Sludge – from waste to secondary fuel

The starting point

The agricultural use of sludge is not ensured in the long run: Apart from heavy metals, sludges can contain organic pollutants (residues of pharmaceuticals, fragrances, plasticisers, etc.) as well as pathogens, of which the effects due to bioaccumulation and combination effects are barely assessable.

In conventional procedures, a thermal removal of the sludge is often accompanied with high costs and pollution, due to the transport of sludge with a high water content (filter cake has a water content of approx. 70-80%) and the use of fossil fuels while drying, among other things.

On the other hand, sludge drying concepts based on the use of low-temperature heat and solar energy are gaining in importance.

Hybrid sludge drying – The process

Hybrid sludge drying – an advancement of the mere solar drying – can make a positive economical and ecological contribution for a sustainable sludge disposal concept.

Hybrid sludge drying uses waste heat and solar energy to dry pre-dewatered sludge (from 18% DM) to over 90% DM continually over the course of the year.

The slow-running, low-maintenance turning and discharge system with building width harrow rakes ensures a careful sludge transport through th drying hall/ solar house.

This enables easily passing through the adhesion phase (with DM-contents of 50-55%) and regardless of drying to >90% DM, there is no dust emission.

The formation of anaerobe zones and the resulting odour emissions are avoided. Additional investments for wheel loaders or similar things used for feeding or clearing the drying hall are eliminated.

The low-temperature heat (55-95 °C) is incorporated into the drying process via underfloor heating. Biomass plants, heat pumps, waste heat from industrial plants, etc. present themselves as heat sources.

Using computer support, the air saturated with water vapour is discharged from the solar house through vent stacks, roof flues, as well as ventilators.

The end product results in a biologically stable, storable and odourless dry granulate.


Dried sludges with a dry matter content of >90% DM can substitute fossil energy sources as secondary fuel; e.g. approx. 370 kg coal can be substituted for one ton of dried sludge (>MJ/kg).

Sludges are classified as biogenic residues and can be combusted CO2-neutrally. This can lead to advantages in the trading of CO2-emission rights; e.g. the trade price in autumn 2005 was approx. 20.00 – 25.00€/t CO2.

Combusting the dried sludge in the clinker kilns of cement factories leads to a residue-free material and energetic utilisation: the mineral proportion of the sludge is completely turned into cement clinker, the organic proportion serves for firing.

Heavy metals are fully and permanently set into the concrete. Quantitative and qualitative regulations (small grain size, DM-content of >90% etc.), which can be met through the process of the hybrid sludge drying, apply to the utilisation due to the procedural requirements.

Conclusion and outlook

With the hybrid sludge drying, it is possible to produce secondary fuel out of sludges. This can be achieved with a very low use of electrical energy (the power consumption is only approx. 20-25 kWh / t filter cake) through the use of solar energy and waste heat from biomass plants by means of underfloor heating and an effective turning and discharge system.

This results in the following advantages:

  • Planning security through long-term stable drying costs
  • Increase of disposal safety due to a storage-stable and transportable dry granulate with >90 % DM (sludges with a solids content of approx. <85% are sticky, pasty and difficult to convey; furthermore, they tend to self-heat, possibly spontaneous combustion)

Thus, the hybrid sludge drying is of interest to many communities, operators of biomass plants, disposal companies, etc. An exact ecological and economical accounting should be performed for each specific case, since further factors such as the situation of the local sewage treatment plant, possibilities of land use, disposal potential, etc. must be taken into account.

In this article, a description of the hybrid dewatering, for thickening wet sludges with a DM-content of 2-5%, has been foregone, since there is no reliable information from the practice so far.

In order to make further statements about this innovative process, AgroEnergien is currently searching for small sewage treatment plants >2,000 EGW for the project planning and operation of hybrid sludge dewatering plants.