Wet oxidation of domestic sludge and process integration: the Mineralis process.

Wet oxidation (WO) in subcritical conditions is a new alternative to usual routes for sewage sludge treatment that complies with environmental standards. This paper presents tests carried out using a batch reactor and continuous pilot and industrial units, treating municipal sewage sludge. The main products after oxidation are CO2, water, VFA and ammonia. The results highlight the considerable influence of the treatment temperature and of the type of sewage sludge which is treated. At temperatures around 240 degrees C, VFA fraction present in WO supernatant is limited to 50% because of the presence of non-degraded fatty compounds and surfactants. Moreover, the COD reduction is limited to 70%. On the contrary, at 300 degrees C, COD removal efficiencies greater than 80% are achieved without any catalyst addition and, in addition, only highly biodegradable compounds remain in the oxidised liquor. In order to treat the residual ammonia nitrogen by biological processes, it is therefore necessary to obtain a VFA fraction as high as possible for achieving denitrification and then to operate the WO process at high temperature and without catalyst addition.

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

Wet oxidation in subcritical conditions is a new alternative to usual routes for sewage sludge treatment and it complies with environmental standards. The paper presents tests carried out on a batch reactor and on a continuous pilot unit, treating municipal sewage sludge. A method is proposed that shows that the oxidation efficiency in a continuous reactor can only be easily predicted from the residence time distribution and batch tests results. Nevertheless, a partial settling of the solid residue in the continuous bubble column reactor is evident, and it increases the solid residence time and then decreases its organic content with respect to a similar batch test. In addition, these results highlight the considerable influence of temperature in the oxidation reactor and of the type of sewage sludge which is treated. At temperatures around 240 degrees C, foaming can seriously impair the operation of the continuous reactor, because of the presence of non-degraded fatty compounds and surfactants. Moreover, the COD reduction is limited to 70%. On the contrary, at 300 degrees C, COD removal efficiency greater than 80% is achieved without any catalyst additive and, in addition, only highly biodegradable compounds remain in the oxidised liquor.