Sweep floe

Freshly formed amorphous Al(OH)3(s) sweep floes have large surface areas which is beneficial for a rapid adsorption of soluble organic compounds and trapping of colloidal particles. Finally, these floes are removed easily from aqueous medium by sedimentation or flotation. 

Add coagulant usually alum in the sweep floe concentration of 20 to 50 mg/L adjust pH to 6 to... 

Colloids present in the water to be clarified, may serve as condensation nuclei for the precipitation process or may become trapped in the precipitate as it settles. For this reason the process is also known as "sweep floe" coagulation. 

Formation of a precipitate that captures particles in a sweep floe... 

The addition and precipitation of multivalent metal salts like alum or ferric chloride can create a dense and readily separable floe that can enmesh and capture particles in a process known as a sweep floe. In addition to physically ensnaring particles, iron and aluminum hydroxide floes also present a large and active siuface area, which is effective in removing certain dissolved chemicals. 

Hgure 10.9 shows a schematic of a wastewater treatment process that is used specifically to treat CMP wastewater. The process is designed and operated primarily to achieve copper removal but also provides a high level of CMP parlicle removal. The lime addition provides a high concentration of divalent cation, Ca, which destabilizes particles and forms a sweep floe that aids particle removal. 

It has also been reported that aggregates produced under sweep floe conditions were more compressible than for charge neutralization conditions, resulting in compaction when the membrane filtration system was pressurized (Antelmi et al., 2001 Cabane et al., 2002). Lee et al. (2000) also reported that the specific resistance was lower with charge neutralization than with sweep floe, attributed to the formation of a less compressible and more porous cake. Judd and HUlis (2001) suggested that floes need to reach a certain... 

A fourth mechanism is called sweep flocculation. It is used primarily in very low soflds systems such as raw water clarification. Addition of an inorganic salt produces a metal hydroxide precipitate which entrains fine particles of other suspended soflds as it settles. A variation of this mechanism is sometimes employed for suspensions that do not respond to polymeric flocculants. A soHd material such as clay is deUberately added to the suspension and then flocculated with a high molecular weight polymer. The original suspended matter is entrained in the clay floes formed by the bridging mechanism and is removed with the clay. 

In oscillatory measurements one carries out two sets of experiments (i) Strain sweep measurements. In this case, the oscillation is fixed (say at 1 Hz) and the viscoelastic parameters are measured as a function of strain amplitude. This allows one to obtain the linear viscoelastic region. In this region all moduli are independent of the appUed strain amplitude and become only a function of time or frequency. This is illustrated in Fig. 3.50, which shows a schematic representation of the variation of G, G and G" with strain amplitude (at a fixed frequency). It can be seen from Fig. 3.49 that G, G and G" remain virtually constant up to a critical strain value, y . This region is the linear viscoelastic region. Above y, G and G start to fall, whereas G" starts to increase. This is the nonlinear region. The value of y may be identified with the minimum strain above which the "structure of the suspension starts to break down (for example breakdown of floes into smaller units and/or breakdown of a structuring agent). 

---