Filtration operations are capable of handling suspensions of varying characteristics ranging from granular, incompressible, free-filtering materials to slime-like compositions, as well as finely divided colloidal suspensions in which the cakes are incompressible. These latter materials tend to contaminate or foul the filter medium. The interaction between the particles in suspension and the filter medium determines to a large extent the specific mechanisms responsible for filtration. In practice, cake filtration is used more often than filter-medium filtration. Upon achieving a certain thickness, the cake must be removed from the medium. This can be accomplished by the use of various mechanical devices or by reversing the flow of filtrate back through the medium (hence, the name backflushing). To prevent the formation of muddy filtrate at the beginning of the subsequent clarification - clarifiers are designed to efficiently remove undissolved substances from wastewater; removal is dependent upon density differences and is often enhanced by chemical means. Clarifiers are tank-like structures that may be either circular or rectangular in shape. When wastewaters enter these treatment areas denser undissolved substances settle out, others rise to the surface. A scraper (rake) moves across the bottom of the clarifier; settled matter (sludge) is moved to a collection area. A skimmer moves across the water's surface collecting floating material.
filtration cycle, a thin layer of residual particles is sometimes deposited onto the filter medium. For the same reason, the filtration cycle is initiated with a low, but gradually increasing pressure gradient at an approximately constant flowrate. The process is then operated at a constant pressure gradient while experiencing a gradual decrease in process rate.
The structure of the cake formed and, consequently, its resistance to liquid flow depends on the properties of the solid particles and the liquid phase suspension, as well as on the conditions of filtration. Cake structure is first established by hydrodynamic factors (cake porosity, mean particle size, size distribution, and particle specific surface area and sphericity). It is also strongly influenced by some factors that can conditionally be denoted as physicochemical. These factors are:
1. the rate of coagulation or peptization of solid particles,
2. the presence of tar and colloidal impurities clogging the pores,
3. the influence of electrokinetic potentials at the interphase in the presence of ions, which decreases the effective pore cross section, and
4. the presence of solvate shells on the solid particles (this action is manifested at particle contact during cake formation).
Due to the combining effects of hydrodynamic and physicochemical factors, the study of cake structure and resistance is extremely complex, and any mathematical description based on theoretical considerations is at best only descriptive. The influence of physicochemical factors is closely related to surface phenomena at the solid-liquid boundary. It is especially manifested by the presence of small particles in the suspension. Large particle sizes result in an increase in the relative influence of hydrodynamic factors, while smaller sizes contribute to a more dramatic influence from physicochemical factors. No reliable methods exist to predict when the influence of physicochemical factors may be neglected. However, as a general rule, for rough evaluations their influence may be assumed to be most pronounced in the particle size range of 15-20 ftm.
In specifying and designing filtration equipment, attention must be given to those methods that minimize high cake resistance. This resistance is responsible for losses in filtration capacity, which in turn impact on operating time and removal efficiency. One option for achieving a required filtration capacity is the use of a large number of filter modules. Increasing the physical size of equipment is feasible only within certain limitations as dictated by design considerations, allowable operating conditions, and economic constraints.
A more flexible option from an operational viewpoint is the implementation of process-oriented enhancements that intensify particle separation. This can be achieved by two different methods. In the first method, the suspension to be separated is pretreated to obtain a cake with minimal resistance. This involves the addition of filter aids, flocculants or electrolytes to the suspension. In the second method, the period during which suspensions are formed provides the opportunity to alter suspension properties or conditions that are more favorable to low-resistance cakes. For example, employing pure initial substances or performing a prefiltration operation under milder conditions tends to minimize the formation of tar and colloids. Similar results may be achieved through temperature control, by limiting the duration of certain operations immediately before filtering such as crystallization, or by controlling the rates and sequence of adding reagents.
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