Aqueous phase of thermo-catalytic reforming of sewage sludge - quantity, quality, and its electrooxidative treatment by a boron-doped diamond electrode

Pyrolysis is a promising conversion technology to produce biofuels from residues such as sewage sludge. In this study, the chemical composition and the amount of the resulting aqueous phase from the thermo-catalytic reforming process of sewage sludge at different process parameters are compared to those of non-catalytic fast pyrolysis processes from literature. Furthermore, electrooxidation using a boron-doped diamond (BDD) electrode was investigated as purification technology for the resulting aqueous phase. The results of this study show that the quantity of the aqueous phase strongly depends on the temperature of the thermo-catalytic reforming process and is presumably governed by the influence of secondary gas-carbonisate reactions like the water gas shift reaction. Higher reaction temperatures result in a reduced amount of the aqueous phase. The chemical composition of the aqueous phase of fast pyrolysis processes was comparable to that obtained at the thermo catalytic reforming process. To reduce the chemical oxygen demand (COD) and the number of refractory sub stances to a level at which the aqueous phase can be discharged into a conventional wastewater treatment plant, electrooxidation experiments with the aqueous phase of the thermo-catalytic process were carried out using a BDD electrode. The treatment efficiency was investigated in galvanostatic operation varying the applied current density and the flow rate of the aqueous phase. A COD removal of 94.6 % was achieved after 0.54 kWh at an instantaneous current efficiency of 0.47 at a current density of 50 mA/cm2 and a flow rate of 3.7 L/min. This indicates that electrooxidation by a BDD electrode can be a promising purification technology for the aqueous phase from pyrolysis of sewage sludge. However, the high energy consumption of the process will need to be addressed in future studies to make the process economically feasible.