Quick answer: Sludge is thickened before it is dewatered because dewatering equipment is sized on the volume of sludge it must process, not on the dry solids in it. Raising a sludge from 1% to 4% solids removes three-quarters of the water and cuts the volume by 75%, so the filter press, its feed pump, polymer dose and cycle time all shrink accordingly. Gravity thickeners, dissolved air flotation, rotary drum thickeners and gravity belt thickeners are the common choices; which one depends on whether the sludge is primary, biological, chemical or mineral. A filter press fed at 3–6% solids runs two to four times more cycles per day than one fed at 1%.
What thickening does and why it comes first
Thickening and dewatering are the same operation at different scales. Both remove water from sludge; thickening takes it from below 1% to 3–8% solids, and dewatering takes it from there to 20–40% solids or more. The reason they are separate steps is that the equipment that does the first job cheaply cannot do the second, and the equipment that does the second job cannot afford to do the first.
Consider a plant producing one tonne of dry solids a day as waste activated sludge at 0.8% solids. That is 125 m³ of sludge. Thickened to 4%, it is 25 m³. A filter press taking the unthickened sludge would spend most of each cycle simply filling with water, and the pump would be pushing 100 m³ of water a day through the cloth for no benefit. Thickened, the same tonnage fits in a press one-third the size running a normal cycle. The same arithmetic applies to belt presses, centrifuges and screw presses.
Thickening also stabilizes the feed. Dewatering equipment performs best on a consistent feed concentration; a thickener smooths out the swings that come from clarifier blow-down and process upsets, and gives a buffer volume so the press can run on a shift schedule while the treatment plant runs around the clock. Our article on how much sludge a wastewater plant produces gives the method for estimating the daily load.
The thickening methods
| Method | Typical feed | Typical thickened solids | Best on | Weak on |
|---|---|---|---|---|
| Gravity thickener (circular tank with picket rake) | 0.5–2% | 3–8% (primary), 2–3% (biological) | Primary, chemical and mineral sludges that settle | Waste activated sludge (goes septic, floats, reaches only 2–3%) |
| Dissolved air flotation (DAF) | 0.3–1% | 3–5% | Waste activated sludge, oily and light sludges | Heavy mineral solids; energy for air saturation |
| Gravity belt thickener | 0.5–1.5% | 4–7% | Biological sludge with polymer; simple and compact | Needs polymer; open to odour; belt washing water |
| Rotary drum thickener | 0.5–1.5% | 5–9% | Biological sludge with polymer; enclosed, small footprint | Needs polymer; screen blinding from fibre or grease |
| Centrifuge (thickening mode) | 0.5–1.5% | 4–8% | Where space is very tight and odour must be contained | Power, maintenance, polymer |
| Static sludge holding tank (settle and decant) | 1–3% | 2–5% | Small plants, chemical and mineral sludges | Slow; biological sludge goes septic |
The pattern is that sludges which settle by themselves, meaning primary, physico-chemical and mineral sludges, are thickened by gravity, while biological sludges that do not settle well are thickened by flotation or by a polymer-assisted mechanical thickener. Our guide to when DAF is needed covers the flotation case in more detail.
How thickening changes the filter press
A filter press is a batch machine with a fixed chamber volume. Each cycle fills the chambers with cake, and the cycle time is the time it takes to build that cake from the feed. Feed solids affect it in three ways:
- Fill time. At 1% solids, a chamber must receive roughly 30 times its own volume of sludge to fill with cake; at 4%, about 8 times. The fill stage shrinks from hours to well under an hour.
- Cloth wear and blinding. Every cubic metre of water pushed through the cloth carries fines into it. A thickened feed means less water per kilogram of cake and cleaner cloths.
- Polymer. Polymer is dosed per kilogram of dry solids, but mixing is better and dose lower when the sludge is concentrated before conditioning rather than after.
The practical result is that a press fed at 3–6% solids completes two to four times as many cycles per day as the same press fed at 1%, so the press that handles the load is smaller and cheaper. In sizing terms, the feed solids figure is the first number we ask for, because it changes the plate count more than any other single parameter. See how to calculate filter press capacity and how many filter plates do I need.
When you can skip the thickener
Not every plant needs a separate thickening stage:
- Chemical and mineral sludges from a clarifier underflow are often already at 3–10% solids and go straight to the press. Electroplating, tannery, textile and mining sludges usually fall in this group.
- Small STPs and ETPs below about 500 m³/day usually thicken in the sludge holding tank by settling and decanting overnight, which is enough to feed a small press at 2–3%.
- MBR plants hold their mixed liquor at 8–12 g/L, so their waste sludge is already close to 1% and a rotary drum thickener or DAF is generally needed before a press; there is no primary sludge to help.
The rule of thumb: if the sludge reaching the press is below 2% solids, thickening will pay for itself in press size; if it is above 3%, a thickener adds little. Between 2% and 3% depends on how many hours a day the press can run.
Thickening in the treatment train
In a typical STP the train is primary clarifier and secondary clarifier → sludge blend → thickener → (digester, if present) → conditioning → dewatering. In an ETP it is usually clarifier or DAF → sludge holding tank → conditioning → dewatering, with the holding tank doing the thickening. In mining and mineral processing the thickener is a large-diameter high-rate unit feeding the press at 30–60% solids, and the roles of “thickener” and “dewatering” are the same as above at a much higher concentration. Our ETP vs STP and mining tailings dewatering articles show where the press sits in each.
Choosing the thickener with the press in mind
Two points matter for the downstream press. First, polymer added in a mechanical thickener conditions the sludge once; adding more at the press for a second conditioning is normal, but the two products must be compatible and the total dose should be checked by jar test rather than assumed. Second, thickened sludge must be pumpable to the press: above 6–8% solids, biological sludge becomes difficult to pump with a centrifugal pump and needs a progressive cavity or piston pump, and the sludge holding tank needs a mixer to stop it stratifying. Our page on wastewater treatment equipment lists thickening and pumping options that pair with our presses, and polymer conditioning for filter press cloth covers the dosing side.
Frequently asked questions
Why is sludge thickened before dewatering?
Because dewatering equipment is sized on sludge volume. Thickening from 1% to 4% solids removes three-quarters of the water, so the filter press, feed pump, polymer and cycle time are all smaller. It also smooths the feed concentration and gives buffer storage.
What solids concentration should I feed a filter press?
For biological and mixed sludges, 3–6% solids is a good target; for chemical and mineral sludges from a clarifier, 3–10% is typical and needs no separate thickener. Below 2% a thickener pays for itself in press size.
Which thickener is best for waste activated sludge?
Dissolved air flotation, a gravity belt thickener or a rotary drum thickener, all of which reach 4–7% solids. Gravity thickening alone reaches only 2–3% on waste activated sludge and the sludge tends to go septic.
Can I skip the thickener?
Often, for chemical and mineral sludges that already leave the clarifier at 3% or more, and for small plants that settle and decant in the sludge holding tank. Dilute biological sludge below 2% almost always benefits from a thickener.
Does thickening reduce polymer consumption?
Polymer is dosed per dry tonne, so the total does not fall automatically, but mixing and floc formation are better in concentrated sludge, and in practice the combined thickener-plus-press dose is usually lower than dosing dilute sludge straight into the press.
How does feed solids affect filter press cycle time?
Strongly. At 1% solids a chamber needs roughly 30 chamber volumes of sludge to fill with cake; at 4% about 8. The press completes two to four times as many cycles per day on thickened sludge, so a smaller press handles the same tonnage.
Tell us your sludge type, daily dry solids and current feed concentration, and we will tell you whether a thickener is worth adding and size the press either way: request a thickening and dewatering proposal.
References: US EPA — Biosolids Technology Fact Sheet: Gravity Thickening, US EPA — Biosolids Technology Fact Sheet: Centrifuge Thickening and Dewatering, US EPA — Biosolids Technology Fact Sheet: Recessed-Plate Filter Press.