The wet air oxidation (WAO) process generates residual solids that can be landfilled 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 composition of the solids. X-ray analysis confirmed that the fine powder (diameter 2–6 μm) mainly comprises aluminium phosphates, kaolin, quartz, calciteand an amorphous fraction which immobilises heavy metals in the form of hydroxides, carbonates and insoluble phosphates. The residual solids contain a low amount of organic matter (1–2 wt.% TOC, depending on the process parameters). They are easily settled and dewatered with dry solids (DS) content between 50 and 60%. The re-use of WAO sludge supernatant as a carbon source in biological denitrification have been investigated. Theinitial results are deemed very satisfactory. The most significant results obtained during process development are reported in this paper.

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