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Thin cylinders of sludge (pelleted, torrefied sewage sludge)Credit: Jakub Raček, Brno University of Technology
Sewage sludge incinerator ashCredit: Katarzyna Zabielska-Adamska, Bialystok University of Technology

Sludge thermochemical treatment

Thermochemical methods employ high temperatures combined with, for some processes, high pressures for the end treatment of sludge.

These processes are all able to recover resources from the sludge, primarily energy but also potentially phosphorus, whilst also substantially reducing the sludge solids.

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Sludge thermochemical treatment − an overview

Thermochemical methods are used for either:

  • significantly reducing the sludge solids content, or
  • pre-treating sludge upstream of anaerobic digestion (AD) to increase the biodegradability of the organic carbon.

When operated at elevated temperatures, i.e. for solids destruction/conversion rather than AD pre-treatment, energy is generated from the carbon content. The energy is recoverable either as heat or a combustible material (solid, liquid or gas), depending on the type of process.

Sludge thermochemical treatment − an overview
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Sludge thermochemical reactor design configurations

A number of different reactor designs have been applied to or trialled for the various sludge thermochemical treatment processes. They differ mainly in the way in which the sludge solids are mixed and heated within the reactor vessel.

The different reactor designs include the multiple hearth (Herreshoff) furnace, ‘bubbling’ and circulating fluidised beds, rotating cones, ablative and auger/screw configurations.

Sludge thermochemical reactor design configurations

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Sewage sludge incinerator ash
Credit: Katarzyna Zabielska-Adamska, Bialystok University of Technology

Incineration of sludge

Incineration − or combustion − of sewage sludge is the most widely-accepted alternative end disposal method after land spreading.

It is an oxidative method, converting the organic carbon, nitrogen, sulphur, nitrogen and phosphorus into gaseous and predominantly mineral (i.e. inorganic) solid products.

Incineration of sludge
Thin cylinders of sludge (pelleted, pyrolysed sewage sludge)
Credit: Jakub Raček, Brno University of Technology

Pyrolysis of sludge

Pyrolysis refers to the decomposition of organic matter at elevated temperatures in the absence of oxygen and under substantially dry conditions.

The process employs temperatures between 300 and 1300 °C, depending on the process type and reactor configuration. The process usually operates at atmospheric pressures. In all cases, oxygen-free conditions are maintained.

Sludge treatment − pyrolysis

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Gasification of sludge

Gasification is an extension of the pyrolysis process, with the conversion of sludge taking place in a partial reducing atmosphere at higher temperatures than those used for pyrolysis.

Whereas pyrolysis gives multiple products, some of which are liquid and form a bio-oil, gasification aims to completely convert the carbonaceous content to a solid ash and a clean combustible gas product for use locally.

Sludge treatment − gasification
Hydrothermal processes and approximate regions of operation with reference to the pressure:temperature water phase diagram. (HTC: Hydrothermal carbonisation, HTL: Hydrothermal liquefaction, HTG/SCWG: Hydrothermal gasification/supercritical water gasification.)

Hydrothermal processes

Hydrothermal processes for treating sludge employ elevated temperatures and pressures under wet conditions and in the absence of oxygen. They can be employed for two different applications:

* pretreatment of sludge at lower temperatures prior to anaerobic digestion, and

* substantial conversion of organic carbon at higher temperatures to useful, combustible products.

Sludge treatment − hydrothermal processes
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