Floc and Flocculation Processes
Flocculation: A Discussion of the Technical Process and Optimum Requirements of this most simple physical mechanism which forms a part of the most frequently used water treatment processes.
The physical separation of the solid phase from the water in order to improve the quality of water by flocculation (the formation of particles known as "flocs" which settle or in flotation processes rise), occurs in both organically contaminated and inorganically contaminated raw water and is the simplest, yet one of the very most important, water treatment process.
Flocculation may occur naturally in liquids. However, when referred to in the context of water treatment processes includes both the physical pollutants present in the raw water and the addition of additional solids by the precipitation of dissolved pollutants. These pollutants are most often organic in nature, such as in sewage (wastewater) treatment.
The precipitation process may also be chemical as in potable water treatment but both produce a solid phase that requires separation, however, the purpose of creating the floc is always to bind together the contaminant for ease of subsequent removal.
To achieve the separation a number of different processes are used and these include sedimentation, filtration (surface and depth) and flotation. The required degree of liquid/solid separation is achieved by choice of the flocculant to enhance key characteristics within the flocculated solids.
These are specific to the separation process chosen and are described as follows:-
- sedimentation (settlement) is aided by large compact flocs which have high densities and low amounts of drag
- flotation which requires small low density flocs with relatively open structures which aid contact between the flocs and the bubbles which will remain caught within the floc structure for long enough to float them to the surface for removal
- filtration is best achieved by large, strong porous flocs with low surface charges
- sludge thickening works best with compact flocs and a high solid to liquid ratio is clearly preferred to aid water content reduction during the process, again low charge surfaces assist.
It is also worthwhile to consider the character of flocculated suspensions by splitting them into four groups by considering size, shape, strength and charge.
Size is the most important feature of a floc as size relates closely to removal efficiency. For maximum efficiency of solids removal it is important to avoid as far as possible the generation of fine particles (less than 10 um) where removal efficiency starts to deteriorate and the charge of the surfaces becomes increasingly important.
Fine particles are created by floc breakage which occurs when the hydrodynamic stress applied at the surface of the floc particle is sufficient to overcome the bonding strength of the connections within the flocs. Two specific mechanisms have been identified and are commonly classified as surface erosion or large scale fragmentation.
Surface erosion relates to the stripping of small particles from the extremities of the floc's surface resulting in an increase in the primary particle concentration.
In contrast, large scale fragmentation relates to the floc being cleaved into a number of smaller flocs without an increase in primary particle concentration.
The strength of a flocculated suspension is usually determined by applying a known shear rate to the systems and measuring the response. A number of techniques have been tried in the literature but the majority of studies use an impeller based system connected to a particle size instrument.
Comparison of the two mechanisms of floc fragmentation suggests that organic flocs are more prone to erosion than large scale fragmentation. The explanation of this is partly due to the difference in the shape of the flocs formed under the different conditions.
The factors we have described in this article form the introductory principles from which the science of flocculation has been developed to enhance this most simple and extremely useful of water treatment processes.
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