Sludge dewatering: screw press, belt press or chamber press
Any treatment plant produces two products: treated water and sludge. The water is discharged, the sludge is hauled away, and hauling almost always turns out to be the most expensive line in operating costs. Dewatering does not make sludge harmless — it makes it many times lighter, and that is what the whole calculation is built on.
You pay the same for water in the truck as for sludge
Sludge from a secondary settler or an aeration tank is a liquid. Its dry solids concentration is around 1 %: a cubic metre of such slurry holds about 10 kg of sludge proper and about 990 kg of water. Waste activated sludge can be five times as dilute — 0.2 % dry solids.
Hauling is charged by mass or by truck volume, and the tariff does not distinguish what exactly is being carried. Dewatering does not reduce the amount of dry solids by a single gram: whatever formed in the biological stage stays. It removes water — and water is the overwhelming part of the raw sludge.
The consequence is simple: the economics of dewatering are set not by the throughput of the machine but by how dry a cake it delivers.
Cake dryness: what the percentages actually mean
Cake dryness is the share of dry solids in the mass of the cake, as a percentage. Moisture content is the same thing in reverse: 20 % dryness means 80 % moisture. Both figures describe the same thing and both appear in specifications, so when comparing machines it is worth making sure you are looking at the same indicator.
The calculation goes like this: the mass of dry solids at the outlet equals the mass at the inlet, and the mass of the cake is inversely proportional to its dryness. A few percentage points more dryness look like a trifle at first glance, but in practice they change the number of truck trips per month.
Achievable dryness depends on three things at once: the nature of the sludge itself, the quality of its conditioning with flocculant and the type of machine. The first is not a matter of choice: raw primary sludge gives up water readily, waste activated sludge does so with difficulty, and oily and biologically "young" sludge is worse still. That is why the specific dryness figure for a site is obtained by testing on the real sludge, not from a table.
By machine type the difference is this: pressure filtration through a cloth gives a cake of low moisture content, because water is squeezed out by the feed pressure in a closed chamber. Continuous machines — the screw press and the belt press — work more gently and on the same sludge deliver a wetter cake, but they do not stop.
Three machines, three different logics
The HLDS screw press squeezes sludge in the channel between a screw shaft and a set of alternating moving and fixed rings. The gap narrows along the screw, pressure rises, water leaves through the slots between the rings, and the cake is discharged at the end. The rings move relative to each other and clear the slots themselves — there is no filter cloth here at all. The range runs from the HLDS-131 with a throughput of about 6 kg of dry solids per hour (about 0.6 m³/h of sludge at 1 % concentration) to the HLDS-404 at ~400 kg DS/h (~40 m³/h). Masses run from 300 kg for the smallest version to 3850 kg for the HLDS-304, and drive power on the small machines is measured in fractions of a kilowatt: 0.2 kW on the HLDS-131, 0.3 kW on the HLDS-132.
The main feature of the screw arrangement lies elsewhere: the machine takes dilute sludge straight from the aeration tank or the settler, without a separate sludge thickener. The thickener need not be built at the plant at all — that removes a whole structure, not just the floor area under equipment.
The DNY belt filter press separates the phases between two permeable belts running over a system of rollers. First comes an extended gravity drainage section, where free water leaves by itself, then the belt folds into an S-shaped path and travels with the sludge between the rollers, so that pressure on the layer builds up gradually. Throughput on dry solids runs from 100–120 kg/h on the DNY 500 to 600–700 kg/h on the DNY 3000. The frame is SUS304 or SUS316 stainless steel, belt tension is held by pneumatics, and the drive is slow-running: on the DNY 1000A and DNY 1500A it is 1.1 kW.
The price of continuity is wash water. The belt has to be washed continuously, and the consumption is noticeable: 11.2 m³/h on the DNY 1000A, 16 m³/h on the DNY 1500A, 25.2 m³/h on the DNY 2500B. This water returns to the head of the works and increases the load on it.
The HL chamber filter press works in cycles. The cloth is stretched over the plates, and when the pack is closed it also serves as the seal, so a chamber forms between each pair of plates. Slurry is fed by a pump through the central channel, the filtrate leaves through drainage channels in the plates, and the solid phase stays on the cloth. When the chambers are full, the pack is opened, the cake is dropped and the cycle starts again. Filtration area in the range runs from 50 to 250 m²; on the HL100 it is 100 m² with a chamber volume of 1475 l and a throughput of 2–4 t/h, on the HL150 it is 150 m², 2063 l and 3–5 t/h.
Mass is a separate point: the HL100 weighs 6628 kg, the HL150 already 10455 kg. Such a machine needs a foundation, headroom for opening the pack and space for a cake receiving hopper underneath.
| Screw HLDS | Belt DNY | Chamber HL | |
|---|---|---|---|
| Mode | continuous | continuous | in cycles |
| Filtering medium | rings, no cloth | two belts | cloth on plates |
| Cake moisture | higher than with pressure filtration | higher than with pressure filtration | low |
| Wash water | little | 11.2–25.2 m³/h depending on version | no constant flow |
| Prior thickening | not required | required | required |
| Space and mass | 300–3850 kg, no thickener needed | SUS304/SUS316 frame, belt from 1000 mm | 6628 kg on the HL100, 10455 kg on the HL150 |
| Noise | quiet operation | low, slow-running drive | set by the feed pump |
| Attendance | automated, no permanent operator | belt tensioning and washing | opening the pack and dropping the cake every cycle, cloth selection |
| Range coverage | ~6–400 kg DS/h | 100–700 kg DS/h | 2–5 t/h on the HL100 and HL150 |
Flocculant: none of the three works without it
Sludge particles are fine and carry the same charge, so in water they are stable and do not stick together on their own. A flocculant is a polymer that binds them into large flocs which release free water. It is prepared in a solution make-up unit and dosed ahead of the machine: screw presses have a flocculation chamber at the inlet for this.
Without flocculation the result is equally poor in all three arrangements, but it shows up differently. In the screw and belt presses the fines simply leave with the filtrate back to the head of the works — the machine turns but there is no cake. In the chamber filter press the fine particles blind the pores of the cloth, the cycle stretches out, feed pressure rises and the cloth has to be replaced more often.
Hence the practical rule: selecting the flocculant and its dose is part of selecting the equipment, not a separate task for later. A machine underfed with polymer will not reach its rated performance under any setting.
What decides the choice on site
- ✓Flow and mode. A small plant where sludge accumulates over days is served perfectly well by a cyclic press. A continuous sludge stream requires a continuous machine.
- ✓Whether there is a thickener. If there is none and none is planned, the screw arrangement takes dilute sludge as it comes. The belt and chamber presses need thickening.
- ✓Where to put the wash water. Returning tens of cubic metres per hour to the head of the works is an extra load that has to be built into the balance.
- ✓Proximity to people. A site next to housing or inside a building leans towards quiet machines.
- ✓Who attends it. An automated screw press works without an operator permanently present. A chamber press needs a person to open the pack and look after the cloth every cycle.
- ✓The cost of hauling. The higher it is, the stronger the argument for pressure filtration: a dry cake pays for the labour.
What next
Collect four figures: the daily amount of sludge in cubic metres, its dry solids concentration, the sludge type (waste activated, raw, mixed, chemical, oily) and the mode — whether it accumulates or arrives as a stream. The fifth thing worth finding out in advance is the current hauling tariff: without it you cannot compare the options in money.
With those figures you can look at the catalogue: HLDS screw presses cover the needs of small and medium plants and of sites without a thickener, DNY belt filter presses cover a continuous sludge stream at high flows. If the sludge type is unusual, plan a test on a real sample: a cake dryness figure obtained on your own sludge is worth more than any table.
