Thermal treatment of sludge

The drying mechanism works in 2 stages:

• evaporation of surface water above 100 °C,
• deep drying after the water has moved from the inside of the sludge micelle to the outer edges.
  
  

Currently the following two applications may be chosen depending on whether the required output dryness for the sticky paste-like sludge is, roughly speaking, below or above 50 to 70 % :

• partial drying is used to raise the dryness level of the sludge to a value which will enable it to spontaneously combust in a sludge incinerator, such as the Pyrofluid™ type, i.e. currently around 35 % (maximum 45 %).
• advanced drying to a dryness level that is generally greater than 90% or at least 65 %, whereby the sludge volume is reduced to its minimum level either for storage or for sludge removal for agricultural reuse, or for incineration in a household waste incinerator or even a cement incinerator (the ash generated from the dry sludge acts as a filler).

Agricultural reuse requires a dryness level greater than 90 % to ensure that the sludge is non-fermentable and can be stored in silos while awaiting its reuse.

In order to facilitate handling, this dry sludge must also be in granular or pellet form. Dry sludge can be added to a household waste incinerator either in the pit or in the hopper. In the latter case, the optimal dryness level is 65 % which is similar to that for household waste.

Drying therefore ensures that the desired dryness level can always be achieved. Energy costs can be reduced even further if you have low cost calories. This occurs when the dryer is combined with the incinerator or when the treatment plant supplies biogas and the condensates can be used to heat the digester.

On the other hand, once the sludge has been incinerated, it will still be necessary to reprocess the gas effluents and make the fly ash inert.

Co-incineration with household waste (HW) is another form of incineration. The aim of co-incineration is to inject the sludge - either in a paste-like or dry form - into an incinerator which was initially meant for HW. The first form will entail the use of an adapted injector (such as the Pyromix™ type), whose role is to add the sludge at a point where it will not adversely affect the combustion process of the HW or its efficiency.
This function can be achieved through the judicious choice of the injection point and by the effect that the injector can have on how the sludge enters the incinerator.

For economical reasons co-incineration can only be considered when the household waste incinerator is near to the treatment plant and when its residual treatment capacity makes it possible. It is also necessary that the incinerator technology allows for the introduction of injectors into its sides.

Incinerating just the sludge still remains the most flexible solution as it is able to adapt to changes in the running of the treatment plant. This is particularly true of the fluidized bed technique which has proved itself for more than 25 years.

Its rapid development will increase all around the world thanks to its inertia (the incinerator comprises a bed of fluidized sand), and the total absence of any mechanical pieces coming into direct contact with the flame. The effect of this specific feature is that it is highly reliable, whilst its operation has now been made even safer thanks to the implementation of particularly effective centralized operation management systems.

However regardless of which solution is chosen, it will still need to be studied from the perspective of its optimal thermal upgrading. If the treatment principle concentrates the heavy metals initially contained in the sludge into fly ash, these can then be reused by Veolia Water Solutions & Technologies' Fluofill™ process for example.

As for other gas by-products from the combustion process, they are purified to prevent the aqueous pollution changing into atmospheric pollution.

This process has been used for some considerable time in the chemical industry.

It entails oxidizing the sludge under humid conditions. This involves heating the sludge to a high temperature and at a high pressure to enable the organic matter contained in the sludge to be oxidized in the presence of oxygen while at the same time remaining in a liquid phase (in order to prevent any evaporation).

This means that contrary to incineration the matter is reduced without any need for combustion or flames, thereby avoiding any possible smoke pollution that would have resulted from incineration.

Main features of the Athos™ process as developed by Veolia Water Solutions & Technologies :

• Liquid Phase Oxidation
• Simultaneous oxidation and cleaning 200 to 300°C - 40 to 150 bar
• Retention Time: 1 h
• No polluting smoke and a biodegradable overflow
• Auto thermie on the thickened sludge (4% SM)
• Inert Solids (Techno Sand)