Application of the wet oxidation process to the treatment of municipal sewage sludge

Wet oxidation (WO) technology is an alternative to normally accepted method s of sewage sludge treatment and discharge. This can comply with environmen tal standards. The results of tests carried out on a continuous-feed pilot unit, treating municipal sewage sludge, highlight the significant influence of the temperature in the oxidation reactor where a COD removal efficiency greater than 80% can be achieved at 300 degreesC without any catalyst addi tion. Tests have also revealed that at temperatures around 240 degreesC, fo aming can seriously impair the operation of the reactor, because of the pre sence of nondegraded fatty compounds and surfactants. Some compounds presen t from the outset or produced in the reactor are resistant to oxidation und er subcritical conditions: such is the case of ammonia whose return to the wastewater treatment plant inlet creates an additional 20 to 30% load in re lation to the nitrogen contained in the raw water. It is therefore essentia l to include specific biological treatment of the ammonia liquid phase or t o adapt the design of the treatment plant for removing the ammonia released from WO. The study has revealed that the reaction temperature has a consid erable effect on the volatile matter content of the final solid waste: it i s evident that, given the expected evolution of regulations governing therm al process, WO units will have to be operated in such a way as to reduce th e impact of these by-products to a minimum, and to study ways in which they might be reused.