Production of useful organic matter from sludge using hydrothermal treatment

The need for readily biodegradable organic matter to support denitrificatio n and enhanced biological phosphorus removal (EBPR) can be satisfied using external or internal carbon sources. This paper examines the production and removal of volatile fatty acids (VFAs), mainly acetic acid, and other hydr olyzed products from sludge using hydrothermal treatment at subcritical (<3 74 degrees C) and supercritical (>374 degrees C) temperatures. Subcritical water oxidation (SubCWO) achieved effective hydrolysis (> 95% as GOD) of th e sludge organic solids but incomplete oxidation (< 95% as GOD) of the orga nic component of sludge. SubCWO also produced GOD-rich liquors, which conta ined as much as 30% wt/wt hydrolyzed GOD, including 10% wt/wt acetate. The contribution of acetate to COD in the liquor increased as treatment progres sed, accounting for as much as 80% of the soluble COD when the oxidation ef ficiency exceeded 80%. While SubCWO resulted in partial COD removal, comple te oxidation (>99% as GOD) of the organic component of sludge, including ox idation of hydrolysis products, was achieved using supercritical water oxid ation (SCWO). Accordingly, hydrothermal treatment using SubCWO and SCWO can provide a useful sludge management option, combining sludge mass and volum e reduction with the production of useful organic matter to support biologi cal nutrient removal. (C) 2000 Elsevier Science Ltd. All rights reserved.