of the impact of three drying processes on the characteristics of Wastewater sludge

Sewage sludge constitute an important by-product of municipal wastewater treatment plants. Their production has grown exponentially during the last decade and was estimated at more than 9 million tons of dry matter in Europe in 2013. Sustainable valorization or disposal options involve direct amendment for crop production or composting. A step of drying is commonly used before, which aims at sanitizing and at reducing the volume of sludge thus lowering the transportation cost. Drying pretreatment processes includes thermal drying, Advanced Oxidation Processes (AOP) or methanation which has the supplementary advantage to produce energy. The objective of this study was to compare the influence of three different processes: thermal drying, electron beam (e-beam) irradiation and anaerobic digestion on the organic matter (OM) characteristics of wastewater sludge (WWS). Indeed the knowledge of the characteristics of this residual biomass is essential to improve their valorization. WWSs OM were investigated at the global scale using elemental analysis, attenuated total reflectance Fourier transform infrared spectroscopy, thermogravimetric analysis and OM fractionation. Double-shot thermochemolysis coupled with gas chromatography and mass spectrometry (GCMS) which has been demonstrated to be an accurate and efficient method for characterizing WWS, was used to compare the diversity and distribution of the molecular contents. A strong influence of thermal drying on lipid and humic-like substances contents was observed through OM fractionation, which traduced a weakening of the OM. The anaerobic digestion induced an increase in lipids for the hydrolysis phase followed by a decrease which correlates with the volume reduction of sludge by about 30%. E-beam induced change in the distribution of the different pools of organic matter depending on the irradiation dose. At the molecular scale, fatty acids, steroids and aromatics were the main thermochemolysis products in all the samples. The thermal drying induced an increase in fatty acids and steroids, probably released from the refractory OM. The anaerobic digestion modified exclusively the amount and distribution of fatty acids while e-beam induced a decrease in all the identified compounds including aromatics. Finally double-shot thermochemolysis-GCMS demonstrated the impact of 3 drying processes on the molecular contents of WWS, which can have consequences on its valorisation.