Catalytic Wet Air Oxidation of Sewage Sludge: A Review

Wet air oxidation (WAO) is an attractive technique for sewage sludge treatment. The WAO process and the factors influencing the process are examined in detail, together with the advantages and disadvantages. Catalytic wet air oxidation (CWAO) is emphasized because it can lower operational conditions, and the commonly-used and new homogeneous and heterogeneous catalysts are introduced. Homogeneous catalysts tend to be more appropriate for the CWAO treatment of sewage sludge, and Cu-based homogeneous catalysts such as CuSO4 are the most popular for industrial applications. Heterogeneous catalysts include non-noble metal catalysts, noble metal catalysts, metal-organic frameworks (MOFs) catalysts, and non-metal catalysts. Non-noble metal catalysts typically contain hetero-elements as in Mo-based, Ce-based, Cu-based, Fe-based catalysts, multi-metal supported catalysts, and polyoxometalates catalysts. In general, Mo-based catalysts and Ce-based catalysts have higher activities than other metal-based catalysts. The commonly-used noble metal elements are based on Ru, Pt, Pd, Rh, and Ir. The MOF catalysts tend to have high catalytic activity, and the non-metallic carbon catalysts may be used in environments that would otherwise be toxic to traditional metal catalysts. To conclude, a summary of the challenges and prospects of WAO technology in sewage sludge treatment is given.