Sludge dewatering is the stage at which water is mechanically separated from the sludge produced by the earlier treatment stages. The stage does not treat water: its purpose is to reduce the volume of waste hauled away and to turn a pumpable mass into a material that is loaded into a skip. The separated water returns to the head of the scheme together with whatever is dissolved in it.
Principle
Dewatering proceeds in two stages, and both are present in each of the three machines. First the free water separates: the sludge enters the drainage zone and water passes through the filtering surface under gravity. The remaining mass is then subjected to mechanical pressure and part of the capillary moisture is squeezed out. It is the second stage that distinguishes the machines: the way pressure is generated governs both the achievable cake moisture and the duty cycle.
The flocculant is added ahead of the machine and is part of the process rather than an improvement to it. Water is held by the extensive hydrophilic surface of fine particles, and the particles themselves are smaller than the filtering gap or the pores of the cloth. A polyelectrolyte neutralises their charge and binds them into large flocs within which the water becomes able to separate. Without a flocculant sludge is not dewatered less well — it is not dewatered at all: the particles pass the filtering surface along with the water, blind it and leave with the filtrate, and no cake forms.
The screw press. A shaft of decreasing pitch turns inside a cylinder built up from alternating fixed and moving rings. The gap between the rings serves as the filtering slot, and as the shaft turns the moving rings clear it, so wash water is needed only intermittently and in small quantity. The shaft turns at a few revolutions per minute; pressure comes from the reducing volume of the flight and from a back-pressure cone at the outlet. Hence the low installed power: 1.65 kW in the catalogue models at a capacity of the order of 100 kg DS/h.
The belt press. Sludge is spread over a moving belt and passes a gravity zone, where free water drains away; a wedge zone, where pressure builds up gradually; and a pressing zone, where two belts with the sludge between them run over rollers of decreasing diameter, pressure and shear both rising as the diameter falls. The belts are washed continuously with pressurised water, otherwise they blind within a single pass — the wash water flow is stated in the specifications and, for the model with a working belt width of 1000 mm, amounts to 11.2 m³/h. That water goes entirely into the filtrate.
The chamber filter press. Plates faced with filter cloth are closed into a stack, forming chambers between them; a pump feeds sludge into the chambers under pressure, the filtrate passes through the cloth and the solid phase stays inside. When the chambers are full the feed stops, the stack opens and the cake is discharged. The cycle is a batch one and ends with discharge, so the machine does not run unattended; in return it gives the driest cake of the three, since feed pressure is held throughout the cycle and pressing time is not limited by the rate at which sludge passes through. The defining figure of the sub-series is filtration area: for the CFP100 model it is 100 m².
Principle
The capacity of a dewatering machine cannot be stated as a single number, and a volumetric figure in m³/h means nothing on its own: the machine is limited by the mass of dry solids it can separate in an hour, not by the volume of liquid passed through it. Sludge at 1.00 % dry solids holds about 10 kg of dry solids per cubic metre, so 10 m³/h of such sludge is 100 kg DS/h; the same machine on sludge at 5 % DS will take about 2 m³/h, and that will be the same 100 kg DS/h. This is why the screw press specifications place the volumetric figure next to the concentration at which it was obtained, and why the figures of the sub-series are not comparable directly: belt presses are rated by dry solids, chamber presses by filtration area and by throughput in t/h.
A dewatering machine pays for itself in haulage rather than in treatment. Sludge at 99 % moisture is almost entirely water by mass, and the contractor hauling it away is hauling water. Reducing moisture from 99 to 80 % without losing dry solids leaves 20 % dry solids in the cake instead of one, and at a density close to that of water a tonne of the original sludge yields about 50 kg of cake and about 950 kg of filtrate — a twentyfold reduction. In real schemes the reduction is more modest and depends on the starting concentration: sludge already thickened to 5 % DS gives a fourfold reduction at the same cake moisture.
The water separated from the sludge does not leave the scheme. The filtrate returns to the head of the works, and it is not clean water: dissolved nitrogen and phosphorus remain in it, along with part of the COD and, where dosing is out of order, carried-over fine solids as well. An internal loop appears: the return load increases sludge production, and more sludge yields more filtrate. Where the return is not built into the design of the main structures, the analyses show a rising load on the biological stage with no visible change in the incoming effluent — and the cause is looked for in the wrong place.
Practice
The stage sits at the end of the scheme but is calculated together with its beginning. Sludge arrives from a flotation unit, a clarifier or the biological treatment structures, and the water separated from it returns to the same place. Dewatering is therefore designed alongside the main structures rather than fitted to structures already built: otherwise those are sized for a load that ignores the return stream.
The flocculant is matched by testing the actual sludge, not by the type of production. Polyelectrolytes differ in the sign and density of their charge and in molecular weight, and sludge of one and the same origin — surplus activated sludge from two neighbouring plants, say — may call for different grades. Choosing by analogy yields either no floc at all or a reagent consumption several times higher than needed.
The choice between the sub-series is governed by the duty of the site as much as by capacity. A screw press runs continuously at low speed, needs little wash water and tolerates swings in concentration — a solution for small and medium flows and for sites without a permanent operator. A belt press is also continuous and takes a larger flow, but requires continuous washing of the belts, that is, water and a route for it. A chamber press works in batches and gives the driest cake, but the cycle ends with discharge, which means somebody has to be on site.
Wash water is a line in the water balance of the scheme, not a detail: for the belt press it is stated in the specifications alongside capacity and power — 11.2 m³/h for the model with a working belt width of 1000 mm. That water has to be supplied at the required quality, since solids blind the wash nozzles, and taken back together with the filtrate.
The mass of the machine and the space for discharge are settled in advance: one of the catalogue screw presses has an operating mass of 3400 kg and the chamber filter press with a filtration area of 100 m² weighs 6628 kg, and both need a floor of matching load-bearing capacity and enough height for the cake to drop straight into a skip. We do not manufacture this equipment: our work is the calculation by dry solids mass, the selection of the machine and the flocculant from a sludge sample, supply, commissioning and service.
Limits
Practice
The cases are typical examples, not site reports.
Innovations
Scheme
Dewatering is never a stage in its own right: it always follows the unit where the sludge arises, and its calculation starts not from the capacity of the machine but from the quantity of dry solids the preceding stage produces per day. A holding tank with mixing is placed between the sludge source and the press: sludge arrives in batches — as the scraper operates, as the clarifier is pumped down, as surplus sludge is withdrawn — whereas the press works to a steady flow and to a shift pattern.
Thickening ahead of dewatering is frequently justified. The higher the dry solids concentration at the inlet, the greater the dry solids throughput the machine takes at the same volume and the lower the flocculant consumption per tonne of dry solids; a gravity or mechanical thickener often makes a smaller size sufficient.
The flocculant make-up and dosing station enters the scheme as equipment of its own. Dry polyelectrolyte has to be dissolved, the solution matured and then diluted before it is introduced into the sludge stream; the concentration of the solution determines the actual dose, so it cannot be prepared without control.
Downstream of the press the scheme divides into two streams again. The cake drops into a skip or onto a storage area, which calls for height beneath the machine and access for vehicles. The filtrate returns to the head of the scheme, and its flow and composition enter the design of the main structures on the same footing as the incoming effluent.
Scheme
Operation
Energy
The installed power of the continuous presses is modest: 1.65 kW in the catalogue screw presses and 1.1 kW of drive power in the belt presses. Referred to capacity, that gives of the order of 0.017 kWh per kilogram of dry solids on a screw press at its rated 100 kg DS/h and about 0.006 kWh per kilogram on a belt press at 200 kg DS/h. The specifications, however, state drive power, not the consumption of the whole unit: the sludge feed pump, the wash water pump, the solution make-up station and, on a chamber press, the high-pressure pump are supplied separately, and the total consumption is appreciably higher. Electricity is not the leading item here in any case: the operating cost of the stage is dominated by flocculant and by haulage of the cake.
Consumables
What goes to drain
The stage produces two streams and both call for a decision. The first is the cake: on continuous presses its moisture on most sludges falls within 75–85 %, and it is lower on a chamber filter press; the actual figure is set by the kind of sludge — mineral sludge gives up water more readily than organic — rather than by the type of machine as such. The cake is stackable, is loaded into a skip and hauled away; its hazard class and the permitted route are determined by the composition of the original effluent, not by the type of equipment. Mechanical dewatering does not stabilise sludge: in prolonged storage it putrefies, so the size of the storage area is limited by the haulage interval. The second stream is the filtrate: it returns to the head of the scheme carrying dissolved nitrogen and phosphorus, part of the COD and, where dosing is disturbed, carried-over fine solids as well. Its flow is close to that of the incoming sludge and, on a belt press, exceeds it by the volume of the wash water.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
Because the machine is limited by the mass of solid phase it can separate, not by the volume of liquid passed through it. Sludge at 1.00 % dry solids holds about 10 kg of dry solids per cubic metre, so 10 m³/h of such sludge is 100 kg DS/h; the same machine on sludge at 5 % DS will take about 2 m³/h and deliver the same 100 kg DS/h. That is why the screw press specifications place the volumetric figure next to the concentration at which it was obtained: without it the figure carries no meaning.
No, and this is not a question of efficiency. Water is held by the extensive surface of fine particles, and the particles themselves are smaller than the filtering gap or the pores of the cloth. Until they are bound into large flocs they pass the filtering surface along with the water, blind it and leave with the filtrate, and no cake forms. The flocculant is part of the process on all three types of machine.
On continuous presses — screw and belt — it falls within 75–85 % on most sludges, and it is lower on chamber filter presses. The actual figure is set first of all by the kind of sludge: mineral and hydroxide sludges give up water more readily, surplus activated sludge least readily of all. The achievable moisture is established by testing rather than chosen from a data sheet, and below a certain limit mechanical dewatering does not proceed at all: bound water is separated only by thermal drying.
On the quantity of dry solids, on the duty of the site and on the required cake moisture. A screw press runs continuously at low speed, needs little wash water and tolerates swings in concentration — a solution for small and medium flows and for sites without a permanent operator. A belt press is also continuous and takes a larger flow, but requires continuous washing of the belts, that is, water and a route for it. A chamber press works in batches, gives the driest cake and ends its cycle with a discharge that calls for a person.
It returns to the head of the treatment works. The filtrate is not clean water: dissolved nitrogen and phosphorus remain in it, along with part of the COD and, where dosing is out of order, carried-over fine solids. Its flow is close to that of the incoming sludge and, on a belt press, exceeds it by the volume of the wash water. The load of the return stream is taken into account when the main structures are designed; where that has not been done, the scheme runs away from its design regime.
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