Your web browser is out of date.

Update your browser for better security, speed and to get the best experience on this website.

Update your browser

Introduction to sludge drying

Posted on
Dried, "noodles" of sewage sludge

Sludge drying refers to processes where moisture is removed from sludge as water vapour. This is to be distinguished from processes where sludge is dried by purely mechanical means, such as thickening or dewatering where moisture is removed as a liquid.

Why dry sewage sludge?

There are three primary reasons why drying sewage sludge may be advantageous:

  • to reduce the sludge water (or moisture) content and, therefore, the volume further than is possible by mechanical means
  • to render the sludge suitable for treatment by certain thermochemical methods, principally incineration and pyrolysis, and
  • to produce a stabilised product, i.e. one which is substantially free of pathogenic organisms and odour, and is not putrescent (i.e. susceptible to decay).

Reducing the moisture content increases the organic carbon concentration and so the calorific content of the sludge, normally expressed in units of mega-Joules per kg sludge dry solids (MJ/kgDS).

Other sludge stabilisation unit operations include anaerobic digestion (AD) and thermochemical methods. Thermochemical methods adopt more aggressive heating conditions than those employed for sludge drying, converting the solids to a stable char solid product alongside gaseous products which may or may not have a calorific value depending on the thermochemical method used.

Drying the sludge also substantially changes its nature and consistency. Sludge dried to >65% dry solids (DS) can exist either as pellets or, at the highest solids concentrations of 90% or more, as a powder. Both these forms are more readily conveyed than the dewatered sludge of higher water content, which exists as large lumps which can only be transported by conveyor belt.


Image credit: Jakub Raček, Brno University of Technology

About this page

'Introduction to sludge drying' was written by Simon Judd

This page was last updated on 19 April 2021


Information on this page may have been supplied by third parties. You are reminded to contact any third parties to confirm information is accurate, up to date and complete before acting upon it. accepts no liability for information provided by third parties, actions taken on the basis of this information or information held on third-party websites.

© Text copyright Judd & Judd Ltd unless otherwise indicated on this page