Sludge is often the desired medium for process waste because solid material is easily separated from liquids, the volume is smaller than other waste mediums (such as brines), and sludge contaminants remain relatively inert.
The production of sludge occurs in many industries, usually as a waste by-product to the main processes; so it is important to understand treatment options that minimise sludge handling and disposal costs.
Sludge is a semi-solid which consists mostly of water (between 95% and 99%) and a high concentration of suspended solids (the remainder). As such the most important treatment for sludge is the process of dewatering.
Dewatering sludge: (left to right) sludge, removed water (filtrate), and dewatered solids (filter cake)
Sludge dewatering minimises both the weight and volume of the sludge by removing most of the liquid component. It makes the sludge significantly easier and cheaper to handle by reducing the expense of transportation, storage, and removal.
Dewatering converts the sludge into a solid cake (above 60% w/w solids), with the removed water usually recycled back to the main process. The below diagram shows the typical position of sludge dewatering in the flowsheet of a water treatment plant:
Sludge dewatering in a typical water treatment plant
Several technologies are available for sludge dewatering – such as geotextile bags, belt and filter presses, and centrifuges. Selecting the correct technology depends on the type of sludge and the amount being produced.
Geotextile bags are a simple, but effective technology that utilises purposely woven polymer fabrics as semi-permeable membranes. The fabrics contain small apertures that allow water – but not solids – to pass through.
The fabrics are stitched into cylindrical bags and the sludge is pumped inside and left to passively dewater for several days. Once a bag has finished dewatering, it is opened and the solids are removed, making space for a new round of filling.
Geotextile fabric tube. Source: ZebraTube
Geotextile bags are an excellent low-cost option for sludge dewatering, particularly for sites in dry environments with enough space. The filling process is easy for operators to perform, and there are no complex moving parts, keeping maintenance requirements to a minimum.
The bags consume very little power and can be stacked to enable efficient use of space; however, the final solids concentration (level of dewatering) is slightly lower than other dewatering technologies.
Belt and Plate Filter Presses
Belt and plate filter presses are mechanical machines that physically squeeze the liquid out of sludge.
Belt presses use conveyor belts to pressurise the sludge, trapping solids on the belt and forcing the liquid to drain away.
Belt filter press. Source: Wikiwayman at English Wikipedia, CC BY-SA 3.0, Wikimedia Commons
Plate presses, by contrast, are hydraulically or pneumatically driven and use pressurised plates to force the liquid into draining ports, leaving behind dewatered solids.
Both types of filter presses are commonly used in many industries; and can treat a wide range of sludge types to produce a highly concentrated solid cake.
Filter presses are expensive pieces of equipment; and maintenance requirements tend to be high due to the filter press’s complex mechanical nature.
Centrifuges exploit the different specific gravities (densities) of the solid and liquid components within a sludge. A rotating chamber forces denser solids to the outer rim of the chamber, while the less dense liquid flows in the opposite direction towards the chamber’s centre.
Centrifuges are effective for most types of sludge and can operate continuously with minimal operator intervention. The dewatering chamber is enclosed, which minimises spillage and odours, and the solid cake produced is highly dewatered.
Large centrifuges are expensive to build and operate, and the driving motors consume a significant amount of power. As such, they are used predominantly for smaller dewatering applications.
An industrial-scale centrifuge. Source: Wikimedia Commons
Selecting the correct sludge dewatering technology can significantly enhance a water treatment plant, by reducing sludge handling costs and recovering additional treated water.