Pumps/Water Treatment The tough get going: An overview of pumps for sludge drying and thickening applications

When handling sludges and slurries, filter presses and pumps have to go the extra mile. From piston powered units to screw designs, engineers have a wide array of pumps to choose from. Yet, each of these technologies has not only apparent strength but also significant weaknesses. But there might be one pump design which ticks all the boxes.

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In many processes, substances occur in aqueous or “muddy” form as sludges or slurries. Since the disposal of such sludges is calculated on weight and volume, thickening and drying is lucrative. During the process, slurries are often treated with chemicals and/or physical processes so that contained substances form flake-like solids. After pH-adjustment to neutral or alkaline, they are dewatered, via gravity or technical methods such as centrifuges and evaporators, or more commonly, chamber filter presses.

These presses consist of a number of plastic frames that are forced together under high pressure. Inside these are hollow chambers, from which the press gets its name, surrounded by filter cloth. When sludge is pressed into the chambers, a “filter cake” forms while the filtrate flows into drainage channels.

The pressure required to fill these presses — usually between 8 and 15 bar at its peak — should be even in order not to destroy the flocs. In addition to the constantly increasing counter-pressure that occurs until the end of the pressing, a further constraint is the eventuality that an empty-running sludge tank can lead to dry running of the pump.

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Pumps and their constraints

To build pressure, different displacement pumps are used, including piston diaphragm pumps for large presses. In these units, diaphragms are hydraulically actuated to feed the sludge into the press via valves.

These pumps require a large and costly overhead, such as an air pressure vessel to equalize the feed rate and a maximum pressure monitor or bypass. Eccentric screw pumps are used to process large slurry quantities or when long fibers prevent the use of valves.

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However, there are constraints in small- and medium-sized plants due to their sensitivity to abrasion and dry running as well as their considerable footprint. The list of pumps also includes hose-diaphragm piston pumps and piston pumps, which generate strong pulsation and require constant lubrication. Both of these pumps are characterized by fairly high installation and maintenance costs.

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