Filter Press or Decanter Centrifuge for Sludge Dewatering?

Dewatering turns liquid sludge with 2–6 % dry solids into a cake that can be transported, landfilled, composted or incinerated. The two machines most often compared for this are the chamber filter press and the decanter centrifuge. Both do the job; they differ in how dry the cake becomes, in how they are operated and in what they cost to run. This guide sets out the differences and shows how to decide.

How the two machines work

Filter press: filtration under pressure, in batches

A feed pump presses conditioned sludge into the chambers between filter plates covered with cloth. The cloth holds back the solids, the filtrate drains away, and the pressure rises to as much as 15 bar while the chambers fill with cake. At the end of the cycle the press opens and the cakes drop out. A cycle takes two to four hours. See our filter presses for sludge dewatering and the matching feed pumps.

Decanter centrifuge: sedimentation in a centrifugal field, continuously

The sludge flows into a horizontal bowl that turns at 2,500–3,500 revolutions per minute. The centrifugal force, 2,000 to 3,500 times gravity, presses the solids against the bowl wall. A screw conveyor inside the bowl, turning slightly faster or slower than the bowl, moves the solids to the conical end, where they leave as cake. The clarified liquid (centrate) leaves at the other end. The machine runs continuously and is fully enclosed.

Cake dryness and what it means for disposal

The filter press produces the drier cake. With municipal sludge, a decanter typically reaches 20–28 % dry solids and a chamber filter press 25–35 %. With mineral and industrial sludges a filter press reaches 40 % and more.

The difference matters because disposal is paid per tonne of wet cake. One tonne of dry solids is:

• 4.5 tonnes of cake at 22 % dry solids,
• 3.1 tonnes of cake at 32 % dry solids.

That is 31 % less mass to transport and dispose of. At a disposal price of 100 € per tonne, the drier cake saves about 140 € for every tonne of dry solids. For many plants this saving decides the choice.

Filter press and decanter at a glance

Typical values. The result with a given sludge can only be confirmed by a test.

When a filter press is the better choice

• Disposal is expensive, so every percentage point of dryness pays.
• The cake must meet a landfill or incineration limit for dry solids.
• The sludge is mineral, inorganic or contains metal hydroxides, for example from metal finishing, mining, stone cutting, chemical plants or water works.
• The filtrate should be clear enough to be reused or discharged without further settling.
• The sludge quantity is small to medium and can be processed in a few cycles a day.
• Abrasive solids would wear a centrifuge quickly.

When a decanter is the better choice

• Large sludge flows have to be dewatered around the clock with few staff.
• Space is limited, or the machine has to stand in an existing building.
• Odour must be contained.
• The sludge is oily, or three phases (oil, water, solids) have to be separated.
• The cake goes to a dryer or digester that needs a continuous feed.

What decides the operating cost

• Disposal — usually the largest item. It falls with every point of cake dryness.
• Conditioning chemicals — a decanter needs more polymer per tonne of dry solids than a filter press. Lime conditioning for a filter press adds mass to the cake but gives a stable, well-structured cake.
• Energy — a minor item for a filter press, a noticeable one for a decanter.
• Staff — a filter press with automatic plate shifting and cloth washing runs largely unattended; simpler presses need an operator at every discharge.
• Maintenance — filter cloths are changed every one to three years; a decanter needs its screw re-coated at intervals that depend on the grit content of the sludge.

The filter press calculator estimates the cake volume and the chamber volume for your sludge quantity. Conditioning stations are described under chemical dosage systems.

Data we need for a proposal

• Type and origin of the sludge, and whether it is digested.
• Quantity per day in m³ and its dry solids content.
• Required cake dryness and the disposal route.
• Operating hours per day and available staff.
• Space and height available at the installation site.

Frequently asked questions

Which machine produces the drier cake?

The filter press. With municipal sludge the difference is typically 5 to 8 percentage points of dry solids; a membrane squeeze adds a further 3 to 8 points.

Is a filter press more work to operate?

A manual press is, because the plates have to be shifted and the cake removed by hand. An automatic press with plate shifter, drip trays and cloth washing needs attention only for checks and cloth changes.

Which is cheaper to buy?

For small and medium sludge quantities the filter press is usually the lower investment. For large continuous flows a decanter needs less building space, which changes the comparison. The total cost over ten years, including disposal, is the figure to compare.

Can a test show which machine suits my sludge?

Yes. A laboratory filtration test or a pilot press run with the real sludge shows the cake dryness, the cycle time and the chemical demand before any investment is made.

What about belt presses and screw presses?

Belt filter presses and screw presses run continuously with little energy and are common for municipal sludge where 18–25 % dry solids is enough. Where the cake has to be drier, the filter press remains the reference.

Send us your sludge data via our contact page. We will size a filter press with feed pump, conditioning and cake conveying for your quantity, and tell you plainly if another process suits your sludge better.