Pii: S0043-1354(99)00222-5

ÐThe need for readily biodegradable organic matter to support denitri®cation and enhanced biological phosphorus removal (EBPR) can be satis®ed using external or internal carbon sources. This paper examines the production and removal of volatile fatty acids (VFAs), mainly acetic acid, and other hydrolyzed products from sludge using hydrothermal treatment at subcritical (<3748C) and supercritical (>3748C) temperatures. Subcritical water oxidation (SubCWO) achieved e€ective hydrolysis (>95% as COD) of the sludge organic solids but incomplete oxidation (<95% as COD) of the organic component of sludge. SubCWO also produced COD-rich liquors, which contained as much as 30% wt/wt hydrolyzed COD, including 10% wt/wt acetate. The contribution of acetate to COD in the liquor increased as treatment progressed, accounting for as much as 80% of the soluble COD when the oxidation eciency exceeded 80%. While SubCWO resulted in partial COD removal, complete oxidation (>99% as COD) of the organic component of sludge, including oxidation of hydrolysis products, was achieved using supercritical water oxidation (SCWO). Accordingly, hydrothermal treatment using SubCWO and SCWO can provide a useful sludge management option, combining sludge mass and volume reduction with the production of useful organic matter to support biological nutrient removal. # 2000 Elsevier Science Ltd. All rights reserved Key wordsÐhydrothermal treatment, subcritical water oxidation, supercritical water oxidation, volatile fatty acids, acetic acid, biological nutrient removal, sludge treatment