1.1 Mechanisms of Chemical Conditioning
There are two main mechanisms involved in the chemical conditioning of sludge. One is neutralisation of particle charge and the other is particle bridging. These two mechanisms are distinguished by the terms coagulation and flocculation, respectively (Dentel, 2001). Lue-Hing (1998) pointed out that such division of these mechanisms could be an oversimplification and therefore a combination of both mechanisms may occur simultaneously or sequentially.
1.1.1 Charge neutralisation
The sludge is visualised as a concentrated suspension of colloidal materials stabilised by the electrostatic repulsion of their negatively charged surfaces, hence incorporating large amounts of water in their structure (Lue-Hing, 1998; Dentel, 2001; Dentel, 2010). According to Mikkelsen and Keiding (2002), the deterioration of sludge dewaterability is contributed by the presence of surface charges due to the biological nature of sludge and also due to the existence of weakly charged extracellular polymeric substances (EPS). Hence the concept of charge neutralisation has been considered as an important aspect of chemical conditioning.
During the neutralisation or destabilisation of particle charges, the layers of electrical charge surrounding the suspended particles are discharged by the addition of cationic conditioning chemicals. The conditioning of biological sludges with inorganic chemicals is predominantly related to neutralisation mechanism (Lue-Hing, 1998). The neutralisation mechanism is commonly described as shown in Figure 1?6. The charge neutralisation mechanism is presented as a ‘patch’ model wherein the localised patches of opposite charge on opposing particles can eliminate the repulsion, while fostering attraction amongst the matching patches. This effect results in the formation of flocs, which settle and compact well (Lue-Hing, 1998).
Figure 1?6: Charge neutralisation (patch) model (Dentel, 2001)
Bridging mechanisms shown in Figure 1?7 is the simultaneous attachment of polymer molecules, which dissolve as a long chain in the sludge, to two or more sludge particles. In this way, polymers bridge the gaps between particles and draw them together in a lattice structure to form floc. The higher the polymer molecular weight, charge density, and geometric length the greater its effectiveness (Lue-Hing, 1998). The high molecular weight polymers provide higher floc strength, permitting cake structure with a less compressibility, which is beneficial in high shear operations such as pumping of the conditioned sludge or centrifugal dewatering.