ATHOS : A NEW HYDROTHERMAL OXIDATION OF SLUDGE
Abstract:To comply with updated stringent environmental and social regulations for sewage sludge and industrial wastewater disposal, thermal destruction of these wastes by wet oxidation appears to be a way that alleviates the need for dewatering, decreases air pollution problems and helps the possibilities of material recycling.
After a review of wet air oxidation technologies, the works performed on a laboratory scale and at design and construction steps are detailed : results of the development studies and description of the ATHOS® process, specifically tailored for its smooth integration within the wastewater treatment plant. The data and experience recovered from the demonstration plant, which capacity corresponds to about the output of 45 000 People Equivalent., installed in Toulouse WWTP in 1998 is presented.
The moderate operating conditions (235 °C, 44 b.g.) result in more than 80 % COD abatement for the liquid phase and self-sufficient thermal operation with about 4 % DS sewage sludge, using catalysts. The treatment of the ammonia-rich supernatant liquor recovered by the standard wet oxidation processes is improved to reach about 70 % removal of nitrogen, allowing the recycling of this easily biodegradable liquor with the plant influent, as a cheaper carbon-containing source for biological denitrification /dephosphatation.
The mineral part of the treated waste shows excellent chemical and physical properties to be considered for further re-use in other industries: the residual solids contain less than 5 % TOC and immobilize all metals which are precipitated within the inorganic matrix. These solids are easily dewatered to about 55 % DS on a filter-press, without any additives. A study performed in co-operation with both local industrial partners and national concerned entities is investigating some opportunities for the reuse of the inert residual solid.
The process vent is discharged into the atmosphere after a polishing treatment consisting in a catalytic oxidation of NH3, CO and VOCs off-gas pollutants. The process using pure oxygen is NOx and dioxin free and low CO2 emissions.
Consumables figures and operating costs confirm both technical and economical competitiveness of this clean technology, applicable from medium to large WWTP and which should be helpful for its social acceptance due to its very low environmental impact.
Document Type: Research Article
Publication date: January 1, 2000
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