New wet oxidation method for sludge does not require incineration

Nov. 12, 2002
Vivendi Environnement has developed a method of sludge clarification using wet oxidation and biological treatment.

Nov. 12, 2002 -- Vivendi Environnement has developed a method of sludge clarification using wet oxidation and biological treatment.

If the present production of sludge in Europe is estimated at 7 million tonnes of dry matter, then the densification of the untreated water collection networks, coupled with the increased number of treatment installations and plants improving depollution outputs, should result in twice this rate of production by 2005.

This increase in sludge production, coupled with collective environmental awareness, which is reinforced by ever-stricter regulations, means that valorisation solutions must be imposed during treatment.

ATHOS® is one of the latest solutions to the problem of sludge clarification, which makes use of the principle of wet oxidation (WO) coupled with a biological treatment. Unlike traditional wet oxidation, ATHOS® was developed as a one-step process for the destruction of sludge in liu of incineration.

Compared with the other methods of eliminating clarified sludge (agricultural valorisation or evacuation in CETs or incineration), wet oxidation is one of the solutions, which best responds to the criteria for reducing the volume of sludge, stabilisation and hygiene.

At present, 200 litres of wastewater generate 5 litres of untreated sludge. Thanks to the wet oxidation technique, this quantity of sludge is now finally reduced to the equivalent of a cup of coffee.

Wet oxidation consists in heating an effluent in a pressurised liquid phase (70 to 150 bar) at a high temperature (250 to 300 °C) in the presence of an oxidising gas, with the aim of transforming, by oxidation, the organic components into a gaseous reject, a biodegradable organic liquid and an essentially mineral solid.

The ATHOS® method is capable of treating sludge with the same efficiency but under more moderate temperature (235°C) and pressure (approx. 45 bar) conditions, in the presence of pure oxygen.

Operating principle

The thickened sludges, with a degree of dryness of between 4 and 7%, are fed into an oxidation reactor by way of a high-pressure pump. Stored in liquid form, the oxygen is vaporised and safely injected into the reactor using a metered flow-rate hydro-ejector.
When running at the set rate, the oxidation reaction releases heat, which maintains the mixture in a reactional state at higher temperatures and serves to pre-heat the thickened sludges before they are introduced into the reactor. This heat exchange is ensured by a heat recovery loop.

The pressure is kept constant in the reactor by evacuating the gases as they are produced. These gases contain the water vapour, carbon dioxide, molecular nitrogen and traces of volatile organic compounds. Following treatment in a catalytic reactor, the gas itself is vented to the atmosphere.

The relatively low oxidation reaction temperature prevents the formation of harmful sub-products, such as dioxins, furanes and nitrogen oxides. Reaction in a liquid medium prevents dust from being released into the atmosphere.

The mineralised sludges extracted from the reactor are cooled again in the heat exchanger before being poured into a decanter.

The liquid in the decanter, containing an easily biodegradable carbonised charge (acetic acids, fatty acids, alcohols), is returned to the head of the station at the water biological treatment stage, whilst the solid part, containing less than 5% of organic material, is conveyed to a dehydration plant, which may be completed by a drying stage, depending on the type of valorisation proposed.

Dehydration takes place without adding any conditioning products. The result is a solid extraction with dryness of at least 50%, referred to as "technical sand".

This technical sand is inert, can be used in road building or in the production of concrete, or even stored. The heavy metals, which may be contained in the sludge, are fixed in the technical sand in a non-lixiviated form.

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