Coagulation particle removal

Filtration is analogous to coagulation in many respects. This is illustrated by juxtaposing the basic kinetic equations on particle removal ... 

With hydraulic residence times ranging from months to years, lakes are efficient settling basins for particles. Lacustrine sediments are sinks for nutrients and for pollutants such as heavy metals and synthetic organic compounds that associate with settling particles. Natural aggregation (coagulation) increases particle sizes and thus particle settling velocities (Eq. 7.1) and accelerates particle removal to the bottom sediments and decreases particle concentrations in the water column. 

It has been observed for more than 30 years that preozonation ahead of particle removal units can improve the efficiency significantly, can induce a lower coagulant demand or allow higher flow rates, e. g. in deep-bed filtration. Ozone gas is added either before or together with the coagulant (ferric or aluminum salts or cationic polymers) at rather low dosages of 0.5-2 mg L l. The terms microflocculation or ozone-induced particle destabilization are used in practice (Jekel, 1998). 

The vector c in Eq. (5) describes the creation in, and/or removal of latex particles from, the system. The creation component may arise from in situ particle formation in Interval I) or from the flow behavior in a continuous stirred-tank reactor system (CSTR) with an arbitrary number of reaction vessels. Particle removal terms may be required if coagulation occurs or in the context of CSTR operation. 

Fiiedlander (1960), Hunt (1980), Filella and Buffle (1993), and others have analyzed the effect of colloid agglomeration by coagulation and particle removal by settling on the shape of the particle size distribution function as expressed by equation 4. The predictions of model calculations are often consistent with the range of values of /3 observed in aquatic systems. 

Figure 14.23. Variables that typically determine the efficiency of coagulation and filtration in natural waters and in water and waste treatment systems, (a) How the variables determine the coagulation efficiency, (b) Marked increase in filtration rate can be achieved by counterbalancing a reduction in contact opportunities by chemically improving the contact efficiency, with similar efficiency in particle removal.

In this chapter, mechanisms of particle removal are limited to coagulation and sedimentation. Predictions of size distributions are obtained that are in reasonable agreement with measured size distributions from oceanic waters and digested sewage sludge. Sensitivity of the predictions to fluid turbulence and fluid density presents a plausible explanation for zones of higher particle concentration observed in the oceanic water column. The analysis does not include zooplankton fecal pellet production, particle breakup, or dissolution, nor does it directly incorporate biological productivity. 

Removal or loss of a particle of a given size from a volume of fluid is assumed to occur through only two physical processes sedimentation and coagulation. Sedimentation removes particles from the volume of fluid, while coagulation transfers many smaller particles into fewer larger particles within the fluid volume. 

Powdered activated carbon may also be coupled to ultrafiltration (UF). This hybrid system, referred to in the literature as the PAC/UF process, incorporates the adsorption capabilities of activated carbon and the microorganism and particle removal ability of the UF membranes [71]. Combined with coagulation as a pre-treatment, tlie coupling has been successfully applied to the treatment of domestic wastewater [72]. Mdtivier-Pignon et al. [73] have proposed a combination of ultrafiltration with activated carbon cloth to treat complex aqueous solutions with suspended solids, colloids and dissolved organic molecules, like highly colored wastewaters from ink manufacture. 

Colloidal particles frequently must be removed from a dispersing medium, as in the removal of smoke from smokestacks or butterfat from milk. Because colloidal particles are too small to be separated by simple filtration, they must be enlarged in a process called coagulation. The coagulated particles can then be separated by filtration or merely allowed to settle out of the dispersing medium. 

Table 10.4 Reported coagulant doses used in silica particle removal... 

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