Drying waste water sludge is an expense that must be minimized. Wastewater treatment plants (WWTP) remove screenings, grit, scum, solids and biosolids, collectively called sludge. Its properties are affected by many factors such as origin, history of treatment, duration and conditions of storage, etc. Drying this sludge is a difficult process.

Figure 1- Dewatered sludge (Grosses Battes WWTP, Angleur, Belgium)
Dewatered sludge (Grosses Battes WWTP, Angleur, Belgium)

Reducing water contents

In wastewater treatment plants (WWTP) the first series of treatment consists of reducing the water content of the sludge by thickening, conditioning and mechanical dewatering. This results in water contents of 18-35 % dry solids (DS), depending on the technology. The use of a drying step is considered essential after mechanical dewatering, prior to agricultural or energy valorization. Conventional combustion technologies require a minimum lower heating value of 6-10 GJ/t to compensate thermal losses and incomplete combustion. This minimum dictates at least partially drying of the sludge.


Sludge drying is also of interest for countries still promoting the use of sludge in agriculture as drying produces pellets which are easy to store, handle and transport, contrary to pasty dewatered sludge. The product is also stabilized due the low water activity and will not produce bad odors in the field. Destruction of pathogens may even be reached provided the drying temperature and the residence time are sufficient.


Most sludge dryers are adaptations of standard driers, but this adaptation is not straightforward due to the unusual stickiness of the sludge (for indirect dryers) or to the bad odors and risk of explosion (for direct dryers). Nowadays, few companies possess expertise and sufficient experience in the thermal processing of sewage sludge. Dryer design relies too often on empirical considerations, sometimes leading to inefficient systems, unable to handle changes in the properties of the sludge due to any modification within the WWTP.

Convective micro-dryer
Convective micro-dryer

A complex material to dry

Sewage sludge is quite different from the products usually dried in industry. It is a heterogeneous mixture of microorganisms, mineral particles, colloids, organic polymers, cations, fibers, etc., where the composition varies considerably depending on the origin, the season, etc. The maximum drying flux in convective drying can vary by a factor of three, depending on the origin.

Impact of sludge origin, indicated by the letters, on the convective drying kinetics
Impact of sludge origin, indicated by the letters, on the convective drying kinetics


The rheological properties of sludge also change dramatically with the water content as shown in the table. The glue phase is the most documented one, as it can dramatically affect the operation of indirect dryers and can lead to damage to equipment. It also affects the hydrodynamic characteristics of the dryer, the gas to solid contact area, and the local solids hold-up.

Other problems in relation to sludge rheology are also encountered, such as pumping and liming. Simply pumping the sludge with a cavity pump completely upsets the drying behavior resulting in a significant decrease of the drying rate. This can be counteracted by a well-known stabilization step called 'liming'.

The rheological properties of sludge also change dramatically with the water content as shown in Table 1

Adding lime to the sludge strengthens the texture and can partially counteract the negative effect of pumping. The way the liming step is carried out, either before dewatering ('pre-liming') or after dewatering ('post-liming'), also has an impact. The texturing effect of lime is partially lost during post-liming due to shear stresses produced by the mixing device. The results obtained with a discontinuous pilot scale convective dryer confirmed earlier observation by industrial dryer operators.

Impact of pumping and liming on sludge drying kinetics
Impact of pumping and liming on sludge drying kinetics
Impact of pre- and post-liming on sludge convective drying kinetics
Impact of pre- and post-liming on sludge convective drying kinetics

The negative effect of mixing has also been recently observed in a project aiming at producing new biofuels by mixing sludge and sawdust. In addition to all these variables, specific phenomena may occur due to the organic content of sludge. Aerobic and anaerobic fermentation will change the product during storage, affecting rheology and drying behavior. Also heating causes a partial breakdown of the organic matter, leading to the emission of volatile organic compounds (VOC) and malodorous substances such as hydrogen sulfide (H2S). The non-condensable parts of the vapor have to be treated in biofilters, biowashers, absorption/adsorption facilities or by thermal oxidation while the condensates are sent back to the head of the wastewater treatment plant. These drying characteristics, along with the legal status of waste influence the energy consumption of drying and the dryer design.

Convective pilot scale dryer (l.) and sludge extrusion (r.)
Convective pilot scale dryer (l.) and sludge extrusion (r.)


Sludge drying is an essential step before incineration or agricultural use. The whole treatment chain, from sludge production, dewatering, drying to energy recovery must be optimized. Each stage and their coupled effects have to be examined for the design of a sustainable 'zero energy wastewater treatment plant'. One of the most challenging parts remains the better understanding of the mechanisms governing sludge drying, depending on sludge origin and previous treatment. Any improvement of the knowledge in this area should lead to the conception of dedicated sludge dryers in a more rational way, contributing to success of the whole valorization process.


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