Effect of the micron-sized silica particles (MSSP) on biogas conversion of sewage sludge

Micron-sized silica particles (MSSP), ranging from 3 μm to 50 μm, have been widely found in the sewage sludge. The inhibition of MSSP to biogas conversion of both excess sludge (ES) and model sludge (MS) are explored in this study. It is observed that with the effect of MSSP, the net cumulative methane production (NCMP) of ES and MS were decreased by 23.5% and 22.3%, respectively, and the apparent activation energy (AAE) of organic solubilisation of ES and MS were increased by 38.7% and 215%, respectively, which implies a crucial role for MSSP in anaerobic sludge digestion. Analysis of physicochemical properties of sludges before and after interaction with MSSP reveals that MSSP can bond with organic matter from sludge on the surface sites to form a larger bioinorganic-floc. Further analysis indicates that MSSP can increase the AAE of sludge organic solubilisation by reducing the surface site density, thus resulting in poor NCMP. Through characterizing the bioinorganic-floc, it is found that the protein in sludge is the main component that bonds with MSSP. Further research show that the interactions between protein and MSSP are mainly enthalpy-driven with exothermic (the enthalpy was about -10.93 ± 0.10 kJ/mol, at 25 °C), indicating that protein is more stable after non-covalent bonding. These findings can provide a new understanding of the characteristics of sludge and important references for the improvement of anaerobic sludge digestion.