Production of useful organic matter from sludge using hydrothermal treatment

The need for readily biodegradable organic matter to support denitrification 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 hydrolyzed products from sludge using hydrothermal treatment at subcritical (<374°C) and supercritical (>374°C) temperatures. Subcritical water oxidation (SubCWO) achieved effective 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 efficiency 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.