Advanced thermal hydrolysis for biopolymer production from waste activated sludge: Kinetics and fingerprints

Abstract

Waste activated sludge (WAS) is the main residue of wastewater treatment plants, which can be considered an environmental problem of prime concern due to its increasing generation. In this study, a non-energetic approach was evaluated in order to use WAS as a renewable resource of high value-added products. For this reason, WAS was treated by thermal hydrolysis, H2O2 oxidation and advanced thermal hydrolysis (ATH) promoted by H2O2. The influence of temperature, H2O2 concentration and dosing strategy on biomolecule production (proteins and carbohydrates), size distribution (fingerprints) and various physico-chemical parameters (VSS, total and soluble COD, soluble TOC, pH and colour) was studied. The results revealed a synergistic effect between TH and H2O2 oxidation, which led to a significant increase in the production of both proteins and carbohydrates. In this sense, the concentration of proteins and carbohydrates obtained during TH at 85 °C for120 min was found to be 1376 ± 9 mg/L (121 mg/gVSSo) and 208 ± 4 mg/L (18 mg/gVSSo), respectively. However, in the presence of 4.5 mM H2O2/gVSSo under the same process conditions, the concentrations of proteins and carbohydrates exhibited a significant increase of 1.9-fold and 3.1-fold, respectively. Besides, the addition of H2O2 promoted the transformation of hydrophobic compounds, such as proteins and or lipids, into hydrophilic compounds, which presented low and medium sizes. An increase in temperature improved the solubilization rate and the yield of biomolecules significantly. Besides, the analysis of the kinetics related to the dosing strategy of H2O2 suggested the existence of two fractions during WAS solubilization, one of them being easily oxidizable, whereas the other one was more refractory to oxidation. Thus, the value of kH2O2 for the first addition of 1 mM H2O2/g VSSo was 0.020 L0.4 mgH2O2−0.4 min−1, while it was 4.3 and 8 times lower for the second and third additions, respectively.