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Small, steel cylindrical sludge sedimentation tank with a conical base and yellow railings on the platform above it
Bubbles in industrial wastewater
Cake being discharged from a sludge dewatering belt filter press

Sludge basics

Sludge characteristics depend on both the original feedwater quality and the type of the unit operation from which the sludge is discharged.

For conventional municipal wastewater treatment, the two main sludge streams are from the primary sedimentation stage, producing primary sludge, and the secondary biological treatment stage, producing waste activated sludge (WAS) if the biological treatment is based on the activated sludge process.

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Sludge water fractions for flocs and particles: surface water, floc water, interstitial water and free water
Credit: Judd Water & Wastewater Consultants

What is sludge?

For conventional municipal wastewater treatment, there are two main sludge streams. The first is from the primary sedimentation stage, producing primary sludge. The secondary stream is from the biological treatment stage, producing waste activated sludge (WAS), if the biological treatment is based on the activated sludge process.

Regardless of its origins, sludge contains water, dissolved organic and inorganic materials and suspended solids. The suspended solids normally make up 2−5% of the sludge for municipal wastewater sludge.

What is sludge?

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Dried, powdered sewage sludge (Jakub Raček, Brno University of Technology)
Credit: Jakub Raček, Brno University of Technology

What are biosolids?

Biosolids are the concentrated, stabilised solids product generated from the treatment of sewage sludge.

The terms 'biosolids' and 'sewage sludge' are often used interchangeably. However, biosolids are usually considered to be the useful fraction of the sewage sludge. They are generally higher in solids concentration and lower in pathogen content, having passed though a stabilisation process such as drying, chemical dosing or digestion.

What are biosolids?

What is sludge treatment?

Sludge is the main waste stream generated from water purification, both water for drinking and wastewater for environmental discharge.

It requires processing to reduce its volume, the risk it poses to public health, its odour, and its stability, in case e.g. it leaches toxic or otherwise harmful components.

There are essentially two methods by which the volume of sludge is reduced: consolidation and destruction.

What is sludge treatment?

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Sludge treatment theory

Two key aspects of sludge solid−liquid separation processes for which fundamental theoretical representations exist are gravitation (or ‘sedimentation’) and filtration.

Sludge treatment theory
Small, steel cylindrical sludge sedimentation tank with a conical base and yellow railings on the platform above it

Gravitation/sedimentation

Sedimentation (or gravitation or settlement) is the separation of particles from water on the basis of their weight. Particles heavier (more dense) than water will sink, while those less dense will rise. If the water is still, then the particles can collect at the base or surface of the water, from where they may then be removed.

Particles less dense than water can be separated by flotation, though flotation commonly employs air to promote the separation of particles which are neutrally buoyant.

Gravitation/sedimentation
Circles/uneven strata of different blues from light blue/yellow on the outside to dark blue/black at the centre

Centrifugation

The theory of centrifugation relates to sedimentation theory, since centrifugation acts to enhance gravity through the centrifugal force (or G force).

Centrifugation
Plot of time/Volume vs Volume passed for filtration of suspended solids, according to cake filtration theory, giving the specific resistance to filtration in the slope.

Filtration

Filtration theory as applied to sludge processing defines the way in which water flows through the bed, which is formed of the sludge solids – known as the cake.

The key parameters of the theoretical description are the resistance of the cake to flow and the change in the cake resistance with time.

Filtration
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