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 centr