Transforming sludge into a recyclable and valuable carbon source by wet air oxidation

The wet air oxidation (WAO) process generates residual solids that can be l andfilled without additional treatment. This practical advantage stems from the intrinsic leaching-resistant behaviour of the solids, which is linked to the treatment conditions in water at high temperature and to the composi tion of the solids. X-ray analysis confirmed that the fine powder (diameter 2-6 mu m) mainly comprises aluminium phosphates, kaolin, quartz, calcite a nd an amorphous fraction which immobilises heavy metals in the form of hydr oxides, carbonates and insoluble phosphates. The residual solids contain a low amount of organic matter (1-2 wt.% TOC, depending on the process parame ters). They are easily settled and dewatered with dry solids (DS) content b etween 50 and 60%. The re-use of WAO sludge supernatant as a carbon source in biological denitrification have been investigated. The initial results a re deemed very satisfactory. The most significant results obtained during p rocess development are reported in this paper.