Course: BFM 203 Water Treatment and Control

Lecture

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Types of Filtration and Filters

Conventional filtration is preceded by coagulation, flocculation, and sedimentation. Direct filtration omits the sedimentation process; flocculation facilities are reduced in size or omitted. A flash mixing step is required in both conventional and direct filtration processes.

Types of filters include

- Gravity filtration (sand, dual media, and mixed media)
- Pressure filtration (mixed media)
- Diatomaceous earth filtration (Precoat)
- Slow sand filtration
- Poly tube and ceramic filtration

Poly tubes and extruded ceramic filters are becoming more popular and are able to consistently produce drinking water with very little turbidity.

The following illustration is an example of a multimedia setup. The density of each layer increases from top to bottom so that natural stratification will occur after the backwash process.

Filtration is both a chemical and a physical process. Removal of turbidity is based on the 1) chemical characteristics of the water being treated, 2) nature of suspended solids, 3) types and degree of pretreatment, and 4) filter type and operation.The level of success in the previous processes directly impacts filtration efficiency. Ineffective coagulation, flocculation, and sedimentation reduces filter run times and forces more frequent backwash cycles. The following images are top views of a multimedia filter. (The white spots you see are likely a type of scum or foam that comes from coagulant chemicals and colloidal floc. This is not necessarily bad, but it could indicate overfeeding of coagulant chemicals. This should be evaluted with other factors like filter run times.)