Understanding filtration › 1. Separation basics

CHAPTER 1.1

Filtration, settling, centrifugation: which process for which sludge

Separating a solid from a liquid always relies on one of two forces: gravity, possibly multiplied by rotation, or a pressure difference that drives the liquid through a porous medium. Knowing which one suits your sludge avoids many disappointments.

Nicolas Ract · 7 min read · October 2026

Two families of processes

Separation by density: settling and centrifugation

In a still liquid, a particle denser than the liquid sinks. For a small isolated spherical particle, Stokes' law gives its settling velocity: it increases with the square of the diameter and with the density difference between solid and liquid, and decreases with the viscosity of the liquid [3].

Two practical consequences. A particle half the size settles four times more slowly: fines settle poorly. And a solid whose density is close to that of water, like many biological sludges, settles slowly whatever its size. Stokes' law is in any case only an approximation: non-spherical particles, concentrated suspensions in which particles hinder each other, and non-Newtonian liquids all deviate from it [3].

The settling tank (or thickener) relies on gravity alone, in a large basin. It clarifies the water and concentrates the sludge, but does not produce a shovelable solid.

The centrifuge multiplies gravity by rapidly rotating a cylindrical bowl; an internal scroll discharges the solid, and the liquid leaves by overflow [1]. It operates continuously, in a closed, compact machine. On the other hand, it consumes a lot of energy, often requires a lot of flocculant and calls for noise protection [1] [2].

Separation by pressure: filtration on a medium

Here the mixture is pushed against a porous medium, most often a textile cloth. The liquid passes through, the solid is retained. The first particles stopped form a layer, the filter cake, which itself becomes the real filter medium: this is "surface" or "cake" filtration, as opposed to depth filtration through a granular bed, such as a sand filter [2].

The driving force of the separation is a pressure difference: feed pump and possibly membranes on a filter press, rollers on a belt filter, vacuum on a disc or drum filter. The settling velocity of the particles no longer plays a part, which makes filtration effective where density is unhelpful, provided the cake remains permeable. Chapter 1.3 explains how to measure this.

What the different machines achieve

For municipal wastewater treatment sludges, the US Environmental Protection Agency publishes fact sheets giving ranges of cake dry solids content (its dry matter content, see chapter 1.2). These are orders of magnitude, for a given type of sludge:

The filter press values include the mineral reagents, which can account for 15 to 40 % of the cake dry matter [1]. Industrial sludges may fall outside these ranges in either direction.

According to the same source, the recessed chamber filter press produces the highest dry solids content and solids capture rate of the three; its dry solids content depends little on the feed sludge concentration. On the other hand: high capital cost, discontinuous operation, a large footprint and cloths to be prepared and washed [1]. A French study of sewage sludge dewatering routes finds the same ranking, belt filter, then centrifuge, then filter press, with wide variability from one site to another [4].

Selection criteria

• Particle size and density. Large, dense particles settle or centrifuge well. Fine and light, they are better separated by filtration, often after flocculation or conditioning that agglomerates them.
• Sludge concentration. A very dilute suspension benefits from thickening before going onto a filter; a concentrated suspension quickly forms a cake. The choice of the filtration technique itself depends on particle size and concentration [2].
• Flow rate and operating mode. A continuous, steady flow suits a centrifuge or a belt filter; a filter press works in batches and often needs a buffer tank.
• Target dry solids content. This is often the deciding criterion when the cake is transported, dried or disposed of by the tonne (see chapter 6.3).
• Filtrate clarity and the value of the product: liquid to be recycled, product to be recovered, discharge limits to be met.
• The working environment: odours, noise, available space, maintenance skills.

These criteria are hard to combine on paper. The classic process engineering approach starts with simple tests on the actual sludge (graduated cylinder for settling, laboratory centrifuge tube, Büchner funnel for filtration) and then moves on to pilot machine trials with the shortlisted suppliers [5].

Where cloth filtration fits

Filtration on a textile medium covers most of the machines described in part 2 of this guide: recessed chamber and membrane filter presses, belt filters, disc filters, candle filters and tower filters. On all of them, the cloth has two roles: retain the first particles until the cake forms, then let the filtrate through as easily as possible and release the cake at the end of the cycle.

Field experience: in industry, the filter press comes into its own mainly when the cake is expensive to remove or when a valuable solid or liquid is to be recovered. The centrifuge tends to win when space is short, the flow is continuous and a slightly wetter cake is acceptable. The two are often found on similar sludges: the right choice depends as much on the site context as on the sludge.