Subcritical wet oxidation of municipal sewage sludge: comparison of batch and continuous experiments

Wet oxidation in subcritical conditions is a new alternative to usual route s for sewage sludge treatment and it complies with environmental standards. The paper presents tests carried out on a batch reactor and on a continuou s pilot unit, treating municipal sewage sludge. A method is proposed that s hows that the oxidation efficiency in a continuous reactor can only be easi ly predicted from the residence time distribution and batch tests results. Nevertheless, a partial settling of the solid residue in the continuous to bubble column reactor is evident, and it increases the solid residence time and then decreases its organic content with respect to a similar batch tes t. In addition, these results highlight the considerable influence of tempe rature in the oxidation reactor and of the type of sewage sludge which is t reated. At temperatures around 240 degreesC, foaming can seriously impair t he operation of the continuous reactor, because of the presence of non-degr aded fatty compounds and surfactants. Moreover, the COD reduction is limite d to 70%. On the contrary, at 300 degreesC, COD removal efficiency greater than 80% is achieved without any catalyst additive and, in addition, only h ighly biodegradable compounds remain in the oxidised liquor.