Page 183 -

8.2 CHAPTER EIGHT

MECHANISM OF FIL TRA TION
Removing suspended solids by high-rate granular media filtration is a complex process
involving a number of phenomena. Attempts to develop theories that quantitatively pre-
dict solids removal performance with sufficient precision and versatility to be of use in
practical filter design have met with relatively little success. Consequently, filter media
selection is often an empirical process. Pilot investigations are common tools for assess-
ing the performance of a particular filter design (see Chapter 28).
In current high-rate granular media filtration techniques, solids removal occurs pri-
marily as a two-step process (Cleasby, 1972). During the initial transport step, particles
are moved to the surfaces of media grains or previously captured floc. Transport is be-
lieved to be caused largely by hydrodynamic forces, with contact occurring as stream lines
converge in pore restrictions. The second step is particles' attachment to either grain or
floc surfaces. Electrokinetic and molecular forces are probably responsible for the adher-
ence of particles on surfaces within the bed (O'Melia and Crapps, 1964; Craft, 1966;
O'Melia and Stumm, 1967). Physical straining through the surface layer of solids and bi-
ological growth (schmutzdecke) is the principal filtration mechanism of a slow sand fil-
ter, but it is generally a minor means of solids removal in high-rate granular media
filters.

DESIGN CONSIDERATIONS

A number of interrelated components are involved in the overall design of a high-rate
granular media filtration system:

• Regulatory requirements
• Pretreatment systems
• Filter media
• Filtration rates
• Depth of the filter box
• Mode of operational control
• Filter washing system
• Filter arrangements
• Underdrain system
• Filter performance monitoring
• Auxiliaries
These components are discussed in detail in the following sections.

Regulatory Requirements
The primary regulation related to the design of filters is control of effluent turbidity. The
ESWTRs establish a maximum contaminant level (MCL) on combined filter effluent
(CFE) at 0.3 ntu in 95% of samples collected in a month, for direct and conventional fil-
tration systems (employing a clarification process). The CFE turbidity limit for slow sand
and diatomaceous earth filtration is 1.0 ntu. Additionally, individual filter turbidity must