BELT PRESS AND GRAVITY TABLE DEWATERING
PROCESS PERFORMANCE - MYTH AND REALITY
Belt Presses and Gravity Tables are being increasingly used for sludge dewatering in the Waste Water Industry. Users and specification writers are often not familiar with the constraints and factors dictating performance, and this paper attempts to provide the basic information, and suggests a method of characterising sludge dewatering properties. The pressure zones of a press system are described, and their influence on the volumetric and solids handling capacity of a particular design. The influence of flocculation on performance is discussed, and it is proposed to establish a sludge characteristic "benchmark" dosage rate, which is the minimum practical rate.
The use of punitive process guarantee specification clauses is discouraged and it is recommended that instead, specifiers could rely on specifying mechanical/ physical design factors, to ensure that the equipment provides the best possible dewatering result, irrespective of variable sludge characteristics.
A Belt Press dewatering system has two capacity factors -a volumetric capacity and a solids handling capacity for a particular type of sludge. One or other of these capacities will be the determining factor, depending on the solids concentration of the feed sludge. The ideal situation is to match the equipment to the feed sludge so that it is running at both its rated volumetric and solids handling capacities simultaneously.
The flocculation and dewatering properties of a sludge are often widely variable, and descriptions such as "anaerobically digested sludge" or "extended aeration waste activated sludge" are not sufficient to describe the dewatering characteristics. It is therefore impossible for a Belt Press manufacturer to accurately predict a particular site operating condition, unless a totally representative sample is available.
Some specifying authorities utilise extremely onerous contract process penalties as a means of ensuring that a particular process performance is met. This approach is in conflict with the fact that the sludge properties are widely variable and often unpredictable, and it is suggested that a more preferable approach would be to specify physical design parameters such as filtration areas, and configurations of zones and pressure rolls. This would ensure that whatever the properties of the sludge, the plant operator has available the best possible process performance attainable.
A Belt Press dewatering system may be characterised by the areas of its three filtration zones, as well as the configuration of these zones. Each of the zones has its own influence on the performance of a press and we will discuss them in turn.
The first essential step in the dewatering process, is the super-flocculation of the sludge. The floccs required are much greater in size (typically 15 to 20mm), than those necessary for the sedimentation process. Figure 1 shows a flocculated sludge. The flocculation produces large floccs, surrounded by free water. In the press, the free water (Fig 1) is drained in the gravity drainage zone, and the capillary water is progressively removed by gradually increasing pressure in the wedge and surface pressure zones.
The actual process of flocculation requires experience and suitable equipment. The choice of injection points, flocculator/mixing device, dilution ratio, and type of flocculator, is more an art form than a science, and experience plays an important role. Usually the diluted polymer solution is injected ahead of a mixer flocculator, which ensures a correct mixing energy is applied, combined with retention time and flocculator action, to ensure large floccs are built, then gently deposited onto the gravity drainage zone.
Zone 1 - Gravity Drainage
The gravity drainage zone should ensure the almost complete drainage of the free water. This zone is simply a screening action through the filter cloth under gravitational pressure only, and may take the form of one of the following:
A section of one of the main press belts.
A separate gravity drainage belt, proceeding into the gravity or wedge zone of the press.
A Rotary Screen Thickener (Trommel), which is a rotating filter cloth screen, which has a high efficiency due to the rotation, which causes washed cloth to be continuously exposed to the sludge.
A gravity drainage section on one of the Belt Press filter belts, is the most common system, often in conjunction with some form of agitation, such as ploughs. A sufficient length of gravity drainage zone is necessary to ensure that the feed is uniformly distributed across the width of the belt, which is essential for reliable belt alignment. In addition, the gravity drainage zone forms the very useful function of providing a visual indication to the operators of the flocculation of the sludge. A poorly flocculated sludge will not drain in the time available.
Zone 2 - Wedge Zone
The wedge zone (refer Fig 2) is formed by the two belts of a Belt Press enclosing the sludge as a sandwich, with the gap between the two belts progressively reducing as they pass through the press.
This reduction in volume produces an initial low pressure dewatering action, and removes a large proportion of the total liquid removed. As the sludge is squeezed between the two belts, water is expelled through the surface of the belts, similar to the squeezing of a sponge, and hence the action of the wedge zone is independent of its orientation. Most high quality presses incorporate some form of seal along the edges of the belt in the wedge zone, which assists in retaining the sludge between the belts. In some designs, these seals are also adjustable towards the centreline of the belt, in order to compensate for low stability sludges.
Zone 3 - Press Zone
Higher pressure and a shearing action to release the moisture from the cake is obtained by wrapping the belt and cake sandwich over a series of rolls in a S shape in order to apply an increasing surface pressure, caused by the tension of the belt and decreasing roll diameter.
Further high pressure maybe obtained by applying a press nip pressure roll, (similar to the wringer roll on old washing machines) or alternatively, an additional belt operating at higher tension. These high pressure devices are not usually appropriate to sewage sludges, due to their soft nature.
The size of the rolls, number off, and ratio in sizes, determines the final cake solids content, and to a degree, the solids handling capacity. Most sludges have a point past which increasing pressure and shear does not release further moisture. We can therefore define as a sludge characteristic- the "Ultimate Cake Solids Content" 1UCS). This characteristic is influenced by the polymer dose rate, and can sometimes be reduced by high polymer dose.
FLOCCULATION CHARACTERISTICS AND POLYMER DOSE RATE
In order to dewater by pressing, the sludge needs to be super-flocculated and the amount of polymer required for this process is a property of the sludge. Currently there is no Australian or International Standard for evaluation of this characteristic.
A suggested practical method involves timing the drainage rate of a fixed volume of flocculated sludge through a filter belt sample. This produces a drainage/time relationship which has rapid drainage at first, followed by progressive reduction as the floccs blind the flow path.
From a series of these curves, a sludge characteristic related to practical application, may be derived, which is the time required to drain approximately 75% of the free water, and this plotted in a curve as shown in the attached figure (3)
From this set of curves, it is clear that dose rates below a minimum, will produce a practically infinite drainage time, and therefore are not suitable for Gravity Belt or Belt Press dewatering. Increasing dose into the next zone, the "transition zone" is where the drainage rate is extremely sensitive to small changes in polymer dose rates. For practical purposes it is impossible to operate a dewatering unit in this zone, unless the sludge feed consistency is absolutely stable.
As the curve flattens out, we enter into the operating zone, and this is the range of polymer dosage rates at which it is practical to operate a Belt Press running at optimum efficiency. This polymer dose rate can be defined as the "Benchmark Polymer Dosage Rate" 1BPDR) sludge characteristic and used to assess the relative performance of a Belt Press.
Volumetric capacity of a particular Belt Press system is determined generally by the areas of the Gravity and Wedge Zones. For thin sludges, eg waste activated sludge at 0.8% TSS, the volumetric capacity maybe increased by adding Gravity Drainage Tables or Rotary Screen Thickeners. For thick sludges, eg digested primary sludge at 7 to 8% total solids, the volumetric capacity is less relevant. The capacity of a Belt Press is not a fixed number, as higher throughputs maybe obtained on any press by increasing the polymer dose rate, running at higher belt speed, and accepting the corresponding reduction in the cake solids. This becomes limiting when the rate of increase in pressure is so great in the pressure zones that the sludge escapes at the sides of the belt.
Polymer dose rate efficiency can be evaluated by comparing the actual dose rate with the BPDR as defined above, and higher dose rates generally indicate that the press system is operating beyond its optimum point.
Throughput and Cake Solids
As discussed above, throughput and cake solids contents are often interchangeable, higher throughputs resulting in lowered cake solids, (typically 2 -3 percentage points), and conversely running at very low throughputs may sometimes yield similar gains in cake solids, if this is desired.
Cake solids content (UCS) is also basically a function of the capacity of the sludge to withstand pressure, and is difficult to predict unless a sample is available for testing.
Capture rates of Belt Press and Gravity Drainage sludge dewatering systems are usually comparatively high, and of the order of 92 - 95%. Most of the losses are not, as may be expected, usually through the sides or mesh of the belt, but rather due to cake sticking to the belt and being lost in the washwater. Again, this is a characteristic of the sludge as well as a function of the selected belt type.
Capture rates are extremely difficult to predict, even when samples are available.
The sludge characteristics will determine the optimum efficiency of Belt Press sludge dewatering through the Benchmark Polymer Dose Rate (BPDR), and the Ultimate Cake Solids Content (UCS). The efficiency of a Belt Press dewatering system can be evaluated by comparing actual results with these optimum figures.
Specifiers and users can ensure that they have the most efficient system by specifying physical design of the press system, including filtration areas, configuration of wedge and pressure zones, and overall mechanical design. This will avoid the need for onerous process penalty contracts, whilst ensuring that the user receives the most efficient plant achievable with state-of-the-art technology.