Treatment of sewage sludge using hydrothermal oxidation - technology application challenges

The objective of this paper is to examine the technical feasibility of hydr othermal oxidation as a sewage sludge treatment option. Hydrothermal oxidat ion involves treatment at temperatures and pressures below and above the cr itical point for water, 374.2 degrees C and 22.1 MPa. Subcritical water oxi dation (SubCWO) achieves incomplete sludge oxidation (<95% COD removal) and produces high-strength liquors containing significant quantities of volati le 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 arid volume reductions. Sup ercritical water oxidation (SCWO) on the other hand can completely oxidize the organic component of sludge (>99.9% COD reduction), produce high qualit y effluents and disposable ashes and air emissions,The major engineering ch allenges associated with developing hydrothermal treatment systems, especia lly SCWO systems, are solids management, corrosion and safety. The solids m anagement challenge relates to scale formation, including deposition of ino rganic salts and transport of suspended particles. The corrosion challenge relates to handling highly halogenated and extreme-pH waste streams. Nevert heless, the available body of technical knowledge supports the design of ef ficient SCWO sludge treatment systems.