Efficient conversion of municipal sludge to practical carbon source via N/M_xO_y in catalytic wet air oxidation

The residual liquid phase after wet air oxidation of sludge has long been recognized as a potential carbon source due to its richness in volatile fatty acids (VFAs). However, nitrogen inevitably accumulates in the form of ammonia during the process, undermining the goal of adding a carbon source to aid bioreactors in nitrogen removal. To address this issue, different N/M_xO_y catalysts were synthesized with noble metal (N=Ru, Pt, Pd) and metal oxide supports (M_xO_y=Al₂O₃, TiO₂, CeO₂, ZrO₂). The entire process comprised the liquefaction of sludge and the subsequent refining of oxidation liquid. The performance of these catalysts was compared using various parameters, including total nitrogen (TN) removal, practical carbon source (PCS) production, and volatile fatty acids (VFAs) production, to ensure both the quantity and quality of the carbon source. Ru/TiO₂ was found to be the best catalyst, achieving a TN removal rate of 94.6 % and a PCS concentration of 2760 mg/L under optimized reaction conditions of 260 °C, 1.5 MPa O₂, 4.0 h, an initial pH of 7.1, and the addition of 3.0 g/L vacuum-dried catalyst. In general, 110.1 g COD carbon source could be produced per kg dry sludge. The reusability of the catalyst was examined, and PCS only decreased 4.7 % after four cycles of usage. The change in other dissolved organic matter was also investigated, with CHON and CHO molecules, as well as lignins and condensed hydrocarbons, being considered the dominant components. This study offers a viable approach for the resource utilization of municipal sludge.