Sludge Drying Overview - Treatment Methods and Applications

Sludge is a residual, semi-solid by-product left from industrial or refining processes. It is a separated solid suspended in a liquid, typically containing major quantities of interstitial water between its solid particles. This material can be dried to cut its volume and reduce most of the moisture content of the biosolids present in the sludge.

Sludge Treatment Methods

Sludge, produced from the treatment of wastewater, basically consists of two basic forms- raw primary sludge and secondary sludge. Below are the methods described that are most commonly used to process the sludge produced during wastewater treatment.     


The sludge produced during waste water treatment has a water content between 97 % and 99.5 %. In sludge thickening, the dry and solid content of sludge is increased by reducing the water content with low energy input. This task is usually accomplished in a tank called a gravity thickener. The process of gravity thickening is one of the easiest ways to reduce the water content of sludge with low energy consumption. The process can reduce the total volume of sludge to less than half the original volume.  


Dewatering aims to reduce the water content further so that the solids content of the sludge remains about 20 %. It can then be handled like a solid material. Dewatering can either be done mechanically using a filter press or by using drying beds. Sludge drying beds, however, provide the simplest method of dewatering. In the process, digested sludge slurry is spread on an open bed of sand after which drying takes place by a combination of evaporation and gravity drainage through the sand.  

Sludge Drying

This is basically a thermal drying process where thermal energy is provided to the sludge to evaporate water. The process of drying sludge reduces volume of the product, making its storage, transportation, packaging and retail easier. 

This process of drying sludge comprises of two methods- direct and indirect. However, these days the indirect thermal drying is becoming more popular as a means of decreasing sludge volume by removing the moisture content and attaining a dry solids content of 90%. The process reduces the environmental impact and produces a stabilized dry granular product that is easy to store, and deliver and suitable for agricultural use.

Sludge Drying Applications

Dried or treated sludge offers a wide range of applications, mainly including agricultural land application. When the drying is completed to produce biosolids, these biosolids act as a fertilizer for crop harvesting. The organic nitrogen and phosphorous found in biosolids are used quite efficiently by crops as these plant nutrients are released slowly throughout the growing season. This allows the crop to absorb these nutrients as the crop grows. 

Besides the application of crop harvesting, the treated sludge can also be used as top dressing on golf course fairways, soil conditioner for construction of parks, and other athletic fields, a soil substitute in final landfill cover, etc.


Related Publications

Experimental Methods in Wastewater Treatment - M.C.M. van Loosdrecht, J. Keller, P.H. Nielsen, C.M. Lopez-Vazquez and D. Brdjanovic
Publication Date: May 2015 - ISBN - 9781780404745

Applications of Activated Sludge Models - D. Brdjanovic, S.C.F Meijer, C.M. Lopez-Vazquez, C.M. Hooijmans, M.C.M. van Loosdrecht
Publication Date: Jun 2014 - ISBN - 9781780404639

Activated Sludge - 100 Years and Counting - David Jenkins, University of California at Berkeley, USA and Jiri Wanner, Institute of Chemical Technology, Prague, Czech Republic
Publication Date: Jun 2014 - ISBN - 9781780404936

Faecal Sludge Management - Linda Strande, Mariska Ronteltap and Damir Brdjanovic
Publication Date: May 2014 - ISBN - 9781780404721


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