A further advantage of high molecular weight flocculants is their propensity for dose efficient performance. The effect of increasing molecular weight on dewatering of sewage sludge. Sewage Sludge Pre-treatment Sewage sludge must be dewatered to facilitate economic disposal. Dewatered sludge decreases the cost of transportation to landfill, or if the sludge is to be incinerated the removal of water combustion. Ultra high molecular weight cationic flocculants are increasingly used for pre-treatment of sewage sludge prior to dewatering because they are particularly effective in the thickening of surplus activated sludges and centrifuge applications.
Centrifuges exert a very large shearing force on flocs as they enter the centrifuge bowl.
The resilience of flocs formed with ultra high molecular weight flocculants, readily explain the usefulness of such flocculants for centrifuge applications. Figure 1 illustrates the performance benefit of higher molecular weight flocculants using a laboratory 'free drainage' test. In the Bayer process, crushed bauxite is digested in concentrated sodium hydroxide in order to dissolve and extract aluminium. An insoluble residue known as red mud is produced, which is composed primarily of iron oxides, quartz, sodium aluminosilicates, calcium carbonate, calcium aluminate and titanium dioxide. To recover the aluminium, the solid red mud must be removed from the liquor.