The work presented in this paper concerns the application of low temperature wet oxidation for volume reduction of settled and floated sludges resulting from the treatment of waste water from a pulp and paper mill in Norway.

The results of batch and continuous pilot-scale tests show that it is possible to obtain reductions of more than 50% of COD and more than 70% of suspended solids by a mild treatment at 200°C and a retention time of only 20 minutes. This retention time is very short compared to conventional wet oxidation treatment applied to conditioning of organic sludge, which may lead to a reduction in reactor size and lowering of construction costs. The product obtained after wet oxidation has excellent dewatering characteristics resulting in a sludge volume reduction of 80% by sedimentation. The dewatered sludge can be further processed by acid treatment for recovery of alumina used for chemical precipitation of the sludge. The results show that it is possible to recover more than 80% of the alumina present in the sludge. Tests of recovered alumina show a flocculation efficiency comparable to that of virgin alumina.