THE CHOICE OF BELT PRESS vs CENTRIFUGE

for sludge dewatering 

Introduction

The choice of Belt Press vs Centrifuge for sludge dewatering, is often a difficult problem for specifiers. Both technologies can produce dewatered cake of approximately the same solids content, but are quite different in a number of aspects.

When sludge dewatering was first introduced into Australian municipal wastewater treatment plants in the late 1970s and early 1980s, a number of trials for comparison between the centrifuge and belt press were carried out. In most cases in the municipal wastewater industry, the belt press was chosen for its simplicity, lower cost, lower power, lower noise and ease of control, as well as sometimes for lower polymer dosage requirements.

The Centrifuge offers one major advantage over the belt press, and that is that high capacities can be achieved in a very compact unit. This has made it the machine of choice for applications where sludge dewatering must be introduced into existing large plants on restricted sites, in heavily populated areas. New generation centrifuges can offer up to 10 percentage points drier cake than the earlier designs, however this is at considerably higher capital and operating cost.

 

The Belt Press is a relatively newer technology than the centrifuge, and as a result, there is a wider variety in the market in the design and quality of belt presses and the sub-systems provided. This factor has sometimes led to incorrect assumptions about the operation of belt presses in comparison to centrifuges, where a poor belt press was compared with a well engineered centrifuge.

Advantages of the Belt Press

  • The Belt Press is a very low speed device, (typically 3 to 10 rpm), requiring very little power consumption.

  • The Belt Press is a low noise machine, the main source of the noise being the washwater sprays, which can have a soothing effect.

  • The simple machine design makes it easy to maintain, so that even a small council workshop can do all maintenance that is required.

  • Accidental damage is limited, and the Belt Press can generally handle a large variety of foreign material passing through it, without major damage.

  • The Belt Press offers the lowest possible polymer dosage rate.

  • The Belt press feature of separation of the Sludge/polymer mixing energy from the dewatering process, and variable control of this energy, allows the optimum polymer flocculation, which can be seen and controlled by the operator.

  • The Belt Press will always have an equal or lower polymer dosage rate in actual operation when compared to a Centrifuge.

  • The Belt Press is easy to control - operators can see the effect of the polymer and the process through the press. Even when enclosed, it is possible to open covers and observe when necessary

  • The Belt Press can never produce 'slops'. The cake formed by a belt press is always in a handleable state, due to the nature of the process.

  • The Belt Press can be enclosed. It is not usual, but it is possible, to fit covers around all the zones of the machine of the Belt Press, so that it becomes enclosed to the same degree as a Centrifuge. Some designs are particularly suitable for enclosure, such as those with large solid steel frames, and a simple one level layout. (EG the Sernagiotto BS model). Filtrate trays can be sealed, and piped to a central collection pipe, including belt washwater.

  • High volumetric capacity available by increasing gravity drainage area..

  • The Belt Press is well suited to thin sludge dewatering as it is not necessary to accelerate a large mass of water containing very little solids as is required for the Centrifuge. As a result, throughputs of up to 180m3/h are easily achieved in a single Belt Press train, when the solids content is less than 1% solids.

Disadvantages of the Belt Press

  • Requires washwater. The press belts are continuously washed, usually by recycled clarified plant effluent water. This disadvantage is minimised by the fact that most plants today have effluent water systems for hose-down and clean-up. At some plants, recycled effluent water is not available, however, belt press gravity zone filtrate may be used instead. Depending on the pressure available, a booster pump may be required. The choice of multi-stage pumps for this application reduces the power consumption, so that even taking into account the washwater pumps, the total power requirements for the Belt Press are still less than the Centrifuge.

  • Greasy sludges are a problem. The Belt Press is not well suited to sludges containing high levels of fats and greases. These greases tend to blind the filter belt, and whilst periodic washing can remove accumulations, this normally degrades the performance of the Belt Press.

  • The Belt Press is sometimes said to produce large amounts of aerosols. This can be true for poor quality units, or badly maintained units, but today's high quality machines generally have close fitting filtrate trays, fully enclosed washboxes, and piped discharges, so that aerosols are negligible.

  • The belt press is not as clean as a centrifuge externally, and requires more labour for cleanup.

  • There are potential nip point hazards, which require proper guarding, safety lanyard switches for emergency stop, and handrails or similar devices to keep personnel at a safe distance. However, the machines move at a very slow speed, and usually the pressures are very low (less than 0.7 bar), and hence it is just possible to remove a caught object if one is fast.

Advantages of the Centrifuge

  • Does not require belt washwater. This is sometimes a good advantage where an effluent reticulation system is not yet available. However, the trend is to have effluent water at most plants.

  • Good for greasy sludges. The Centrifuge is excellent for dewatering or separation of fats, oils and greases.

  • Large capacity in small space. The Centrifuge can offer a large solids handling capacity in a very small space. This is advantageous in large plants.

Disadvantages of the Centrifuge

  • Centrifuge is a high speed rotating machine. As such, it requires careful maintenance and is not suitable for overhaul by inexperienced personnel.

  • High power consumption. The Centrifuge is powered by a large single motor which has a high power demand. This makes it less suitable for a mobile plant application, as many small wastewater treatment plants do not have sufficient spare capacity. In this case, it is often necessary to provide an additional diesel generator, which generates noise, maintenance, etc.

  • High noise level. Centrifuge has a much higher noise level than the Belt Press due to its high speed operation. In addition, the large size of the motor has a significant motor noise factor as well.

  • The Centrifuge is not totally enclosed even though it appears to at first glance. The cake discharge is open to atmosphere, and some sludges tend to liberate more gases due to the centrifugal action, and these gases are released to atmosphere through the discharge opening of the centrifuge.

  • Polymer mixing not in control. The mixing action of the Centrifuge cannot be optimised separately for flocculation of the sludge. It is for this reason, that in many cases, the Centrifuge requires a higher polymer dosage rate than the Belt Press.

  • Difficult to control manually. The centrifuge is a "black box" from which sludge and centrate are emitted, providing very little visual clues as to the process. Modern automatic torque sensing controls go a long way to improving this problem. In comparison, the Belt Press has available different zones, and much more information regarding the characteristics of the sludge, in order for the operator to evaluate controlling action.

  • The lack of visual control information of the centrifuge can be a disadvantage for a mobile unit, which may be rapidly moved from site to site or from sludge to sludge, as the “settling down” time is extended.

  • Can be catastrophically expensive. Without accidents, the Centrifuge can operate for many years without problems. However, in the event of a malfunction of lubrication, or entry of large tramp material or unusually abrasive sludge, a major failure can occur, which will require the Centrifuge to be returned to the manufacturer for overhaul. This can be extremely costly and inconvenient to the plant.

Conclusion

The Belt Press is a simple, robust machine, suitable for plants run by less skilled personnel, where low noise and ease of operation, flexibility and low maintenance are critical. It can economically handle large quantities (more than 100 m3/hr) of sludge less than 1% solids.

The Centrifuge is a high speed rotating machine, suitable for large plants with well screened sludges, where cost, noise and high power levels are not of primary importance. The latest high performance units can offer a drier sludge, and a compact package, especially for thick sludge throughputs greater than 50 m3/hr.