Filtration Processes

Filtration is often employed in wastewater treatment, with or without prior treatments by coagulation-flocculation and sedimentation, for removal of flocs (or bioflocs) from primary and secondary wastewater treatment processes, solids remaining in effluents from primary and secondary wastewater treatment processes, and precipitates from physicochemical treatment of phosphate from the advanced wastewater treatment stage. Earlier applications of filtration for wastewater treatment borrowed heavily from design and operational experience with potable water treatment; the wastewater treatment adaptation has been perfected ever since. Filtration of sludge on rotary vacuum filters for dewatering is a common application of filtration in wastewater. Other applications of filtration are dewatering of digested sludge on sand beds and wastewater treatment through deep granular filters (sand, dual-medium, and multimedia) after most of the solids are removed.

Wastewater treatment using filtration usually is designed to get rid of microorganisms, reduce turbidity and color, remove odors, reduce the amount of iron, and remove most other solid particles that remain in the water. Water is sometimes filtered through activated carbon particles (see also the section "Adsorption," below), which removes refractory organic particles.

Filtration in wastewater treatment is a unit operation that mirrors the natural system that treats impaired waters; groundwater is filtered through layers of sand and/or soil (underground strata) and potable quality of water may be obtained in wells deep beneath the surface. It is no surprise that filtration media used in wastewater treatment are sand, crushed anthractite coal, diatomaceous earth, perlite, and powdered or granular activated carbon. Combinations of different media are commonly used; a dual media may consist of coal over sand and a multimedia filter may be layered with garnet sand, silica sand, and coal.

Filtration involves complex mechanisms because these mechanisms depend on the physicochemical characteristics of the suspension and the filter medium, the rate of filtration, and the composition of wastewater. Deep granular filters consisting of 46 to 76 cm (18 to 30 in) of filter medium are supported on an underdrainage system; the filter may or may not be open to the atmosphere. A closed filter usually involves pressure and is called a pressure filter; open filterss are termed gravity filters. Precoat filters comprise a number of porous septa in a filter housing connected to a collection manifold. The septa hold a thin layer of filter medium that is deposited hydraulically on the outside of the septa at the beginning of the filtering cycle. The filter septa are either open to the atmosphere while they are submerged in a tank or totally enclosed in a pressure tank. They are sometimes called vacuum filters. The characteristic designs of different types of filters contribute to the mechanisms of filtration in the systems. For example, precoats remove impurities at the surface through a formation of a filter cake made of the impurities; therefore, it is a mechanism of straining water. With deep granular filters with coarse materials, on the other hand, removal of particulate materials from wastewater is primarily within the filter bed as wastewater wiggles through the spaces formed by the filter medium. This type of filtration, as seen in some deep granular filters, is called depth filtration. However, actual removal of impurities involves several mechanisms: interstitial straining, gravitational settling, diffusion, interception, hydrodynamic interactions, and attachment/adsorption due to electrostatic interactions, chemical bonding, or specific adsorption, which may be affected by the type and dosage of the coagu lant used prior to filtration. Removal results in many deep granular filters may be dominated by a combination of surface cake removal and depth removal mechanisms.

The flow of wastewater through a filter at the rates commonly used in wastewater treatment is hydraulically the same as flow through underground strata in a natural system of groundwater treatment. The rate of flow through a filter may be expressed as follows (Equation 3.7):

Driving Force

Filter Resistance where the driving force represents the pressure drop (head loss) across the filter.

Historically, single-phase wastewater flow through porous media has been modeled using either the linear Darcy's law or some empirical nonlinear relationship between the pressure gradient and the Darcy velocity as an approximation to momentum conservation. For example, head loss through granular materials in the laminar range may be described by Darcy's law (Equation 3.8):

where Kp is coefficient of permeability, hf is head loss, and L is depth (or length) of filter. The coefficient of permeability can be determined experimentally. Theoretical relationships have been developed to calculate coefficient of permeability from measured physical characteristics. Nonlinear relationships between the head loss and the Darcy velocity v have been used to illustrate important variables attributing to the head loss such as the following (Equation 3.9):

dp p where J is a constant of approximately 6 in a laminar flow, e is the porosity ratio of the filter bed, dp is the diameter of the particles, and as is the shape factor. Values of shape factor range from 6.0 for spherical particles to 8.5 for granular materials such as anthracite coal.

Filter effluent quality patterns vary with the characteristics of the filter medium, the solids, the water chemistry, and operating conditions of the filter. For precoat filters, the effluent quality is usually excellent, particularly after the filter cake is formed, which acts as an additional filtering medium. Removal of impurities by granular filter usually occurs within the cavities and interstices of the filter medium. Formation of cakes in these filters may contribute to the filtering of solids. Because the filtering occurs within the granular bed, the burden of removal shifts gradually from the upper layers to the lower layers; it is observed that a granular filter usually starts with high removal efficiency but degrades steadily over time as layers of the filter medium progressively are saturated with pollutants.

Filtration in wastewater treatment is a unit operation that frustrates theoreticians of wastewater engineering design because of the inherited variability in the characteristics of the feed stream to be filtered. This is not only a testament to the diversity of wastewater sources but also the fact that the filtration process is often employed as a supplemental process to the primary or secondary wastewater treatment processes. Thus, the filtration process is subject to changes in the degree of coagulation-flocculation of colloidal dispersion in the settling basins or tanks because these changes lead to variability in particle size and distribution. It is often necessary to conduct pilot plant studies to ensure that the filter configuration selected for a given task will perform adequately for the treatment objective.

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