Processing Magazine

What are operational process controls for overall effective sludge dewatering?

How sludge gets dry

January 8, 2014

Press filtration using plates or belts and centrifugation separation are common methods for dewatering sludge. Someone recently asked, “What are operational process controls for overall effective sludge dewatering?”

Press filtration using plates is a batch sludge-dewatering process using a plate-and-frame unit.

The plate-and-frame filter press consists of vertical plates held rigidly in a frame and pressed together. Sludge is fed  into  the  press  through  feed  holes  along  the  length  of  the press. A filter cloth is mounted on the face of each individual plate. As filtration proceeds, water (filtrate) passes through the cloth fibers, is collected in drainage ports at the bottom of each press chamber and discharged. Sludge solids are retained on the filter cloths and build up until the cavities between the plates are completely filled with solids (cake).

As the cake builds up, resistance to flow increases because the water has to pass through thicker solids layers. As resistance increases, the sludge volume fed to the filter and consequently the filtrate volume decreases. When filtrate flow is near zero, the feed is shut off and the plates are disengaged. The plates are pulled away from each other and the retained cakes are discharged by gravity, falling into a hopper or conveyor.  After the cakes are discharged, the plates are pulled back together and the feed is restarted.

Press filtration with belts is a continuous-sludge dewatering process using a belt-press unit.

Belt-filter presses consist of two endless belts that travel continuously over a series of rollers. Variations in belt-filter designs are available, but the basic principles are the same. Sludge to be dewatered is preconditioned, usually with polymers applied to the free water drainage zone of the filter belt. This portion of the belt allows most of the free water to drain through the filter and collect in a trough on the belt underside. Some presses use in-line polymer mixing where the polymer is added directly to the feed line and mixed with sludge by passing the flow through a restriction to create mild turbulence. With this type of chemical mixing system, the conditioned sludge is applied to a horizontal drainage zone.

Cylindrical mixing chambers can also be used to ensure adequate polymer and sludge contact. They slowly rotate to allow the polymer to mix with the sludge. When the conditioned sludge moves out of the mix chamber, it can be applied directly to a horizontal drainage zone or it can be delivered to a cylindrical reactor chamber. The reactor chamber replaces the horizontal drainage zone and includes a screen around the outside edge to allow most the free water to drain out.

The partially dewatered solids are then carried to a point on the unit where they are trapped between two “endless” belts. The solids are further dewatered as they travel over a series of perforated and un-perforated rollers. This zone is known as the "press" or "dewatering zone." Here, the entrapped solids are subjected to shearing forces as they proceed over the rollers. Water is forced from between the belts and collected in filtrate trays, while the retained solids are scraped from the two belts where they separate at the discharge end of the press. The two endless belts then travel through washing chambers to remove fine solids that could lead to plugging.

Centrifugation separation is a continuous process of sludge dewatering with a centrifuge unit.

A number of process and machine variables must be adjusted to achieve optimum centrifuge performance. Some of these variables, such as bowl speed, conveyor differential, and pool depth are major adjustments that should be made during the initial optimization of the centrifuge and at frequent intervals thereafter, especially when major changes in the slurry quality occur. Others, such as polymer feed rate, dilution water flow, and slurry feed rate are adjusted as needed for low-cost optimized results.

Increasing bowl speed is often thought of as being a “good” adjustment, as it leads to drier sludge cake or clearer water leaving the unit. But surprisingly, increasing bowl speed doesn’t necessarily improve performance. It does, however, always increase power consumption.

The fact is higher speeds do increase centrifugal forces. They tend to create larger shear forces, which in turn, can break up fragile agglomerates of flocculated particles. Some materials reach a certain compaction level and will not compact much more -- if at all -- regardless of the applied centrifugal force.

However, it is not possible to accurately predict how bowl speed relates to performance. If there is a question, it can best be resolved by evaluating performance of several speeds during equipment startup. Select, as the optimum, the lowest speed that achieves the desired cake dryness and centrate water quality.

The conveyor’s differential speed impacts centrifuge performance. A lower differential speed allows more time of compaction, usually leading to cake with a higher dry solids content. Lowering the differential also decreases pool turbulence, reducing the problem of solids suspension, and allowing more quiescent settling conditions. As a result, lower differentials usually mean improved centrate water quality.

Two factors limit the minimum differential speed. First, the low scrolling rate will result in a high depth of cake solids. At a certain differential, these may infringe on the clarified pool and be carried out with the centrate water, causing a marked decrease in centrate water quality. Second, plugging of the centrifuge may occur, if the solids input rate exceeds the scrolling capacity of the conveyor. A plugged centrifuge may have to be dismantled, if it is not cleared quickly by stopping the feed.

Pool depth is controlled by adjustable weir plates located on the bowl headwall opposite to the feed end of the machine. The effect of varying pool depth is difficult to predict and will depend on the settling and compacting characteristics of the sludge solids. However, in some applications, the effect of pool depth can be critical to good performance, especially when the pool level is almost at the solids discharge point. As a general rule, it is necessary to experiment with different pool depths to evaluate the effect on your specific sludge.

In theory, lowering the pool should increase cake dryness because a longer length on the conical part of the bowl (beach area) is out of the liquid. This increases the time that the solids can drain before they are discharged from the machine. Centrate water clarity should suffer, if the pool is lowered too much, since settling time is reduced and unsettled solids then exit with the centrate water. Also, lowering the pool depth can make the solids difficult to convey due to the increased length of the beach area.

Again, in theory, increasing the pool depth should cause cake moisture to increase, since raising the pool depth reduces the length of beach area. A deeper pool depth, however, usually improves centrate water clarity, since the liquid retention time is increased, giving the lighter particles time to flocculate and settle out. Deepening the pool also makes it easier for the solids to be conveyed.

The actual volumetric sludge feed rate to the centrifuge will also affect centrifuge performance. The residence time in the centrifuge is inversely proportional to the sludge feed rate. As a result, decreasing the feed volume will increase the residence time and should improve centrate water clarity. Lower volumetric feed rates will mean lower dry solids feed rate as well, which permits lower conveyor differentials and will possibly enhance cake dryness. Unfortunately, there is a down side to lower feed rates. Electrical consumption, in terms of kilowatt hours per ton of dry solids processed, will increase as the feed rate is decreased.

In Summary:

Operational process controls identified above to dewater sludge using press filtration or centrifugation separation should satisfy sludge dewatering requirements in many circumstances. However, if you have specific sludge-handling concerns including sludge holding, dewatering or disposing or other wastewater queries, please submit a question.