Treatment of sewage sludge using hydrothermal oxidation – technology application challenges

The objective of this paper is to examine the technical feasibility of hydrothermal oxidation as a sewage sludge treatment option. Hydrothermal oxidation involves treatment at temperatures and pressures below and above the critical point for water, 374.2°C and 22.1 MPa. Subcritical water oxidation (SubCWO) achieves incomplete sludge oxidation (<95% COD removal) and produces high-strength liquors containing significant quantities of volatile fatty acids (VFAs). Controlled liquor recycling into the treatment plant can provide a useful carbon source to support biological nutrient removal (BNR). SubCWO also achieves efficient destruction of the organic component of sludge solids, resulting in significant mass and volume reductions. Supercritical wateroxidation (SCWO) on the other hand can completely oxidize the organic component of sludge (>99.9% COD reduction), produce high quality effluents and disposable ashes and air emissions. The major engineering challenges associated with developing hydrothermal treatment systems, especiallySCWO systems, are solids management, corrosion and safety. Thesolids management challenge relates to scale formation, including deposition of inorganic salts and transport of suspended particles. The corrosion challenge relates to handling highly halogenated and extreme-pH waste streams. Nevertheless, the available body of technical knowledge supports the design of efficient SCWO sludge treatment systems.